Disturbance-mediated trophic interactions and plant performance

Disturbances, such as flooding, play important roles in determining community structure. Most studies of disturbances focus on the direct effects and, hence, the indirect effects of disturbances are poorly understood. Within terrestrial riparian areas, annual flooding leads to differences in the arthropod community as compared to non-flooded areas. In turn, these differences are likely to alter the survival, growth, and reproduction of plant species via an indirect effect of flooding (i.e., changes in herbivory patterns). To test for such effects, an experiment was conducted wherein arthropod predators and herbivores were excluded from plots in flooded and non-flooded areas and the impact on a common riparian plant, Mimulus guttatus was examined. In general, the direct effect of flooding on M. guttatus was positive. The indirect effects, however, significantly decreased plant survival for both years of the experiment, regardless of predator presence, because of an increased exposure to grasshoppers, the most abundant herbivore in the non-flooded sites. Leafhoppers, which were more abundant in the flooded sites, had much weaker and varying effects. During 2000, when the leafhopper herbivory was high, arthropod predators did not significantly reduce damage to plants. In 2001, the mean herbivory damage was lower and predators were able to significantly reduce overall leafhopper damage. The effects of predators on leafhoppers, however, did not increase plant survival, final weight, or the reproduction potential and, thus, did not initiate a species-level trophic cascade. Overall, it was the differences in the herbivore community that led to a significant decrease in plant survival. While flooding certainly alters riparian plant survival through direct abiotic effects, it also indirectly affects riparian plants by changing the arthropod community, in particular herbivores, and hence trophic interactions.