Carabid specialists respond differently to nonnative plant invasion in urban forests

Forests within urban areas are important for the survival of some native plant and animal communities. Urban forests are negatively affected by human-mediated disturbances, including those that increase nonnative plant invasion. Nonnative plants alter forest structure, and can contribute to dense understory vegetation, more open canopy structure, and less leaf litter volume. This modified vegetation structure alters resources urban forests provide for native forest species. Carabid beetles (Coleoptera: Carabidae) are used globally to understand community-level responses to changes in ecosystem quality. We sampled carabids in 24 urban temperate deciduous and mixed forests in two cities, Raleigh, North Carolina and Newark, Delaware, to understand how carabid communities in different regions change in response to nonnative plant invasion. We predicted forests invaded by nonnative plants would have denser understories and carabid communities with less forest specialist species and more open area specialist species. We focused on forest specialist carabids, species that depend on intact forest canopies, and open area specialist carabids, species that depend on open or no canopies, to determine whether urban forests support species indicative of intact forest ecosystems. We found greater native ground cover was associated with greater forest specialist diversity, greater canopy openness increased open area specialist capture and diversity, and understory vegetation density reduced open area specialist diversity. However, relationships were dependent on regional plant and carabid community composition. This study suggests abundant nonnative plants and associated dense vegetation structure are potential mechanisms explaining differences between rural and urban carabid communities and provides a unique example of how nonnative plants influence a guild of arthropods other than obligate herbivores. Our results indicate urban forests are not negligible ecosystems, but rather a reservoir for native biota. We suggest urban forest managers promote native species by managing nonnative species invasion in the understory of urban forests, to improve ecosystem quality for native forest specialist species.