Species Competitive Ability and Position Along a Natural Stress/Disturbance Gradient

We tested the prediction that plant species that grow in undisturbed, nutrient—rich habitats tend to have higher competitive abilities than those found in disturbed or nutrient—poor habitats. The distributions of seven species (Eriocaulon septangulare, Rhynchospora fusca, Hypericum ellipticum, Juncus pelocarpus, Lysimachia terrestris, Dulichium arundinaceum, and Drosera intermedia) were measured along a gradient of exposure to wave action on the shore of Axe Lake, Ontario. The exposure gradient incorporates disturbance, through the removal of plant biomass, and stress, through the creation of a gradient in sediment organic content, nutrient concentrations, and fine particle sizes. Species distributions on the exposure gradient were quantified by determining the mean sediment organic content of the quadrats containing each species. Competitive abilities were measured as relative increase in dry mass per plant, in a field experiment in which species were grown together in all pairwise combinations (N = 10 replicates). Species had significantly heterogeneous competitive abilities (P < .01). Species found on exposed, nutrient—poor shores (e.g., E. septangulare) had low competitive abilities, while those growing on sheltered, nutrient—rich shores (e.g., D. arundinaceum) had high competitive abilities. Competitive ability was significantly correlated with mean position on the exposure gradient.