Herbivore

**Herbivores** are animals that only eat plants. They are *herbivorous* animals.

Most modern herbivores (such as deer, elephants, horses) are grazers with teeth that are adapted to grind grass. The main present-day vegetation in most continents is grassland. Some herbivorous animals do eat eggs and sometimes other animal protein.

Animals who eat mainly fruit are called frugivores. Browsers eat mostly leaves and sometimes small tree branches. Animals that eat mostly grass are grazing animals.

The diet of some herbivorous animals changes with the season. In the temperate zones, some seasons are hot and some are cold, so different plants are available at different times of the year.

## Digesting cellulose 
Plant cell walls are mostly made up of cellulose. No animal can digest cellulose by itself. They make use of gut flora, some of which produce an enzyme called cellulase. This is an example of symbiosis.

## Herbivore-plant interactions 
According to the theory of predator-prey interactions, the relationship between herbivores and plants is cyclic. When prey (plants) are numerous, herbivores increase in numbers. Reducing the plant population causes herbivore number to decline. Eating a second plant type helps herbivores’ populations stabilize.

People used to think herbivores increase plant diversity by avoiding dominance. Dominant species tend to exclude subordinate species. However, the effects on plant diversity caused by variation in dominance could be beneficial or negative. Herbivores do increase bio-diversity by consuming dominant plant species, but they can also prefer eating subordinate species according to plants’ palatability and quality. In addition to the preference of herbivores, herbivores' effects on plant diversity are also influenced by other factors, defense trade-off theory, the predator-prey interaction,  and inner traits of the environment and herbivores.

One way that plants could differ in their susceptibility to herbivores is through defense trade-off. Defense trade-off theory is commonly used to be seen as a fundamental theory to maintain ecological evenness. Plants can make a trade-off response to resource allocation, such as between defense and growth. Defenses against herbivory on plant diversity can vary in different situations. It can be neutral, detrimental or beneficial for plant fitness. Even in the absence of defensive trade-offs, herbivores may still be able to increase plant diversity, such as herbivores prefer subordinate species rather than dominant species.

The predator-prey interaction, especially the “top-down” regulation. The predator-prey interaction encourages the adaptation in plant species which the predator prefers. The theory of “top-down” ecological regulation disproportionately manipulates the biomass of dominant species to increase diversity. The herbivore effect on plant is universal but still significantly distinguish on each site, can be positive or negative.

In a highly productive system, the environment provides an organism with adequate nutrition and resources to grow. The effects of herbivores competing for resources on the plant are more complicated. The existence of herbivores can increase plant diversity by reducing the abundance of dominant species, redundant resources can then be used by subordinate species. Therefore, in a highly productive system, direct consumption of dominant plants could indirectly benefit those herbivory-resistant and unpalatable species. But the less productive system can support limited herbivores because of lack of nutrients and water. Herbivory boosts the abundance of most tolerant species and decreases the less-tolerant species’ existence which accelerates the plant extinction. And the adaptation of plant species to avoid predators can also adjust the vegetation structure and increase diversity. Larger herbivores often increase plant diversity. They use competitively dominant plant species, and disperse seeds and create disorder of the soil. Besides, their urine position also adjusts the local plant distribution, and prevent light competition.

Therefore, the mechanisms of herbivores’ effects on plant diversity are complicated. Generally, the existence of herbivores increases plant diversity. But varies according to different environmental factors, multiple factors combined to affect how herbivores influence plant diversity.

## Related pages 
* Carnivore
*Omnivore
*Balanced diet
## References