The influence of sea otter predation has several interesting historical dimensions. Faunal remains in Aleut kitchen middens show that sea urchin size distributions during most of Aleut prehistory were similar to those of modern systems lacking sea otters, thus suggesting that aboriginal humans, by limiting sea otters, influenced coastal ecosystems long before modern humans arrived on the scene. Evolutionary influences of sea otter predation appear over even longer time scales. The control of herbivores by sea otter predation has decoupled a coevolutionary arms race between kelps and their herbivores, thus resulting in the development of poorly defended plants and unresistent herbivores in the North Pacific Ocean. This hypothesis has been tested through a comparison with Australasian kelp forests, which lacked a predator of comparable influence to the sea otter and which contain well-defended macroalgae and herbivores that are more resistant to those defenses. This coevolutionary scenario may explain why Northern Hemisphere kelp forests have been so devastated by sea urchin grazing following decimation of their predators. It also may explain why Steller sea cows radiated from the tropics into the North Pacific Ocean, but not elsewhere, and why the world's most diverse and largest abalones occur in the North Pacific. The sea cows were sustained by their consumption of kelp, and abalone more diffusely benefited from the rich resources provided by the kelp environment.
The sea otter–kelp forest system has changed remarkably as killer whales entered the coastal ecosystem and began preying intensively on sea otters after their normal prey populations (seals and sea lions) declined. Since the early 1990s, killer whale predation has driven otter numbers downward by approximately an order of magnitude across large areas of western Alaska. The loss of sea otters has caused sea urchin numbers to increase and kelps to decline (Figure 5). This example illustrates that predator–prey interactions, acting through trophic cascades, influence herbivore–plant interactions in a manner consistent with the predictions described earlier for odd- versus even-numbered food chains (Figure 4). It further indicates a role for predators in linking ecosystems over large areas. This ecological chain reaction may have been set off by post World War II industrial whaling, which deprived killer whales of an important food resource and may thereby have forced them to broaden their diet to include these smaller species of marine mammals.
Make this shorter like its a caption of a food web.