In 1978, the National Park Service established a marine reserve at Anacapa Island in southern California. The reserve was expanded in 2003 and a fished MPA was added. Throughout 2 decades of monitoring, the reserve has had a healthy kelp forest, while nearby fished areas frequently have been urchin barrens.

 

The difference is caused by the effects of fishing cascading through the food web. Outside the reserve, fishing reduced the number of spiny lobsters, which eat sea urchins. As a result, urchins were over 13 times more abundant than in the reserve, and they ate vast quantities of kelp. Meanwhile, lobsters became 6 times more abundant in the reserve because there was no fishing. By keeping urchins in check, lobsters enabled kelp forests to flourish in the reserve.

 

In the 1970s, all monitoring sites inside and outside the marine reserve had kelp forests. After the stressful climatic El Niño events in 1982-1983, however, some sites outside the marine reserve became barren, while sites inside still had kelp forests. The figure at right shows a period of 20 years, spanning 4 El Niño events, when a fished area (blue line) alternated between kelp forest and urchin barrens, but the marine reserve (green line) remained kelp forest. These findings suggest that kelp forests in the reserve may be more resilient to climatic stress than kelp forests in unprotected sites.