Dispersal evolution can speed invasions despite exacerbating a mate-finding Allee effect

Background/Question/Methods

A key characteristic of whether a newly introduced species will be invasive is the rate at which it is able to spread in a new environment. Spread rate (or invasion speed) depends in large part on dispersal ability, which may differ among individuals (e.g. according to sex). For sexually reproducing organisms, another factor influencing spread is the ability to mate and reproduce successfully. Difficulty finding mates can cause an Allee effect where population growth decreases at low densities. Individuals on the edge of an invasion front are by definition at low density, and therefore likely subject to mate-finding Allee effects. However, most invasion models do not explicitly account for males and females separately and typically ignore the role of mate-finding Allee effects in invasions. Here we develop a two-sex model to determine how mating system, strength of mate-finding Allee effect, and dispersal evolution interact to influence the invasion speed.

Results/Conclusions

We show that mating system has a strong effect: in the extreme it can cause an order of magnitude difference in invasion speed. We find that allowing dispersal to evolve in a spreading context can sometimes alleviate the mate-finding Allee effect and slow the rate of spread. However, we show that when resource competition among females is high, evolution actually acts to speed up to rate of spread, despite simultaneously exacerbating the Allee effect. Our results indicate that although dispersal evolution in the presence of a mate-finding Allee effect can indeed slow an invasion, it may be less likely to do so than previously expected.