Return of diversity: Wetland plant community recovery following purple loosestrife biocontrol

Abstract

Spread of non-native species can be important drivers of biodiversity declines, leading to precautionary management based on assumptions that (1) non-native biota have negative impacts and are “guilty” of causing harm and (2) reducing a non-native species' abundance will reduce these negative impacts, in turn, benefiting native species. However, we frequently lack data to gauge both negative impacts of non-native species and success or failure of chosen management interventions to benefit native species. Addressing these knowledge gaps is critical to improving management outcomes for native species while maintaining public trust to sustain funding of management activities. Here, we investigated the response of Lythrum salicaria (purple loosestrife) and associated plant communities to implementation of biological control in more than 10 wetland sites in New York State for up to 28 years. Introduced to North America from Europe in the 1800s, L. salicaria is a prime example of a non-native species with a continent-wide distribution that could not be suppressed by mechanical and chemical treatments. In the 1980s, waterfowl biologists, wetland managers, and conservationists alike worried about the loss of diverse wetland plant communities associated with the rapid expansion of L. salicaria. In response, after careful assessments of safety, and potential costs and benefits, four highly host-specific insect herbivores were released in North America in the early 1990s to reduce L. salicaria abundance and its negative ecological impacts. In a companion paper, Blossey et al. documented reduced L. salicaria occupancy and stem densities following insect releases over time (i.e., biological success), irrespective of site-specific differences in starting plant communities or L. salicaria abundance. Here, we show that reduced abundance of L. salicaria leads to the ultimate goal of non-native plant management: increased cover, abundance, and diversity of species, often of native species (i.e., ecological success). We also conduct analyses to provide inference about which plant species are most sensitive to L. salicaria, including changes in L. salicaria stem density. Overall, we provide an important conservation success story: our findings emphasize that biocontrol of non-native plants can be effective and safe, allowing native species to recover as a dominant non-native species gradually declines.