More than half of the global population lives in urban areas that cover less than 3% of the earth’s surface. As human populations grow and development expands, urban forests are threatened by habitat loss and fragmentation. Forests contribute to the preservation of native species and habitat, carbon sequestration, and atmospheric cooling. Increased urbanization and nonnative plant invasion change the composition of ecological communities and alter forest function.
Nonnative plant invasion makes forests denser, specifically the ground layer and shrub layer strata. Therefore, we asked how plant composition and nonnative plant invasion affects ground dwelling arthropods. We predicted nonnative plant invasion would reduce leaf litter and coarse woody debris volume and change the ground-dwelling beetle community. We then asked how vegetation density and nonnative plant invasion affect understory-dwelling arthropods. We predicted that understory vegetation density would have a different effect than increased nonnative plant invasion on spider communities.
We studied plant and arthropod communities in 24 urban forest fragments in two cities along the eastern coast of the United States. We sampled two cities (Newark, Delaware and Raleigh, North Carolina) to understand if interactions change under different temperatures, providing insight to how a warming climate may affect urban systems. Urban forest sites ranged in amount of urbanization and size (1 to 25 hectares). Within each forest, we sampled plants and arthropods at 10 locations. We quantified the vegetation community with metrics of density, canopy cover, and species abundance. Ground arthropods were sampled using pitfall traps and all carabid beetles were identified to species. Understory arthropods were sampled with a bug vacuum and spiders were identified to family, then dried and weighed for biomass.
Results/Conclusions
Forests with established canopies were less colonized by nonnative plants. We collected more carabids in forests with less nonnative plant species and more coarse woody debris, and these sites had more predatory carabids. Smaller and younger forests had more nonnative plant species and more generalist carabids. The forest plant community affects which species make up the urban carabid community. Forest vegetation complexity did not predict community trends in spider diversity or biomass. Increased nonnative plant richness correlated with increased spider biomass, but not diversity. This research combines knowledge of urban communities with background temperature differences to provide management recommendations for future urban forest conservation along the east coast. We recommend conserving urban forests regardless of size and improving forest quality for native communities by removing nonnative invasive plants.