By Christopher Wills
As my blood thins with age, I tend to SCUBA dive in the tropics. But in July of 2010, loaded with twenty-four pounds of lead weights to overcome the buoyancy of my thick wet suit and the dense salty water of the frigid Japanese Current, I found myself plunging into cold water to investigate an ecological success story off California’s Channel Islands. I wanted to see what happens when a damaged ecosystem recovers. Can it ever return to its former self?
A sheephead, Semicossyphus pulcher, cruises through a kelp forest off the coast of California’s Anacapa Island. Such large and tempting fish are among the first to disappear in coastal ecosystems. If they can be persuaded to return, can such ecosystems revert to their undamaged past?
In my forthcoming book I take the reader to different parts of the world and explore their long evolutionary history. But my cold-water dives will form part of a projected sequel to the book, in which I ask whether evolutionary and ecological forces can overcome centuries or millennia of damage to our planet by humans.
One of these damaged regions is the kelp forest ecosystem of southern California, which has been overfished for more than a century. I had picked what I thought would be a warm July to visit these forests, but the summer turned out to be the coldest along the Southern California coast in thirty years. When I swam down about 10 meters I hit the thermocline, and the water temperature suddenly dropped more than ten degrees. The cold pierced me to the marrow. Even the smaller fish, dancing in rays of sunlight near the surface among the fronds of brown algae, seemed to prefer the warmer water!
But the scenes that greeted me, both above and below the thermocline, were lively and healthy. Rockfish were common near the shore, and the bottom was rich in sea slugs and sea hares. Lobsters swarmed in the shallow submerged caves that line the coasts of the islands.
Perhaps most critically, during my three days of diving I saw several large black sea bass, including one that appeared to be (even after discounting a diver’s tendency to exaggerate) well over a meter long. The largest caught have been over two meters in length. Along with the rarer white sea bass, sea lions, seals, porpoises and the occasional shark, these fish are top predators in this ecosystem. Black sea bass are highly endangered because of overfishing — the catch plummeted from hundreds of tons to just a few tons in the 1970s and 1980s. Fishing for these giants of the sea was banned in California in 1981, but they still get tangled in nets and fishermen are still permitted to catch a few in Mexican waters.
Black sea bass, Stereolepis gigas, are among the top predators in the kelp forest. They move quickly, so this is the best of my pictures! Scars like the one you see on the fish’s side are common, but it is unclear what causes them.
The waters around the Channel Islands were declared a marine sanctuary in 1980 by President Carter, though fishing is allowed in some areas. Populations of large fish such as the sea bass have bounced back in the subsequent thirty years, but some smaller fish have declined in numbers in the reserves. This might seem to be worrying, but it can also be interpreted as a return to a healthier ecosystem.
Ecologists have shown, for terrestrial ecosystems, that predators that occupy the top of a food chain are responsible for maintaining the ecosystem’s diversity. Because predators tend to switch their attention to the prey species that happens to be commonest at the moment, no single prey species can multiply and overwhelm the others. In a terrestrial ecosystem, in which these “top predators” tend to be mammals or birds, each predator needs a lot of energy. As a result, there are few of them. In a marine ecosystem, cold-blooded top predators with lower energy requirements can be more numerous. At the undisturbed Palmyra Atoll in the central Pacific, sharks swarm and their prey are relatively rare.
The Channel Islands may be moving slightly in that direction as the cold-blooded top predators multiply. But the recent gains are fragile, and pressure from fishing in the waters outside the reserve is increasing.
A kelp rockfish, Sebastes atrovirens, peers from the base of the kelp forest. Healthy diversity, aided by the re-emergence of top predators, is characteristic of the Channel Islands Marine Reserve.
Christopher Wills is Professor of Biological Sciences at the University of California. His research interests include the maintenance of genetic variability in human populations, the forces that maintain variation in complex ecosystems such as rainforests and coral reefs, the evolution of diseases, and the evolution of our species. His forthcoming book is The Darwinian Tourist: Viewing the world through evolutionary eyes. The photos in this post are the author’s own.