LAUSR

Significance Kelp forests are declining worldwide due to varied combinations of environmental change and the trophic downgrading of urchin-controlling predators. These processes have increased the frequency and extent of rapid, nonlinear shifts to so-called urchin barrens whose ecological functioning and services are reduced relative to those of kelp forests. Understanding the factors that regulate kelp-forest tipping points and switches between states is key to their management. Here we demonstrate that substrate complexity (surface rugosity) determines both the existence of and dynamic transition between community states around San Nicolas Island, CA. Kelp-forest conservation and restoration efforts are growing internationally and may benefit from the consideration of substrate complexity in their strategies. The factors that determine why ecosystems exhibit abrupt shifts in state are of paramount importance for management, conservation, and restoration efforts. Kelp forests are emblematic of such abruptly shifting ecosystems, transitioning from kelp-dominated to urchin-dominated states around the world with increasing frequency, yet the underlying processes and mechanisms that control their dynamics remain unclear. Here, we analyze four decades of data from biannual monitoring around San Nicolas Island, CA, to show that substrate complexity controls both the number of possible (alternative) states and the velocity with which shifts between states occur. The superposition of community dynamics with reconstructions of system stability landscapes reveals that shifts between alternative states at low-complexity sites reflect abrupt, high-velocity events initiated by pulse perturbations that rapidly propel species across dynamically unstable state–space. In contrast, high-complexity sites exhibit a single state of resilient kelp–urchin coexistence. Our analyses suggest that substrate complexity influences both top-down and bottom-up regulatory processes in kelp forests, highlight its influence on kelp-forest stability at both large (island-wide) and small (<10 m) spatial scales, and could be valuable for holistic kelp-forest management.