Zombie Urchins, Sea Otters, and the Hidden Reach of Restoration

"All men are created equal," Jefferson wrote (perhaps these days he would write, "All men and women are created equal"), but the same cannot be said for all species within a natural community. The interplay among species is subtle, interwoven, a "tangled bank," maddeningly confusing for a restorationist trying to bring back ecological function. We strive to fill up landscape space and to reestablish the major players of a destroyed or heavily degraded habitat. However, the uncommon and even small species may be critical for project success. Understanding complexity in biology is an intellectual pleasure but a practitioner's nightmare.

Ecological theory has detailed many studies of trophic cascades and keystone species. Some species have an inordinate effect on the surrounding biotic community even though they may be small or uncommon. "Little things mean a lot" is not just a sweet habit of romance, it is an axiom of restoration ecology. Recent studies (Recommended Reading, below) have shown that the impact of an important, albeit oddball, species can affect much more than species richness. This brings us to the playful sea otter (Enhydra lutris) and the troublesome sea urchin (with the fabulous Latin binomial, Strongylocentrotus purpuratus).

The role of sea otters on Pacific Coast inshore communities has been well established by Estes and his collaborators. The sea otters eat urchins by the thousands. The urchins are herbivores and graze on kelp beds. When human hunters overexploited the sea otter populations, the urchin population exploded and destroyed many kelp beds. The enormous kelp forests are the home for a riot of marine organisms. Consequently, the human market for sea otter pelts caused the crash of many species on the site of the then destroyed kelp beds. Curiously, the urchins, having lost their kelp food supply, do not always die but enter an almost dormant state, termed zombie urchins, who can recover when a food source reestablishes.

As the kelp forest communities were restructured, the economic relationships between the human and marine world also shifted. Of course, sea otter pelts were once very valuable, but hunting them is now outlawed. The urchins themselves support the human economy as urchin roe is a prized item for sea food connoisseurs. With the sea otters rare, urchins become abundant on the seafloor—a "purple rug" it is called. However, starved sea urchins contain little roe and are useless to the marketplace. In time, as the sea otter populations increase, the urchins are eaten, the kelp beds can reestablish, and the marine food web starts to recover. The sea otters are the keystone species that can control this magnificent biodiversity at the North Pacific shore and add diversity to the culinary world.

This is a well-known case study of species interactions and the critical role that humans as well as sea otters play in the continuity of the marine biota. The study is important for restoration ecology as a warning that ignoring less common species, sea otters are still sparse, can destroy our high hopes for sustainable living communities.

New studies have shown that this one example has even deeper chords that enrich its ecological music. First, kelp beds can play a significant role in storing carbon. The massive kelp macroalgae have one of the highest growth rates of any plant species and sequester carbon both in their tissue while growing and on the seafloor when the kelp strands detach and fall to the bottom. Think of kelp as swirling, flexible plants analogous to the Douglas fir onshore: huge plants that suck up carbon from the atmosphere and hold it in living or dead tissue for many, many decades. In this sense, sea otters play a role in controlling climate change! The otters eat the urchins. This action protects the kelp, yielding an increase in carbon sequestration. Restoration of kelp beds through a policy of sea otter management was not mentioned in the recent United Nations conference on climate change but it is one part of the solution.

Second, the foraging by sea otters plays a role in the genetic diversity of the plant communities where they live. This is another type...