Changes in spider community composition are associated with urban temperature, not herbivore abundance

Urban forests provide many ecosystem services. The urban heat island effect can alter these services, in part by increasing arthropod herbivore abundance. Natural enemies, such as predators and parasitoid wasps, play a crucial role in controlling street tree herbivores, but their responses to urban warming are almost completely unknown. In this 2-year study, we investigated how urban warming and herbivore abundance affected arthropod natural enemies in street trees using an urban warming gradient and herbivore removal treatments. Spiders were by far the most abundant natural enemy group, but spider abundance did not increase with urban warming as herbivore abundance often does. In addition, spider and herbivore abundances were not related, either when herbivore abundance increased with warming or decreased with horticultural oil applications. Instead, urban warming was associated with altered spider community structure. This was due in part to loss of Anyphaenids—ghost spiders, which are known to control herbivorous arthropods in other systems—at the hottest urban sites in a relatively hot year. Together, the decline of this important spider group and the lack of increase in spider abundance with urban warming or prey abundance may contribute to abundant street tree herbivores in the hottest areas within cities. Our study is the first to manipulate a combination of biotic and abiotic factors to understand the forces driving natural enemy composition, abundance, and diversity in cities.