This research will apply resource competition theory to rigorously evaluate the underlying mechanism that has allowed an invasive plant species to displace a widespread, dominant plant species. It will specifically test the hypothesis that an invader is successful because it can outcompete a native plant, via the invader's ability to deplete the level of the shared limiting resource (Tilman's R* theory). It will also test the practical hypothesis that a biological control agent will only be effective if it prevents the invader from depleting the key limiting resource (Grover's keystone herbivory theory). These theories will be examined using the invasive purple loosestrife, its primary native competitor, broad-leaved cattail, and the biocontrol agent Galerucella calmariensis, a leaf-feeding beetle. The work will involve a rigorous, dual experimental approach that combines the use of carefully controlled mesocosms and in situ field manipulations. Prior funding has allowed the establishment of both the mesocosm and field experiments, which have now completed their third and first full field season, respectively. This award will allow the treatments to be maintained and the collection of response data for two additional field seasons, to achieve competitive exclusion, which is the critical response to definitively test these theories.
