Positive interactions expand habitat use and the realized niches of sympatric species

Background/Question/Methods

Niche theory, the oldest, most established community assembly model, predicts that in sympatry, the realized niche will contract due to negative interspecific interactions, but fails to recognize the effects of positive interactions on community assembly. The stress gradient hypothesis predicts that positive interactions expand realized niches in stressful habitats. We tested the predictions of the stress gradient hypothesis in a cobble beach model system across both physical and biological stress gradients. We transplanted seven common littoral species within, adjacent to, and below cordgrass stands in control, cage control, caged, shaded, and shaded cage treatments to test the hypothesis that cordgrass expands intertidal organism habitats.

Results/Conclusions

At high cobble beach elevations common fauna and flora are restricted to cordgrass patches largely due to thermal and desiccation stress buffering by cordgrass. At lower elevations, predation generally limits organisms to cordgrass habitats protected from biotic stress. On cobble beaches, cordgrass expands the distribution and realized niches of species to habitats in which they cannot live without facilitation, suggesting that niche theory and species distribution models should be amended to accommodate the role of positive interactions in community assembly.