An herbivore is an animal that eats plants as its primary source of sustenance. Examples of herbivores include large mammals such as cattle, deer, sheep, and kangaroos, as well as smaller creatures such as leaf-eating insects and crustaceans that graze upon aquatic algae. However, many animals are not exclusively herbivorous. In addition to feeding mostly upon live plants, omnivorous animals such as pigs and bears may also kill and eat other animals, opportunistically feed upon dead creatures, or eat dead plant biomass.
In the language of trophic ecology, herbivores are known as heterotrophic creatures, which means that they must ingest biomass to obtain their energy and nutrition. In contrast, autotrophs such as green plants are capable of assimilating diffuse sources of energy and materials, such as sunlight and simple inorganic molecules, and using these in biosynthetic reactions to manufacture complex biochemicals. Herbivores are known as primary consumers, because they feed directly on plants. Carnivores that feed on herbivores are known as secondary consumers, while predators of other carnivores are tertiary consumers.
A fact of ecological energetics is that within any ecosystem, herbivores are always much less productive than the green plants that they feed upon, but they are much more productive than their own predators. This ecological reality is a function of the pyramid-shaped structure of productivity in ecological food webs, which is itself caused by thermodynamic inefficiencies of the transfer of energy between levels.
However, this ecological law only applies to production, and not necessarily to the quantity of biomass (also known as standing crop) that is present at a particular time. An example of herbivores having a similar total biomass as the plants that they feed upon occurs in the open-ocean, planktonic ecosystem, where the phytoplankton typically maintains a similar biomass as the small animals, called zooplankton, that graze upon these microscopic plants. In this case, the phytoplankton cells are relatively short-lived, but their biomass is regenerated quickly because of their productivity. Consequently, the phytoplankton has a much larger total production than the longer-lived zooplankton, even though at any particular time their actual biomasses may be similar. Similarly, the densities of animals are not necessarily less than those of the plants that they eat, as occurs, for example, if insects are the major herbivores in a forest of large trees.
Following further along the above line of reasoning, because herbivores eat lower in the ecological food web, there is a relatively large quantity of food resource available to sustain them, compared with what is available to sustain carnivores. This fact has implications for humans, which can choose to sustain themselves by eating various ratios of food obtained directly from plants, or from animals that feed upon plants (such as cows, pigs, sheep, or chickens). In a world in which food for humans is often present in a supply that is less than the demand, at least in some regions, many more herbivorous (or vegetarian) people could be sustained than if the predominant feeding strategy was carnivorous.