Nonnative grass litter enhances grazing arthropod assemblages by increasing native shrub growth

Abstract

Recent theory and research have highlighted how the brown (detritus-based) world may control the trophic structure of the green (grazing) world. Detritus can alter bottom-up control of green webs by affecting autotroph biomass and quality through its ability to alter ecosystem properties, including soil moisture and nutrient cycling. Additionally, the role of detritus as the food resource base of brown webs may subsidize omnivorous predators that can provide top-down control of green webs. Brown-green connections may be especially important following plant invasions, which often lead to increased detritus and altered food webs. I combine field experiments, observational data, and path analysis to understand how nonnative grasses impact native arthropod communities in a semiarid shrub system. Theory and correlative evidence predict that decreased shrub growth and nutritional quality, and increased feeding of detrital predators on the grazing web, would decrease the abundance of shrub arthropods. In contrast, I found nonnative litter increased shrub growth via increased soil moisture and produced a strong bottom-up increase of the grazing arthropod web; effects of detrital predators and plant quality were comparatively unimportant. I link these findings to the apparent lack of overlapping predators between the brown and green webs, and to the important abiotic role of litter in this xeric system, which increased native plants and the abundance and richness of arthropods on them.