Using invasion history to test the assumption of environmental equilibrium in over 250 introduced plant species in North America

Species distribution models are a widely used approach in ecology and conservation biology. Despite the implications for the predictive accuracy of these models, the assumption that species are in equilibrium with their environment (i.e., they occur in all suitable areas) is poorly tested. This assumption is even more likely to be violated for introduced species that are still in the process of colonizing suitable environments. In the limited cases where this assumption is tested, researchers do not typically incorporate a temporal analysis of an invading species (i.e. invasion history), but rather evaluate the assumption from a snapshot in time. Here, we quantify the equilibrium status of over 250 introduced plant species (i.e., trees, shrubs, and forbs) in North America over their invasion history. Using models that compare the density of occurrence records of a species between two contexts, we evaluated the change in the proportion of available environmental space occupied over the invasion history, relative to the native range. For species showing evidence of having reached equilibrium, we estimated how long this took.

For most species (74%), the change in occupied environmental space over time was best explained by an asymptotic model, suggesting early rapid filling into available environmental space followed by a declining rate of invasion as they approached an apparent equilibrium. In contrast, there was a much smaller group of species (17%) for which there was no indication that the rate of invasion into available environmental space was slowing. Of the species with asymptotic curves, 66% have reached equilibrium in the past century while 34% appear to be close to reaching equilibrium now. For the former group of species, we found that it took on average 147 years, with a range of 60 to 244 years across species, after introduction in North America for the filling of available environmental space to stabilize. Our results highlight the importance of incorporating a species’ invasion history when evaluating the equilibrium of biological invasions, and provide new insights into conflicting assessments of the prevalence of niche shifts.