Measuring Diffuse Competition Along an Environmental Gradient: Results from a Shoreline Plant Community

We propose a method for measuring variation in diffuse competition along an environmental gradient. This approach has two advantages over pairwise-competition experiments conducted in homogeneous environments. First, it may be more realistic, since individuals in nature are usually confronted with a variety of neighbors, that is, with diffuse competition. Second, this approach allows us to test whether the variation in diffuse competition is correlated with gradients in environmental factors. We used a field experiment to test whether diffuse competition is correlated with standing crop and organic-matter content in the soil of a lakeshore plant community. Diffuse competition was correlated significantly and positively with both of these factors. Further, standing crop was correlated positively with organic-matter content in the soil, suggesting that a general measure of habitat productivity may be indirectly related to the intensity of diffuse competition. These results support models of species diversity and adaptive strategies that assume a predictable variation in the importance of competition within a community.