The Shrub Ephedra Californica Facilitates Arthropod Communities Along A Regional Desert Climatic Gradient

Arthropods underpin arid community dynamics and provide many key ecosystem services. In arid ecosystems, the key habitat components that influence arthropod community structure are relatively understudied. Ephedra californica is a locally abundant shrub now restricted to highly fragmented populations with established positive effects on plant and vertebrate animal communities within the drylands of Southern California. The capacity for these positive effects to further support ground arthropod communities has not been examined. We tested the hypothesis that the physical structure and cover vegetation enhance key measures of arthropod community assembly at nine Californian desert sites that comprise an extensive regional aridity gradient. We contrasted the effects of shrub canopies with ground-covering vegetation on structuring ground-active arthropod communities by surveying ground-active arthropods with pitfall traps and collecting vegetation on the soil surface in the form of residual dry matter (RDM). We collected a total of 5820 individual arthropod specimens for a total of 159 morphospecies. Arthropod abundance and morphospecies richness and RDM biomass and cover were significantly greater beneath the canopy of E. californica throughout the region. Total biomass of RDM did not significantly influence arthropod communities, but cover of RDM on the soil surface negatively influenced arthropod abundance. Neither climatic aridity nor downscaled evaporative stress estimates were significant mediators of the arthropod-vegetation association patterns. Vegetation thus likely has direct and indirect physical effects on arthropod communities. These canopy vs. soil surface vegetation differences will refine sampling of fine-scale patterns of arthropod diversity in drylands. Regional land managers can support arthropod diversity by maintaining populations of foundation shrub species such as E. californica.