Interhemispheric comparison of scale-dependent spatial variation in the structure of intertidal rocky-shore communities

In rocky intertidal habitats, the pronounced increase in environmental stress with elevation, caused by tides, generates significant vertical variation in community structure. Along coastlines, environmental changes generate horizontal biological variation, which, when measured at large sampling intervals, generally surpasses vertical biological variation. It is unknown, however, how vertical biological variation compares with horizontal biological variation when the latter is measured in environmentally similar habitats. We tested the hypothesis that, for sites experiencing similar environmental conditions along the shore, horizontal biological variation does not surpass vertical biological variation even when horizontal variation is measured at large sampling intervals along the coast. We compared vertical and horizontal variation in intertidal communities by surveying habitats experiencing the same wave exposure on the NW Atlantic and SE Pacific coasts. We measured biological variation based on differences in species richness, occurrence, and abundance between quadrats from low and high elevations (vertical variation) and between quadrats at three horizontal scales of sampling interval on both coasts: local (tens of cm between quadrats), meso- (similar to 100 km between quadrats), and regional (similar to 200 km between quadrats). We measured biological variation for all species combined, separately for sessile and mobile species, and for the numerically dominant species. The data analyses indicated that horizontal biological variation was never higher than vertical biological variation, not even at the regional scale, providing support for our hypothesis. Overall, our findings suggest that studies comparing spatial scales of biological variation should consider the underlying environmental variation in addition to simply scale alone.