A forthcoming study examines two important issues: interactions of pathogens’ spread and changing climate, and invasive species threats to tropical islands’ forests.
Underwood et al. (in press) analyzed how an introduced vascular wilt pathogen — Leptographium calophylli – is likely to affect a tree endemic to Madagascar’s already threatened mid-level elevation humid & subhumid forests, Calophyllum paniculatum (sorry; I can find no photographs of the tree species).
Climate change is expected to cause substantial shifts in temperature and precipitation patterns on the island. These temperature and moisture regimes in turn govern pathogen sporulation, infection efficiency, and survival. They also affect the host’s levels of stress and defenses. The direction of change is not certain, however. In some cases, warming and other changes to the climate might facilitate a pathogen’s spread, allowing it to track shifts in the host’s range and expand into previously unoccupied refugia. In other cases, these changes might erect environmental thresholds that limit the pathogen’s survival and spread, thereby creating spatial refugia for the host.
Environmental change increases the area of suitable landscape, that is, it weakens climatic barriers to establishment. Continued anthropogenic movement of some vector (biological or not) generates multiple introductory events over time. As a result, the likelihood of a successful establishment also increases, even if the probability per individual introduction is unchanged. Underwood et al. say that invasion outcomes thus become increasingly dependent on propagule pressure.
On many other tropical islands the threat from climate change is exacerbated by deforestation. On Madagascar, clearing driven by slash-and-burn agriculture and fuelwood harvesting has already reduced natural forest cover to less than 10% of its original extent. [For more on this topic, see e.g., Mittermeier et al. (2011).] Underwood et al. cite a determination by the ForestAtRisk model that humid forest in Madagascar could be almost entirely lost by 2100.
Loss of Madagascar’s forest has global implications. The island is one of 36 global biodiversity hotspots for both flora and fauna (e.g., lemurs). Its flora exceeds 12,000 plant species, of which 83% are endemic. In this case, the host tree species — Calophyllum paniculatum — is already considered vulnerable by the International Union for the Conservation of Nature (IUCN). Thus it is of global importance to understand the relative importance of several threats so that conservations can adopt the most effective countermeasures.
While they do not say so explicitly, it appears that Underwood et al. worry that too few of the conservationists active on Madagascar are paying attention to the possible impact of introduced pathogens. They note that pathogen-driven mortality of dominant or functionally unique trees can rapidly alter community structure and ecosystem function, potentially triggering local extinctions and cascading ecological consequences. For example, if an infection removes mature trees, their loss reduces fruit and nectar availability and so depresses populations of dependent wildlife. The trees’ death also diminishes above-ground carbon stocks and litter inputs. In combination, these impacts can shift community composition toward disturbance-tolerant states and heighten susceptibility at forest margins. These changes difficult to reverse once thresholds crossed.
This threat is not hypothetical. Since 2016 mature C. paniculatum at one site – a National Park – have been dying from a vascular wilt disease caused by a species in the Leptographium genus, probably Leptographium (formerly Verticillium) calophylli. While the species hasnot yet officially been recorded in Madagascar, it is established on neighboring Indian Ocean islands and across much of mainland Africa. Various species in the fungal genus are known to cause disease in other woody hosts. Underwood et al. suggest it was probably transported to Madagascar on infected wood, although they present no data.
Inside forests, Leptographium spp. are vectored by bark beetles in the Cryphalus genus. At least 25 Cryphalus species occur on the African Continent; some are vectoring disease on Seychelles and Mauritius.
The analysis by Underwood et al. indicates that future climatic conditions are likely to worsen the Leptographium calophylli infection over coming decades. The causal agent is likely to retain two-thirds of its current probable distribution and expand into previously uninhabited regions. The suitable habitat is expected to stretch across the entire north-south humid belt – the entire distribution of the host tree. Underwood et al. (in press) say it is even possible that the pathogen might remain in the forest, subsisting on other hosts, after C. paniculatum becomes functionally extinct across its range.
Meanwhile, that host – Calophyllum paniculatum – is projected to experience severe range shifts, with an overall net contraction across all climate change scenarios. It is forecast up to 67% of its current area by 2100. This range contraction will be compounded by fragmentation and dispersal limitation resulting from from deforestation. The refugia will be few and geographically isolated by late in the 21st century.
Are conservationists considering the implications of Leptographium calophylli’s probable persistence? Underwood et al. imply they are not; they say the impact of this and related pathogens on Madagascar & nearby islands is “still an unknown to the conservation community”. They urge their colleagues to conduct a set of research actions to identify, monitor, & limit the fungus’ spread – – and thereby improve the effectiveness of conservation efforts.
- Host range & other targets: determine whether L. calophylli infects other taxa in Madagascar – especially the endemic species and genera. They suggest systematic field sampling of multiple species across sites within the core probable range of L. calophylli. A trained pathologists should be consulted to officially identify the pathogen.
- Determine the spread phase of the pathogen. They suggest random sampling of species & sites within & outside of the fungus’ probable distribution, mapping the possible start point & dispersal patterns, including both anthropogenic & natural spread routes.
- Assess applicability of IPBES tools & suggestions for invasive species management to the case of a fatal pathogen in the context of tropical islands’ characteristics. How might Madagascar implement prevention, early detection & rapid response systems?
I applaud Underwood et al. for trying to alert the conservation community active on tropical islands to the simultaneous impacts of multiple global & regional change drivers on vulnerable species. Probably other host-pathogen systems are experiencing the same diverging trajectories that might intensify their biodiversity loss, particularly when compounded by deforestation.
SOURCES
Mittermeier, R.A., E.E. Louis Jr., M. Richardson, C. Schwitzer, O. Langrand, A.B. Rylands. 2010. Lemurs of Madagascar. Conservation International, Arlington, USA. ISBN 9781934151235
Underwood, E.L., K.A Brown, A. Ronnfeldt, M. Mulligan, N. Walford, R. Allgayer. In press. Climate change facilitates fungal pathogen expansion while driving endemic host range contractions in a tropical biodiversity hotspot. Research Square.
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
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