Category: invasive species

Plant invasions grow everywhere

invasion of Chinese privet (Ligustrum sinense)

A decade ago I posted a blog reporting that 39% of forests surveyed under the Forest Inventory and Analysis (FIA) system were invaded by one or more invasive plants (Oswald et al. 2015). By regions, Hawai`i had the highest invasion intensity – 70%. The second highest density was in the eastern forests – 46%. Forests in the West ranked third, with 11% of plots containing at least one of the monitored invasive plant species. Finally, forests in Alaska and the Intermountain regions both had 6% of plots invaded.

I rejoice that US Forest Service scientists have continued to analyze their data on plant invasions. Analysis of the most recent data shows alarming increases in invasions everywhere since 2015. However, the scientists could not determine a nation-wide percentage because many areas in the West had not yet been surveyed anew. They did determine that the number of inventory plots containing invasive plant species rose in 58.9% of surveyed counties. Furthermore, in 73.2% of the counties the plots experienced an increase in species richness of invading plant species. While increases were observed in all regions, they were greater in the East than in the West — and in the USFS Southern region compared to the Northern region. Specifically, the proportion of forest plots in the East (USFS Southern and Northern regions) invaded has risen from 46% to 52.8%. In the Rocky Mountains they rose from 6% to 11%. In Hawai`i plots having invasive plants grew from 70% to 83.2%. Surveys in the Pacific Coast states have not yet been completed so this region is not included in the analysis (Potter et al. 2026). It is not clear to me how the current boundaries of the western regions – which are based on Bailey’s ecosystem boundaries relate to the 2015 boundaries, which were based on USFS official regions. Hawai`i is clearly the same.  

Porter et al. (2026) concluded that in the forests of the East plant invasions are so extensive that elimination of their impacts is practically impossible.  Their spread to new areas is unhindered now and, I would add, is likely to remain so without heroic counter measures.

Forests in the East have a greater mean richness of invasive plant species than do western forests. In particular, there is a profusion of shrubs and vines as well as trees. The West has a greater diversity of invasive forbs. The diversity of invasive grasses is high in both regions.

kudzu (Pueraria montana) spreading from edge into forest in Virginia; photo by F.T. Campbell

Potter at al. (2026) worry that the apparently lower level of plant invasions in the West might be an artifact of a higher proportion of plant species being at an earlier stage of invasion. That is, the species have not yet established sufficiently widely to be classified as invasive.

thicket of guava (Psidium cattleianum ) replacing ohia killed by ROD; Hawai`i Island; photo by F.T. Campbell

Of course, the situation in Hawai`i is much worse. Another, more detailed, discussion of invasive plant species in Hawai`i pointed out that relying on data reflecting canopy-level trees obscures the real picture. While “only” 29% of large trees across the Islands are non-native, about two-thirds of saplings and seedlings are. Potter et al. (2023) expected that plant succession will result in non-native tree species taking over the canopy. This likelihood exists regardless of the impact of rapid ‘ohi’a death since ‘ohi’a lehua (Metrosideros polymorpha) is not reproducing even when seed sources are plentiful and people remove invasive forbs and grasses Potter et al. (2023).

The nation-wide analysis of Potter et al. (2026) does not include forests on U.S. Caribbean islands, i.e., Puerto Rico and the Virgin Islands. See here for a description of this situation. In summary, 33 of 57 (58%) of non-native tree species tallied by FIA surveyors are actual or potential high-impact bioinvaders. Furthermore, 21 (38%) of the non-native species occurred on at least 2% of the FIA plots – far above the seven species fitting this description in the continental U.S.

As these sources, and those with a broader perspective, demonstrate that we should not ignore invasions of our forests by non-native plants. These species erode forest productivity and provision of the full range of ecosystem services, hinder shifting (?) forest uses, and degrade biodiversity and habitat.

These invasions also impose extensive financial costs from lost or damaged resources (Potter et al. ( 2022). Potter et al. (2026) note that these negative outcomes depend on interactions between the traits of the non-native plants and the biomes being invaded. These impacts are greatly exacerbated in Hawai`i because more than 95% of native species on the Islands are endemic. This includes 67% of the large trees still present in the forests. As Potter et al. (2023) point out, extirpation of any of these species is a global loss.

ʻōhiʻa lehua (Metrosideros polymorpha); photo by F.T. Campbell

Data issues

Potter et al. (2026) note that in the Northern region only about 20% of plots were surveyed for invasive plants. They state that these difference in sampling intensity does not affect statistical analyses across broad scales.

The regional lists of invasive plants were developed by experts. They include those species thought at the time to be most damaging. Of course, there are other non-native plant species that might be present – and some might prove to be invasive over time (Potter et al. 2026). I have been unable to determine whether the regional lists are updated periodically. Because of this structure of the FIA system, these surveys can assess only spread of already-established species. It is not suitable for early detection of new species entering the forest.

For all these reasons, the analyses in Porter et al. (2026) probably underestimate the total abundance of non-native plant species in U.S. forests. Indeed, the time lag between introduction or even identification of invasive species and their eventual ecological and economic impact obscures their full impact. This ever-increasing invasion debt probably contributes to decisions not to implement effective countermeasures.  

Recommendations

How do we set priorities for responding to nearly unmanageable situations? We sharpen our focus on the most damaging pathways of introduction, the most vulnerable regions, and the most at-risk species.

The high-risk pathways are imports of plants for planting and wood – including but not limited to crates, pallets, and other forms of packaging.

Vulnerable regions start with the Hawaiian Islands, Puerto Rico, and the Virgin Islands; and include many biodiversity-rich areas on the continent. We should enhance monitoring of these vulnerable regions by federal, state, and tribal agencies, conservation organizations, citizen scientists, and others. Surveys must report all non-native plant present, not just those already known to be invasive. These data will improve detection of new species and better inform us about factors affecting species’ spread.

Also, I support Potter et al.’s (2026) emphasis on the wildland-urban interface as an area of high human-environment conflict.These include, but are not limited to, plant invasions. The authors point out that we need new policy, management, and scientific tools to address threats in these vulnerable and too-often ignored social and ecological zones.

This increase in available information must be paired with management of the factors that facilitate invasion. Some of these are associated with ecosystems. But the key target must be plant species being brought into the region by people for various purposes. This is often for ornamental horticulture.

lesser celandine (Ficaria verna) dominating herb layer in a Virginia forest; photo by F.T. Campbell

We must ask state legislatures and Congress to empower  regulatory agencies – e.g., their state departments of agriculture and USDA’s Animal and Plant Health Inspection Service – to be far more more assertive and pro-active. For example, they must give higher priority to the full range of ecological and economic impacts of invading plants, not just damage to agriculture.

Evans et al. (2024) urged prioritizing for state regulation those species in the ornamental trade that are projected to remain or become abundant under evolving climate conditions. Beaury et al. (2023) called for regulating the nursery trade at the national level – reflecting the scope of sales.   

SOURCES

Beaury, E.M., J.M. Allen, A.E. Evans, M.E. Fertakos, W.G. Pfadenhauer, B.A. Bradley. 2023. Horticulture could facilitate invasive plant range infilling and range expansion with climate change. BioScience 2023 0 1-8 https://doi.org/10.1093/biosci/biad069

Evans, A.E., C.S. Jarnevich, E.M. Beaury, P.S. Engelstad, N.B. Teich, J.M. LaRoe, B.A. Bradley. 2024. Shifting hotspots: Climate change projected to drive contractions and expansions of invasive plant abundance habitats. Diversity and Distributions 2024;30:4154

Potter, K.M., C. Giardina, R.F. Hughes, S. Cordell, O. Kuegler, A. Koch, E. Yuen. 2023. How invaded are Hawaiian forests? Non-native understory tree dominance signals potential canopy replacement.  Landsc Ecol 2023 https://doi.org/10.1007/s10980-023-01662-6  

Potter, K.M., B.V. Iannone III, K.H. Riitters, Q. Guo, K. Pandit, C.M. Oswalt. 2026. US Forests are Increasingly Invaded by Problematic Non-Native Plants. Forest Ecology and Management 599 (2026) 123281

Potter K.M., K.H. Riitters, and Q Guo.  2022. Non-native tree regeneration indicates regional and national risks from current invasions. Frontiers in Forests & Global Change Front. For. Glob. Change 5:966407. doi: 10.3389/ffgc.2022.966407  

Potter, K.M., K.H. Riitters, B.V. Iannone, III, Q. Guo and S. Fei. 2024. Forest plant invasions in eastern US: evidence of invasion debt in the wildland‑urban interface. Landsc Ecol (2024) 39:207   https://doi.org/10.1007/s10980-024-01985-y

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

Or

https://fadingforests.org

Invasive plants threaten integrity of eastern U.S. forests

garlic mustard (Alliaria petiolata); photo by Katja Schulz via Wikimedia

I welcome a recent series of studies documenting the extent of plant invasions in forests of the eastern United States and the socio-economic conditions that contribute to a state of affairs increasingly recognized as a crisis. I wish, however, that the authors had devoted more attention to the role of deliberate planting of non-native species and the resulting propagule pressure.

I summarize here findings of two studies written by largely the same scientists and relying on the same underlying data: surveys of forest plots conducted under the Forest Inventory and Analysis (FIA) program. In this blog, if focus on the extent of invasive plant presence in the forests of the eastern United States. In an accompanying blog I will summarize the status of plant invasions in forests nation-wide.

As I have noted in earlier blogs, link a decade ago one or more invasive plant species had already invaded 46% of FIA plots in the eastern U.S. (Oswald et al. 2015). This situation has worsened. Updated data show that 52.8% of these plots contain invasive plants. In the USFS Southern Region, invasive plants have been documented on 55.3 million ha. In the Northern Region, they are found on 36.9 million ha. (Only ~20% of FIA plots in the Northern Region were surveyed for invasive plants.) In some counties of the 37 states constituting these two USFS regions, 80% of inventoried forest plots contain invasive plants. Areas with lower levels of invasion are found in parts of New England, the Great Lakes states, southern Appalachians, southeastern coastal plain, and western Texas and Oklahoma (Potter et al. 2026). Spread of these bioinvaders is largely unchecked – either throughout the East or “just” in the South. In any case, the extent and intensity of these invasions are so great that their complete removal – or elimination of their impacts – is “practically impossible” (Potter et al., 2024; Potter et al. 2026). [It is not clear whether the scientists mean “nearly” or “in practice”. Or that this difference is important.]

[In comparison, in the West less than 30% of FIA plots are invaded, on average. In Hawai`i, more than70% are (Potter et al. 2026).]

The scientists analyzing the FIA data warn that the extent and impact of plant invasions in eastern forests is undoubtedly worse than these data indicate. The records include only some of the non-native plant species present — those considered to be the worst invaders at the time regional lists were compiled – apparently in the first years of the 21st Century (Potter et al. 2026).

Japanese honeysuckle (Lonicera japonica) photo by Chuck Bargeron

The scientists emphasize the role of disturbance in promoting plant invasions. They cite various studies as well as the FIA data to document that forest edges facilitate non-native plant establishment and spread into forests. They stress various aspects of suburban development, including roads and other transportation corridors. It follows that invasion rates are highest in the “wildland-urban interface (WUI).” [The wildlife-urban interface is the zone of transition between unoccupied land and human development; the zone where structures meet or intermix with undeveloped land and its vegetation.] They worry that the WUI is growing faster than any other land use type in the country – and especially rapidly in the East. As a result, the scientists expect more and worse invasions in the future (Potter et al., 2024 and Potter et al. 2026).

I appreciate that they highlight the uniqueness of WUI ecosystems. Housing development in the WUI has numerous effects on natural ecosystems, including habitat modification and fragmentation followed by diffusion of the direct and indirect effects of anthropogenic activities into neighboring ecosystems at different scales. As regards specifically non-native plants, this transmission occurs through a combination of (1) human-driven disturbances to native ecosystems that promote plant invasion and (2) providing a source of non-native plant propagules in their yards and gardens. These plants can then spread into and establish in nearby ecosystems (in this case, forests). [I note that tree-killing arthropods and pathogens also can be introduced in the WUI.] (Scroll below “Archives” to “Categories”, click on “forest pests” and “wood packaging”.)

They also found that plant invasions are more strongly related to older, than more recent, land-cover changes. Survey plots that have been located in the WUI since 1990 or earlier had on average 2.6% more invasive plant cover and 0.33 more invasive plant species than those that were classified as being in the WUI in 2000 or 2010. Their explanation is that the WUI forests experienced decreased spatial integrity, increased forest-developed area edges, and falling proportions of forest in the surrounding landscapes. In addition, the human population in the vicinity might have grown. All these factors that would increase forest fragmentation and the plots’ susceptibility to invasion.

The other side of the coin is propagule pressure. Both Potter et al (2024) and Potter et al. (2026) note that the flora of residential landscapes – rural as well as suburban – is typically dominated by non-native plant species. Still, I think these studies downplay the impact of this ubiquity of non-native plants in all anthropogenic landscapes.

In discussing the higher invasion rates found in survey plots located in WUIs dating from the 1990s they made no mention of human activities that promote plant invasions. There are several. Plants growing in those older yards had one or two more decades to flower – and for their fruits and seeds to be transported into the forest by birds, wind, or water. Residents might have decided to beautify their neighborhood by planting shrubs or flowers in the woods. Maybe they succumbed to the temptation to dump yard waste in the woods – thinking it would be absorbed by “nature”. Since plant invasions take time to unfold, these additional years of human-mediated exposure are highly relevant. Another factor is that people who choose to live in wooded surroundings probably choose horticultural plants that thrive under such conditions – exactly those best able to establish beyond the property line.

Another opportunity to discuss these factors came from the discovery that plant invasion rates are higher in association with “interface” rather than “intermix” WUI forests. [“WUI interface forests” are those where settled areas abut wildlands. In “WUI intermix forests” the structures are scattered.] They speculate about reasons. Potter, et al. (2024) mention that invasions originating from older housing developments have had more time to establish (or at least to be detected) given the well-known lag associated with plant invasions.

I wish they had focused more on the probable difference in suburban development across time. While I was growing up in expanding suburbs in the 1950s, I observed that the earlier housing developments were either built on land that had been cleared to support agriculture or the builders cleared the forest to make construction easier and cheaper. More recently, wealthier buyers have sought residences on more wooded sites – so creating an “intermix” WUI. Potter et al. (2024) speculate that locations in the “interface” WUI are closer to high-density urbanization so have higher exposure to non-native plants. They do not discuss whether the “interface” WUIs are older, thus giving associated plantings longer years to proceed through the stages of bioinvasion.

burning bush (Euonymus alatus) invading a forest in Virginia; photo by F.T. Campbell

The Role of Deliberate Planting?

I recognize that these authors analyzed mountains of data. However, I wish they had incorporated the findings of numerous scientists who have analyzed the role of deliberate planting – especially ornamental horticulture – in facilitating introduction and spread of invasive plants. (Scroll below “Archives” to “Categories” and click on “invasive plants”. Also See Reichard and White 2001 and Mack 2000).

As I hope USFS scientists are aware, recent studies confirm the continuing role of ornamental horticulture in plant invasions. Kinlock et al. (2025) blog 440 found that more than 1,600 plant species sold by nursery and seed catalogs over 200 years had “naturalized” somewhere in the continental 48 states. They do not discuss what proportion of these species are truly damaging invaders. Fertakos and Bradley (2024) found that species were likely to establish if they were introduced to as few as eight locations. Beaury et al. (2024) found that half of 89 plant species recognized as invasive are sold in the same locations where they are invasive. Another 25 species are sold by one or more nurseries located in an area that is currently unsuitable for those species, but that will become more suitable for invasion as temperatures warm.

Potter et al. (2026) acknowledge that the ornamental plant trade is likely to continue introducing new plant species into U.S. forests. However, they recommend only updating the lists of invasive plants to be included in future surveys. Apparently these lists have not been updated since 2004.

Potter et al. (2024) go farther, urging efforts to encourage homeowners to plant more native and environmentally friendly private landscapes. They note that such advocacy is complicated by the fact that non-native – even invasive – species provide valued ecosystem and cultural services.

I add that the nursery industry and their customers enjoy enormous lobbying clout.

Many associations – native plant societies, regional or state invasive plant councils, etc. – are pursuing this approach. To research these efforts, visit the websites for the state native plant societies and the Southeast Exotic Pest Plant Council, Mid-Atlantic Invasive Plant Council, and Midwest Invasive Plant Network. These voluntary efforts have yielded some success. But they have not resulted in adequate protection for our ecosystems. Dr. Douglas Tallamy points out that even non-invasive, non-native plants disrupt food webs.

The insufficient attention to the role of the plant trade in articles intended to be comprehensive has crucially important impacts. As both Potter, et al. (2024) and Potter et al. (2026) affirm, determining which factors are most important in facilitating plant invasions of eastern American forests is the necessary foundation for identifying and implementing the most efficient and effective counter measures.

These scientists are employees of the U.S. Department of Agriculture. If departmental leadership interpret their studies as justifying inaction on regulating plant sales, USDA’s regulatory agencies will not respond. And we will continue failing to curtail introduction and spread of damaging plant invasions.

I agree with the authors on the need for enhanced monitoring and management of WUI zones in the East to detect new species or new locations of invasion and the need to develop better tools for these purposes. However, I ask all stakeholders to follow Evans et al. (2024), who urge prioritizing for state regulation those species in the ornamental trade that are projected to remain or become abundant under evolving climate conditions. Or, more aggressively, follow Beaury et al. (2023)’s call for regulating the nursery trade in a manner consistent with the scope of the horticultural trade at the national level. That would require legislation, since the Federal Noxious Weed Act does not currently address long-established, widespread species. Beaury et al. (2023) also note that existing state restrictions are outdated, tend to include only a few weeds that plague agriculture rather than those that invade natural systems, and are irregularly enforced.

orchids in Everglades National Park; photo by F.T. Campbell

I conclude by agreeing with the scientists that managing the disturbance component of plant invasions points to protecting particularly forests of high conservation value. They suggest adoption of land-use planning rules aimed at this goal. However, as they point out, such action will be extremely unlikely given the magnitude of predicted land-use changes in the country and powerful demographic factors driving them. I would add other barriers: the lobbying clout of the real estate industry and homeowners plus the local nature of zoning decisions.

SOURCES

Beaury, E.M., J.M. Allen, A.E. Evans, M.E. Fertakos, W.G. Pfadenhauer, B.A. Bradley. 2023. Horticulture could facilitate invasive plant range infilling and range expansion with climate change. BioScience 2023 0 1-8 https://doi.org/10.1093/biosci/biad069

Evans, A.E., C.S. Jarnevich, E.M. Beaury, P.S. Engelstad, N.B. Teich, J.M. LaRoe, B.A. Bradley. 2024. Shifting hotspots: Climate change projected to drive contractions and expansions of invasive plant abundance habitats. Diversity and Distributions 2024;30:4154

Fertakos, M.E. and B.A. Bradley. 2024. Propagule pressure from historic U.S. plant sales explains establishment but not invasion. Ecology Letters 2024;27:e14494  doi: 10.1111/ele.14494

Kinlock, N.L., D.W. Adams, W. Dawson, F. Essl, J. Kartesz, H. Kreft, M. Nishino, Jan Pergl, P. Pyšek, P. Weigelt and M. van Kleunen. 2025. Naturalization of ornamental plants in the United States depends on cultivation and historical land cover context. Ecography 2025: e07748 doi: 10.1002/ecog.07748

Oswalt, C.M., S. Fei, Q. Guo, B.V. Iannone III, S.N. Oswalt, B.C. Pijanowski, K.M. Potter. 2016. A subcontinental view of forest plant invasions. NeoBiota. 24:49-54 http://www.srs.fs.usda.gov/pubs/48489

Potter, K.M., K.H. Riitters, B.V. Iannone III, Q. Guo and S. Fei. 2024. Forest plant invasions in the eastern United States: evidence of invasion debt in the wildland‑urban interface. Landsc Ecol (2024) 39:207 https://doi.org/10.1007/s10980-024-01985-y

Potter, K.M., B.V. Iannone III, K.H. Riitters, Q. Guo, K. Pandit, C.M. Oswalt. 2026. US Forests are Increasingly Invaded by Problematic Non-Native Plants. Forest Ecology and Management 599 (2026) 123281

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

Or

https://fadingforests.org

Bird nesting habitats – why no mention of invasive species or deer?

ovenbird (Seiurus aurocapilla); photo by Rhododentrities via Wikimedia

Studies of forest ecosystems in eastern North America that claim to be comprehensive still too often make no reference to invasive species – pests, earthworms, or plants. I try here to bridge these gaps.

Akresh et al. (2023) conducted a meta-analysis of bird species’ use of forests as nesting habitat. They applied the Partners-in-Flight to evaluate the community-wide bird conservation values of unmanaged forests compared to various levels of tree removal by harvest. Because of the decline of many bird species that prefer shrubland or early-successional stands, their process gave highest ranks to management approaches that retained 40%–70% of the canopy trees.

Their study notes that habitats for shrubland birds comprise only about 6% of forests in the eastern U.S. They don’t provide data for southeastern Canada. But hasn’t this scarcity of open upland, non-wetland, habitats in this region been true for thousands of years?

The type of forest that undoubtedly has shrunk significantly in recent centuries is “virgin” (or old-growth or late-seral) forests. As Akresh et al. (2023) report, contemporary closed-canopy forests in eastern North America are predominantly structurally homogeneous, mid-seral, even-aged, stands that have regenerated on land previously cleared for either agriculture or timber. These forests are much younger from a forest developmental perspective than precolonial forests; they lack the latter’s range of tree fall gap sizes and multiple age-classes. The tiny fraction of eastern forests that are in the late-seral stage might have higher species richness and conservation value for birds, but since they are usually not under management, Akresh et al. (2023) did not include that question in their analysis.

Akresh et al. (2023) list the bird species whose density appears to be closely linked to various tree canopy densities. For example, ovenbirds and brown creepers promptly decline in abundance in response to any amount of tree harvesting. Two other species — wood thrush and cerulean warbler — have declined steeply range-wide in recent decades. Nesting densities of three of these four species (excluding the warbler) are significantly higher in areas harvested in ways that retain a greater percentage of trees. Densities of another five bird species (Acadian flycatcher, hermit thrush, black-throated green warbler, and red-breasted nuthatch) are also higher in areas with a greater proportion of trees retained.

Another nine species had a more complex relationship with tree densities but still had lower densities in stands with low tree retention. These were blue-gray gnatcatcher, blue-headed vireo, blackburnian warbler, black-throated blue warbler, eastern wood-pewee, least flycatcher, red-eyed vireo, scarlet tanager, and yellow-bellied sapsucker. They found little relationship between bird density and tree retention for five putative mature-forest species (American redstart, great-crested flycatcher, hooded warbler, veery, and yellow-rumped warbler).

scarlet tanager (Piranga olivacea); photographed in scrub at Edwin B. Forsythe (Brigantine) NWR by F.T. Campbell

Akresh et al. (2023) claim that silviculture approaches can be used to restore aspects of the structural and compositional conditions found in old-growth forests to second-growth systems, providing a potential pathway for rapidly increasing the conservation value of these areas for bird species. They advocate reducing canopies moderately via variable retention harvests, shelterwood establishment harvests, and irregular shelterwood systems. This strategy can increase understory vegetation density, which they assert can then increase foraging and nesting opportunities for both many mature-forest bird species and many shrubland birds.

I am skeptical; it is much easier to create openings in the canopy than to “create” large trees supporting cavities and associated fauna and flora utilized by some bird species. The authors do advise managers that late-seral, unharvested stands can provide important habitat for old-growth-dependent taxa and any intensive forestry should also take into account other factors.

old-growth hemlock stand in Cook Forest State Forest, Pennsylvania; photo by F.T. Campbell

In addition, often the understory vegetation that responds to the more open environment will be invasive non-native plants. Already about half of eastern U.S. forests have been invaded by non-native plants (Oswalt et al. 2016; Kurtz 2023). Many of these are shrubs: honeysuckles, privets, roses, buckthorn. Management of these plants is difficult – especially when opening the canopy to allow light to reach the forest floor. (at www.nivemnic.us, scroll down to “categories”, click on “invasive plants”.) So the question arises, do the non-native plant species adequately substitute for native shrubs in providing resources needed by those birds?  

Maybe. Gleditsch and Carlo (2014) found that a shrub layer dominated by non-native honeysuckle shrubs (Lonicera species) does support nesting populations of several common species, especially catbird (Dumetella carolinensis), American robin (Turdus migratorius ), and northern cardinal (Cardinalis cardinalis). However, they did not consider the species of concern to Akresh et al. (2023) – the rare species that prefer open-canopy, early-successional communities. So they do not inform us whether these high-priority species can utilize shrublands dominated by non-native species. Gleditsch and Carlo (2014) apparently did not find nests of several species considered to be associated with mature forests. So, again, these forests’ value for conservation remains unclear. Gleditsch and Carlo (2014) do counter earlier fears that these non-native shrubs are “traps” for nesting passerine birds. (The concern was that the plants’ structure facilitated nest raiding by predators.) They say, instead, that these plants’ effects are species-specific, context-dependent, and often a mix of both positive and negative outcomes.

invasive shrub honeysuckle; photo by Kevin Casper via public.domain.pictures.net

Akresh et al. (2023) also do not address the impact of browsing by super-abundant deer. Others (at www.nivemnic.us, scroll down to “categories”, click on “deer”.) have demonstrated that interactions of deer predation with invasive plants is especially damaging to native flora. Considering forests from Virginia to Maine, Miller et al. (2023) advise opening the canopy or subcanopy of forests to promote tree regeneration where deer and invasive shrubs overlap only where deer are controlled.

I have seen no recent analyses of the impact of widespread pest-caused tree mortality beyond some early efforts focused on eastern hemlocks and on high-altitude whitebark pines.

SOURCES

Akresh, M.E., D.I. King, S.L. McInvale, J.L. Larkin, and A.W. D’Amato. 2023. “Effects of Forest Management on the Conservation of Bird Communities in E North America: A Meta-Analysis.” Ecosphere 14(1):e4315. https://doi.org/10.1002/ecs2.4315

Gleditsch, J.M. and T.A. Carlo. 2014. Living with Aliens: Effects of Invasive Honeysuckles on Avian Nesting. PLOS One. September 2014. Volume Nine Issue Nine. E107120

Miller, K.M., S.J. Perles, J.P. Schmit, E.R. Matthews, M.R. Marshall. 2023. Overabundant deer and invasive plants drive widespread regeneration debt in eastern United States national parks. Ecological Applications. 2023;33:e2837. https://onlinelibrary.wiley.com/r/eap

Oswalt, C.M., S. Fei, Q. Guo, B.V. Iannone III, S.N. Oswalt, B.C. Pijanowski, K.M. Potter. 2016. A subcontinental view of forest plant invasions. NeoBiota. 24:49-54 http://www.srs.fs.usda.gov/pubs/48489

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

Or

https://fadingforests.org

It Zigs, We Zag: Following the Invasion of Elm Zigzag Sawfly in North America

elm zigzag sawfly larvae feeding on an elm leaf; photo by Delaney Serpan

As one of the newest – and most unique – invasive insects, elm zigzag sawfly (EZS; Aproceros leucopoda) has been making headlines across the eastern U.S. and Canada since 2020. The defoliating pest was first confirmed in North America in Québec, Canada and has since spread rapidly across many states and provinces. As its name suggests, EZS larvae feed primarily on elm in a distinctive zigzag pattern. Moving inwards from the leaf edge, the larvae can eventually consume nearly the entire leaf, leaving nothing but the midrib and a few lateral veins behind. Defoliation from EZS can range from nearly undetectable to 100% canopy defoliation of a mature tree.

(See earlier blog here.)

adult zigzag sawfly; photo by Delaney Serpan

Elm zigzag sawfly biology

EZS is a multivoltine insect, meaning it can have multiple generations in a single growing season. In Europe, where EZS has been invasive since 2003, 1 to 4 generations are common though up to 6 generations have been recorded. In the U.S., many regions document up to 5 generations per year.

In the early spring, EZS emerges from the soil where it has overwintered. They reproduce parthenogenetically- a form of asexual reproduction- allowing them to lay eggs immediately following adult emergence. Each individual is able to lay up to 49 eggs, drastically increasing EZS reproductive potential. Once the eggs hatch, the larvae begin feeding on the foliage until they are ready to pupate. At that point, the larva may build a summer cocoon, attached to a nearby object such as a branch or fence post. Four to 7 days later, an adult will emerge. The entire life cycle only takes 3 to 6 weeks. Alternatively, the larva could drop to the soil beneath the tree’s canopy where it will build its winter cocoon and overwinter, waiting to repeat the cycle the following spring. A small portion of each generation create overwintering cocoons.

EZS summer cocoons attached to the underside of a leaf with evidence of larval feeding; photo by Delaney Serpan

Where is EZS now?

As of the end of 2025, EZS can be found in 15 states and 4 provinces as far west as Minnesota and Manitoba and as far south as North Carolina and Tennessee.

map of states/provinces with official EZS detections;

Invasion pathways in North America are currently unknown; however, EZS has been documented attaching its summer cocoons to truck wheel wells and other objects which may be moved. The subsequent movement of these objects can potentially contribute to EZS spread. It has also been suggested that infested elm nursery stock or potted soil of any plants could be a potential pathway for EZS, but more research is needed to fully understand this.

EZS cocoons on truck – under side mirror & wheel well; photos by Jared Beach, adapted from Oten et al. 2025

How does EZS affect the trees?

Defoliating pests typically decrease the aesthetic value of trees but leave the host largely unharmed. Across Europe and its native range of eastern Asia, EZS defoliation is relatively minimal, with the occasional severe outbreak resulting in total defoliation of a tree. Resulting branch dieback is even more uncommon.

When EZS was first found in North America, particularly North Carolina and Virginia, there were initial concerns about the implications of a warmer climate accelerating development. Like most insects, EZS development is related to temperature; a warmer climate allows for faster insect development. It was hypothesized that a longer growing season could allow for faster population growth and potentially more damage to host trees. At this point, it is still unclear if this will consistently occur in the southern extent of the range. In North Carolina, reported damage has varied widely since it was found there in 2023. Some trees have been 75% defoliated or more multiple years in a row and are exhibiting upwards of 20% branch dieback after just 3 years. However, trees with less than 10% defoliation and no branch dieback have also been recorded.

Since its first detection in North America, researchers have been working to better understand how this pest will affect stakeholders. They’ve been conducting research on the phenology and voltinism of EZS, exploring novel host associations, and evaluating management techniques. Here’s what they’ve learned so far.

A severely defoliated American elm in Surry County, N.C. Photo by Delaney Serpan

First, the bad news.

Elm zigzag sawfly has recently been found to feed on Japanese zelkova (Zelkova serrata), another common ornamental planting within the Ulmaceae family. However, it is important to note that Japanese zelkova is likely not a preferred host. It is suggested that while EZS can complete its life cycle on Japanese zelkova, it will do so only when no other suitable host is present. Researchers are continuing to explore this novel host association.

But help is on the way! There are management recommendations to control elm zigzag sawfly.

Research conducted at North Carolina State University has determined that soil injections of imidacloprid or dinotefuran at label rate are effective methods to significantly reduce larval populations on infested trees. Both active ingredients are easily accessible to landowners and can provide at least one year of protection against EZS. There is ongoing research to explore more treatment options, including augmentative biocontrol.

What can you do about elm zigzag sawfly?

If you are in an EZS-infested region, check vehicles or outdoor items before moving them.

And if you find EZS, report it! To best manage and prevent the spread of EZS, forest health professionals need to know where it is. Elm zigzag sawfly is the only insect that feeds in the unique zigzag pattern on elm trees. If you see the diagnostic feeding pattern, take a picture of it and contact your county’s local Extension agent or state forestry agency to report it.

Invited blog posted by Delaney Serpan

Delaney Serpan is a second-year Ph.D. student in the Forest Health Lab at NC State University, where she studies elm zigzag sawfly biology and management. She first began working with elm zigzag sawfly as an undergraduate researcher shortly after it was detected in North Carolina for the first time. Working with a novel invasive species on the leading edge of its invasion has been incredibly rewarding. Her work aims to provide accessible management techniques to stakeholders while also protecting elms, an already imperiled species, from further damage.

CISP welcomes comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

Or

https://fadingforests.org

New thinking on how non-native plants invade forests

invasive privet in Georgia

It used to be thought that closed-canopy forests are resistant to bioinvasion because of the low light availability and relatively infrequent disturbance. Yet many are badly invaded! (On this site, scroll down past the Archives, choose “invasive plants” category.)

Nor is it just temperate forests in North America. Subtropical and tropical forests have also been invaded, as have the temperate forests of South America and, to a lesser extent, Europe. Temperate forests in Asia are less invaded; boreal forests very little (Fridley et al. 2025; see full citation at the end of this blog).

Fridley et al. (2025) have proposed a conceptual model to explain how this happens: “superinvaders” – a special class of woody plants – that achieve competitive dominance across a wide range of forest conditions. The “superinvaders” pose especially grave threats to native biodiversity because they use life-history strategies unlike those of early successional native species.

The result is that existing invasion and succession theories poorly predict which forests are most invasible and by which species. This failure undermines pest risk analyses and early detection.

Fridley et al. have raised lots of interesting ideas – some of which cannot yet be demonstrated by observations.

Temperate forests of North and South America are increasingly dominated by non-native deciduous and semi-evergreen shrubs and trees that combine fast growth rate in high light and high survivorship in forest interiors. These traits enable them to outcompete the native species. Many invaders also produce many more seeds than co-occurring native species. Similar traits are found in the successful non-native plants in subtropical and tropical forests.

Amur honeysuckle via Flickr

Fridley et al. stress that shade tolerance alone does not endow the invasive plants with sufficiently large advantages in their competition with native species. The forest “superinvader” phenotype must combine this ability to persist in shade with high maximum growth rate and high fecundity when conditions become favorable for reproduction.

They explain the invader’s competitive advantage as the result of their experiencing relatively fewer carbon costs because of enemy release in the novel environment, recent environmental changes that alleviate some stress formerly present in the novel environment, or phylogenetic constraints on the local flora that limit natives’ resource-use efficiency. The non-native plant species enjoy this advantage regardless of whether they also possess other competitive mechanisms, e.g., production of allelopathic compounds, denser growth or shading, greater apparent quantum yield. However, Fridley et al. concede that they lack sufficient evidence to incorporate these other competitive mechanisms into their model.

Since any reduction in carbon costs will enhance both shade tolerance and growth rate when light levels are high, these “superinvaders” can outcompete native species in either situation.

To support these concepts, Fridley et al. note that increased abundance of invaders following disturbance is more pronounced in forests than other habitats. They suggest this is because of the much greater magnitude of change in light levels in forests than in open habitats such as grasslands.

They propose that an analogous situation applies to the presence or absence of mutualist microbial associations, although existing studies are insufficient to reach conclusions about the role of carbon allocation to mycorrhizae in the “superinvader” phenotype. The extent to which these forest invasions alter ecosystem-level carbon dynamics, especially soil processes and litter decomposition is also largely unknown.

Fridley et al. emphasize the role of carbon costs in driving both growth rate and whole-plant light compensation point. This point is defined as the light level at which carbon gain through photosynthesis balances carbon losses from tissue respiration (maintenance and growth) and turnover (shedding and loss from disturbance and herbivory).

To survive in low-light conditions, plants must minimize tissue respiration and turnover. The traits that enable those behaviors have been thought to prevent rapid growth and competitive dominance in high-light conditions. However, the “superinvaders” defy this trade-off by growing faster than most co-occurring native species when light is abundant. Fridley et al. say this is because the plants’ reduced carbon costs enhance competitive advantage in both shade and adequate light conditions.

Fridley et al. name several reasons why a native plant’s carbon costs might exceed those of an introduced species. First on the list is either herbivory or investment in defensive traits. Native plants might be challenged by rising abundance or consumption rates of native or introduced herbivores, such as deer or seed predators, that avoid the introduced species.

A second factor is that the non-native species expends fewer resources to sustain adaptations that confer resistance to other stresses, such as drought or freezing. If a long-standing stress is weakened by global change processes (e.g., atmospheric CO2 levels, growing season duration, precipitation levels and seasonality, suppression of fire, atmospheric nitrogen deposition), a non-native plant that lacks defenses against that now-weakened stress will have a lower carbon cost and therefore an advantage. In some cases, the non-native species might benefit directly from these changes, e.g., droughts.

In some regions phylogenetic constraints have limited evolution of adaptive solutions to various biotic and abiotic stresses. This is most obvious on tropical oceanic islands. Fridley et al. report that native trees in Hawaiian montane rain forests are less energy-efficient conducting photosynthesis than are the invaders. However, this phenomenon also occurs on continents. Two continents’ floras might experience different climatic histories even when at they are at similar latitudes. For example, Eurasian woody species leaf out earlier and senesce later than North American trees and shrubs – possibly as a result of more predictable spring and autumnal climate across Eurasia. They name as one example Norwegian maple (Acer platanoides) in North America.

The future is uncertain

Fridley et al. consider enemy release to be a key factor in these shrub invasions of closed-canopy forests. Therefore, if enemy release decays over time because the introduced plant species accumulate pests, or the forest environment shifts to favor more stress-tolerant phenotypes of some native species, then the dominance of superinvaders will decline. If, on the other hand, resource enrichment continues, e.g., nitrogen deposition and elevated CO2, the impacts of woody invaders – present or newly introduced – might continue to rise. The likelihood that additional introductions of more resource-efficient species will continue to damage floras of oceanic islands. 

Implications for risk assessments and management

Fridley et al. warn that habitat-matching criteria might be unreliable predictors of forest invasiveness. Among several examples of species that are invasive in interior forest systems in a novel region that do not exhibit this trait in their native range is red oak (Quercus rubra). It is locally dominant in both natural and managed forests in central Europe while in North America, red oak struggles to regenerate in closed-canopy forests. They suggest that Q. rubra in Europe has escaped seedling pathogens present in its native range in North America. 

red oak sapling in swampy forest in Virginia; photo by F.T. Campbell

Fridley et al. call for research on traits they have identified as important but that are rarely measured in invasion studies. These include rates of tissue loss and respiratory processes above- and below- ground, plant carbon allocation to tissues and processes, and the whole-plant light compensation points of native and invasive plant species.

The Fridley et al. hypothesis has been supported explicitly by Kinlock et al. (2025). This article says that consistent findings have been reported by earlier small-scale studies in U.S. forests.

I ask for your input on how well the Fridley et al. hypothesis explains shrub and tree invasions in American forests – including those on tropical islands! Is it helpful? Is APHIS incorporating these ideas into plant risk assessments? –

Fridley et al. take pains to reiterate the long-accepted importance of ornamental horticulture in explaining invasive plants’ entry and establishment. They do so in the context of concurring that ruderal traits are not universally advantageous; traits’ benefits depend on the landscape into which the species was being introduced.

SOURCES

Fridley, J.D., P.J. Bellingham, D. Closset-Kopp, C.C. Daehler, M.S. Dechoum, P.H. Martin, H.T. Murphy, J. Rojas- Sandoval, D. Tng. 2025. A general hypothesis of forest invasions by woody plants based on whole-plant carbon economics.

Kinlock, N.L., D.W. Adams, W. Dawson, F. Essl, J. Kartesz, H. Kreft, M. Nishino, Jan Pergl, P. Pyšek, P. Weigelt and M. van Kleunen. 2025. Naturalization of ornamental plants in the United States depends on cultivation and historical land cover context. Ecography 2025: e07748 doi: 10.1002/ecog.07748

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

Or

https://fadingforests.org

Urban centers as plant invasion “hotspots” – do global data fail to focus on most important?

Leucanthemum vulgare (ox-eye daisy); ranked by EICAT as “major impact”; photo via picryl

Because urban centers are “hotspots” of species introductions and reservoirs supporting their spread into areas less altered by human activity, a global group of scientists (Richardson et al. 2025) sought to determine whether the same plant species naturalize in urban areas around the world and – if so – where most of those plant species originate.

They chose to pursue this question because urban areas share many interacting environmental and biotic features that they thought might partially overcome the distinct biomes of the continents. These shared features include the prominence of impervious surfaces; increased habitat heterogeneity; eutrophication; fragmentation of any remaining semi-natural habitats; complex human-influenced disturbance regimes; diverse opportunities for dispersal; novel biotic assemblages and interactions; and human facilitation of non-native species’ colonization and local species’ extinction. In addition to the similarities of the receiving ecosystems, these commonalities are facilitated by shared introduction pathways – although Richardson et al. to not pursue this aspect.

The scientists consulted global invasive plant databases to compile a list of 7,792 plant species recorded as naturalized in one or more of 553 urban centers on all six continents (all except Antarctica). Just over 300 species (4%) were reported on all six continents. They call them the “omnipresent” taxa. Further refinement resulted in a list of 96 species that are particularly widespread, defined as being present in more than half of the urban centers of Oceania, North and South America, and Europe. These 96 species are present in a lower proportion of cities in Asia and Africa. Richardson et al. proposed that these species be folded into a new ecological category, the “urban florome”.

I wonder whether this set of species tells us more about biases in the data than the actual “urban florome”. First, 87% of the 96 “most widespread” species (n= 84) are annual or perennial herbs. Only seven tree, six vine, and six shrub or subshrub species were included among the 96 species. In other words, global lists of invasive species are heavily slanted toward species that thrive in disturbance. Is this surprising? As another study (Kinlock et al. 2025) notes, disturbance is ubiquitous!

Second, only a third of the “urban florome” species have been formally evaluated using the Environmental Impact Classification for Alien Taxa (EICAT) system. Of these 32 species, only six were categorized as having a “major” or “massive” impact. Richardson et al. (2025) conclude that many of the species on the most widespread list are human commensals that have few or negligible known impacts.

Still, this finding might underestimate their impacts. First, as noted, two-thirds have not been evaluated. Second, impacts important in urban systems might not be those that increase a species’ rank based on impacts to natural systems (Richardson et al.). Those with substantial nuisance value in the urban setting still require management. Of course, some of the species have severe impacts in both natural and urban ecosystems. For example, Ailanthus altissima causes major infrastructural damage and pollen allergies, while Robinia pseudoacacia alters soil fertility. Both reduce species richness.

Ailanthus

I note that these examples are both trees – which constitute only 7 of the 96 species. Fridley et al. report that trees and shrubs have severe impacts in closed forest systems. I suggest that since many of the urban areas in temperate, subtropical, and tropical regions are probably located in formerly forested areas, remnant (semi-)natural stands and even recreational parks have probably been invaded by these high-impact species. Surely that is more important – at least as regards the level/intensity of the non-native plant species’ impact on biodiversity – than the annual weeds growing along highway verges.

Richardson et al. fear that many cities also have substantial invasion debt. The note specifically that due to the heat island effect, species that can now survive only in cities are likely to spread into surrounding rural and natural areas as temps increase. Thus, these species amplify the urban source effect of plant invasions.

Generalities

Richardson et al. call attention to certain parts of the world acting as ‘factories’ for the evolution of plant species that are well equipped to become invasive when intro to new regions. They name Australian woody flora — although only one species, Melia azedarach, is included among the 96 most widespread species. They also name African grasses and Europe (no taxa specified).

Richardson et al. say that while non-native species in urban areas have usually been described as “passengers” taking advantage of anthropomorphic environmental change, bioinvasions are increasingly recognized as drivers of secondary changes that alter the capacity of these ecosystems to deliver key ecosystem services, or even create disservices. These modifications occur in urban as well as more natural environments.

Regional Differences

Richardson et al. developed lists of the most widespread naturalized urban species for each continent (‘continental lists’). Eighty-seven percent of the 96 “most widespread” species are present in cities of North America, 80% in cities of Oceania, and 34% in European cities. Only 17% of the “widespread” species are present in cities of South America, 13% in cities of Africa or Asia.

While there is considerable overlap regarding species found on several continents, Europe’s urban florome differed significantly from those of the other continents.

The principal source region for these naturalizing species was temperate Asia (145 records); followed by Europe (128 records) and Africa (121 records). Lower numbers came from tropical Asia (95 records); South America (54) records; North America (53 records); and Oceania (8 records). Europe has received 50% of its widespread urban invasive species equally from temperate Asia and North America. Africa has received 75% of its widespread urban species from the two Americas equally.

According to these data, Oceania has been a significant contributor only to South America. I am surprised given the publicized problems caused by Australian Acacia and Hakea in South Africa. I guess these trees are more invasive in the vicinity of urban areas rather than in the cities themselves.

Richardson et al. note a highly skewed relationship between North and South America: while 15.4% of species naturalized in South American cities come from North America, only 2.7% of naturalized species in North American cities are from South America.

Lepidium didymum – brassica from South America introduced widely, including throughout California; photo by Miguel A.C. via Pl@ntnet

Richardson et al. found a distinct division between the “Old” and “New” Worlds (defined by whether the soil was historically cultivated by plough vs. hoe). The latter has more naturalized species (9,905 taxa vs 7,923 taxa), although the “Old World” covers a larger area. Citing di Castri (1989), they suggest that the much longer history of intense human-mediated disturbances in Europe might have allowed its flora to adapt to coexist w/ humans. I wonder, however, whether it is just too difficult to distinguish introductions that occurred millennia ago.

Richardson et al. also found an “echo” from European colonization — strengthened by activities of acclimatization societies. The result is that the continents with longer histories of European colonization, i.e., South and North America and Oceania, have more widespread naturalized plant species than do Africa and Asia.

SOURCES

Fridley, J.D., P.J. Bellingham, D. Closset-Kopp, C.C. Daehler, M.S. Dechoum, P.H. Martin, H.T. Murphy, J. Rojas- Sandoval, D. Tng. 2025. A general hypothesis of forest invasions by woody plants based on whole-plant carbon economics.

Kinlock, N.L., D.W. Adams, W. Dawson, F. Essl, J. Kartesz, H. Kreft, M. Nishino, Jan Pergl, P. Pyšek, P. Weigelt and M. van Kleunen.  Naturalization of ornamental plants in the United States depends on cultivation and historical land cover context. Ecography 2025: e07748 doi: 10.1002/ecog.077

Richardson, D.M., L.B. Trotta, M.F.J. Aronson, B. Baiser, M.W. Cadotte, M. Carboni, L. Celesti-Grapow, S. Knapp, I. Kühn, A.C. Lacerda de Matos, Z. Lososová, D. Li, F.A. Montaño-Centellas, L.J. Potgieter, R.D. Zenni, P. Pyšek. 2025. Here, There and Everywhere: Widespread Alien Plants in the World’s Urban Ecosystems. Global Ecology and Biogeography, 2025; 34:e70159  https://doi.org/10.1111/geb.70159

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

Or

https://fadingforests.org

The invasive risk of Eucalypts

Eucalyptus grandis (in Australia); photo by Poyt448 Peter Woodard via Wikimedia

Deus et al. 2025 (full citation at the end of this blog) have published a review of current knowledge on the invasiveness of trees in the Eucalyptus genus. They report that eucalypt plantations cover more than 30 million ha globally; they could not determine the actual extent more precisely. The area is expanding at an estimated 4% per year. Eucalypts are so popular as timber trees because of their fast growth, ease of management, wood quality and environmental tolerance.

Until recently, trees in the Eucalyptus genus were thought to pose a low invasion risk. This was because these trees have limited seed dispersal, high juvenile mortality, and were expected to lack compatible ectomycorrhizal fungi in novel environments. However, several risk assessments and reports of ongoing invasions in some locations have raised questions. So Deus et al. undertook a literature survey to try to resolve the issue.

One of the risk assessments concerns the United States; see Gordon et al. (2012). This study – completed a dozen years before Deus et al. undertook their literature survey – cited several other sources documenting harmful invasiveness of nearly a dozen species, including Eucalyptus globulus, E. camaldulensis, E. grandis, and  E. tereticornis.

Deus et al. found that the limitations listed above actually can be overcome, so they do not prevent invasions:

  • seeds can disperse farther than 100 meters from parent plants;
  • high recruitment densities can compensate for the high juvenile mortality; and
  • ectomychorrhizal fungi can be found in the root systems of introduced eucalypt plants.

In fact, several Eucalyptus species meet criteria defining invasiveness in the Australian Weed Risk Assessment system. Still, Deus et al. found that existing studies cover too few plantations and species to allow an in-depth comprehensive understanding of eucalypts’ invasion ecology.

One reason that eucalypt trees’ invasiveness remains unresolved is that the countries which have established most large Eucalyptus plantations (Brazil, India and China) have not conducted many studies. Instead, most studies have been done in Iberia and South Africa, which together host less than six percent by area of the world’s estate of eucalypt plantations.

Deus et al. say that several possible reasons have been proposed to explain why Eucalyptus is considered to pose an invasion risk by scientists in Iberia and South Africa, but not in Brazil. 

  1. The few studies in Brazil were conducted in intensively managed plantations with very short rotations, which are probably less prone to invasion than plantations managed at low intensity levels.
  2. The Brazilian plantations were established 40 to 50 years ago, whereas those in Iberia were introduced ~ 200 years ago.
  3. Iberia experiences recurrent forest fires.
  4. In Brazil, leaf eating ants attack the trees; this might reduce trees’ vigor.
  5. In Brazil, native forests dominate the environs.

Deus et al. say that these hypotheses have never been tested.

Since studies have been conducted in only a few countries, they have evaluated only a few of the species used in plantations. At least 372eucalypt species have been introduced outside their native range; nine species are planted widely. Yet most of the studies reviewed by Deus et al. covered just two species, Eucalyptus globulus (46% of the studies), and E. camaldulensis (33% of the studies). Still, these two widely cultivated species received the highest invasiveness ranking of all species reviewed (65 and 72, respectively). According to Deus et al., these scores are higher than the average score for 32 species of Acacia – a genus considered to be one of the most invasive tree genera in the world.

Other, potentially invasive species, have not received adequate attention. Deus et al. note that E. tereticornis, which is widely planted in China, India and other regions of Southern Asia, has an invasiveness score of 66, placing it second highest in the evaluation. However, only 12 of 140 articles analyzed by Deus et al. addressed this species.

These eucalypts’ high scores result from their potential to hybridize, to naturalize outside their natural habitat, and from high flammability. Other contributing factors are high seed production and ability to resprout after cutting or fire.

The analysis determined that the major drivers for Eucalyptus invasions are soil disturbance, availability of moisture (essential for seedling establishment), and fire. Recruitment density increases with harvesting and tree age; it decreases when the understory is managed. This partially explains why the abandonment of plantations might promote invasions by eucalypts.

Deus et al. fear that there might be a large “invasion debt” in the regions where few studies have been conducted. Assessments for California and Iberian Peninsula indicate that the best areas for cultivation – under either current conditions or expected new environments linked to climate change – are also those most prone to invasion. A further complication is that in some regions it might be difficult to distinguish plants escaping from small plantations from the plantations themselves. They suggest ways to overcome this difficulty: 1) surveys of recruitment along roadside, where trees would not have been planted; 2) genetic analysis of seedlings and possible parents

Another weakness is that that none of the studies considers changes in fire regime, which probably increases the areas prone to invasion.

Deus et al. think it is unlikely that eucalypt invasions will turn out to be as damaging as those of acacias or pines, but that further invasions involving more species and more regions are very likely.

Deus et al. call for considering eucalypt species’ potential invasiveness when developing strategies for the sustainable management of these plantations, including how to manage those that are no longer economically viable.

Status in the United States

The risk in the United States was evaluated by Gordon et al. in 2012. At the time, there were proposals to plant 5,000 to 10,000 ha/year in the Southeast over the next decade.

Gordon et al. adapted the Australian weed risk assessment system to evaluate 38 Eucalyptus taxa then being tested and cultivated in U.S. for pulp, biofuel, and other purposes. Their analysis concluded that 15 of these taxa posed a low risk; 14 taxa posed a high risk; and 9 taxa could not be ranked without further information. The four taxa cultivated most extensively –  E. globulus, E. camaldulensis, E. grandis, and E. tereticornis – all had high risk outcomes, as did several other taxa. Gordon et al. thought that these differences reflected both new data and differences in how the assessors reacted to insufficient data.

Gordon et al. warned that novel genotypes with unknown invasiveness were being propagated in the search for increased cold tolerance. This meant that the taxa they had assessed might not indicate of the actual long-term invasion risks associated from this genus. A major source of uncertainty is the long lag time in appearance of evidence of a tree species’ invasiveness. Only one study (as of 2012) had quantified lag time for introduced tree species; it found an average of 170 years from the time introduction to identification of the taxon as invasive. Propagule pressure also influences the lag time and the probability of invasion.

Since the bulk of expanded cultivation was expected to be in the southeast, Gordon et al. recommended that a regional assessment be conducted to more precisely specify the effects of possible differences in phenology, age at reproductive maturity, seed viability, and cold tolerance. 

Gordon et al. suggested several actions to reduce the invasion risk. First, selection and breeding strategies could aim to minimize relevant traits – especially eliminating seed production. Second, plantations could be so managed by avoiding cultivation near waterways, harvesting stems before seeds can mature, and restricting the extent of cultivation of any one taxon. More broadly, a fund could be established to cover control costs; growers would contribute the money.

What has happened in the dozen years since the analysis was published? My Google search led to publications from 2013 and earlier. I hope this indicates that no one has funded major expansions. Dr. Gordon reports that most Eucalyptus pulp is imported. ArborGen continues to breed Eucalyptus in Brazil – as I noted earlier, scientists there are not pursuing studies of possible invasiveness of eucalypts.

Still, the regional risk assessment has not been conducted. Worse, Dr. Gordon reports that the Florida Department of Agriculture and Consumer Services has exempted several species [E. amplifolia, E. benthamii, E. dorrigoensis, E. dunnii, E. grandis, E. gunni, E. nitens, E. smithii, and E. urograndis (E. grandis  E. urophylla)] from a requirement that growers obtain Non-Native Species Planting Permits. So if the market does take off, there will be no regulation by the state.

At the end of December 2025, Dr. Gordon received information from Florida Division of Plant Industry that no one has applied for a permit to grow Eucalyptus in the state other than under USDA research auspices. So my worst fears have not (yet) come to pass.

I note that in 2022, Potter, Riitters, & Guo ranked Eucalyptus grandis & E. globulus as potentially highly invasive. Their criterion was that at least 75% of stems detected by USFS Forest Inventory and Analysis (FIA) surveys were saplings or seedlings.

SOURCES

Deus, E., D.M. Richardson, F.X. Catry, F.C. Rego, J. Gaspar, M. Nereu, M. Larcombe, B. Potts, J.S. Silva. 2025. Invasion ecology of eucalypts: a review. Biol. Invasions (2025) 27:239  https://doi.org/10/1007/s10530-025-03695-1

Gordon, D.R.,S.L. Flory,.L. Cooper, and S.K. Morris. 2012. Assessing the Invasion Risk of Eucalyptus in the United States Using the Australian Weed Risk Assessment. International Journal of Forestry Research Volume 2012, Article ID 203768, 7 pages doi:10.1155/2012/203768

Potter K.M., Riitters, K.H. & Guo, Q. 2022. NIS tree regeneration indicates regional & national risks from current invasions. Frontiers in Forests & Global Change  

doi: 10.3389/ffgc.2022.966407

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

Or

https://fadingforests.org

Welcome high-level attention to bioinvasions – although key issues remain unresolved

Japanese knotweed (Reynoutria japonica) – one of the worst invaders around globe. Photo by Will Parson, Chesapeake Bay Program via Flickr

On 23 October, Science published a five-page, data-packed analysis of bioinvasion impacts on terrestrial ecosystems!!! 

Thakur, Gu, van Kleunen, and Zhou (full citation at end of this blog) analyzed 775 studies with the goal of improving understanding of factors contributing to invasions’ impacts – as distinct from “invasibility” (ability to establish). This knowledge is essential to assessing the risk posed by introduced species and setting priorities for management. They analyzed five ecological contexts—diversity of native species and introduced species in the recipient systems, latitude, invader residence time, and invader traits.

They concluded that ecological factors commonly used to explain invasion success do not consistently translate into strong predictors of invasion impacts. Impacts vary in response to the context of the invasion.

[In January 2026, the authors announced changes in details of the article due to some errors in the database and their understanding of it. (Science 8 Jan 2026 Vol. 391 Issue 6781) They conclude that the corrected analysis did not alter the trends described or the overall conclusions.]

limber pine (Pinus flexilis) – one of the species killed by Cronartium ribicoli; photo by F.T. Campbell

Among the studies available for analysis, reports on plants dominated: 605 focused on plant invasions, 114 on animal invasions, and only 56 on microbial invasions. Among the animals were one study of Adelges tsugae (hemlock woolly adelgid), two studies of Agrilus planipennis (emerald ash borer) and one study each of Lymantria dispar (spongy moth), and Ips pini (North American pine engraver). Studies also addressed earthworms, ants, rats, and feral hogs. Microorganisms included Cronartium ribicoli (white pine blister rust) and several Phytophthora species, including P. agathidicida (kauri dieback), P. alni (affects alders), and P. ramorum (sudden oak death).

Thakur et al. note the skewed taxonomic coverage and say that the low number and narrow taxonomic/ecological variety in the animals and microorganisms probably limit their ability to reach robust conclusions about the impacts of such invasions.

The most consistent negative impact they found is reductions in native plant diversity. While this is not surprising given the studies analyzed, I think it is still important since it counters the widespread sense that plant invasions are somehow less deserving of a robust response.

The authors also detected some broader ecosystem impacts of plant invasions. Plant invasions increased soil organic carbon; soil nitrogen (ammonium and nitrate), and available phosphorus; soil moisture, litter biomass; and emissions of carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4). The changes in biogeochemical properties might reinforce impacts on native plant communities. The reported increase in greenhouse gas emissions might reflect a bias in the studies so Thakur et al. call for more research to solidify this finding.

High native plant species richness had only a weak overall effect on ecosystem-level impacts. While plant invasions often resulted in higher overall plant species richness, when considering only native community responses, the gain in species numbers did not necessarily indicate conservation benefits. Native plants’ biomass increased after invasion. This might reflect short-term increases in productivity in response to altered resource conditions or structural facilitation, rather than a long-term reversal of competitive exclusion. Finally, the longer the invasive [plant] species had been present, the greater the negative effects on native diversity. However, soil abiotic property impacts weakened over time. In fact, the initial increase in soil organic carbon and total nitrogen disappeared after 6 to 10 years. This development might reflect fertilization of ecosystems by long-established nitrogen-fixing invaders such as non-native legumes.

Traits of non-native plant species related to growth and resource acquisition were overall weak predictors of ecosystem impacts. Thakur et al. consider that this finding reflects the relatively narrow range of specific leaf area exhibited by the plant species studied most commonly.

Consequently, Thakur et al. urge managers to focus on containment and impact mitigation, and to prioritize persistent losses of native plant diversity. When considering abiotic responses that might lessen over time, managers should apply “adaptive monitoring” (which is not defined).

Thakur et al. had greater difficulty determining the impacts of animal and microorganism invasions because of the smaller number of studies. They could not determine the effect of native species richness. The observed decline in soil organic carbon they thought was attributable to the large proportion of studies (9 out of 114) that focused on introduced earthworms. Earthworms reduce organic matter by consuming litter. Mammals were also found to reduce soil organic carbon. Introduced insects had no significant ecosystem effects on soil organic carbon. Non-native animals also increased soil emissions of carbon dioxide and nitrous oxide. The microorganisms included in reviewed studies decreased soil ammonium and increased nitrate, consistent with elevated nitrification. While data on body size of invasive animals were sparse, the authors could determine that larger-bodied species tended to increase soil nitrate while reducing effects on total soil N.

Applying the Results

Thakur et al. report that residence time outperformed other factors as a predictor of invasion impacts. The authors regret the scarcity of long-term studies, especially in the Global South, that could increase our understanding of whether these impacts persist or shift under sustained invasion pressure.

How can scientists apply this information in risk assessments evaluating not-yet introduced species or in deciding what is the appropriate intensity of immediate response to newly detected incursions. Should they give greater weight to others’ studies that focus on long-established invasions by the species in question? Otherwise, this finding seems to largely duplicate the long-established “invasion curve”.

I hope scientists will note that observational studies generally showed stronger impacts than experimental ones, particularly in the case of plant invasions. Perhaps this is true because observational studies better incorporate environmental heterogeneity and longer time spans.

Agrostis stolonifera – one of the plants invading on Prince Edward Island, an Antarctic region island under South African jurisdiction. Photo by Stefan Iefnaer via Wikimedia

Thakur et al. note that one factor they analyzed, “latitude”, incorporates several ecological and anthropogenic components relevant to invasion impacts. One element is the greater native bioidiversity in warmer, lower-latitude, regions. According to the “biotic resistance” hypothesis, greater diversity might make these systems more resistant to bioinvasion. However, the situation is complicated by the fact that temperate regions have also often experienced longstanding and intensive land-use modifications — which are believed to facilitate invasive species establishment and spread. I regret that the authors make no attempt to separate the effects of factors that are anthropogenic from those arising from immutable conditions, e.g., latitude, topography, weather patterns, etc.

Thakur et al. call for more studies that cover a wider geographic range. In addition, the studies should include more experimental designs and explore the relationship between invaders’ traits and impacts — especially regarding animals and microbes.

SOURCE

Thakur, M.P., Z. Gu, M. van Kleunen, X. Zhou. 2025.  Invasion impacts in terrestrial ecosystems: Global patterns and predictors. Science 23 October 2025

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

Or

https://fadingforests.org

Threats to Spring

Erythronium americanum dominating herb layer in woods owned by the Institute for Advanced Studies, Princeton, in the 1970s; photo by F.T. Campbell

I fell in love with spring ephemerals in the woods of the Institute for Advanced Studies in Princeton. While the degree I was pursuing had no relationship to birding in the swamp, I spent a lot of time enjoying the woods. At that time, more than 50 years ago, the herbaceous layer was dominated by spring beauties (Claytonia virginica), trout lilies (Erythronium americanum), and violets (Viola species).

Beyond the beauty that delights us (or at least, me!), spring ephemerals are important ecologically. They support specialist pollinators and reduce nutrient losses at a time of year when vegetation cover is low and leaching and runoff rates high.

In the decades since I left Princeton, scientists and nature lovers have observed declines in native understory plant communities. These are predicted to continue due to invasion by plants and worms, worm blogs herbivore pressure by deer, E NPS blog, Blossey blog land use changes, and climate change.

Where I live, in the suburbs of the District of Columbia, these forces are clear. The formerly glorious riparian forests where I walk are overrun by invasive plants. The herb layer is dominated by Japanese stiltgrass (Microstegium vimineum) and – increasingly — lesser celandine (Ficaria verna = Ranunculus ficaria). (I found it interesting that Ficaria began taking over floodplain forests only in the last decades of the 20th century, although it was introduced more than 100 years earlier.) Many invasive shrubs (Rosa multiflora, various Lonicera species. …) and vines (Ampelopsis sp, Orbiculatus, Lonicera japonica, Hedera helix …) compound the problem. While I am not sure whether most earthworms here are native or not, high deer populations certainly are a factor.

Ficaria invasion in Fairfax County, Virginia in 2023; photo by F.T. Campbell

So I rejoice that scientists are studying how one taxon of spring ephemerals, trout lilies – Erythronium species – are coping with individual and combined threats. Gutiérrez and Hovick (full citation at the end of this blog) investigated how two species of Erythronium performed in the absence of a leaf litter layer – with and without competition by Ficaria. They chose to manipulate leaf litter as a proxy for impacts from invasive earthworms and non-native shrubs, especially those with rapidly decomposing leaves.  They refer to others’ studies focused on different spring ephemerals.

Gutiérrez and Hovick found that the absence of leaf litter reduced asexual reproduction (corm biomass) in both Erythronium albidum and E. americanum species by 30%. That is, the absence of leaf litter alone reduced the native plants’ performance. This is alarming because persistent leaf litter has been reduced across much of the deciduous forests of eastern North America as a result of action by invasive earthworms and the rapid decomposition of the leaves of most invasive shrubs.

Trout lilies’ performance declined even more when litter absence was coupled with direct competition from Ficaria. Under those conditions, corm biomass declined by 50%. Impacts by lesser celandine occurred despite these plants’ being smaller than counterparts in nearby woodlands. The reduced size of Erythronium corms was sufficient, in their view, to reduce the likelihood that Erythronium would flower to nearly zero. This has clear implications for the long-term population viability of Erythronium andtheir specialist pollinators.

 Gutiérrez and Hovick conclude restoration of these floodplain forests’ herb layer must incorporate management strategies that not only reduce Ficaria’s presence but also restore leaf litter.

Erythronium albidum along Accotink Creek in Fairfax County, Virginia; photo by F.T. Campbell

Underlying Factors

Native spring ephemerals in eastern North America evolved to emerge through litter layers in early spring. The litter layers impose both costs and benefits. In response to shading by leaf litter, Erythronium produces larger petioles compared to same-sized leaves, thus reducing the proportion of resources allocated to building photosynthetic tissue. In these cases, the corms that both perpetuate the individual and carry out asexual reproduction are smaller.

On the other hand, leaf litter increases moisture retention and reduces frost damage by buffering soil temperatures. While these results were seen in their experiment, Gutiérrez and Hovick believe the benefits are greater in nature than demonstrated in the study using potted plants. Leaf litter also increases nutrient availability, directly by increasing supply and indirectly by facilitating fine root growth. In this context, they note that their experiment used litter composed of just two tree species — red oak (Quercus rubra) and red maple (Acer rubrum). This narrow sample probably failed to capture the varied properties of other tree species’ litter and associated microbial activity.

Erythronium americanum along Pohick Creek; photo by F.T. Campbelle

Plants in the Erythronium genus reproduce primarily asexually through producing runners that form corms. The parent corm and runners disintegrate before summer dormancy; the offspring corms persist. Some individuals do not reproduce asexually; they simply replenish their own corm.

The few previous studies give mixed results regarding lesser celandine’s impacts on co-occurring native herbaceous plants (see the summaries in Gutiérrez and Hovick). The authors do not explicitly say whether lesser celandine is usually associated with low litter levels, but that appears to be the implication. They do say that it is not clear whether lesser celandine drives leaf litter loss by altering soil physiochemistry and microbial activity. Or, rather, that it simply performs well when leaf litter is absent.

Where lesser celandine and Erythronium co-occur at high densities, the former’s biomass per square meter can be more than an order of magnitude higher than Erythronium. Gutiérrez and Hovick suggest that competition between the species is primarily belowground. They cite their finding that by the time Erythronium shoots matured, lesser celandine roots occupied most of the belowground pot volume. They expect belowground competition in forests to be even more pronounced because of accumulated lesser celandine root biomass.

Aboveground, the principal factor appears to be the necessity for trout lilies to grow longer petioles to raise their leaves above lesser celandine rosettes, perhaps starving leaf formation. Since leaves are the plant’s photosynthetic organ, this tradeoff could ultimately result in fewer resources returned to the corm for future growth and reproduction. Although Gutiérrez and Hovick also mention that lesser celandine competition might delay Erythronium emergence and flowering, they do not discuss that.

A factor not mentioned by Gutiérrez and Hovick is the probability that Ficaria verna is allelopathic. See the article by Kendra Cipollini listed as a source at the end if this blog.

one of the few floodplains in Fairfax County still dominated by native herbs – Pohick Creek in the Burke area of Fairfax County, Virginia. Note the prevalence of beech in the canopy and subcanopy! photo by F.T. Campbell

Details of Impaired Performance of Erythronium

At the time of senescence, Erythronium plants grown in pots with leaf litter were nearly twice as large as those grown in bare soil conditions. One-third of their offspring corms grew to be larger than the putative biomass threshold for flowering. Only 9% of corms of plants grown in bare soil and 2% (one individual) of those grown with lesser celandine did. As noted above, corms developed by Erythronium grown in the presence of Ficaria actually lost biomass. This is the basis for their conclusion that there would be almost no sexual reproduction the following year where litter was absent and lesser celandine present.

Gutiérrez and Hovick think the principle mechanisms by which leaf litter affects performance of Erythronium plants is by buffering temperature ranges and increasing moisture retention. Indeed, they found that daily temperature ranges and maxima of soil in pots with bare soil or lesser celandine plants were both higher than temperatures under leaf litter. Reducing temperature maxima could be especially important with the increasing frequency and intensity of late-spring heatwaves associated with climate change. Absence of leaf litter advanced trout lily’s shoot emergence, flower emergence, and petal opening by 14 or more days.

This change might expose the plants to increased risk of frost damage. These dynamics will be system-specific, especially with complications added by climate change. However, Therefore, Gutiérrez and Hovick encourage future research to explore species-specific litter effects on spring ephemerals.

Broader Implications

Their findings regarding these two species of spring ephemerals prompt Gutiérrez and Hovick to assert that negative impacts from invasive plant species might be especially underestimated in spring ephemeral communities due to the combination of their short period of annual aboveground activity and tendency towards long lives. Changes might be very subtle over short timeframes.

They add that it is important to learn the role different conditions might play in the futures of related species. The two species’ ranges largely overlap, but E. americanum extends into the extreme southeast and northeast, E. albidum into the prairie states. Although these species’ respond to loss of leaf litter and lesser celandine invasions in similar ways, the fact that E. albidum occurs in areas of higher soil moisture makes it more vulnerable to negative population-level impacts from lesser celandine invasions.

Note about additional threats

Most of the photos of Erythronium americanum in this blog were taken along a particular creek in Fairfax County, Virginia. Ficaria has just begun to invade this area (see photo above); deer are plentiful. These plants face another bioinvasion: beech leaf disease has arrived. Widespread mortality of the predominantly beech understory will presumably open areas to more light, probably spread of the extant invasive plants.

beech in Fairfax County, Virginia with symptoms of beech leaf disease; photo by F.T. Campbell

SOURCES

Cipollini, K. and K.D. Schradin. 2011. Guilty in the Court of Public Opinion: Testing Presumptive Impacts and Allelopathic Potential of Ranunculus ficaria” 

Gutiérrez, R.G. and S.M. Hovick. 2025. Compounding negative effects of leaf litter absence and belowground competition from an invasive spring ephemeral on native spring ephemeral growth and reproduction. Biol Invasions (2025) 27:213 https://doi.org/10.1007/s10530-025-03668-4

Call for new approach to biological conservation – integrating bioinvasion

whitebark pine in Glacier National Park killed by white pine blister rust

The Kunming-Montreal Global Biodiversity Framework (KMGBF) is a major global policy driver around the world for more effective action to preserve biodiversity from current and future threats. (However, the United States has not joined the underlying treaty, the Convention on Biological Diversity (CBD). So its importance is probably less in the United States than in countries that take part.) This relatively new Framework was adopted at the 15th Conference of the Parties (COP) of the CBD in December 2022 after four years of negotiations. However, cynics note that the 196 countries that are parties to the CBD have rarely met previous ambitious goals set at earlier COP.

Hulme et al. have just published a paper [full reference at the end of this blog] addressing how invasive species and this Framework’s target may interact. They note that conserving biodiversity costs money. Many of the countries hosting diverse and relatively intact ecosystems lack sufficient resources, capability, or robust governance structures for this conservation.

The Kunming-Montreal Global Biodiversity Framework sets out ambitious global targets to reduce biodiversity loss by 2030 so as to maintain the integrity of ecosystems and their constituent species. Of the 23 targets, one – Target 6 – addresses bioinvasion. Countries endorsing the CBD have committed to eliminating, minimizing, reducing and/or mitigating invasive species’ impacts on biodiversity and ecosystem services. This is to be accomplished by identifying and managing introduction pathways; preventing introduction and establishment of priority invasive species; reducing rates of introduction and establishment of known or potential invasive species by at least 50% by 2030; and eradicating or controlling invasive species, especially in priority sites.

I rejoice that the CBD parties have recognized invasive species as a major driver of biodiversity loss in terrestrial and marine ecosystems. I wish conservation organizations’ and funders’ activities clearly reflect this finding.

This is the challenge raised by Hulme et al.: countries must integrate efforts to counter bioinvasions into overall conservation programs. Success in curbing bioinvasion depends upon achieving almost all other KMGBF targets. And this is a two-way street: the more holistic approach offers greater likelihood of successful biodiversity conservation.

The same authors point out that some of the 22 other targets address rapidly evolving introductory pathways e.g.,

  • Target 15 – increasing international and domestic tourism;
  • Target 12 – encroachment of urban areas near protected areas;
  • Target 10 – development of intensive agriculture or aquaculture systems near protected areas;
  •  Target 7 – species rafting on plastic marine pollutants; and
  • Target 8 – growing risk from species shifting ranges in response to climate change.  
pallet graveyard behind camp store & snack bar art Lake MacDonald, Glacier National Park; photo by F.T. Campbell

Other targets relate to management of established invasive species, e.g.,

  • Target 1 – planning and priority-setting for allocation of limited resources among the various threats to biodiversity;
  • Identifying factors that pose risks to highly-valued species, e.g., threatened species (Target 4) and species that provide important ecosystem services (Target 11);
  • Target 19—obtaining necessary financial resources.  

A final group of targets are intended to guide all conservation efforts. These goals include integrating biodiversity concerns in decision-making at every level (Target 14); reducing harmful economic incentives and promoting positive incentives (Target 18); and several targets addressing issues of equity, benefit sharing, and access to information.  Hulme et al. assert that the threat posed by bioinvasions must be incorporated into policies, regulations, planning and development processes and environmental impact assessments across all levels of government.

Hulme et al. decry an imbalance as to which KMGBF targets have been the focus of attention from governments, conservation organizations, and media. These stakeholders have concentrated on

  • Target 3, which calls for extending legal protection to 30% of lands and waters by 2030;
  • Target 4, which promotes maintaining genetic diversity within and among populations of all species;
  • Target 7,  which encourages reducing harmful pollution;
  • Target 15, which urges businesses to decrease biodiversity risks arising from their operations; and
  • Target 21, which advocates ensuring equitable and effective biodiversity decision-making.

Even when stakeholders have looked at Target 6, they have focused primarily on how to quantify the numbers of species being introduced to novel ecosystems. Hulme et al. argue that conservationists should instead concentrate on the challenge of achieving the target. They note that bioinvasion is worsening despite implementation of many long-term management programs. As they note, numbers of introduced species globally have increased, these species are occupying larger geographic areas, and the species’ measured impacts have risen to astounding levels (see my previous blog about new cost estimates). This same point was made two years ago by Fenn-Moltu et al. (2023) [full citation at the end of this blog]; they found that the number of invasive species-related legislation and treaties to which a country adheres did not relate to either the number of insect species detected at that country’s border or the number of insect species that had established in that country’s ecosystems.

As conservationists, Hulme et al. remind us that not all damages are monetary: invasive species threaten more than half of all UNESCO World Heritage Sites.

Hulme et al. say achieving Target 6 presents several scientific challenges – most of which have been discussed by numerous other authors. Introduction pathways are changing rapidly. There is great uncertainty regarding current and especially future propagule pressures associated with various pathways. Information about particular species’ impacts and where they are most likely to be introduced is insufficient. Management costs are routinely underestimated. Perhaps most challenging is the need to judge programs’ effectiveness based not simply on outputs (e.g., number of acres cleared of weeds) but on outcomes in relation to reducing the subsequent impact on biodiversity and ecosystem services.

I note that several environmental organizations endorsed a “platform” that discussed this last point a decade ago. [I have rescued the NECIS document from a non-secure website; if you wish to obtain a copy, contact me directly through the “comment” option or my email.] Unfortunately, the coalition that prepared this document no longer exists. Even when conservation organizations have invasive species efforts, they are no longer attempting to coordinate their work.

APHIS inspecting imported plants

I greatly regret that Hulme et al. continue a long-standing misrepresentation of international border biosecurity controls as consisting primarily of inspections — of imported commodities, travellers, and associated transport conveyances. I have argued for decades that inspections are not effective in preventing introductions. See Fading Forests II Chapter 3 (published in 2003); Fading Forests III Chapter 5 (published in 2014); “briefs” describing pathways of introduction prepared for the Continental Dialogue on Non-Native Forest Insects and Diseases – in 2014 and in 2018.    

 The weaknesses of visual inspection are especially glaring when trying to prevent introductions via wood packaging material and living plants — also here.

Hulme et al. propose a politically astute approach to finding the resources to strengthen countries’ efforts to curtail invasive species’ spread within their borders.  Recognizing that no country has unlimited resources to allocate to managing invasive species, they suggest concentrating slow-the-spread efforts on preventing damage to legally protected areas. Furthermore, authorities should avoid designating as new “protected areas” places that are already heavily invaded – or at risk of soon becoming so. As they note, programs aimed at protecting these areas often engage conservation stakeholders, decision-makers, even potential non-governmental donors. In other words, there is a foundation on which to build.

To buttress their argument, Hulme et al. cite evidence that bioinvasions threaten these areas’ integrity. For example, Cadotte et al. (2024) found that bioinvasion is one of most frequently identified threats identified in a survey of 230 World Heritage sites; and that they pose a greater degree of concern than other threats to biodiversity. They reiterate that managing invasive species is one of the most effective interventions aimed at protecting biodiversity.

The task remains complex. Hulme et al. note that accurate information about pressure caused by invasive species is not easily quantified using remote sensing. It requires expensive on-the-ground data collection. Even current methods for ranking invasive species have crucial gaps regarding species’ potential impact and the feasibility of their control. Choosing management strategies also requires assessing potential unintended effects on biodiversity and other GBF Targets, e.g., pollution from pesticides (Target 7).

Still, the context remains: successful management of bioinvasions to support the integrity of protected areas depends on the integrative approach described above.

Hulme et al. note a contradiction within the Kunming-Montreal Global Biodiversity Framework: Target 10 calls for the agriculture, aquaculture, and forestry industries to adopt sustainable practices, but doesn’t raise the issue of these sectors’ role in the introduction and spread of invasive species. They say guidelines have been developed for sustainable forestry production. These guidelines recommend that commercial plantation forests not plant non-native tree species within 10 km of a protected area. Hulme et al. also suggest applying a “polluter pays” fine or bond to forestry businesses that use invasive species without sufficient safeguards to prevent escape. These funds could be accessed to support invasive species management in protected areas, particularly surveillance. (Target 19 mandates obtaining more funds for this purpose).  They add that these aquaculture, agriculture, horticulture and forestry sectors should take action to prevent the local feralization of alien crops and livestock.

Target 8 calls for minimizing the impacts of climate change on biodiversity. Hulme et al. note numerous scientific challenges here, including understanding how specific ecosystems’ and native species’ are vulnerable to altered climates, along with how specific invasive species’ are responding to an altered climate regime.

These same authors provide specific recommendations to the global conservation community to put in place a more holistic perspective. Some recommendations deal with data integration. Others call for major undertakings: i.e., developing a protected area management toolkit at a global scale. This action will require significant investment in capacity-building of protected area managers plus international cooperation and technology transfer (Target 20). Hulme et al. suggest funding this effort should be a priority for any resources leveraged from international finance (Target 19).

Hulme et al. also propose changes in the conservation approaches advocated by the CBD and IUCN. Specifically, they call for more explicit consideration of current and future impacts of bioinvasions and their management — on protected areas. The needed activities fall into six areas:

(1) reduce risks associated with various pathways;                                 

(2) plan for range-shifting invasive species;

(3) mitigate invasive species’ impacts on biodiversity and (4) on ecosystem services;

(5) ensure new protected areas (including urban green spaces and infrastructure corridors) are largely free of established (“legacy”) invasive species; and

(6) provide managers sufficient resources to take effective action.

SOURCES

Fenn-Moltu, G., S. Ollier, O.K. Bates, A.M. Liebhold, H.F. Nahrung, D.S. Pureswaran, T. Yamanaka, C. Bertelsmeier. 2023. Global flows of insect transport & establishment: The role of biogeography, trade & regulations. Diversity & Distributions DOI: 10.1111/ddi.13772

Hulme, P.E., Lieurance, D., Richardson, D.M., Robinson, T.B. 2025 Multiple targets of Global Biodiversity Framework must be addressed to manage invasive species in protected areas. NeoBiota 99: 149–170. https://doi.org/10.3897/neobiota.99.152680

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

Or

https://fadingforests.org