The Impact of Exotic Purple Loosestrife (Lythrum salicaria) on Wetland Bird Abundances

The exotic invasive wetland plant purple loosestrife (Lythrum salicaria) is often considered to have negative impacts on native plant and animal species, but this is debated. Clarifying its influence would provide insight into appropriate management actions following invasion. We investigated the influence of L. salicaria cover and density on abundances of wetland bird species that are associated with a variety of vegetation structures. We found evidence of relationships between L. salicaria measures and abundance for most species we examined, but these relationships did not always agree with our predictions based on species' habitat associations. Some bird species positively responded whereas others negatively responded to increasing L. salicaria cover or density. Response curves varied in complexity and included linear and quadratic relationships as well as interactions. Our results suggested that L. salicaria did not categorically decrease habitat quality for all wetland bird species, and it may have had a positive influence on quality for some species. This ambiguity is not unique to L. salicaria invasion but applies to many changing habitat features. Therefore, there is likely no single appropriate strategy for managing L. salicaria when the goal is to maintain a diverse avian community in which species have divergent habitat preferences.