The pattern of effects of a single consumer species on other trophic levels in a food chain. Many different patterns can be described, involving predators, herbivores, and primary producers such as plants and algae. However, a common pattern stems from the effects of a predator controlling the abundance of its herbivore prey, which in turn impacts lower down the food chain. For example, off the Aleutian Islands and southeastern Alaska, the sea otter (Enhydra lutris) feeds on sea urchins, which in turn graze the holdfasts of large algae (kelp). At sites where otters have historically been absent due to hunting by humans, the overabundance of sea urchins leads to sparse kelp beds, and relative paucity of other organisms that live in these beds. When otters recolonize such sites, urchin populations are controlled and the kelp beds recover, benefiting the entire ecosystem.