Non-native mammals are homogenizing island faunas worldwide
Human actions have drastically altered the global distribution of organisms. Increased trade and travel have resulted in the introduction of species into areas beyond their native ranges and direct consumption, overexploitation and habitat destruction have driven species to extinction. The result of these actions is biotic homogenization, or the gradual replacement of native biotas with non-native generalists. In order to determine the extent and drivers of homogenization, we created a database of the native and non-native mammals on 48 continental and oceanic islands worldwide at multiple points in time and analyzed how human actions have changed that composition. We divided the mammalian assemblages into two time periods: the prehistoric period (first human settlement to first contact with Europeans) and the historic period (first European contact to the present) and calculated the change in Jaccard’s index of similarity for all 1,128 possible island pairs to measure the degree of similarity of mammal populations during both time periods. We analyzed several factors that might drive changes in the degree of similarity and examined differences in trends between continental and oceanic islands and during the two time periods. Finally, we made projections for future similarity levels assuming that past trends continue.
Island mammal communities have significantly homogenized since human settlement with a mean increase in similarity of 0.1256. In spite of 60 recorded species’ introductions and 55 extinctions during the prehistoric period, there was little measured change in mammal community similarity across continental or oceanic islands during this timeframe since most of these events seem to have been unique. In contrast, during the historic period, mammalian communities have clearly become homogenized with a mean increase in Jaccard’s index of similarity of 0.0954, particularly as a consequence of widespread species introductions. This homogenization has been most pronounced on oceanic islands, which experienced 971 I02 biotic turnover events, with the same non-native species being introduced to both islands in a pair. We show analytically that if current trends continue, islands will further homogenize, although the future level of homogenization will be greater overall on oceanic island pairs than on continental island pairs. Using these analyses, we can prioritize future management plans to halt homogenization and protect biodiversity, with a focus on preventing future introductions of mammals into island ecosystems.