How can a sea otter help a forest grow underwater?

A sea otter eating a sea urchin may seem to be doing only one thing: finding a meal. In some coastal ecosystems, that meal can influence an entire kelp forest. Urchins graze on kelp, so changes in their grazing can affect the tall algae that shelter many other animals.

Kelp grows from the rocky seafloor toward sunlight. Its underwater fronds provide surfaces and hiding places for a wide variety of organisms. Sea urchins can eat kelp, including tissue near the base that helps maintain the larger plantlike body. Where large amounts of grazing persist, a kelp forest may be reduced to a much more open area.

Otters are predators of urchins, although they also eat many other invertebrates. By consuming urchins, otters can reduce pressure on kelp. The effect crosses more than one feeding link: predator affects grazer, and grazer affects the algae. Ecologists call a chain of such indirect effects a trophic cascade. Trophic refers to feeding relationships.

This helps explain the term keystone species. A keystone species has a large effect on community structure relative to its abundance. The idea does not mean that one species performs every job or that all ecosystems have a single leader. It points to relationships in which changing one population can cause unusually broad consequences.

The otter–urchin–kelp relationship is not an automatic three-part switch. Storms, warming water, nutrient supply, disease, other predators and the condition of the urchins also matter. A place with otters will not necessarily have the same kelp cover as another place with otters. Scientists compare these interacting influences when they investigate why a forest expands or shrinks. The importance of the effect depends on how strongly each feeding link operates in that particular place and time.

The drawing isolates one important pathway so that its direction is clear: more effective predation can mean less grazing pressure, which can allow more kelp to persist. It does not claim that otters deliberately protect a forest. The revealing consequence is that an animal's ordinary feeding behavior can change habitat for organisms it never directly meets. A food web is also a web of effects on living space.