Herbivory

Herbivory is the process of consuming plants as food. Herbivores are organisms that eat living plants for nourishment. A monarch butterfly caterpillar eating milkweed leaves, a field sparrow eating seeds, snails grazing on lettuce in a vegetable garden, a deer browsing on woodland shrubs, Canada geese relishing the grass on a golf course, and minnows grazing on algae in a stream are all engaging in herbivory. Through their feeding activity, herbivores can have a powerful influence on biodiversity, both species numbers and distribution.

Herbivores can be selective feeders, able to survive on only one type of plant. For example, monarch butterfly caterpillars will eat only milkweed plants. Other plant eaters can be generalists, thriving on a variety of plant materials, though they may have their favorites. White-tailed deer will eat many different kinds of herbs and shrubs. Gypsy moth caterpillars prefer oaks but will do well on other trees. Most herbivores remain plant-eaters throughout their lives, but some are herbivorous only during one life stage. For example, frogs are herbivorous as tadpoles, grazing on algae in ponds; as adults, however, they are carnivorous, feeding on insects. Cecropia moth caterpillars feed on elderberry and other plants as larvae, yet do not feed at all as adults!

Herbivory can change the structure and species composition of entire plant communities, affecting biodiversity in a variety of ways. The nature of this impact will depend on the particular ecosystem and its characteristics, the species interacting in that system, and the densities of each of those species.

In terrestrial systems, herbivory can slow succession. Grazing by rabbits, voles, and other plant eaters in an old field, for example, will prevent the field from "growing up" or succeeding into forest. In aquatic systems, herbivores can accelerate change to other plant communities. This happens, for example, when grazing by marine snails clears growing space on submerged rocks along the shore, allowing later suc-cessional seaweeds to take hold.

Heavy grazing pressure can remove so much plant material from an ecosystem that diversity is severely reduced. In the northeastern United States, for example, browsing by high numbers of white-tailed deer has resulted in the disappearance of many native plants from the woodlands, reducing forest diversity. In many cases, what remains after such heavy grazing are plants that are unpalatable—those that aren't eaten because they are covered with spines or are toxic. In this way, grazing by deer can also change the plant species composition of the woodlands. That in turn affects other animals in the forest. Many bird species depend on certain woodland plants for food and as nesting sites, and these are the same plants that may be removed by the grazing deer.

If not controlled by herbivore grazing, some plants actually out-compete other plants in the area by growing over them and shading them out. This may result in lowered species diversity in a particular environment. On the other hand, a modest amount of herbivory can result in an increase in diversity in an ecosystem by removing some of the competing plants and allowing more species to find space to grow. This is particularly true if the grazing animal is selectively feeding on the competitively dominant plant species in that ecosystem.

In response to herbivory, many plants have developed antigrazing tactics. Some produce toxic or distasteful chemical compounds in their leaves to deter grazing, such as the tannin in oak leaves. Others develop leaves with hairs, thorns, or spines (for example, mullein, Japanese barberry, cactus). Still other plants have evolved strategies for seed dispersal to deter predation on seeds. Over evolutionary time, many intricate plant and herbivore relationships and strategies have coevolved, ensuring the survival of both plant and animal.

—Elizabeth A. Johnson

See also: Botany; Carnivora; Communities; Food Webs and Food Pyramids; Succession and Successionlike Processes

Bibliography

Harper, John L. 1977. Population Biology of Plants. New York: Academic; Morin, Peter Jay. 1999. Community Ecology. New York: Blackwell Scientific.

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