Spatial scale and evolutionary history determine the degree of taxonomic homogenization across island bird assemblages

It is widely documented that human activities have elevated the extirpation of natural populations as well as the successful introduction to new areas of non-native species. These dual processes of introduction and extirpation can change the similarity of communities, but the direction and magnitude these changes take are likely to depend on the manner in which introductions and extirpations occur, the spatial scale at which the changes are measured, and the initial similarity of the communities before the human-induced drivers occurred. Here, we explore patterns of extirpation and introduction and their influence on the similarity of global oceanic island bird assemblages from four different Oceans (Atlantic, Caribbean, Indian, Pacific). We show that different historical patterns of introduction and extirpation have produced varying trends in compositional similarity both between islands within archipelagos and between islands across different archipelagos within the same ocean. Patterns of bird assemblage convergence (i.e. taxonomic homogenization) or divergence (i.e. taxonomic differentiation) among islands depended on the scale of examination, the evolutionary associations among species of the region, and the cultural history of human colonization. These factors are all likely to be leading to a series of multiple interacting processes that are shaping the complex compositional changes observed among global island bird faunas over time.