Articles by Evan Fricke and colleagues remind us to look more broadly at bioinvasion to consider the impact on ecosystem function and evolution. They focus on animal interactions with plants in the shared environment, especially animals’ role as seed dispersers.
The authors also remind us that natural barriers explain why there are different species in different areas and thus how evolution and speciation follow different paths in different places. Think of Galapagos finches evolving in isolation from a few ancestors that somehow made it over the ocean from mainland South America.
These points are made in two recent articles.
In the first, Fricke and Svenning 2020 (full citation at end of this blog) note that about half of all plant species depend on animals to disperse their seeds. Animal seed dispersal is influenced by several drivers of global change, including local or generalized extinction (= defaunation); bioinvasion; and habitat fragmentation. The decline of large vertebrates has a particularly important role in these interactions.
Their study focused on fleshy-fruited plants that are dispersed by animals. (The study does not include nuts, e.g., acorns, which are presumably subject to some of the same pressures.) They expect evolution of the affected plants and animals to proceed differently as a result of the new partnerships, but they did not study any such interactions.
Their study covered animal seed-dispersal interactions with plants at 410 locations. The data encompassed 24,455 unique animal-plant pairs involving 1,631 animal and 3,208 plant species. Three quarters of the animals were birds; most of the rest were mammals, primarily bats and primates. Only 1% were in other animal groups – lizards, tortoises, or fish.
They found that introduced plants and animals are twice as likely as native species to interact with introduced partners. The resulting interactions are likely to amplify biotic homogenization in future ecosystems. Already, introduced species have largely replaced missing native frugivore species in some places. In fact, mutualisms in which either or both the plant and animal is an introduced species are now about seven times higher than decades ago.
These mutual-benefit interactions of introduced species are even more prevalent in areas where human modification of the environment is greater. The proportion of introduced species and of novel interactions caused by introduced plant or animal species was higher for oceanic island systems than for continental bioregions. This finding adds a new dimension to the already recognized heightened susceptibility of remote islands to invasion and their loss of native species. Continental bioregions’ networks typically had few introduced animals and a greater prevalence of intro plants than animals.
Fricke and colleagues think plant-frugivore networks are likely to increasingly favor a relatively few introduced generalists over many native species, reducing the uniqueness of future biotas. The result might be to reduce resilience of terrestrial ecosystems by, first, allowing perturbations to propagate more quickly; and, second, by exposing disparate ecosystems to similar drivers. They called for giving higher priority to managing increasing ecological homogenization.
In the second article, Fricke, Ordonez, Rogers, and Svenning (2022) note that climate change requires many plant species to shift their populations hundreds of meters to tens of kilometers per year to track their climatic niche. Earth is also experiencing the formation of novel communities as species introductions and shifting ranges result in co-occurrence of species that do not share co-evolutionary history. They conclude that the novel mutualistic interaction networks will influence whether certain plant species persist and spread.
These authors examined four scenarios to assess how current long-distance dispersal has been affected by past defaunation and invasion and how it is threatened by species endangerment. These scenarios are as follows:
1st scenario (current scenario) = natural and introduced ranges of extant species today.
2nd scenario (natural scenario) = mammal and bird ranges as they would be if unaffected by extinctions, range contractions, or introductions.
3rd scenario (extinction scenario) = those bird and mammal species listed as vulnerable or endangered by the IUCN go extinct.
4th scenario (extirpation of introduced species scenario) = introduced species are extirpated.
Fricke and colleagues estimate that extinction of at least local populations of seed-dispersing mammals and birds has already reduced the capacity of plants to track climate change by 60% globally. The effect is strongest in temperate regions and regions with little topographic complexity. Two examples are eastern North America and Europe. These regions face a double threat: rapid climate change and loss of large mammals that provided long-distance dispersal.
The extinction scenario is most evident in Southeast Asia and Madagascar. The remaining animal seed dispersers are already threatened or endangered. Fricke and colleagues project that future loss of vulnerable and endangered species from their current ranges would result in a further reduction of 15% in the capacity of plants to track climate change.
The contrary situation is found on islands which have few native mammals. Introduced species are now important long-distance seed dispersers. In some cases, the introduced animals are dispersing invasive plant seeds, e.g., on Hawai`i feral hogs are spreading the invasive plant strawberry guava (Psidium cattleianum).
People’s actions have resulted in ecoregions disproportionately losing the species that provide long-distance seed dispersal function, i.e., large mammals. In other words, human activities have caused not only rapid climate change—requiring broad-scale range shifts by plants—but also defaunation of the birds and mammals needed by plants to do so. Habitat fragmentation and other land-use changes will likely amplify existing constraints on plant range shifts.
Fricke and colleagues say their findings emphasize the importance of not only promoting habitat connectivity to maximize the functional potential of current seed dispersers but also restoring biotic connectivity through the recovery of large-bodied animals to increase the resilience of vegetation communities under climate change.
SOURCES
Fricke, E. C., & Svenning, J. C. (2020). Accelerating homogenization of the global plant–frugivore meta-network. Nature, 585(7823), 74-78. https://www.nature.com/articles/s41586-020-2640-y
Fricke, E. C., Ordonez, A., Rogers, H. S., & Svenning, J. C. (2022). The effects of defaunation on plants’ capacity to track climate change. Science, 375(6577), 210-214. https://www.science.org/doi/full/10.1126/science.abk3510
Posted by Faith Campbell
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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