When natural habitat is cleared, some species go extinct immediately, but others only after a period of time—the latter constitute an “extinction debt”. More habitat loss generally leads to greater species loss and greater extinction debt. But does the spatial pattern of habitat fragmentation matter? This issue is currently the topic of fervent debate in the ecological literature (see, e.g., here and here and here).

In a new Ecology Letters paper led by Sam Thompson, our recently graduated Imperial-NUS PhD student, we developed analytical methods for calculating extinction debt after habitat fragmentation in a spatial neutral model, i.e., a model that treats all species equally. Sam’s paper built on previous work by our lab looking at the immediate response of fragmentation to species richness, i.e., before extinction debt has been paid. Our new methods for estimating extinction debt in a neutral model are accurate and efficient to compute. They involve first calculating two key metrics, termed effective area and effective connectivity, and then plugging these into formulas.

Modelling approaches such as these are invaluable for understanding species loss with habitat fragmentation, because of the difficulty of carrying out large-scale habitat fragmentation experiments. One general insight from our analysis is that for a fragmentation metric to be biologically meaningful, it should be based on the way that the affected species interact with the landscape, rather than on what looks “fragmented” to the human eye.

Overall, we found that even in a neutral model the effect of habitat fragmentation on species loss is non-trivial and varies with spatial scale, temporal scale, and the degree of fragmentation. If this is true even in a neutral model, surely it must be true in reality. We suggest that this degree of subtlety is sometimes missing from the ongoing fragmentation debate, and that any discussion of fragmentation and species richness should be informed by rigorous modelling.

The paper was also coauthored by James Rosindell, Sam’s advisor at Imperial College London.

Update: Our paper has been highlighted on Faculty Opinions.

Thompson, S. E. D., R. A. Chisholm, and J. Rosindell (2019). Characterising extinction debt following habitat fragmentation using neutral theory. Ecology Letters (in press)

Trajectories of species loss following habitat clearing under different model scenarios for tropical forest trees corresponding to an area of ~5 km². The shaded grey area to the right of each graph indicates extinction debt in every simulation, with light red and light blue areas additionally indicating the range of extinction debt across all simulations.