Some grazing animals are territorial, in that they occupy, patrol, and defend a patch of habitat in which they live, feed and, often, breed. In some cases, the direct or indirect consequence of this behavior encourages the growth of particular plants, so that the grazing species is farming or gardening its supply of food.
Examples are widespread, but one example of each of a vertebrate and an invertebrate grazer will illustrate some general features. First, pomacentrid or damsel fishes on coral reefs live in areas that are overgrazed by a large diversity of other fish and many invertebrates. Several species in the family Pomacentridae occupy territories in which they threaten and attack intruding fish. In these territories, there can be extensive growth of algae which are absent from adjacent, undefended areas. The territorial resident depends for its food on being able to graze on these algae, a supply of resources created by its activities. Some species of fish are very selective in what they allow to grow within their territories. They weed out unwanted species of seaweeds by biting them off the substratum and carrying them out of the territory. As a result, the territorial fish can have profound influences on local diversity of algal species.
Despite vigilance and rapid responses to intruders, the occupant of a territory can, however, be overwhelmed by the sheer weight of numbers of intruders if large numbers arrive at the same time. This is almost certainly a strong influence on how large a territory an individual can defend. If a school of grazing fish arrive in a territory, no amount of threat, harassment, or attack by the occupant will prevent removal of some of the algal food in the territory. Territorial defenses do, however, serve to ensure a sustained biomass of some algae that would otherwise be completely grazed away.
The second example concerns grazing by limpets on rocky shores. Most limpets wander around grazing over the surface of the rock using a special feeding organ or toothed 'tongue', called a radula. This scrapes over and into the surface of the rock, removing grains of rock and any attached bacteria or microscopic plants, which are then swept into the animal's mouth. In many parts of the world, grazing by these and other gastropods (snails) removes such a large proportion of the algae that the animals generally suffer from a shortage of food and often show evidence of considerable amounts of competition for the food that is present.
In some areas of the world, such competition has resulted in aggressive behavior to defend a territory. On Californian intertidal shores, the owl-limpet (Lottia gigantea) reaches a few centimeters length as its maximal adult size. It patrols an area of rock about 1000 cm extent. When it encounters an intruding limpet or grazing snail, the resident limpet attacks by ramming the front of its shell against the intruder. Sometimes this dislodges the intruder which is then rolled away by waves. Sometimes, the attack, or even the threat of attack as the resident approaches, causes the intruder to leave the territory.
The outcome is a very reduced number of grazers inside compared to outside the territories. When a resident limpet is experimentally removed, the number of other, smaller grazers increases. Often, a resident that has been removed is replaced by a smaller individual of L. gigantea from a nearby area. If an owl-limpet is introduced to an area, it will eventually drive away many of the resident grazers.
The result of this activity is the growth inside the territory of a film of visible macroalgae, which are a major source of food for the limpets. When a territorial limpet is removed, smaller grazers enter the territory and, because they scrape much closer to the surface of the rock while feeding, these remove all the algal film. Obviously, the activities of the limpets are necessary to allow the existence of the visible films of algae, which are thus being protected, as if in a garden.
There are variations on this theme. On rocky shores in South Africa, the limpet Patella longicosta also lives in defended territories. These limpets have pointed extensions radiating from their shells, which are used to dig under and flip over intruding grazing limpets. As a result of ousting intruders and reducing the intensity of grazing, an encrusting seaweed, Ralfsia sp., grows on the rock in the territory. Grazing by these limpets is sufficiently harsh that any small limpets, including juveniles of their own species, are crushed. The only place juvenile P. longicosta can survive and gain access to Ralfsia as food is on the backs of adults. There, the tiny limpets can graze in safety from attack by the territory holder. When they reach about 1014 mm, the juvenile limpets can no longer gain sufficient food on the shell and start to climb down from the adult's shell in order to graze over the rock. The adult, territorial limpet then treats them as an intruder and pushes the juveniles out of the territory, to take up home elsewhere. By now, the small limpets are large enough to survive by scraping over the encrusting, calcareous algae found outside territories made by limpets. Eventually, it is presumed that the small limpets will remove crusts, exposing bare rock on which Ralfsia starts to grow, thus beginning the establishment of a new territory.
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