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Becoming common: Lack of competitors and of herbivores is not enough to explain invasiveness in Lamiaceae, the mint family
Research Mentor and Department
After habitat loss, exotic species are considered the greatest threat to biodiversity. However, some studies show that exotic species have minimal effects on biodiversity at large spatial scales. Here, we ask whether there is a fundamental difference between the reason exotic and native species are common. To control for evolutionary history, we select species from five plant families: one common native and one established exotic species from each, all common in Missouri. We experimentally manipulate the presence of competition and herbivory on these species to evaluate hypotheses implicating competitive ability and lack of herbivory in the success of exotic species. Besides control, each species experiences competitor removal, in which we remove all competitors around the focal species, and herbivore removal in which we apply Malathion to the focal species to deter insect herbivores. We determine the impact of the two treatments on fitness by measuring the difference in survivorship and fecundity for each treatment and the control, then by calculating a unit-less effect size for each treatment versus control, then by comparing the effect sizes of native and exotic species in each family. If (H0) there is no difference in the reason exotic species and native species are successful, then native and exotic species will show the same effect size. However if (H1) one of our treatments addresses a mechanism (competition or herbivory) that is unique to exotic species invasion, then the effect size will be greater for the native species. We find mixed results, possibly due to different mechanisms of success for exotic plant species, or to the short-term nature of the experiment. Next year, we will maintain plots established this year, allowing us to use demographic modeling to assess lifetime fitness of plants to better determine whether exotic species are successful for the same reasons as native species.