mediates this trophic cascade. In Southern California, kelp forests persist without sea otters and the control of herbivorous urchins is instead mediated by a suite of predators including lobsters and large fishes, such as the California sheephead. The effect of removing one predatory species in this system differs from Alaska because redundancy exists in the trophic levels and other predatory species can continue to regulate urchins. However, the removal of multiple predators can effectively release urchins from predator pressure and allow the system to follow trajectories towards kelp forest degradation. Similar examples exist in Nova Scotia, South Africa, Australia, and Chile. The relative importance of top-down versus bottom-up control in kelp forest ecosystems and the strengths of trophic interactions continue to be the subject of considerable scientific investigation. The transition from macroalgal (i.e. kelp forest) to denuded landscapes dominated by sea urchins (or ‘urchin barrens’) is a widespread phenomenon, often resulting from trophic cascades like those described above; the two phases are regarded as alternative stable states of the ecosystem. The recovery of kelp forests from barren states has been documented following dramatic perturbations, such as urchin disease or large shifts in thermal conditions. Recovery from intermediate states of deterioration is less predictable and depends on a combination of abiotic factors and biotic interactions in each case. Though urchins are usually the dominant herbivores, others with significant interaction strengths include seastars, isopods, kelp crabs, and herbivorous fishes. In many cases, these organisms feed on kelp that has been dislodged from substrate and drifts near the ocean floor rather than expend energy searching for intact thalli on which to feed. When sufficient drift kelp is available, herbivorous grazers do not exert pressure on attached plants; when drift subsidies are unavailable, grazers directly impact the physical structure of the ecosystem. Many studies in Southern California have demonstrated that the availability of drift kelp specifically influences the foraging behavior of sea urchins. Drift kelp and kelp-derived particulate matter have also been important in subsidizing adjacent habitats, such as sandy beaches and the rocky intertidal. Patch dynamics Another major area of kelp forest research has been directed at understanding the spatial-temporal patterns of kelp patches. Not only do such dynamics affect the physical landscape, but they also affect species that associate with kelp for refuge or foraging activities. Large-scale environmental disturbances have offered important insights concerning mechanisms and ecosystem resilience. Examples of environmental disturbances