The ecological interactions between grasshoppers, predators, and resources that can limit the population growth of grasshoppers are poorly understood. A number of field 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 same habitat. 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 significantly affected by avian predation, indicative of weaker top-down effects. The effects of predation varied among years and between the two sites. Avian predators modified body size composition of grasshopper populations through size-selective predation on medium-bodied grasshoppers. Even in years when avian predators 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.
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