The importance of coexistence mechanisms for the magnitude and time scale of extinction debt

Background/Question/Methods

Current biodiversity loss is driven largely by habitat loss. Ecological theory and experiments suggest that this habitat loss, and associated habitat fragmentation, will cause additional species loss above and beyond initial extinctions, i.e. that we have an 'extinction debt'. Some studies estimate the magnitude of this extinction debt to be quite large relative to immediate extinctions. Ecological theory of extinction debt is relatively simplified and is ripe for further development. Missing in particular is consideration of how the mechanisms of competitive coexistence--in particular the relative prevalence of niche and neutral coexistence, and the strength of stabilization under niche-based coexistence--can influence the magnitude and time scale of extinction debt. Here we develop a community assembly model framework for studying the influence of coexistence mechanisms on the magnitude and time scale of extinction debt, and report on preliminary results arising from the study of this model.

Results/Conclusions

We develop a spatial metacommunity model in which species compete with one another based on their positioning along a trait axis. This model can be used to consider a range of coexistence scenarios from neutral to niche-based, including a combination of niche and neutral coexistence in the form of emergent groups of similar species arising at separated locations in the trait axis, by varying the functional dependence of competition on trait distance and other model parameters. We also illustrate how to vary the strength of stabilization among niche-differentiated species by varying model parameters. We also add species-specific asynchronous environmental fluctuations in this framework, as a recent study suggests such fluctuations are prevalent and should be incorporated into community assembly theory. Our preliminary explorations of this model suggest coexistence mechanisms, as well as the magnitude of asynchronous environmental fluctuations, do have a critical influence on the magnitude and time scale of extinction debt, as well as on the functional role of species to be lost. In particular, increasing the number of species in the community that are niche differentiated reduces the magnitude, and increases the time scale, of extinction debt, but also increases the complementarity of the species lost.