Extinction rates under nonrandom patterns of habitat loss

Abstract

Most models that examine the effects of habitat conversion on species extinctions assume that habitat conversion occurs at random. This assumption allows predictions about extinction rates based on the species-area relationship. We show that the spatially aggregated nature of habitat conversion introduces a significant bias that may lead species-loss rates to exceed those predicted by species-area curves. Correlations between human activity and major compositional gradients, or species richness, also alter predicted species extinction rates. We illustrate the consequences of nonrandom patterns of habitat conversion by using a data set that combines the distribution of native vascular plants with human activity patterns in California.habitat-conversion ͉ California ͉ species-area ͉ conservation ͉ biodiversity H umans have caused numerous species extinctions (1-9) primarily through the conversion of natural habitat into land dominated by agriculture and other anthropogenic activities (3,(10)(11)(12)(13)(14). Despite attempts to conserve global biological diversity, habitat conversion rates are accelerating, particularly in tropical countries (14). Policies for the preservation of global biodiversity rely on accurate predictions of the effects of habitat conversion on species distributions (15-17).Attempts to estimate the relationship between habitat loss and extinction rates have focused on species-area relationships (6,(18)(19)(20)(21)(22) and metapopulation models (23)(24)(25)(26)(27)(28). Although these methods differ in their predictions about the types of species that are most at risk and the delay between conversion of habitat and extinction (i.e., the ''extinction debt'') (29), both approaches assume random habitat loss (26,29). This assumption ignores two related biases. First, the distribution of species is influenced by underlying habitat characteristics such as rainfall, soil quality, and elevation. Secondly, these characteristics influence the rate at which areas are converted to human-dominated land uses. Specifically, humans have clear habitat preferences for coastal or other low-lying lands with adequate supplies of water (30, 31). The rate of habitat conversion is significantly faster in these areas than in areas less suitable for agriculture (26). Consequently, habitat-loss rates may be correlated with gradients of species composition and diversity (Fig. 1).Furthermore, extinction rates will be sensitive to the specific distribution of species across the landscape as well as the spatial configuration of habitat conversion (32-34). A correlation between habitat-loss rate and species richness may cause extinction rates to deviate from the predictions of the species-area relationship. If species-rich areas are lost first, we would expect that initial species extinction rates to exceed those predicted under the assumption of random habitat loss.The systematic nature of anthropogenic habitat conversion also changes the relationship between species extinctions and habitat loss, because species turnover ...

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