Grazing and Indirect Interactions among Species

Indirect interactions occur when the presence or activities of one species influence the interactions among other species. There are, in general, two major types of indirect interactions: (1) an interactive chain or (2) a modifying interaction. Grazing can be involved in either of these. An interactive chain is the situation where one species of plant (A) is a superior competitor for space or light or nutrients compared with a second plant (B). Left to their own devices, A would outcompete B in any shared patch of habitat. If, however, grazers arrive which preferentially consume the superior competitor, there is a resulting indirect, positive effect on B, the inferior competing plant. Thus, grazers have a direct, negative effect on A, their food and an indirect, positive effect on B because of the reduction in competition.

A modifying interaction occurs when one species indirectly influences the direct interaction between two others. An example involving grazing is the situation where a grazer (A) consumes a plant (B). There are, however, other plants (C) which are well-defended from attacks by grazers, because they have spikes, thorns, etc., which discourage the grazers from foraging near them. Where plant species B and C grow together, C causes an indirect, modifying interaction by reducing the effectiveness of grazing on B and, thus, has a positive, indirect effect on B and, usually, a direct negative effect on the grazers (A), which have their foraging reduced.

An example of an interactive chain is grazing of seaweeds on the rocky shores of New England (USA) by littorinid snails. In some areas, the snails consume green algae, such as Enteromorpha intestinalis. On the open shore, these green algae can be eliminated by grazing, resulting in a reduction in the number of species present. In contrast, in shallow rock-pools, the green algae can grow over and reduce the cover of encrusting species of seaweed. The encrusting species are tough and difficult for the snails to eat, in contrast to the soft-bodied E. intestinalis. As a result, the effect of grazing on E. intestinalis is to increase the survival or abundance of the encrusting species.

This is a classically known result for nonselective grazing or lawn-mowing of terrestrial vegetation. This was noted a very long time ago by Darwin, who commented that mowing lawns causes the persistence of more species of plants than is the case where plants are allowed to grow undisturbed. In the absence of mowing, a few species of plants eventually dominate the space, light, and resources in the soil. Competition eliminates the others. Lawn-mowers operate in an unselective manner, cutting back all the species ofplants and thereby preventing competitive dominance and maintaining a greater diversity of plants.

Most grazers are more selective in their choice of food, but can have important effects on the diversity of plant species. A long-known example (and one of the early sustained experimental ecological analyses of the influences of grazing) concerns rabbits as grazers in British grasslands. Where rabbits were experimentally excluded from plots of grasses, the experimental areas became dominated by a reduced number of species. Rabbits were generally consuming competitively dominant plants, such as clover.

This situation is, however, made more complex because grazers such as rabbits can exist in very large numbers. Very intensive grazing then occurs and selectivity of species of food breaks down, so that less-preferred species are also eaten in large quantities. Under these circumstances, large numbers of rabbits cause reduced diversity, as a direct effect of the increased intensity of grazing.

By the 1920s, it was known that the number of species of plants was strongly related to the intensity of grazing by rabbits. When there is no grazing, the direct influences of competition among plants cause there to be few species present. Under intermediate amounts of grazing, competitively dominant species are consumed selectively and inferior competing species persist. Diversity is greater. At very great intensity of grazing, many species of plants are consumed, some to extinction, thus reducing the diversity of species.

Such influences on diversity of grazing (or predation or physical disturbances) are these days described by the model of 'intermediate disturbance'. Where grazing or disturbance is mild or nonexistent, competition results in loss of species. Where grazing or disturbance is frequent or intense, it causes direct elimination ofspecies. Diversity of species is greatest at intermediate levels of grazing or disturbance, where these act selectively to reduce the abundances of competitively dominating species, thus allowing inferior competitors to survive.

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