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 affect 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 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.