Distinct community regimes characterize many systems. Possible drivers of spatial patterning include environmental heterogeneity and biotic interactions, and recent work shows that interaction strength can depend on density-dependent behavioral changes. This raises the question of whether behavior can affect large-scale community patterns. For example, on temperate rocky reefs, longstanding empirical evidence documents distinct kelp forest and urchin barren regimes that greatly vary in spatial extent among regions. Forests and barrens span large (>500m) patches in California but co-occur across adjacent depth zones along the coast of New Zealand. This difference might arise from differences in depth-dependent wave stress on kelp and/or from differences in urchin grazing, which intensifies at low kelp densities. We quantify the extent to which environmental heterogeneity versus scale-specific feedbacks in urchin grazing behavior explain the different community scales by comparing the fit of dynamical models incorporating each feature to data from large-scale field surveys.
Results/Conclusions
We find that when behavior mediates the scale of strong consumer-resource interactions, it may also govern larger-scale community patterns. Large kelp forest and barren patches in California are most consistent with large-scale grazing feedbacks, wherein urchins actively graze whenever low kelp densities across large (>500m) scales lead to insufficient subsidies of kelp fronds to the bottom. By contrast, community zonation in New Zealand is best explained by a combination of depth-dependent wave stress and local (1-5m) grazing feedbacks, in which urchins intensively graze wherever plant densities are insufficient to physically deter them. Given a strong role of biological feedbacks and environmental gradients, our best-fit models predict that rapid large-scale shifts among distinct community states can occur when feedbacks span large (California) but not local (New Zealand) scales. Our best-fitting models incorporate well-established forms of density-dependent grazing thought to produce alternative stable grassland states, suggesting that behavioral feedbacks might also underlie community patterns in many terrestrial ecosystems.