Expansion and contraction of range borders ultimately determine the distribution of a species and whether it is spreading, stable, or declining. Species’ borders are currently at the forefront of ecology due to increased concern about the impacts of global change on the distributions of native and nonnative species. Shifts in a species’ range can promote hybridization, influence community interactions, and alter ecosystem structure. Changes in climate and human land use patterns can alter border dynamics by opening new habitats to invasion or imposing environmental barriers to further spread. Understanding why species ranges expand, contract, or remain stable is critical to managing species invasions, preventing extinctions, and maintaining biodiversity in a changing world.
The classic approach to studying range limits and invasion potential treats a species’ range as a fundamental niche, where the current or native distribution is overlaid on environmental maps to characterize a species’ habitat space. This approach is useful for general predictions about range shifts, but does not always provide insight into underlying multi-scale processes and the resulting dynamics of edge populations. An alternative framework determines the quasi-equilibrium state of a species border from the combination of probabilistic colonization and extinction events. Recent studies have used this framework to test a broad range of processes that shape the dynamics of species borders, such as predator-prey interactions and dispersal limitation. These studies tend to focus on mechanisms at a single spatial scale , and a general challenge is to link models of species’ range processes across spatial scales.
In the Johnson Lab, we use a conceptual framework integrating physiological, evolutionary, ecological, and anthropogenic processes that shape invasion front populations to understand the dynamics of spatial spread. We examine multi-scale drivers of range edge dynamics and invasion rates using the nonnative gypsy moth, Lymantria dispar (L.), perhaps the most extensively monitored invasion in the world. Using this system, we connect range-defining mechanisms operating across local, landscape, and regional scales and construct invasion models to test the separate and combined effects of population and spatial processes on spread dynamics.
Virginia Commonwealth University is near the southern edge of the gypsy moth invasion front, a strategically ideal location to study the moth’s invasion patterns. I am involved in a multi-collaborator effort to look at the potentially interacting effects of climate, human movement, landscape characteristics, Allee effects, and natural enemies on gypsy moth invasion. We are studying physiological limitations, gene flow, local adaptation, and anthropogenic movement of gypsy moths.
In particular, we study the role of Allee effects in limiting gypsy moth population spread. Dynamics in low density populations are critical to the trajectories that drive persistence and extinction in rare species, as well as colonization and range expansion in pest species. The Allee effect is noted for its profound effects on persistence and spread (e.g., Johnson et al. 2006 Nature). Beyond the gypsy moth, despite theoretical evidence suggesting that Allee effects may be widespread, empirical evidence remains elusive, partially because masking effects in time series data (Walter et al. In Submission). I am currently developing Bayesian methods to overcome the difficulties in detecting Allee effects. This work will provide a new approach to studying Allee effects and result in important insights into the processes structuring the dynamics of species at low population densities, and informs both the management of endangered species facing declining population sizes and spread of invasive species.