Data from: Cascading effects of grazing on predatory arthropod and parasitoid densities
Abstract
An important goal in arthropod conservation is to understand how arthropods are affected by anthropogenic activities. Livestock graze 29% of the land area in the U.S., which can result in both top-down and bottom-up effects on ecosystems that are grazed. Grazing often reduces species richness and abundance of arthropods; however, these trends depend upon context and taxa, making generalization difficult. Grazing can either increase or decrease species richness and abundance of arthropods, depending on context and taxa, making generalization difficult. The impacts of grazing on different taxa may also be indirect, depending upon trophic interactions with other members of the community. We propose that considering trophic relationships will help clarify the effects of grazing on arthropods. Here, we study the effects of grazing by ungulates on pompilid wasps (Hemipepsis and Pepsis hawk wasps) and tarantulas (Aphonopelma hentzi). We do this by comparing the cover of forbs (flowering plants that provide nectar to hawk wasps), and the densities of hawk wasps and of tarantula burrows in areas grazed by cattle to areas with light grazing by wild ungulates in the shortgrass prairie. Grazed areas had lower cover of flowering plants, fewer parasitoids, and more tarantulas, while lightly grazed areas had higher cover of flowering plants, more parasitoids, and fewer tarantulas. We propose that hawk wasp abundance may track floral resources, enabling parasitoids to exert strong top-down pressure on tarantulas in lightly grazed areas. Thus, grazing may benefit tarantulas by reducing the abundance of their parasitoids.