<P> Multiple kelp species often co-exist within a forest; the term understory canopy refers to the stipitate and prostrate kelps . For example, a Macrocystis canopy may extend many meters above the seafloor towards the ocean surface, while an understory of the kelps Eisenia and Pterygophora reaches upward only a few meters . Beneath these kelps, a benthic assemblage of foliose red algae may occur . The dense vertical infrastructure with overlying canopy forms a system of microenvironments similar to those observed in a terrestrial forest, with a sunny canopy region, a partially shaded middle, and darkened seafloor . Each guild has associated organisms, which vary in their levels of dependence on the habitat, and the assemblage of these organisms can vary with kelp morphologies . For example, in California, Macrocystis pyrifera forests, the nudibranch Melibe leonina, and skeleton shrimp Caprella californica are closely associated with surface canopies; the kelp perch Brachyistius frenatus, rockfish Sebastes spp., and many other fishes are found within the stipitate understory; brittle stars and turban snails Tegula spp . are closely associated with the kelp holdfast, while various herbivores, such as sea urchins and abalone, live under the prostrate canopy; many seastars, hydroids, and benthic fishes live among the benthic assemblages; solitary corals, various gastropods, and echinoderms live over the encrusting coralline algae . In addition, pelagic fishes and marine mammals are loosely associated with kelp forests, usually interacting near the edges as they visit to feed on resident organisms . </P> <P> Classic studies in kelp forest ecology have largely focused on trophic interactions (the relationships between organisms and their food webs), particularly the understanding and top - down trophic processes . Bottom - up processes are generally driven by the abiotic conditions required for primary producers to grow, such as availability of light and nutrients, and the subsequent transfer of energy to consumers at higher trophic levels . For example, the occurrence of kelp is frequently correlated with oceanographic upwelling zones, which provide unusually high concentrations of nutrients to the local environment . This allows kelp to grow and subsequently support herbivores, which in turn support consumers at higher trophic levels . By contrast, in top - down processes, predators limit the biomass of species at lower trophic levels through consumption . In the absence of predation, these lower - level species flourish because resources that support their energetic requirements are not limiting . In a well - studied example from Alaskan kelp forests, sea otters (Enhydra lutris) control populations of herbivorous sea urchins through predation . When sea otters are removed from the ecosystem (for example, by human exploitation), urchin populations are released from predatory control and grow dramatically . This leads to increased herbivore pressure on local kelp stands . Deterioration of the kelp itself results in the loss of physical ecosystem structure and subsequently, the loss of other species associated with this habitat . In Alaskan kelp forest ecosystems, sea otters are the keystone species that 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 . </P> <P> 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 . </P> <P> 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 . </P>

Kelp forests like the one shown above are part of which biome