Selectivity of terrestrial gastropod extinctions on an oceanic archipelago and insights into the anthropogenic extinction process

Anthropogenic impacts have led to widespread extinctions of species on oceanic islands but the nature of many of these extinctions remains poorly known. Here we investigate extinction selectivities of terrestrial gastropods from the Ogasawara archipelago in the northwest Pacific, where anthropogenic threats have changed over time, shifting primarily from the effects of habitat loss to predation by a variety of different predators. Across all of the islands, extinct species had significantly smaller geographic ranges compared with species that are still alive, but among the surviving species, ranges of those that are currently declining due to human impacts do not differ significantly from those that are not threatened. Extinctions were selective with respect to spire index (SI) of shells, a trait of potential functional importance, but the relationship between body size and extinction vulnerability varied among extinction agents, some of which were strongly size selective, whereas others were not. Overall, whereas anthropogenic impacts have resulted in nonrandom losses of phenotypic diversity, the patterns of selectivity are complex, vary among islands, and with the type of threat. As extinction agents have changed historically, so has the pattern of loss. Because of the changing nature of anthropogenic impacts, resiliency to one type of threat does not guarantee long-term survival of species and future patterns of biodiversity loss on these islands are likely to be different from those in the past.