The Impact of Alien Plants on Native Insect Herbivores

Graduate Research Assistants: Chris Phillips (MS Applied Ecology); Erin Reed (MS Entomology); Brian Cutting (MS Entomology); Christy Beal (PhD. Entomology); Meg Ballard (PhD Entomology)

Collaborators: Vince D’Amico, Judy Hough-Goldstein, Greg Shriver; Kimberley Shropshire, Karin Burghardt

Alien plants are rapidly replacing native species in both natural and anthropogenic landscapes throughout North America. Large-scale replacement of native vegetation has a number of biotic and abiotic consequences but one that has received little empirical attention is the effect of alien plant species on insect herbivores and the insectivores that eat them. Theory predicts that specialists should be unable to grow and reproduce on plants with which they share no evolutionary history. Because the majority of insect herbivores are thought to specialize on one or a few plant lineages, alien invasions should reduce local biodiversity, which, in turn, is predicted to reduce ecosystem stability and productivity. Specialists may, however, be able to adopt alien congeners of their native hosts with few negative effects. Generalists, in contrast, are predicted to be relatively unaffected by aliens and may even compensate for the loss of specialists in terms of insect biomass production. Existing data are few but they support neither of these predictions. Preliminary experiments suggest that specialists adopt alien congeners of their hosts as food plants only sporadically and most generalists appear to avoid most aliens. In view of the critical role of herbivorous insects in transferring energy from plants to higher trophic levels, it is essential to understand exactly how alien plants affect the production and diversity of our native insect herbivores and their natural enemies. This study addresses this need by quantifying 1) the degree to which specialists switch to alien congeners of their native hosts; 2) the comparative success of generalists on aliens with no native congeners; and 3) the change in insect community structure that occurs during the course of an alien plant invasion. These three projects are explicitly designed to measure the impact of alien plants on the production of insect biomass, and the structure, richness, and diversity of insect communities, essential information if we hope to manage natural and anthropogenic landscapes for biodiversity. The local and global effects of alien plant invasions on consumer diversity will be anticipated accurately only if we clearly understand the degree to which alien invaders are the ecological equivalents of the plants they replace. The project explicitly addresses this need.