Habitat Destruction and the Extinction Debt Revisited

A very important published analysis of the problem of habitat destruction (Tilman et al. 1994 (Nature 371:65-66)) concluded that such destruction may lead to an extinction debt, which is the irreversible, preferential loss of competitive species following a prolonged transient or delay after the habitat destruction. We analyzed this model using analytic and simulation techniques. Relating this analytic model to real-world situations shows that it applies to scattered permanent devegetation of small patches and to across- the-board decreases in fecundity such as could be caused by pollution. For repeated spatially random disturbance, we develop a new model that shows an even more severe extinction- debt effect. For larger fragments of remnant vegetation, such as forest woodlots, we argue that the assumptions of the model are violated but that an extinction debt nevertheless occurs due to gradual stochastic elimination of species that are very rare and isolated in these fragments. For habitat destruction on regional scales (reduction in ecosystem area without disturbance in remnant areas), one must, in contrast, apply species-area relations based on the distribution of different habitat types (e.g., elevational and rainfall gradients; physiographic and edaphic variability). Such an analysis predicts rapid, not delayed, loss of all types of species, not just competitive types. We conclude that the extinction-debt effect is real and arises in three different models, but relating the models to real-world conservation problems must be done with care.