Things can get even more complicated. Predator-prey interactions may be felt across a number of food-chain levels, resulting in what's known as a "trophic cascade." For instance, populations of sea otters in the waters of Alaska have declined in several areas, with the result that their principal prey, sea urchins, have increased dramatically. All these sea urchins have caused a ten-fold decline in kelp, which acts as a breakwater against wave action, and this has resulted in increased erosion of the shoreline. And why the decline in sea otters? It appears that it's due to increased predation by killer whales, which may be finding fewer of their regular prey-sea lions and harbor seals-which are perhaps declining due to a reduction in forage fish such as pollock, a result of heavy commercial fishing pressure. Gets involved, doesn't it?