Indirect Effects of Biological Control on Plant Diversity Vary across Sites in Montana Grasslands

Abstract:  Biological control with specialist, nonindigenous, herbivorous insects is an important option for controlling invasive exotic plants in wildlands and nature reserves. It is assumed that biological control agents will reduce the dominance of the target weed, thereby increasing the native diversity of the associated plant community. However, this hypothesis has rarely been tested. We introduced Aphthona nigriscutis into grassland sites infested with the invasive exotic species Euphorbia esula L. on a nature reserve in Montana (U.S.A.). Two sites with better soil had been treated previously with herbicide, whereas two other sites had not. We measured the density and biomass of Euphorbia vegetative and flowering stems and number of native and exotic shrubs, grass-like plants, and forbs in 48 microplots in Aphthona release and control macroplots at each site. After 5 years, Aphthona release was associated with a 33–39% decline in Euphorbia aboveground biomass compared with controls at all sites. Other effects of the biocontrol depended on the site. Biocontrol slowed the recovery of species diversity at the sites previously treated with herbicide but slowed the loss of diversity at sites without a history of herbicide. Biocontrol introduction was not associated with a disproportionate increase in nontarget exotic species. Release of Aphthona caused a decline in the biomass of flowering stems relative to controls at good-soil, previous-herbicide sites but was associated with a relative increase in flower stem mass at poor-soil, no-herbicide sites. Our results suggest that biocontrol reductions in weed dominance will not always be associated with increased species diversity. More emphasis should be placed on conserving desirable communities and less on simple weed control. Monitoring of community-level effects should accompany biocontrol introductions on nature reserves.