A Test of the Enemy Release Hypothesis Using a Congeneric Pair of Lespedeza Species

The enemy release hypothesis (ERH) postulates that invasive species are released from the effects of the herbivores, predators, pathogens, and other enemies that control population sizes in the native ranges of the invasive species. In a one-year common garden experiment, I compared the native species Lespedeza capitata to the invasive species Lespedeza cuneata. I examined relative fitness and performance over the first growing season and manipulated arthropod abundance using insecticide applications. While L. capitata had higher germination and survivorship than did L. cuneata, it also sustained more natural enemy damage. Arthropod reduction increased height and apparent survivorship for L. capitata. Consistent with the predictions of the ERH, arthropod reduction did not significantly effect damage, fitness, or performance of L. cuneata because natural enemy damage was relatively low for this species as compared to L. capitata. I also tested for associational susceptibility, in which one species decreases the fitness of the other by attracting herbivores. Neither species differed in damage when grown together compared to when grown alone, suggesting that associational susceptibility is not a factor for these species. While the predictions of the ERH were supported, ERH may not be the most important factor allowing L. cuneata to invade. L. cuneata produced an average of 31 seeds per plant, but seed production for L. capitata was virtually nonexistent. The comparatively high seed production of L. cuneata may contribute more strongly to enhancing population growththan does the increase in survivorship that L. capitata experiences when herbivory is reduced.