Forests of giant kelp (Macrocystis pyrifera) have been a lifelong passion, or perhaps addiction, of mine. They were my object of study in graduate school, and I was fortunate enough to be able to create a living kelp forest exhibit at the California Science Center in Los Angeles.
Giant kelp forests can be found around the world in temperate latitudes, those areas on the globe between about 30° and 60° north and south of the equator, where ocean temperatures are on the cool side, and sunlight and nutrients are plentiful. In the southern hemisphere, giant kelp forests are found in Chile, Australia, South Africa, and on the various sub-Antarctic islands found in between. But in the northern hemisphere they are found only along the coast of North America from just south of San Francisco to about the middle of the Baja California Peninsula. How this odd geographical distribution occurred, being circum-global in the southern hemisphere, and restricted to one location in the northern, must have been some trick of plate tectonics and climate change in the distant past.
Giant kelp forests along the coast of California are known to harbor at least 800 species of fish and invertebrates that are associated strictly with kelp. That means when kelp forests disappear, a large number of other species disappear with them.
Giant kelp can disappear for at least a couple of reasons. These include sea urchin outbreaks and displacement by other species of algae. Sea urchins dine exclusively on algae, and particularly like giant kelp because of its abundance. These hungry herbivores are normally kept in check by predators like sea otters (Enhydra lutris), sheephead fish (Semicossyphus pulcher), and lobsters (Panulirus interruptus). But when predators are removed, urchin populations can explode. In central California, all three of these predator species are found, and, in general, these kelp forests don’t suffer from the appearance of urchin barrens – areas that excessive numbers of urchins have stripped of all algae. But in Southern California, where otters have been absent for over a century, and populations of sheephead and lobster have been heavily fished, the biological control of sea urchin populations has been greatly reduced. As a result, the occurrence of urchin barrens caused by urchin population explosions are much higher than along the central coast.
The accidental introduction of a species of Japanese algae in 2003 has also caused the loss of kelp forests along the coast and on the northern side of Catalina Island, although the ultimate outcome has yet to be determined. Sargassum horneri was most likely introduced via ballast water into Long Beach Harbor, California. From there it has spread north and south, and out to some of the Channel Islands. Catalina Island has been particularly hard hit. Much of the northern, or front side of the island has been overrun. In the process, many areas normally supporting giant kelp have been converted to supporting S. horneri. In some places the kelp has fought back, but in others it hasn’t. The reproductive and growth patterns of these two species will eventually determine the new normal for kelp forests in the area.