Herbivory alters competitive interactions between two invasive aquatic plants

We compared the effect of herbivory by two congeneric specialist weevils (Neochetina eichhorniae and Neochetina bruchi) on competition between their host Eichhornia crassipes and another floating aquatic plant (Pistia stratiotes) by experimentally manipulating plant densities, presence of either or both herbivore species, and nutrient levels. Measurements of E. crassipes yield per original plant included biomass, clonal expansion, and flower production. Without herbivory, intraspecific competition among E. crassipes individuals was 41 times more intense than interspecific competition in terms of biomass yield. Herbivory shifted competitive outcomes, bringing intraspecific competition in line with interspecific competition. Both weevil species reduced E. crassipes biomass and flowering, but N. bruchi also lessened clonal expansion (rosette density) whereas N. eichhorniae did not. Nutrient limitation reduced plant performance but did not alter the pattern of competitive interactions. However, low nutrients moderated differences between weevil species such that both seemed equally damaging. We conclude that while herbivory directly and indirectly affected plant performance by altering competition between these two invasive plant species, the competitive response varied depending upon the herbivore species and availability of nutrients. The influence of herbivores on competitive interactions may thus be an important consideration for evaluating the invasive potential of exotic plant species as well as for predicting the potential efficacy of biological control agents, but interactions between herbivory, competitors, and nutrient availability create complexities that must be considered.