Drivers of specialist herbivore diversity across 10 cities

Urban areas are an increasingly prevalent feature in contemporary landscapes and can play an important role in conservation. In urban areas, planting and maintenance of native plants is practiced to promote multi-trophic native communities; however, the effect of an urban habitat on the native foodwebs that use these plants is poorly understood. Here, we asked (1) whether the composition and diversity of specialized herbivore communities on native trees remains intact in urban areas as compared to reference natural areas and (2) what factors, from patch to landscape, correlated with variation in composition and diversity in urban areas. Specialized herbivore (gall wasp) communities on native trees (Quercus lobata) were surveyed in 10 paired urban and natural areas across the tree's natural range. Communities of specialized herbivores remained partially intact in cities. Species richness was greater in natural areas; community composition, however, varied by region but did not differ between urban and natural areas. Within urban areas, species richness was higher when there was a greater density of habitat patches; patch quality factors also affected specialist herbivore diversity. Overall, our results indicate that urban environments are not deterministically removing individual specialist herbivores species. Rather, dispersal limitation and patch quality drive stochastic losses of herbivores on urban trees. Communities of urban specialist herbivores remain largely intact and cities can be suitable habitat for these groups. Simple management practices in cities that can be conducted on the scale of individual land parcels may increase diversity of specialist herbivore communities to reference levels.

► Survey of specialized herbivore communities on native trees in 10 cities and paired natural areas.
► Determined patch and landscape factors driving community structure.
► Stochastic species losses lower diversity in cities while regional variation in composition is maintained.
► Specialized plant–herbivore systems represent practical urban species conservation target.