Another way in which herbivores can influence communities is to produce a shift in dominance of the community to plants which are defended from grazing. For an example of this phenomenon I turn once again to the Northwest Atlantic subtidal zone, which you will recall is generally heavily grazed by sea urchins. I have studied the role of the food preferences of sea urchins, wave action, and scouring by sea ice in structuring the subtidal community along the east coast of the island of Newfoundland. Ignoring the effects of ice for now, we can consider the basic pattern of the subtidal community in terms of two factors; the abundance of sea urchins, and exposure to wave action.

The presence of dense populations of sea urchins means that wave action exerts its greatest influence through its effect on urchin grazing, which is greatly reduced by large waves. When exposure to wave action is low, even small populations of sea urchins can maintain a community where only crustose coralline algae persist. These algae are defended from grazing by virtue of the hard calcium carbonate deposited in their cell walls. Annual or ephemeral (short-lived, but fast growing) algae can also be important in this heavily grazed community because they can grow in small patches with reduced urchin feeding, but they are eventually grazed by the urchins. On the other hand, when wave action is extreme, even very high densities of urchins cannot effectively remove large perennial seaweeds lacking defenses against urchin grazing. At intermediate levels of wave action and urchin grazing, species with chemical but no mechanical defenses against grazing occur, along with the mechanically defended coralline algae.

These effects occur along a depth gradient along most of the coast, where there is some exposure to ocean swells. There is a shallow community dominated by fleshy algae which have no defense against urchin grazing. This community exists, despite high urchin densities, because wave action reduces their foraging ability in the surge zone. Below this zone, urchin abundance is high, and the effects of waves is reduced. The seaweeds in this community are mainly the hard, grazing-resistant coralline algae, together with a few seaweeds which escape grazing in space (patches with reduced grazing) and time (brief periods with storm waves). Below this zone is another community, which is characterized by a lower urchin density, and a dominance by two species of algae (colander weed and red sea-feather) which are chemically defended against urchin grazing.

At intermediate levels of urchin grazing in the northwestern Atlantic, the colander weed, a species with chemical but no mechanical defenses against grazing dominates the community.

The kelp beds which characterise the west coast of South Africa probably exist in part because heavy wave action keeps sea urchins from grazing the kelp plants. At the same time, waves provide the urchins with a ready diet of uprooted kelp plants.