Using Invasion History to Quantify Equilibrium in Over 250 Invasive Plant Species in North America

Abstract

Plant invasions pose an increasing global threat to ecosystem function and native biodiversity. Understanding the current and potential future distributions of harmful invaders is the key to developing efficient and effective management strategies. One widely used approach to achieve this is the use of species distribution models. 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 intro-duced 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. In this study, I used patterns of expansion in climate space over time to determine if an in-vading species has reached stable climate distribution (i.e., climatic equilibrium) in its introduced range, and estimated how long this process took relative to its first reported collection in North America. To quantify the expansion pattern of each species, I compared the climates occupied by a species in their native and introduced ranges throughout their invasion history. I found that only a small subset (15.2%) of species were actively expanding at a constant rate, indicated by single-phase, linear pattern on expansion through time. Further, the equilibrium assumption (defined as ≥30 years of no significant change in expansion) was only supported for less than half (42.3%) of the species analyzed. Of these species, the time it took to reach equilibrium varied between 20 and 120 years relative to its first reported collection in North America. I also found that, on average, species at equilibrium only occupy 95% of the suitable climates available in their invaded range. This could indicate that some species may be experiencing temporary equilibria caused by intrinsic or extrinsic barriers in the invaded range. My findings suggest that a temporal approach is neces-sary to evaluate equilibrium, and that it is possible that many North American plant invaders have not yet stabilized their climatic distribution. The methods outlined in this study lay the groundwork for developing a standardized approach to evaluating equilibrium, and not just in invasive plants. To my knowledge, this study represents one of the first to use a temporal approach to describe the equilibrium status of a large group of introduced species.