Historically and architecturally a keystone is the trapezoidal block at the peak of an arch that maintains the stability and integrity of its structure. Remove the keystone and the arch collapses.

In ecology, we often speak of keystone species when discussing extinctions or ecosystems in peril. Much like the keystone of an arch, any given species might be the keystone within a web of trophic interactions. For example, consider the humble sea otter (Enhydra lutris). This cuddly mustelid of the sea is much more than just eye-candy. In the late 1970's the sea otter population was relatively large and the kelp bed "forests" of the Pacific were expansive. Overfishing depleted the seas of fish, and seal populations began to wane. Interestingly, orca whales were forced to switch from feeding on seals to consuming the healthy otter stock. As the otter population collapsed, the sea urchin population spiked. The expanding urchin populations decimated the kelp beds and thus the resident the fish nurseries. The depleted fisheries now lost the opportunity to recover from years of overfishing.

So, who's the keystone in this story? It turns out that otters eat sea urchin when the urchin population hit a critical mass. In the absence of the otters, the prolific urchin population exploded, and the kelp-bed ecosystem collapsed.

The image pictured was taken in the Elkhorn Slough near Monterey, CA. Fortunately, the sea otter is now protected species by the Marine Mammal Protection Act. As a result, the California coastal ecosystem is recovering because of the important role this species plays within its ecosystem.