THE ROLE OF VERTEBRATE AND INVERTEBRATE PREDATORS IN A GRASSHOPPER COMMUNITY

We conducted experiments at the National Bison Range, Montana to compare grasshopper densities in cages with and without spiders, and inside and outside avian exclosures to examine the impacts of the most common invertebrate and vertebrate predators. Spider predation did not reduce grasshopper populations. Avian predation reduced the abundance of large- and small-bodied grasshopper species, but total grasshopper abundance increased in the presence of birds, because medium-sized grasshopper species (250-500 mg adult size) became more abundant. The decrease in abundance of-large-bodied grasshoppers was due to birds preferentially killing them; however, predation did not explain declines in small-bodied grasshoppers or increases in medium-sized grasshoppers. These changes in abundance may be due to avian predation modifying competitive interactions. Medium-sized species increase in abundance, when the competitively superior large-species are reduced; this in turn increases competition by the superior medium-sized species on the small species. Therefore, avian predation produced indirect effects on competitive interactions and acted in a ''keystone'' fashion.