Habitats are complexly balanced communities that rely on the interrelationships of different species. Some interactions however can have a greater affect on the overall structure of the habitat than others. One example is the interaction between different species urchins and sea otters.
Herds of Herbivores: Sea Urchins and Their Effects on Kelp Forests
Sea urchins are invertebrates that belong to the same phylum as sea stars. They live in the benthic zone of the kelp forest and are voracious herbivores. The main species of urchins that are found in kelp forests are the Purple, Red, Green, White, and Coronado Urchins. They represent the genera Strongylocentrotus (purple, red, and green urchins), Lytechinus (white urchins), and Centrostephanus (coronado urchins). Urchins can be found all over the world and are not limited to the kelp forest.
They can also live in the intertidal areas where they can spend a considerable amount of time out of the water during low tide. Some species even live on the deep ocean floor, an area that was once thought to be devoid of life.
Urchins of the benthos will spend most of their time living and foraging around the same few square feet, and urchins in the intertidal will tend to inhabit the same hole in the rock throughout their entire life cycle.
Because of their tendency to feed mainly on the base of the kelp, they tend to detach the whole plant from the substrate by eating through the stipe so that it will float away.An over abundance of urchins can lead to heavy grazing that results in barren areas that are free of kelp. These empty spots are aptly referred to as "urchin barrens".
Keeping the Populations in Check: Role of Sea Otters in Maintaining a Healthy Kelp Forest
The main predator of the urchins is the sea otter, Enhydra lutris. Sea otters are large marine mammals that live in a broad range from Alaska to Northern California. They feed mainly on invertebrates and urchins are a major portion of their diet.
Because of their feeding habits, sea otters act as a keystone species in the kelp forest. Keystone species are features of an ecosystem that are essential in maintaining its balance.
With sea urchins feeding on the kelp in a way that causes the plants to be completely lost, the control of their population becomes essential in maintaining the physical structure of the kelp forest. As a keystone species, when sea otters become scarce, the usually thriving kelp forest can shift to the barren condition from over grazing by the urchins. This has been a major concern in areas where, historically, sea otters were hunted for their pelts.
By 1911, in California, the sea otter populations were reduced from nearly 16,000 down to almost extinct. With such a drastic decline in sea otters, the kelp forests were shifted to urchin barrens and were not able to effectively recover until the urchin populations were reduced.
The first step to sea otter protection came in 1911 when the United States was one of the countries that signed the Treaty for the Preservation and Protection of Fur Seals, which imposed a ban on the hunting of sea otters. Federal protection from the Marine Mammal Protection Act of 1972 afforded even stricter rules against the harvesting of sea otters.
As the sea otter populations showed signs of recovery through the 1980's and 1990's, the urchin populations were able to be naturally balanced so as to again encourage the growth of kelp and the return of the lush kelp forest.
Esther Eder - Born and raised in Salem, Oregon, Esther graduated from Oregon State University in the spring of 2007 with a Bachelor of Science degree in ...
