The fact that species are being lost at an unprecedented rate is not in dispute, but how can conservation biologists who are trying to create protected areas account for extinctions which are occurring today because of events in the past?
Extinction is a natural process, but the current rate of species loss – at least 100 times what would be expected under normal conditions1 – is anything but natural. It is common knowledge that species are being lost as a result of human activities, but the fact that extinctions can occur because of historical events as well as contemporary pressures is less well known. This phenomenon is called extinction debt2 and has been documented by researchers working in a range of habitats3, 4, 5.
When an area of habitat becomes fragmented, the isolated patches which remain are not able to support the same array of species as the intact site due to a reduction in the amount of available resources. Over time, many of the species trapped within the patch will die off until, eventually, a new equilibrium is reached and the patch is only occupied by the species that it is able to support. The time taken for equilibrium to be reached is known as “relaxation time”3, because the habitat patch is relaxing back to equilibrium after a considerable disturbance.
In 2012, researchers from Japan studied the diversity of butterflies in a range of habitat patches scattered across Tokyo. They discovered that existing species richness was more closely correlated with the habitat conditions of 1971 than present day habitat conditions5, an indication of an extinction debt that has yet to be paid. They also mapped the predicted extinction debt of different habitat patches and found that the loss of species from large patches is likely to be lower than the loss from small patches5.
Maps which indicate where biodiversity is likely to decline in the future can be used to inform prioritization. Instead of focusing limited resources on the protection of areas that are projected to lose biodiversity, conservation practitioners can more optimally focus their efforts on areas where species are more likely to persist into the future. Prioritization can even be based around the newly emerging concept of “conservation credit”6, which accounts for the colonization of newly suitable habitat by species from other areas.
However, it is important to remember that the disturbances and fragmentation leading to species loss through extinction debt are, in the majority of cases, the result of human activities. Extinction debt doesn’t just illustrate the complexity of the current biodiversity crisis; it emphasizes the importance of protecting habitat in the present in order to secure biodiversity into the future.
1. Pimm, S.L., and C.N. Jenkins. 2005. Sustaining the Variety of Life. Scientific American 293: 66 – 73
2. Tilman, D., R.M. May, C.L. Lehman, and M.A. Novak. 1994. Habitat destruction and the extinction debt. Nature 317: 65 – 66
3. Diamond, J.M. 1972. Biogeographic kinetics: estimation of relaxation times for avifaunas of southwest Pacific islands. PNAS 69: 3199 – 3203
4. Krauss, J., R. Bommarco, M. Guardiola, R.K. Heikkinen, A. Helm, M. Kuussaari, and R. Lindborg. 2010. Habitat fragmentation causes immediate and time‐delayed biodiversity loss at different trophic levels. Ecology Letters 13: 597 – 605
5. Soga, M. and S. Koike. 2012. Mapping the potential extinction debt of butterflies in a modern city: implications for conservation priorities in urban landscapes. Animal Conservation 16: 1 – 11
6. Vellend. M., and H.M. Kharouba. 2013. Setting conservation priorities when what you see is not what you get. Animal Conservation 16: 14 – 15