Direct and Indirect Effects of Avian Predation on Grasshopper Communities in Northern Mixed-grass Prairie

The ecological interactions between grasshoppers, predators, and resources that can limit the population growth of grasshoppers are poorly understood. A number of Þeld experiments have shown that top-down control by avian predators can limit grasshopper populations, but the effects of avian predators on grasshopper populations can differ between years and between sites in the samehabitat. I conducted experiments examining grasshopper populations in avian exclosures and control plots for 3 yr at two locations in eastern Montana. Avian predation had variable direct and indirect effects on grasshopper communities at the two locations. Grasshopper population densities, species richness, and diversity at the two sites were not consistently signiÞcantly affected by avian predation, indicative of weaker top-down effects. The effects of predation varied among years and between the two sites. Avian predators modiÞed body size composition of grasshopper populations through size-selective predationonmedium-bodied grasshoppers. Even in yearswhen avianpredators did not limit grasshopper populations, selective predation seemed to indirectly mediate competitive interactions among grasshoppers. Birds reduced the proportion of presumably competitively superior medium-bodied grasshoppers, and small-bodied grasshoppers increased in abundance.