The populations of big-cat prey including the red coati (Nasua nasua), the agouti (Dasyprocta variegata), and the paca (Agouti paca) are about ten times higher than on Cocha Cashu, Peru, where big cats still live. However, this increase may result from natural population variability rather than the lack of jaguars and pumas. The extreme removal of herbivores and frugivorous mammals would drastically affect forest regeneration, altering tree species composition, but the effects of modest changes in densities are less clear.
The sea otter (Enhydra lutris) is a keystone predator par excellence (Duggins 1980). A population of around 200,000 once thrived on the kelp beds lying close to shore from northern Japan, through Alaska, to southern California and Mexico. In 1741, Vitus Bering, the Danish explorer, reported seeing great numbers of sea otters on his voyage among the islands of the Bering Sea and the North Pacific Ocean. Some furs were taken back to Russia and soon this new commodity was highly prized for coats. Hunting began. In 1857, Russia sold Alaska to the United States for $7,200,000. This cost was recouped in forty years by selling sea otter pelts. In 1885 alone, 118,000 sea otter pelts were sold. By 1910, the sea otter was close to extinction, with a world-wide population of fewer than 2,000. It was hardly ever seen along the Californian coast from 1911 until 1938.
The inshore marine ecosystem changed where the sea otter disappeared. Sea urchins, which were eaten by the otters, underwent a population explosion. They consumed large portions of the kelp and other seaweeds. While the otters were present, the kelp formed a luxuriant underwater forest, reaching from the sea bed, where it was anchored, to the sea surface. With no otters to keep sea-urchins in check, the kelp vanished. Stretches of the shallow ocean floor were turned into sea-urchin barrens, which were a sort of submarine desert.
Happily, a few pairs of sea otters had managed to survive in the outer Aleutian Islands and at a few localities along the southern Californian coast. Some of these were taken to intermediate sites in the United States and Canada where they were protected by strict measures. With a little help, the sea otters staged a comeback and the sea urchins declined. The lush kelp forest grew back and many lesser algae moved in, along with crustaceans, squids, fishes, and other organisms. Grey whales migrated closer to shore to park their young in breaks along the kelp edge while feeding on the dense concentrations of animal plankton.
Keystone predators sometimes are more effective within certain parts of their range. The sea star (Pisaster ochraceus) is a keystone predator of rocky intertidal communities in western North America (Paine 1995). This starfish preys primarily on two mussels Mytilus californianus and Mytilus trossulus. A study along the central Oregon coast showed that three distinct predation regimes exist (Menge, B. A., and T. L. Freidenburg. 2001). Strong keystone predation occurs along wave-exposed headlands. Less strong predation by sea stars, whelks, and possibly other predators occurs in a wave-protected cove. Weak predation occurs at a wave-protected site regularly buried by sand.
If keystone species do exist, how do the links within a system dominated by such a