Counterintuitive outcomes of interspecific competition between two grasshopper species along a resource gradient

Interspecific competition has long been implicated as a force structuring t he distribution of organisms along environmental gradients. The research pr esented here uses a survey, foraging observations, and a manipulative field experiment that test the included-niche competition hypothesis as a mechan ism structuring the distribution of a generalist grasshopper, Melanoplus fe murrubrum, along a food-resource gradient. A field survey of 10 old fields revealed a step-like reduction in M. femurrubrum abundance as the proportio n of grass cover in the fields exceeded 60%. Furthermore, M. femurrubrum ab undance and the abundance of a potential competitor, the grass specialist C horthippus curtipennis, were negatively correlated among the fields. Experi mental foraging observations showed that M. femurrubrum was a polyphagous f eeder that preferred grass and consumed less grass in the presence of C. cu rtipennis. The pattern of grasshopper distribution and foraging data suppor t an included-niche competition hypothesis. However, in a field experiment that manipulated C. curtipennis presence and absence and the composition of vegetation in which competition might occur, growth and mortality rates in M. femurrubrum did not respond to C. curtipennis competition. Performance was predicted to be lowest in grass-dominated conditions, those correspondi ng to the locations when M. femurrubrum does not exist. Instead, growth rat es were highest and mortality rates were lowest in grass-only treatments. W hile the survey and foraging observations support the included-niche hypoth esis, the experiments suggest that interspecific competition in adult grass hoppers is not the cause of the grasshopper distribution pattern. Similar e xperiments in western North American prairies indicate that interspecific c ompetition can be important and that foraging ecology can be a good indicat or of the interaction. This study contributes to the increasing body of kno wledge about interspecific competition's role in organism distributions. It demonstrates that geographic similarity in natural history need not lead t o a similar importance of limiting factors.