Comment on "Avian extinction and mammalian introductions on oceanic islands".

A key advance in conservation biology has been our ability to evaluate the relative importance of multiple drivers of extinction (1). Under this modern paradigm, species extinctions can no longer be explained solely by habitat loss, human hunting, climate change, disease, or species invasions operating alone, but rather by synergistic combinations of extrinsic factors operating on species with diverse sets of intrinsic traits (2–4). Nevertheless, Blackburn et al. (5) recently concluded that much of the variation in avian extinction rates across 220 oceanic islands could be explained by a strong positive correlation with the number of exotic predatory mammal species established since European arrival. Furthermore, this conclusion was made without explicitly quantifying the degree to which other potentially intercorrelated drivers of extinction risk (notably habitat conversion) covaried with introduction success. This is surprising, as these authors have independently assessed the importance of habitat conversion for global avian biodiversity loss (6). Rolett and Diamond (7) estimated habitat loss rates for 69 Pacific islands, providing an opportunity to test whether avian extinctions also correlate with habitat loss across islands. For the 44 islands common to that study and the Blackburn et al. study (5), avian extinction rates are significantly positively correlated with pre-European deforestation scores (pre-European extinctions: Pearson_s r 0 0.414, n 0 44, P 0 0.005; total extinctions: r 0 0.395, n 0 44, P 0 0.008) (Fig. 1A) (8, 9). There is likely to be spatial autocorrelation in extinction and deforestation rates among islands within archipelagos, but our conclusion still holds if the relation is analyzed at the archipelago level (total extinctions: r 0 0.573, n 0 14, P 0 0.033). Furthermore, although total mammalian introductions are not significantly correlated with pre-European deforestation (r 0 0.161, n 0 44, P 0 0.296), they are significantly correlated with the Rolett and Diamond forest replacement index (7) (r 0 –0.621, n 0 44, P G 0.0001) (Fig. 1B). Again, the same conclusion holds at the archipelago level (r 0 –0.744, n 0 14, P G 0.002). This suggests that the same environmental and biogeographic factors that control native forest replacement (7) may also determine the success of mammalian introductions (5). Unfortunately, the deforestation and forest replacement scores are coarse measures of habitat conversion and are available only for a small subset of islands. Thus, there is no statistical power to test the interaction between habitat conversion, species invasion, and biogeographic factors such as island area. However, our objective is not to dispute the importance of mammalian predators in causing some avian extinctions on islands or to discount the importance of island biogeographic factors, but rather to show that habitat conversion is strongly intercorrelated with both extinction and introduction rates, leaving absolute causality equivocal for avian extinctions in general. Given the strong intercorrelation between habitat conversion and species invasion (10), the relative importance of mammalian introductions cannot be determined without explicitly quantifying habitat loss rates across islands. Although quantitative habitat loss rates are lacking for all 220 islands, previous studies using Pimm_s species loss function (11) have shown that habitat loss is a remarkably accurate predictor of past extinction rates (11, 12) and current extinction threat (13) on 31 island archipelagoes (12–14). In fact, whenever accurate habitat loss data are available, they are invariably so highly correlated with observed extinction rates that they confound any attempt to interpret extinctions in the light of species invasions alone. For example, habitat loss rates are sufficient to predict observed extinction rates within one island archipelago, New Zealand, for which comprehensive avian extinction and habitat loss data are available. Pimm_s species loss function (11) predicts that New Zealand should have lost 31.5 to 41.1% of its original species following È78% habitat conversion (12), yet the archipelago has lost only 28.6% (70 of 245 species) of breeding birds (including 37.9% of endemic species) (Table 1) (15, 16). Of course, extinction rates of the most severely affected terrestrial bird families, on the most heavily invaded islands, are almost twice as high as for the archipelago as a whole (Table 1). However, there has also been far greater habitat loss and fragmentation on large islands, such that extinction rates within islands are still only marginally (1 to 4%) higher than predicted from absolute forest loss (Table 1). This is not to say that habitat loss is the sole explanation for a high rate of bird extinctions in New Zealand, but it is certainly an important factor to quantify in addition to mammalian introductions. Given unequivocal evidence that predation by introduced mammals has driven some island species to extinction (2, 5, 15, 17) and equally incontrovertible evidence that habitat loss causes population declines to TECHNICAL COMMENT