The Gopher Tortoise (above and right) is a classic example of a keystone mutualist. It excavates large burrows which may extend 10 meters or more, and which are almost 1 foot in diameter (with some larger chambers as well, so the turtle can turn around. A number of other species including burrowing owls, gopher frogs, indigo snakes, and a number of invertebrates are highly dependent on these burrows; they often live in the burrow alongside the tortoise (benefits to the tortoise of this arrangement are not clear).
Studies of the Purple Sea Star (Pisaster ochraceus) have shown it to be a keystone predator - it preys preferentially on species - such as certain mussels - which would otherwise outcompete all the other species trying to gain a foothold on the rocks. By reducing the number of mussels, the sea stars open up habitat for other species and thus increase the overall diversity of the ecosystem (note that the sea stars are a predator to the mussels, not a mutualist!).
The Red Mangrove, below, has long stilted roots that arch down to the water at the edge of tropical shores. These roots stabilize the soil, protect coastal areas from erosion, and provide hiding places for many animals, including the young of many coral reef fish. In this way the Red Mangrove is a keystone mutualist like the Gopher Tortoise.
The American Alligator, left, excavates depressions in its habitat that fill with water. During dry times, these gator holes may be the only places with water. Thus, to all the organisms whose survival depends on the water in those holes the alligator is a keystone mutualist. Of course, the gator might eat a few of those things that come to live in its wallow.
Beaver are well-known for building dams. These dams create relatively large areas of still water where there once was a small stream. For organisms that live in such still waters the beaver is a wonderful keystone mutualist; for animals that like flowing water it's not such a good deal.
This article expands on the concept of keystone species.
