Studies of biotic homogenization have focused primarily on characterizing changes that have occurred between some past baseline and the present day. In order to understand how homogenization may change in the future, it is important to contextualize the processes driving these changes. Here, we examine empirical patterns of change in taxonomic similarity among oceanic island plant and bird assemblages. We use these empirical cases to unpack dynamic properties of biotic homogenization, thereby elucidating two important factors that have received little attention: 1) initial similarity and 2) the influence of six classes of introduction and extinction events. We use Jaccard's Index to explore the interplay among these factors in determining the changes in similarity that have occurred between human settlement and the present. Specifically, we develop general formulas for changes in similarity resulting from each of the six types of introductions and extinctions, so that the effect of each event type is formulated in terms of initial similarity and species richness. We then apply these insights to project how similarity levels would change in the future if the present patterns of introductions and extinctions continue. We show that the six event types, along with initial similarity, can show dramatically different behavior in different systems, leading to widely variable influences on similarity. Plant and bird biotas have homogenized only slightly to date, but their trajectories of change are highly divergent. Although existing patterns of colonization and extinction might not continue unchanged, if they were to do so then plant assemblages would show little additional change, whereas bird assemblages would become much more strongly homogenized. Our results suggest that moderate changes in similarity observed to date mask the potential for more dramatic changes in the future, and that the interaction among initial similarity and differential introduction and extinction regimes drives these dynamics.

Aim We documented how the similarity of mammal assemblages on continental and oceanic islands has changed since initial human colonization, since European arrival and overall. We investigated how levels of similarity might change in the future. Location Continental and oceanic islands worldwide. Time period Human settlement of islands to the present, as well as projections for the future. Major taxa studied Mammals. Methods We used mammal occurrence data on islands to calculate the change in similarity using a pairwise approach based on Jaccard's index and a multisite approach based on Jaccard's and Sørensen's measures. We divided the mammal assemblages into two time periods, before and after island colonization or trade began with Europeans. We unpacked the mechanisms driving changes in similarity, exploring how initial similarity interacts with seven types of species turnover events to determine overall change. Finally, we calculated how future similarity levels will change if past trends in introductions and extinctions continue. Results Mammals, on both continental and oceanic islands, show one of the most pronounced cases of homogenization ever observed, and on oceanic islands mammals show the largest increase in homogenization ever observed for a terrestrial group. Most of the homogenization observed to date has been driven by recent historical changes, not by changes that occurred before European arrival. If current patterns of species introductions and extinctions continue, then oceanic islands will experience little additional homogenization, whereas continental islands will homogenize greatly beyond current levels. Main conclusions Mammal assemblages on oceanic islands show nearly an order of magnitude greater change in similarity than plant and bird assemblages. Projections of future similarity indicate that continental and oceanic islands are on different trajectories of change. These trajectories could be altered by management actions, but in some cases those actions that would be impactful run counter to current conservation norms.