Grazer behaviour can regulate large‐scale patterning of community states

Ecosystem patterning can arise from environmental heterogeneity, biological feedbacks that produce multiple persistent ecological states, or their interaction. One source of feedbacks is density‐dependent changes in behaviour that regulate species interactions. By fitting state‐space models to large‐scale (~500 km) surveys on temperate rocky reefs, we find that behavioural feedbacks best explain why kelp and urchin barrens form either reef‐wide patches or local mosaics. Best‐supported models in California include feedbacks where starvation intensifies grazing across entire reefs create reef‐scale, alternatively stable kelp‐ and urchin‐dominated states (32% of reefs). Best‐fitting models in New Zealand include the feedback of urchins avoiding dense kelp stands that can increase abrasion and predation risk, which drives a transition from shallower urchin‐dominated to deeper kelp‐dominated zones, with patchiness at 3–8 m depths with intermediate wave stress. Connecting locally studied processes with region‐wide data, we highlight how behaviour can explain community patterning and why some systems exhibit community‐wide alternative stable states. Fitting mechanistic models to field data on hundreds of temperate rocky reefs, we find that changes in grazer behavior can underpin an iconic example of community spatial patterning: rocky barrens where urchins graze actively, and kelp forests where urchins reduce grazing activity. We show how regional differences in grazer behavior determine whether community regimes form local mosaics along a depth gradient in wave stress (New Zealand, left panels) or highly distinctive, kilometer‐scale patches spanning entire reefs (California, right panels). Our best‐supported models also project that feedbacks in grazer behavior form forests and barrens as alternative stable states at a large proportion of reefs (30–40%) in both regions.