Abiotic Stress and Herbivory Interact to Set Range Limits Across a Two-Dimensional Stress Gradient

Range boundaries are a fundamental expression of a species' relationship with other species and with the abiotic environment. In this study, I examined the roles of biotic and abiotic factors in setting the local range limits of Mazzaella parksii (formerly Mazzaella cornucopiae). In Washington State, this turf-forming red alga occurs almost exclusively in a discrete high intertidal band on north-facing, wave-exposed shores. I investigated the mechanisms underlying these distributional patterns via a series of intra- and extra-limital transplant experiments coupled with manipulations of interacting species and abiotic stress. The upper limit of Mazzaella was set by physiological stress; turfs transplanted above the natural Mazzaella zone bleached and died over the course of a few months. Mazzaella's lower limit was set by herbivory; turfs transplanted below the natural zone were consumed in treatments accessible to grazers but thrived in herbivore-exclusion plots. South-facing transplants at the same intertidal height as the natural north-facing Mazzaella zone declined regardless of interspecific interactions, indicating that Mazzaella's upper limit is lower on south-facing shores. However, the position of the herbivore-determined lower limit was similar between aspects. South-facing transplants could survive below the natural Mazzaella zone if the grazer-determined lower limit was extended downshore by herbivore exclusions or at Mazzaella's natural intertidal height (i.e., the height at which it persists on north-facing surfaces) if the physiologically determined upper limit was extended upshore by experimental shades. In the absence of such experimental modifications, Mazzaella was effectively excluded from south-facing shores because its upper limit had converged on its lower limit. Thus, abiotic factors and biological factors interact to set the horizontal distributional limits of this alga. This conceptual model is consistent with Mazzaella's distribution at a variety of spatial scales and has broad implications for both spatial patterns and temporal trends in the distribution and abundance of species. Corresponding Editor: S. G. Morgan.