Urchins have a profound effect on the ocean bottom community. When urchin predators like cod are unable to perform their function of keeping urchins at a sustainable level, the ecosystem changes. In this case the urchin population grows resulting in overgrazing and disappearance of kelp beds. In the subtidal zone of the Gulf of Maine, the algal community and its resident animals are structured to a large extent by the grazing activity of urchins. Urchins can eat so much kelp that they create areas known as "urchin barrens" where only low growing algae can survive.

However, the balance of the ecosystem can shift the other way. This happened when the urchin industry began. The harvesting of urchins during the 1990s has led to the regrowth of kelp beds. This has allowed crabs, lobsters, and other small crustaceans to thrive in these new beds because of the abundant food and protective cover. Can you find the urchins in the image above?