In the fall and winter the canopy can become extremely thick and extensive, creating considerable shade beneath it. The consequences of shading to other photosynthetic organisms in the kelp forest are similar to those in terrestrial forests. The growth of understory and turf algae that live beneath the canopy is subdued and the germination of new plants is suppressed. The shading effect of Macrocystis along with the great length they can attain, has apparently created a selective pressure to evolve a way of transporting photosynthetic products from the blades in the canopy to the rest of the plant below. Kelp forests are subject to dynamic changes induced by herbivory.
Sea urchins are the major grazer on kelp, climbing up the holdfasts and chewing through the stipes.
They also eat drift algae that accumulates on the ocean bottom. Forests of the giant kelp, Macrocystis pyrifera , have become one of the primary symbols of the Monterey Bay and of the Monterey Bay National Marine Sanctuary . The significance of kelp forests can be described in terms of providing both biological and commercial resources for the local ecosystem and community. It is well known by people in many fields that kelp forests are capable of high productivity and create habitat for a wide variety of mammals, birds, fish, invertebrates, and other algae. The influence of kelp forests to tourism and quality of living, along with their ecological importance described above, create a need for understanding the factors that influence where, when, and how long kelp forests will persist in a given area.