It is well recognized that invasive plant species affect native communities and ecosystems. It is less understood how co-occurring invasive plants might interact with one another, and how these interactions might change their impacts. Invasion rates and the likelihood of encountering multiple invasive species within a habitat are increasing. Yet most invasive impact studies focus on single species and few have attempted to understand the impacts when multiple invasive plants co-occur. This study addresses the question: How does the community structure vary between patches dominated by single versus pairs of invasive species?
I studied soil, plant and arthropod communities within naturally occurring patches of two co-dominant, common invasive shrubs Ligustrum sinense (Chinese privet) and Lonicera maackii (Amur honeysuckle) across three sites in eastern Tennessee. Both shrubs are listed as “Severe Threats” by the Tennessee Exotic Pest and Plant Council and are routinely included in invasive management plans. Using a block design, I located plots with one of four vegetation types during the summer of 2010: L. sinense monoculture, L. maackii monoculture, mixture, and control. I sub-sampled plants, collected ground-dwelling arthropods, and analyzed soils for potential extracellular enzyme activity as a proxy for soil community.
Across all plots and taxa, I found that community composition was not driven by dominant vegetation but instead by site and microhabitat (block). A total of 2,652 arthropod individuals spanning 218 morphospecies and 1,643 plants spanning 80 species were sampled throughout the summer. Soil enzyme levels (β-glycosidase, β-N-acetylglucosaminidase, and phosphatase) and plant and arthropod abundance, richness and diversity all varied significantly across microhabitats. Ordinations of community structure for all three taxa were significantly related to site but not to dominant vegetation. These results indicate that the contemporary impact of these two invasive shrubs on current community structure is neutral, yet cannot speak to the historical impacts or site conditions prior to invasion by these species. Regardless of these limitations, this study highlights the need for impact studies or management efforts to account for site and/or microhabitat where invasive species are present.