Adaptation and propagule pressure determine invasion dynamics: insights from a spatially explicit model for sexually reproducing species

Propagule pressure is often considered to be one of the key factors determining the probability that a species becomes invasive. However, there has been relatively little work, either empirical or theoretical, that has tested this assertion. Here, we develop a patch occupancy model for diploid organisms that reproduce sexually, and use it to investigate the dynamics of an introduced species. The model produces some interesting insights: If the introduced individuals are already well-adapted to the wild conditions, then the number of escapees is always larger when there are more introductions. However, when the introduced individuals are relatively poorly adapted to the natural conditions, this is often not the case. Under these conditions, the most rapid invasion occurs for an intermediate number of introductions. When the number of introductions is high, adaptation to local conditions is dramatically slowed and in some cases a well-adapted population never establishes. Thus, high propagule pressure does not necessarily lead to greater invasability. We believe that the model framework described in this paper can be a powerful tool for biologists interested in the genetics and evolution of invasive species, and spatially structured populations in general.