The dispersal paradox: Are good dispersers poor invaders?

Are certain life history characteristics favorable for exotic invasion? One paradigm is that high dispersal ability is important for a species to be a successful invader. In this study we provide evidence that high dispersal ability can be detrimental to species invasion, particularly at the establishment phase, but also at the spread phase of invasion.  The establishment phase of invasion is usually characterized by a single low-density population. Strong Allee effects, defined as negative population growth rates at low population densities, thus, may be important barriers to establishment. We demonstrate a negative relationship between dispersal ability and both establishment and spread in a population model with strong Allee effects. Specifically, high dispersal resulted in negative population growth because the diffusive effects of dispersal caused population density to drop below the Allee threshold, resulting in a failure to establish. Conversely, a species with low dispersal ability maintained population densities above the Allee threshold, resulting in a positive population growth rate and establishment. Model simulations also showed that high dispersal ability can result in a slower rate of spread, stasis (invasion pinning), or range contraction. Which of these patterns emerges is dependent on the strength of the Allee effect (i.e. the slope of the relationship between population size and growth rate around the Allee threshold). The dispersal paradox may explain counterintuitive invasion failures and successes.  For example, why the European gypsy moth, which has flightless females, has so successfully invaded North America.  In contrast, the Asian gypsy moth, which has females capable of flight, has been a much less successful invader despite its greater dispersal ability.  We suggest that a meta-analysis of the dispersal abilities of biocontrol successes and failures, similar to that of Hopper & Rouch (1993), would provide a good test of the dispersal paradox.