Communities which can exist in different forms for very long periods, such as kelp beds or urchin-grazed bottom, are said to have alternate stable states. While ecologists spend a great deal of time arguing over what is meant by stability, and how it is measured, the concept of alternate states is useful in understanding the impact of herbivores in communities. In the example referred to above, the alternate states were sea urchin-dominated bottom and kelp beds, while the triggers which flipped the community between alternate states were urchin-grazing and disease. Similar alternate states exist in marine communities off the south and west coasts of South Africa. The huge kelp beds, which are a familiar sight along the west coast, probably exist because sea urchins are prevented from grazing by continuous wave action. In areas with little wave action, such as parts of St. Helena Bay, the urchins are able to graze effectively and so the kelp beds are reduced or absent. In some places, for example Miller's Point, Simonstown, where wave action is significant, but less than on the open west coast, both alternate states exist together. At Miller's Point, the kelp beds are found close to the shore, and the urchin grazed state is found seaward of the kelp.

Many marine communities exist indefinitely in a heavily grazed state, with low plant abundance, and no natural alternate state is known. The extensive kelp beds of the Atlantic coast of Nova Scotia are unusual in the Northwest Atlantic, where most of the coast exists only in the urchin-grazed state. In the intertidal zone of South Africa, grazing by limpets maintains a community that often looks like bare rock to the naked eye. Only when limpets are prevented from grazing, for example by heavy sand movement or intensive predation by oyster catchers, does an extensive algal cover develop.

Many marine communities exist indefinitely in a heavily grazed state, for example, the Patella cochlear zone, characteristic of the South Coast of South Africa.