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

Spread of non-native species can be important drivers of widespread
biodiversity declines, which often leads to precautionary management based
on assumptions that (1) introduced biota have negative impacts and are
therefore “guilty” of causing harm, and (2) reducing an introduced
species’ abundance will reduce these negative impacts, in turn, benefiting
native species. However, we frequently lack data to gauge both the
negative impacts of introduced species and the success or failure of
chosen management interventions. 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 the 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 an introduced 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. 2024),
we 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 introduced 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
introduced plants can be effective and safe, allowing native species to
recover as a dominant non-native species gradually declines.