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Predicting invasions: Propagule pressure and the gravity of the Allee effect.
Leung, Brian*,1, Drake, John1, Lodge, David1, 1 Dept. Biol., Univ Notre Dame, Notre Dame, IN, USA
ABSTRACT- Invasions by non-indigenous species impose large environmental and economic costs. In order to prevent invasions and target monitoring efforts most effectively, we need to forecast locations at the greatest risk of new invasions. To accomplish this, we need to estimate propagule pressure (inoculum size) and consider population processes, possibly including Allee effects. Here, we develop a method to estimate the probability of establishment, based on survival analysis and maximum likelihood techniques. We demonstrate theoretically the validity of this approach, considering environmental heterogeneity, estimation error, and non-linearity. We then apply this method to zebra mussel (Dreissena polymorpha) invasions of Michigan inland lakes. We fitted our model using presence-absence data between 1992-1996 and propagule pressure estimates from gravity models. Using our fitted model, we estimated the probabilities of establishment and demonstrated that Allee effects were present in the zebra mussel system. We validated our model using invasion data from 1997-2001, which was not used to parameterize the model. Using the validation data set, we correctly predict up to nine times as many invasions as the null (random) model. Further, the Allee model assigned average probabilities 4.5 times higher for lakes that became invaded compared to uninvaded lakes, whereas the non-Allee model only predicted probabilities two times as high. Thus, our model demonstrates the importance of considering the Allee effect and improves predictions of future invasions.
Key words: incidence model, gravity model, risk assessment, survival analysis