Limitations of Gravity Models in Predicting the Spread of Eurasian Watermilfoil

: The effects of non-native invasive species are costly and environmentally damaging, and resources to slow their spread and reduce their effects are scarce. Models that accurately predict where new invasions will occur could guide the efficient allocation of resources to slow colonization. We assessed the accuracy of a model that predicts the probability of colonization of lakes in Wisconsin by Eurasian watermilfoil (Myriophyllum spicatum). We based this predictive model on 9 years (1990–1999) of sequence data of milfoil colonization of lakes larger than 25 ha (n =1803). We used milfoil colonization sequence data from 2000 to 2006 to test whether the model accurately predicted the number of lakes that actually were colonized from among the 200 lakes identified as being most likely to be colonized. We found that a lake's predicted probability of colonization was not correlated with whether a lake actually was colonized. Given the low predictability of colonization of specific lakes, we compared the efficacy of preventing milfoil from leaving occupied sites, which does not require predicting colonization probability, with protecting vacant sites from being colonized, which does require predicting colonization probability. Preventing organisms from leaving colonized sites reduced the likelihood of spread more than protecting vacant sites. Although we focused on the spread of a single species in a particular region, our results show the shortcomings of gravity models in predicting the spread of numerous non-native species to a variety of locations via a wide range of vectors. Resumen: Los efectos de las especies no nativas invasoras son costosos y daninos ambientalmente, y los recursos para disminuir su dispersion y reducir sus efectos son escasos. Los modelos que predigan con precision donde ocurriran nuevas invasiones podrian guiar la asignacion eficiente de recursos para disminuir la colonizacion. Evaluamos la precision de un modelo que predice la probabilidad de colonizacion de Myriophyllum spicatum en lagos de Wisconsin. Basamos este modelo predictivos en datos de 9 anos (1990–1999) de la colonizacion de M. spicatum en lagos mayores a 25 ha (n =1803). Utilizamos datos de colonizacion de 2000 a 2006 para probar si el modelo predijo con precision el numero de lagos que fueron colonizados entre 200 lagos identificados como los mas probables de ser colonizados. Encontramos que la probabilidad de colonizacion pronosticada no se correlaciono con el hecho de que un lago estuviera colonizado. Debido a la baja predictibilidad de colonizacion de lagos especificos, comparamos la eficacia de prevenir que M. spicatum saliera de sitios ocupados, lo cual no requiere predecir la probabilidad de colonizacion. La prevencion de la salida de organismos de sitios colonizados redujo la probabilidad de mayor dispersion mas que la proteccion de sitios desocupados. Aunque nos concentramos en la dispersion de una especie en una region particular, nuestros resultados muestran las deficiencias de los modelos de gravedad para la prediccion de la dispersion de numerosas especies no nativas invasoras hacia una variedad de localidades por medio de una amplia gama de vectores.