Increases in the diversity of island faunas and floras following human occupation

Background/Question/Methods

Human activities have caused many species extinctions, leading to a decline in the total number of species found globally. In contrast, to global declines, local and regional increases in the number of species have occurred in some cases as a result of human activities, i.e., whenever the total number of species introduced within a region exceeds the number of species that have become extinct. Documenting the number of extinctions, introductions and remaining native species within a region can be difficult in many cases due to a lack of historical records and uncertainty regarding the native status of many species. Islands, both oceanic and continental, provide discrete areas where such determinations can often be made with a high degree of certainty. Previous work on islands has shown that vascular plant and freshwater fish richness have both increased (as the number of introductions has exceeded the number of extinctions), but remained stable for land birds (as the number of introductions has largely matched the number of extinctions). Here we update this previous work and expand the number of taxonomic groups examined to provide a synthesis of changes in species richness on islands for vascular plants and terrestrial vertebrates.

Results/Conclusions

Species richness has changed in qualitatively and quantitatively different ways for different taxonomic groups on oceanic and continental islands, but in no cases do we find evidence for regional declines in richness. Instead, some groups have shown increases in richness, while others have shown largely stable richness, characterized by species turnover, but not net loss. The magnitude of change has generally been larger on oceanic than continental islands, but within taxonomic groups the qualitative nature of those changes is congruent. Vascular plants, freshwater fishes, mammals and herpetofauana have shown large increases in the total number of species. Birds have shown relatively stable net richness of species. Documenting these changes is important for understanding how diversity is changing at subglobal scales and how this varies among taxonomic groups. Indeed, these changes might have important implications for the provisioning of ecosystem services. Nevertheless, the loss of native species, and the potential homogenization of island biotas, provides a large and difficult challenge for the conservation of native species and ecosystems worldwide.