Grazer behavior can regulate large-scale patterns of community states

Ecosystem patterning frequently arises either from environmental heterogeneity or in studies of homogeneous systems where biological feedbacks produce multiple persistent ecological states. However, density dependent changes in behavior can strongly regulate species interactions, raising the question of whether behavior can also affect large-scale community patterns. We resolve this empirically by fitting dynamical models to large-scale survey data on temperate rocky reefs predominated by herbivory and environmental variation. We find that increased urchin grazing activity at low kelp densities best explain observed community patterns. In our best-fitting models this feedback creates large-scale, alternatively stable kelp- and urchin-dominated states at 37% of reefs in California. In New Zealand, a more localized behavioral feedback and stronger gradients in wave stress on kelp limit this phenomenon to intermediate depths (3-8m), with only single states stable in shallower and deeper areas and long-term reef state depending little on initial kelp abundance. Our results highlight that behavior can predominantly pattern herbivore-dominated communities despite strong environmental variation, and emphasize that alternative stable states detected locally (here, 1-5m2 samples) may not be relevant at larger scales.