community ecology Grazing

The word “grazing” conjures up images of large mammals moving through seas of grass. Grazing, however, is a form of interspecific interaction that has been adopted by a number of other groups as well. A grazer is defined as any species that moves from one victim to another, feeding on part of each victim without actually killing it outright. The “victim” is to the grazer as prey is to the predator. Hence, grasshoppers that jump from plant to plant, chewing a portion of the leaves of each one they visit, are grazers, as are caterpillars that crawl from one plant to another during development rather than remain as parasites on an individual plant. The grazing lifestyle differs from the parasitic lifestyle in a few important ways. Individuals can vary their diets with different foods, and, by not remaining attached to a single individual for long periods of time, their victims do not have time to develop induced specialized defenses, such as an immune response that a host can develop against a parasite.

Grazing is more commonly perpetrated on plants than animals because plants have a modular structure that allows a part of them to be lost without the whole individual being destroyed. In contrast, most animals that lose a part of the body to an antagonist die immediately or soon afterward, rendering the interaction an act of predation rather than grazing. An exception to this rule occurs in species that can disconnect body parts—some lizards and salamanders are able to detach their tails if they are attacked by a predator.

On most continents, reciprocal evolutionary changes, or coevolution, between grasses and large grazing mammals have taken place over periods of millions of years. Many grass species have evolved the ability to tolerate high levels of grazing, which is evident to anyone who regularly mows a lawn. Simultaneously, they have evolved other defenses, such as high silica content, which reduces their palatability to some grazers. A number of herbivorous mammals have responded to these defenses by evolving the ability to specialize on grasses with high silica content and low nutritional value. Many large grazing mammals such as elephants have high-crowned (hypsodont) teeth that are constantly replaced by growth from below as the crowns are worn down by the silica in their food. Many of these species also have complicated digestive systems with a gut full of microflora and microfauna capable of extracting many of the nutrients from the plants.

Not all grasslands, however, are adapted to grazing by large mammals. In North America, although the grasslands of the Great Plains coevolved with large herds of bison, the grasslands of the upper Intermontane West (which roughly includes eastern Washington and Oregon) have never supported these large grazing herds. The Great Plains had grasses that formed sods and could withstand trampling by large-hooved mammals. These sods were so tightly interwoven that early European settlers cut them to use as roofs for their houses. The grasses of the Intermontane West, however, were tuft grasses that did not form sods and quickly died when trampled. Consequently, when cows replaced bison as the large, grazing mammal of North America, the grasslands of the Great Plains sustained the grazing pressure, whereas those of the Intermontane West rapidly eroded. Similar problems have arisen in other parts of the world where cattle have been introduced into grasslands that did not have a history of coevolution with large grazing mammals.

Plants have evolved more than 10,000 chemical compounds that are not involved in primary metabolism, and most of these compounds are thought to have evolved as defenses against herbivores and pathogens. Some of these chemical compounds are defenses against grazers, whereas others are defenses against parasites. Most of the chemical compounds that make herbs so flavourful and useful in cooking probably evolved as defenses against enemies. These compounds, called allelochemicals, are found in almost all plant species, and their great diversity suggests that chemical defense against herbivores and pathogens has always been an important part of plant evolution.