An herbivore is an animal that mainly eats plants. Herbivores vary in size from small, like bugs, to large, like giraffes.

An animal’s diet determines where it falls on the food chain, a sequence of organisms that provide energy and nutrients for other organisms. Each food chain consists of several trophic levels, which describe an organism’s role in energy transfer in an ecosystem. Herbivores are primary consumers, which means they occupy the second trophic level and eat producers.

For each trophic level, only about 10 percent of energy passes from one level to the next. This is called the 10 percent rule. Because of this rule, herbivores only absorb around 10 percent of the energy stored by the plants they eat.

Not all herbivores eat the same, however. While some herbivores consume a wide variety of plants, others consume specific plant parts or types. For example, frugivores eat fruit, granivores eat seeds, folivores eat leaves, and nectarivores eat nectar.

Herbivores have various physical features evolved specifically for their diet as well. Many herbivores have large, flat molars for grinding tough plant matter. Additionally, herbivores often have multiple stomach chambers and a specialized digestive system. For example, cows have a stomach with four chambers. The food a cow consumes first passes through two stomach chambers before returning to the mouth for additional chewing. This returned food is called cud. Once the cow rechews and swallows the cud, it passes through the third and fourth stomach chambers for further digestion.

Herbivores play an important role in maintaining a healthy ecosystem by preventing an overgrowth of vegetation. Additionally, many plants rely on herbivores such as bees to help them reproduce. By the same token, herbivores rely on plants not just for food but also for habitats and shelter.

Herbivores also serve as a food source for meat eating carnivores, which keeps plant eaters from overpopulating and overgrazing an ecosystem. For example, in 1995, wolves were reintroduced into Yellowstone National Park to control the elk population. Through their overgrazing, the elk had damaged trees, increased erosion, and spoiled trout streams. In the following years, the wolves helped stabilize the elk population and restored balance to the Yellowstone ecosystem.