Using biological control agents to restore native habitats degraded by exotic plants should decrease the abundance of the invaders but should also result in a return toward preinvasion levels of native diversity. However, there are few long-term studies documenting changes in native biological diversity with the decline of an invader. We introduced a biocontrol flea beetle into three Montana grassland sites dominated by leafy spurge and monitored changes in leafy spurge abundance and frequency of associated vascular plants in 48 permanent microplots, in a 530 or 1,960 m2 macroplot immediately before and 14 yr after the release. Density and mass of leafy spurge declined 60 and 69%, respectively, over the 14 yr of the study across the three sites. Total species richness increased by 1.2 species/microplot (21%) between 1994 and 2008 across all three sites, but the increase differed among sites. Mean richness of exotic species was virtually unchanged across the three sites over the course of the study. Graminoid species richness was virtually unchanged across the three sites over the course of the study; most of the increase in diversity was due to the increase in forb richness at all three sites. Release of the biocontrol insects and a subsequent large reduction of leafy spurge were associated with an increase in native diversity after 14 yr, although causality cannot be confidently inferred from these associations because there were no controls. The increase in native diversity was small relative to the decline in leafy spurge abundance, suggesting that significant increases of native alpha diversity in semiarid grasslands may require many decades. Our results also suggest that the response to a decline of an invading species may depend on site quality and history.
Nomenclature: Flea beetle, Aphthona nigriscutis Foudras; leafy spurge, Euphorbia esula L.