Scale insects contribute to spider conservation in urban trees and shrubs

Urbanization filters arthropod communities and selects for species tolerant of urban conditions. Spiders are key generalist predators in urban ecosystems, but certain spider families are rare in cities compared to rural areas. The unique arthropod communities found in different tree species likely affects their ability to conserve spiders by providing different prey resources. If arthropods disperse from trees to plants growing below trees, the conservation benefits of the arthropod communities found in trees may also extend to plants growing beneath them. Certain urban tree species can host high densities of scale-insects and other arthropods that may provide important prey resources for spiders. To assess the conservation value of different arthropod communities in urban trees, we collected spiders from scale-infested and scale-uninfested trees and from shrubs under these trees. We also used hanging cup traps to collect spiders that fell from both tree types. Spider abundance was greater within, and in shrubs below, scale-infested compared to scale-uninfested trees. Scale-infested trees hosted more orb web weaving spiders than scale-uninfested trees. Shrubs under scale-infested trees hosted more hunting, orb web weaving, and space web weaving spiders than shrubs under uninfested trees. Our findings suggest that scale-infested urban trees, and the robust arthropod communities they support, conserve certain spider guilds, and these benefits extend to other plants in the landscape. Implications for insect conservation: The ability of urban trees to conserve spider communities is in part attributable to the abundance of potential prey available within trees. Therefore, tolerating pests such as scale insects in urban trees can conserve spider communities both within trees and in shrubs planted below these trees.