A new framework for investigating biotic homogenization and exploring future trajectories: oceanic island plant and bird assemblages as a case study

Abstract

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.

The Homogocene: a research prospectus for the study of biotic homogenisation

“…Stemming from the fact that the mechanisms responsible for shaping beta-diversity vary from local to regional spatial scales (McGill et al 2015), a more explicit consideration of spatial scale is called for in future investigations of biotic homogenisation. Of additional concern is the fact that past investigations of biotic homogenisation have largely focused on reconstructed species pools from published lists of native, extinct and/or non-native species, thus limiting comparisons to be made between some "historical" baseline and the present day (Rosenblad and Sax 2017). This consequently hampers the ability to estimate rates of changes and detect transitory states in both increases (homogenisation) and decreases (differentiation) in similarity.…”

“…Although the reporting of average similarity change continues today, it is reassuring that both univariate and multivariate tests of significance are increasingly deployed (e.g. Liu et al 2017, Rosenblad and Sax 2017, Strecker and Brittain 2017 and the use of null models appears to be fruitful despite being under-utilised in the context of homogenisation.…”

Effect of distance, area, and climate on the frequency of introduction and extinction events on islands and archipelagos

“…Two main frameworks are used to categorize the types of introductions and extinctions between paired assemblages Poff 2003, Rosenblad andSax 2017; Appendix S1: Fig. S1).…”

“…S1). Unlike Olden and Poff (2003) who use 14 potential scenarios to predict levels of biotic homogenization or differentiation, Rosenblad and Sax (2017) and Longman et al (2018) provide a more concise framework with just seven types of introduction and extinction events to characterize similarity dynamics. These seven types may be summarized as follows: (1) I01, introduction of a species to one site; (2) I02, introduction of a species to both sites; (3) I12, introduction of a species present on one site to the other site; (4) E10, extinction of a species present on only one site; (5) E20, extinction of a species from both sites; (6) E21, extinction of a species present on both sites from one site; and (7) E1/I1, extinction of a species from one site and the introduction of the same species to the other site.…”

“…S1). S1), Rosenblad and Sax's (2017) framework with only seven event types represents a fundamental classification of introduction and extinction events, which allows us to disentangle the factors associated with each type of event. S1), Rosenblad and Sax's (2017) framework with only seven event types represents a fundamental classification of introduction and extinction events, which allows us to disentangle the factors associated with each type of event.…”