Description:
<p>Interspecific competition has long been implicated as a force structuring the distribution of organisms along environmental gradients. The research presented here uses a survey, foraging observations, and a manipulative field experiment that test the included-niche competition hypothesis as a mechanism structuring the distribution of a generalist grasshopper, Melanoplus femurrubrum, along a food-resource gradient. A field survey of 10 old fields revealed a step-like reduction in M. femurrubrum abundance as the proportion of grass cover in the fields exceeded 60%. Furthermore, M. femurrubrum abundance and the abundance of a potential competitor, the grass specialist Chorthippus curtipennis, were negatively correlated among the fields. Experimental foraging observations showed that M. femurrubrum was a polyphagous feeder that preferred grass and consumed less grass in the presence of C. curtipennis. The pattern of grasshopper distribution and foraging data support 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 corresponding to the locations where M. femurrubrum does not exist. Instead, growth rates were highest and mortality rates were lowest in grass-only treatments. While the survey and foraging observations support the included-niche hypothesis, the experiments suggest that interspecific competition in adult grasshoppers is not the cause of the grasshopper distribution pattern. Similar experiments in western North American prairies indicate that interspecific competition can be important and that foraging ecology can be a good indicator of the interaction. This study contributes to the increasing body of knowledge about interspecific competition's role in organism distributions. It demonstrates that geographic similarity in natural history need not lead to a similar importance of limiting factors.</p>