Extinction debt and the species–area relationship: a neutral perspective
To estimate the magnitude of delayed relative to imminent extinctions and to assess the importance of delay as a potential source of error in forecasts of extinction. To formulate a simple mechanistic model using neutral theory that links extinction debt with the species–area relationship (SAR).
We use the neutral model of biodiversity to describe how a community subject to immigration responds to an insular contraction. We investigate the species richness at different times after the habitat-loss event. We compare this with observed species losses in avian studies.
From the model, two SARs emerge: one with a shallow slope for a habitat area before habitat loss and another with a steeper slope for the habitats that remain after habitat loss. From these curves, the first predicts imminent extinctions while the second predicts total extinctions. The difference between the two curves gives the delayed extinctions, namely the number of species that are lost during the relaxation of the community to equilibrium. The model agrees well with observed relaxation rates in communities of birds. The lag times for relaxation are often very large, with half-lives in the order of thousands of years for remnant areas above 5000 km2. In many parameter combinations explored, the majority of extinctions are delayed extinctions, and may exceed imminent extinctions by orders of magnitude.
Extinction debt is a major reason for failures to observe extinctions following habitat loss. Our modelling approach supports the view that a significant proportion of extinctions are delayed, so that the predictions of SARs (as currently applied) are liable to underestimate total extinctions. SARs are a valuable instrument for conservation but must be used with caution.