Beetle Folivory Increases Resource Availability and Alters Plant Invasion in Monocultures of Goldenrod

One way that insect herbivores can influence plant community structure is by altering the ambient availability of resources required by plants. To examine the importance of this mechanism, I tested the following three hypotheses in a field experiment in 1990: (H₁) Folivory by the leaf beetle Trirhabda canadensis (Coleoptera: Chrysomelidae) increases availability of light, water, and N in monocultures of goldenrod (Solidago missouriensis). This hypothesis was partially supported. Folivory by Trirhabda reduced leaf, root, and total biomass of goldenrod and increased light penetration, soil water content, and soil nitrate concentration. However, Trirhabda grazing did not affect overall soil N availability. (H₂) The invasion of goldenrod monocultures by other plant species and the response of goldenrod to folivory reflects an increased availability of resources in monocultures grazed by Trirhabda. This hypothesis was supported. Folivory by Trirhabda raised the probability that a species would invade the experimental monocultures and increased the species richness of the invading plant assemblage. Relative growth rate of goldenrod and production of aboveground biomass by invaders were higher in monocultures grazed by Trirhabda than in ungrazed ones. Prostrate and creeping forbs increased in relative abundance following folivory by Trirhabda. (H₃) Reduced N uptake by goldenrod or increased N mineralization accompanies increased N availability in grazed monocultures. This hypothesis was also supported. Net N mineralization and nutrification were higher in monocultures grazed by Trirhabda than in ungrazed monocultures. Folivory by Trirhabda larvae reduced N uptake by goldenrod, but that of adults did not. The impact of herbivores upon plant communities may be effected through increased availability of important plant resources.