Allee effects, aggregation, and invasion success
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Understanding the factors that influence successful colonization can help inform ecological theory and aid in the management of invasive species. When founder populations are small, individual fitness may be negatively impacted by component Allee effects through positive density dependence (e.g., mate limitation). Reproductive and survival mechanisms that suffer due to a shortage of conspecifics may scale up to be manifest in a decreased per-capita population growth rate (i.e., a demographic Allee effect). Mean-field population level models are limited in representing how component Allee effects scale up to demographic Allee effects when heterogeneous spatial structure influences conspecific availability. Thus, such models may not adequately characterize the probability of establishment. In order to better assess how individual level processes influence population establishment and spread, we developed a spatially explicit individual-based stochastic simulation of a small founder population. We found that increased aggregation can affect individual fitness and subsequently impact population growth; however, relatively slow dispersal—in addition to initial spatial structure—is required for establishment, ultimately creating a tradeoff between probability of initial establishment and rate of subsequent spread. Since this result is sensitive to the scaling up of component Allee effects, details of individual dispersal and interaction kernels are key factors influencing population level processes. Overall, we demonstrate the importance of considering both spatial structure and individual level traits in assessing the consequences of Allee effects in biological invasions.
KeywordsBiological invasion Allee effect Spatial structure Individual-based simulation
We would like to thank Michael Buhnerkempe and the rest of the Webb Lab, Dan Ryan, Ben Bolker, Greg Dwyer, and three anonymous reviewers for helpful comments and advice. ARK acknowledges funding from NSF-IGERT Grant DGE-#0221595 administered by PRIMES at CSU, travel grants from the Global Invasions Network NSF-RCN DEB-#0541673 for facilitating this collaboration, and by the National Institute for Mathematical and Biological Synthesis, an Institute sponsored by NSF, the U.S. Department of Homeland Security, and the US Department of Agriculture through NSF Award #EF-0832858, with additional support from The University of Tennessee, Knoxville. RDH and MB thank the University of Florida Foundation for support.
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