Allee effects, aggregation behavior, and invasion success

Background/Question/Methods
Understanding the factors that influence successful colonization can help inform ecological theory and biogeography, and aid in the management of invasive species.  Spatial structure coupled with demographic rates are strong determinants of the establishment and persistence of migrants/invaders.  When founder populations are small, individual fitness may be negatively impacted by component Allee effects through inverse density dependence (e.g., ability to identify mates).  Reproductive and survival mechanisms that suffer due to a lack of conspecifics (i.e., component Allee effects), often scale up, manifesting in a decreased per capita population growth rate (i.e., demographic Allee effects).  Phenomenological models based on mean-field assumptions and demographic Allee effects may not adequately capture how component Allee effects scale up when heterogeneous spatial structure influences these behaviors.  Hence, general conclusions based on the dynamics of demographic Allee effects alone, may fail to account for the importance of spatial structure and inaccurately predict establishment.  In order to better assess the probability of establishment, we developed a spatially-explicit individual-based stochastic simulation. 

Results/Conclusions

While our model generally describes demographic consequences of component Allee effects, emergent characteristics of successfully established populations provide insight into the ecological conditions that produce such results.  In particular, we found that the formation of clusters (i.e., spatial heterogeneity) in homogeneous, continuous landscapes significantly affects the population-level reproductive rate, which is a strong predictor of the probability of establishment.  Since this reproductive rate is sensitive to the scaling up of component Allee effects, details of the dispersal and interaction kernels are important drivers of the probability of establishment.  Overall, we demonstrate the importance of considering spatial structure and individual-level traits in assessing the consequences of Allee effects in biological invasions.