The paradox of anthropogenic enrichment: homogenization and downsizing of insular mammals

Oceanic islands are global hotspots of biodiversity – many of them harboring marvels of evolution in isolation. Unfortunately, insular biotas are also highly susceptible to extinction, especially following colonization by humans. Here, we assess the influence of humanity on the diversity and biological distinctiveness of mammals inhabiting 37 oceanic islands. We compiled lists of mammals inhabiting these islands prior to and then following colonization by hominids (including Homo erectus and H. sapiens ). We then quantified the dynamics in diversity (as measured by species richness) and distinctiveness (measured as beta-diversity) among islands. We compared mammalian assemblages on islands prior to humanity, following colonization by early hominids and then following colonization by H. sapiens (in the latter case, separating assemblage dynamics resulting from extinctions of native species from the effects of species introductions). As expected, early hominids hardly influenced mammalian diversity or distinctiveness. In contrast, colonization by H. sapiens was initially followed by numerous extinctions and substantial declines in species richness, which then however rebounded to exceed pre-humanity levels. These post-humanity increases in species richness were paradoxically accompanied by substantial declines in distinctiveness among islands. This paradox of anthropogenic enrichment is readily resolved by observing that species introductions to the islands (the sources of the post-humanity surges in species richness) were comprised of a highly redundant set of small species (primarily rats and house mice), resulting in the homogenization and downsizing of island life.