What is the future of biotic homogenization?

A red junglefowl, (Gallus gallus) on Rarotonga in the Cook Islands, where G. gallus is naturalized.

By Kyle Rosenblad

Since McKinney and Lockwood’s (1999) seminal work, a growing body of research has confirmed that human activity is increasing taxonomic similarity among species assemblages worldwide through introduction of non-native “winner” species and extinctions of locally endemic “loser” species. This pattern persists across taxonomic groups, time scales, and spatial scales. However, despite growing concerns regarding the conservation implications of increasing biotic similarity, little effort has been devoted to exploring how the patterns of change observed in the past might inform our understanding of future changes.

In this investigation, we used oceanic island bird and vascular plant assemblages as a case study to unpack the key factors that have driven past changes in taxonomic similarity, and to explore how similarity could change in the future if these factors continue behaving in the same way. We found that although both oceanic island birds and vascular plants have experienced comparably low levels of homogenization to date, if the observed patterns of species introduction and extinction continue in the future, then their trajectories will diverge, with birds homogenizing much more drastically, and plants homogenizing only slightly more before stabilizing at relatively low levels. This finding—that moderate past changes in similarity can mask the potential for drastic changes in the future—only becomes apparent when we unpack the factors that have driven the past changes.

A Kereru (aka New Zealand pigeon, Hemiphaga novaeseelandiae) perched in a ti kouka (aka Cabbage tree, Cordyline australis) on Banks Peninsula, South Island, NZ. Both the Kereru and the Cabbage tree are endemic to New Zealand.

To elucidate the drivers of past homogenization in our study system, we focused on two factors that have received little attention to date: initial similarity and the relative frequencies of six types of introduction and extinction events. (The six event types are defined according to whether the same or different species are being introduced or going extinct between assemblages in the system.) We developed formulas for the effect of each event type on taxonomic similarity based on the initial similarity level of the system. For some event types, different initial similarity levels will strengthen or weaken their homogenizing or differentiating influence, whereas other event types do not change in strength. When we applied this framework to our study system, we found that island plant and bird assemblages have homogenized by comparably small amounts since first human settlement (about 3% for plants and 4% for birds), but that these comparable levels of change were driven by starkly different introduction and extinction regimes. Homogenization of plant assemblages was primarily driven by exotic species that were introduced to two or more islands, which increase compositional similarity by giving their receiving assemblages more species in common. In contrast, homogenization of bird assemblages was primarily driven by extinction of native species that were not present on other islands—i.e., losses of species that made assemblages unique. Both plant and bird assemblages also experienced many unique introductions (i.e. exotic species that were not introduced to other islands in the system), which differentiate assemblages rather than homogenizing them, but at the low initial similarity levels observed in this study (about 5% for plants and 2% for birds), the differentiating effects of these introduction events are overridden by the stronger homogenizing influences of other events.

Although the different introduction and extinction regimes observed in the past have driven roughly the same degree of homogenization for plants and birds, we found that those same regimes would yield drastically divergent trajectories if they continue in the future. Using the formulas we developed for each event type, we calculated what similarity end-points our plant and bird assemblages would reach (or approach asymptotically) if the same types of introduction and extinction events we observed in the past continue occurring at the same rates over time into the future. We found that plant assemblages would only homogenize about 50% again as much as they already have, whereas bird assemblages would homogenize roughly 500% more. On a scale of 0 to 1, these changes would bring plants to an average similarity level of about 0.1, and birds to about 0.7.

Our results demonstrate that, in order to understand patterns of biotic homogenization, it is necessary to examine not just the numbers of introductions and extinctions that are occurring, but also the context—whether the same or different species are appearing or disappearing across assemblages—and the initial similarity level in the system where change is occurring. When these key factors are examined, it is possible to extrapolate future changes in similarity as a thought experiment—i.e., what will happen if the same process observed continue to operate? In our system, this thought experiment revealed that similar patterns of past change can hide marked differences in future trajectories. There are many reasons why the assumption of unchanging introduction and extinction regimes might not always hold in the future, and subsequent studies could build on our framework to incorporate the possibility of dynamic introduction and extinction regimes that do not necessarily remain constant in the future. Our framework represents a first step toward better understanding of the mechanisms underlying past changes in biotic similarity, and how those mechanisms could play out into the future.

References:

McKinney, M. L. and Lockwood, J. L. 1999. Biotic homogenization: a few winners replacing many losers in the next mass extinction. – Trends Ecol. Evol. 14: 450–453.

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