Insect community response to plant diversity and productivity in a sagebrush–steppe ecosystem

Abstract

In natural systems, insect diversity and abundance are often positively correlated with plant diversity; however, these relationships may be obscured or complicated by other characteristics of plant communities that influence insect distributions. For example, diversity and abundance of insects may also be related to structural complexity, nutrition, or productivity of plants. In arid systems, primary production is generally enhanced by supplemental moisture, which may provide food resources of greater abundance and quality for herbivores, resulting in greater insect diversity and abundance. Although the association of insect distributions with sites of either high plant diversity or productivity has been studied extensively, the relative importance of these two factors in influencing insect diversity and abundance remains unclear. We examined how insect communities in sagebrush steppe responded to two plant community treatments (a crested wheatgrass monoculture and a diverse community of native shrubs, grasses, and forbs) and three irrigation regimes (summer irrigation, fall/spring irrigation, and ambient precipitation only) on an experimental site designed to test protective cap designs for isolating buried hazardous waste. In both 2000 and 2001, insect diversity and abundance were positively correlated with both plant diversity and irrigation early in the summer; however, by the end of the summer, insect distributions were more strongly influenced by irrigation treatment. Our results suggest that during the dry season in this semi-arid shrub–steppe system, plant diversity may be secondary to moisture effects in influencing insect distributions.