Dispersal polymorphism in an invasive forest pest affects its ability to establish

Given the increasing number of biological invasions, there is a crucial need to identify life history traits that promote invasion. Invasiveness reflects capabilities for both establishment after introduction and spread following establishment. In this paper, we explore, via simulation, the interacting effects of dispersal and Allee effects on both invasion processes. Dispersal capability is a trait that has been widely recognized to facilitate invasions. However, dispersal dilutes local population densities in isolated populations and this could strengthen Allee effects, ultimately promoting extinction of invading populations. A spatially explicit, stochastic individual-based model was used to simulate dispersal, mating, and growth in isolated, newly arrived invading populations. We used the invasion of North America by the gypsy moth, Lymantria dispar, as a case study because: (1) a great amount of biological data on the species is available; (2) Allee effects caused by mate location failure are known to play an important role in its establishment and spread; and (3) a dispersal polymorphism has previously been identified (i.e., in some populations adult females are fully capable of flight, but in other populations females are not able to fly). We considered the introduction of a hypothetical number of eggs at a single location, originating from populations with varying female dispersal capabilities, and we then used simulation models to evaluate the population growth rate over two generations as well as spread distance. Nondispersing populations had the highest growth rates and inclusion of even limited dispersal capabilities caused population growth rates to be greatly diminished. The Allee threshold was 700 eggs for nondispersing populations and 1400 eggs for the long-distance dispersing populations. Thus, for an intermediate number of eggs initially introduced, nondispersing populations would most likely establish, whereas dispersing populations would likely become extinct. Spread distance increased linearly with the number of eggs initially introduced in both dispersing and nondispersing populations but rapidly reached a limit for nondispersing populations. Though species capable of long-distance dispersal may invade a larger area than nondispersing species, their growth rates are likely to be considerably lower, and eradication should be easier. Following these results, strategies for managing invasions should be adjusted for the interactions between Allee effects and dispersal.