Rocky intertidal community structure in oceanic islands: scales of spatial variability

There is a clear bias in the literature on island ecology towards terrestrial rather than marine systems, which have remained comparatively poorly studied. Marine populations are typically open, and local production may have little impact on local recruitment, such that long-distance dispersal is an important determinant of population ecology. Since oceanic islands form discrete patches of habitat surrounded by a structurally different environment, we tested the general hypothesis that processes operating at the scale of islands have a greater influence on these populations than the processes operating at smaller, intra-island scales. A hierarchical design examined the patterns of abundance and distribution of conspicuous taxa at 3 tidal heights at a range of spatial scales, ranging from a few meters to hundreds of kilometres apart in the rocky intertidal of the Azores. Both uni- and multivariate analyses showed that at the largest scale (islands), significant variation was detected in the lower and mid-shore communities, but not on the upper shore. Along the vertical gradient of immersion there was a trend for increasing small-scale patchiness towards the top of the shore. The potential role of local environmental stress gradients and broad-scale oceanographic patterns of recruitment in structuring these assemblages is discussed. This study corroborates the suitability of the analytical tools used here to examine patterns of distribution over a range of spatial scales and its applicability in the field of island marine ecology.