Drought and herbivory interact on populations of the invasive weed Tamarix sp

Environmental stress often exacerbates the effects of herbivory on plant fitness in unpredictable ways. We aimed to test two popular hypotheses regarding the outcome of plant environmental stress and herbivory: the Compensatory Continuum Hypotheses (CCH) and the Limited Resource Model (LRM). The CCH predicts that a plant’s tolerance to herbivory should be greater in high-resource environment whereas the LRM predicts that plant fitness is dependent upon the particular types of stressors and herbivores and will not be necessarily favored in a high-resource environment. In a common garden, we subjected potted individuals of the invasive weed Tamarix sp. to a 2x2 factorial treatment design of drought and herbivory by biological control agent, Diorhabda carinulata (Coleoptera: Chrysomelidae). Since plants in the genus Tamarix hybridize readily in North America, we designed our experiment to detect differences in responses to herbivory and drought among six populations representing a latitudinal gradient. The size, growth rate and vigor of 120 plants were measured over a 17-week period. Results from five of six response variables show that plants grown in drought conditions have higher tolerances to herbivory than those grown under well-watered conditions, which supports the LRM. For the population effect, plants from southern latitudes were more susceptible to stress by both drought or herbivory than were plants from northern latitudes. Our hope is that a greater understanding of interactions among resource availability, herbivory, and populations will lead to more successful weed biological control programs.