Shrub-annual facilitation complexes mediate insect community structure in arid environments

In dryland systems, shrubs often increase the productivity, abundance, and diversity of understory plants, however these positive interactions can also scale to other trophic levels. The facilitative effect of Larrea tridentata was hypothesized to extend beyond plants to the local insect community in the Mojave Desert. Pan traps were placed under shrubs and in open microsites to test the following predictions: 1) shrub-annual facilitation complexes increase insect abundance, richness, and diversity; and 2) specific insect families or functional groups respond to the shrub-annual facilitation complex more strongly due to increased resources. Abundance, richness, and diversity of the insect communities associated with shrubs were significantly greater in shrub compared to open microsites. The families Sphecidae, Formicidae, Bradynobaenidae and Lauxaniidae were positively associated with shrubs while Scarabaeidae was associated with open microsites. However, there was no difference in the relative abundances of major functional groups, suggesting that the primary pollinators for this ecosystem are not sensitive to differences in floral resources at this scale. This study demonstrates that shrubs facilitate local insect communities and supports the hypothesis that plant-plant facilitation can extend to other trophic levels. Management of desert shrubs is thus an effective means to enhance many components of insect biodiversity.