Luz Silverio

HERBIVORY RESPONSE AND DEFENSE DIVERSITY WITHIN THE LESPEDEZA GENUS. Luz Silverio¹, Tiffany Knight¹, Michelle Schutzenhofer², Biology Department, Washington University, St. Louis, MO¹, Biology Department, Saint Louis University, St. Louis, MO².

Invasive species are widely recognized as serious threats to both ecosystem integrity and conservation. Many studies investigating invasive species have supported the Enemy Release Hypothesis: a theory that postulates that exotic species become invasive as they thrive under the absence of natural predators, allocating their herbivory defense resources towards increased growth vigor in a new habitat. Although it has been shown that plants receiving less herbivory experience an increased growth rate, no research has been done to investigate whether certain species are more resistant than others to the negative effects of herbivory, and whether or not this trait is a contributing factor to a species’ fitness in a novel or native habitat.

This study is a congeneric investigation of the effects of herbivory on ten different exotic, invasive, native, and endangered species of the Lespedeza genus. Growth response to herbivory was investigated in the field through insecticide application and growth rate measurement of seven species of Lespedeza. To measure growth response to a controlled level of herbivory, herbivory was simulated using four treatments of leaf clipping on nine species of Lespedeza within a greenhouse. Leaf resiliency (g/cm²) was measured for each field species to estimate a species’ original herbivore defense.

Although both leaf resiliency and growth response to herbivory values are unique to each species within the Lespedeza genus, neither of these traits correlated to the relative fitness of a species in a novel or native habitat. In the field, growth rate did not differ between species. Insecticide treatments marginally improved growth of both native and exotic species within the field, but did not preferentially improve one category over the other. Although leaf resiliency did not correlate to a species’ growth response to herbivory, native species were found to have a significantly higher leaf resiliency than those that have been introduced to the continental United States.

Leaf resiliency and growth response to herbivory may be indicative of specific mechanisms for a species’ success, but neither are necessarily a major contributor to a species’ general fitness or invasibility. The high leaf resiliency of native species compared to exotic species may indicate a preferential resource allocation to leaf quality and herbivore determent, but this differential allocation did not appear to make a significant difference in a species’ response to either controlled herbivory or natural herbivory. Because species in the field achieved similar growth rates regardless of insecticide treatment or exotic or native status, it is postulated that exotic species within the Lespedeza genus receive equal herbivories as their native counterparts. Natural interspecific herbivory will be investigated in the future to evaluate this hypothesis.

Herbivory may affect other important plant success factors than growth. Further research could investigate herbivory effects on plant reproduction, pollinator interaction, and resource allocation. It is important to understand the mechanisms influenced by the Enemy Release Hypothesis; knowing why certain species become invasive or endangered is an important step in learning how to control or support them.