In 1980, the US Fish and Wildlife Service installed six permanent monitoring stations in the nearshore subtidal surrounding San Nicolas Island. Ongoing biannual surveys (since 1995, by the U.S. Geological Survey) have produced a spatially explicit 36-year time series of community dynamics at each site. The trajectories of subtidal community structure have widely varied among sites, with some sites flipping back and forth between urchin and kelp dominated states, and others exhibiting relatively little community change over time. The complexity of the benthic substrate of these sites varies considerably, ranging from flat sandstone to high-relief pinnacles. To characterize the relative influence of habitat variation on subtidal community structure, I quantified each site’s habitat complexity (surface rugosity/relief) as a function of spatial scale using a distance-wheel roller, and included these as predictor variables in nonmetric multidimensional scaling analyses of community structure over time.
Results/Conclusions
A two-dimensional ordination of species abundances across space and time accounted for 80% of the variation in community structure (axis 1: r2 = 0.52, axis 2: r2 = .28). Axis 1 predominately reflected a gradient between urchin-dominated versus kelp-dominated community states. Axis 2 predominately reflected a gradient between low and high site complexity. Habitat complexity was not correlated with axis 1 (r2 = 0.007) but was strongly associated with axis 2 (r2 = 0.467). More specifically, sites with high complexity were tightly clustered along axis 2, while sites with low complexity were widely spread. These results indicate that high-complexity sites exhibit relatively little community change over time, while low-complexity sites exhibit large shifts in community structure. These results suggest that habitat complexity is altering community dynamics, affecting species by modifying the consequences of physical perturbations such as storm damage, and/or controlling the effects of species interactions in southern California kelp forests.