INFLUENCE OF HABITAT FRAGMENTATION AND CROP SYSTEM ON COLUMBIA BASIN SHRUBSTEPPE COMMUNITIES

Abstract

Crop production in much of the western United States has caused extensive fragmentation of shrubsteppe and other grasslands and has increased regional levels of disturbance through repeated tillage of crop fields and applications of agrochemicals. Although there have been numerous studies of the effects of fragmentation on arthropods in forests, old fields, and cropland, there have been very few studies of fragmentation effects on grassland arthropods. I conducted a two-year study in the Columbia Basin region of the U.S. Pacific Northwest to determine the effects of fragmentation and regional disturbance on shrubsteppe communities. I sampled plants and arthropods in 20 shrubsteppe fragments associated with annual crops, 20 fragments associated with perennial crops, and 20 large sections of shrubsteppe distant from crop fields. Fragments were corners of crop irrigation circles, and only sites that were undisturbed, with an apparent high coverage of shrubs, were included in the study. Results indicated no significant differences in coverages of shrubs, perennial grasses, total perennial forbs, total annual/biennial forbs, and species richness of grasses and shrubs between fragments and large sections of shrubsteppe. Mean percent coverage of the annual grass Bromus tectorum was 18.6% greater in small fragments than in large sections of shrubsteppe distant from crop fields, and mean coverage of bare ground was 22.9% higher and coverage of biological soil crusts was 41.5% lower in the fragments than in large shrubsteppe sections. Shrubsteppe fragments had significantly lower abundances of obligate herbivores, omnivores, total herbivores, obligate predators, total predators, plant detritivores, total detritivores, and butterflies, as well as fewer individuals of numerous other taxa within these trophic groups. Fragmentation had very little effect on species richness within trophic groups, probably because fragments were not that isolated from each other and large sections of shrubsteppe. Shrubsteppe communities that were next to highly disturbed annual crops had greater coverages of total annual/biennial forbs, and lower coverages of total perennial forbs and biological soil crusts when compared to shrubsteppe near perennial crops. Abundances of obligate herbivores, omnivores, total predators, plant detritivores, vertebrate-detritus feeders, and bees were similar in shrubsteppe near annual crops and near perennial crops. However, within these trophic groups, shrubsteppe near annual crops supported fewer obligate shrub-feeding grasshoppers, herbivorous ground beetles, harvestmen, antlions, darkling beetles, and butterflies than did shrubsteppe near perennial crops. The results suggest that a reduction in resources within fragments and regional disturbances caused by the production of annual crops were responsible for changes in arthropod trophic structure in fragments. Changes in trophic structure were probably not due to reductions in habitat heterogeneity or habitat isolation because plant community structure was similar in fragmented and unfragmented shrubsteppe, and fragments were not isolated from large sections of shrubsteppe.