Herbivore

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.

Herbivores are often subdivided into such groups as: frugivores (primarily fruit eaters); folivores (leaves); granivores (seeds); mucivores (plant fluid); nectarivores (nectar); palynivores (pollen); rhizophages (roots); and xylophages (wood).

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.

See also Carnivore; Ecological productivity; Food chain/web; Heterotroph; Omnivore.

Bill Freedman

Herbivore

Herbivores are animals that eat only plants. Since animals cannot make their own food, they must obtain their energy either by being herbivorous (eating plants), carnivorous (eating other animals), or omnivorous (eating both plants and animals). Herbivores are called primary consumers on the food chain since they are the first organisms to consume the energy stored by primary producers (plants). As plant eaters, herbivores have certain physical characteristics that make them different from carnivores and omnivores.

Since green plants are the only organisms capable of producing their own food, they are at the beginning of the world's food chain. This food chain or food path connects species in terms of how food and energy is passed from one species to another. Food chains or webs are divided into organisms that produce energy and those that consume it. Producers, who use energy and make their own food, are called "autotrophs." Plants are autotrophs since they make their own food by converting the Sun's light energy into chemical energy via photosynthesis. Animals cannot do this and must eat other living organisms in order to survive. They are called "heterotrophs" since they are forced to obtain their energy and nutrients from the food they ingest or eat. Animals are considered to be primary, secondary, or tertiary consumers according to the order of who eats whom. A caterpillar is a primary consumer because it eats only plants. A pigeon would be a secondary consumer when it eats a caterpillar. A fox would be a tertiary consumer when it eats a pigeon.

Herbivores have developed many traits that are specialized only for plant eaters. Cows and sheep are examples of herbivores that possess several physical characteristics because of their diet. An examination of their skulls shows that they have no canine teeth, or the two long, pointy teeth in the front of a carnivore's mouth that it uses for ripping and holding its prey. Since these herbivores are grazing animals and have no need to hunt and catch their food, they have instead chisel-like front teeth called incisors to break off blades of grass. Behind the incisors and the back molars is a gap or a space called a diastema, providing the necessary "give" in a jaw that moves side-to-side. These herbivores' molars are flat teeth with ridges on the surface that serve as powerful grinding tools. Sheep and cows also have loose joints in their jaws so they can chew side-to-side, which improves the grinding action of their teeth. After herbivores chew the leaves or grass into a pulp with their molars, it passes into a highly specialized digestive system that is very different from that of a carnivore. The best example is that of a cow, since it has four chambers in its stomach. Grass is very difficult to digest because of its tough cell walls. Sometime after the pulp enters a cow's first chamber or rumen, it is regurgitated or coughed up into its mouth to be chewed again. This helps break the grass down even more and is what we describe as a cow "chewing its cud." The grass later passes on through the remaining chambers of a cow's stomach where it is digested even more. Since herbivores are able to get only a small amount of energy from each mouthful of their vegetarian diet, they must eat enormous amounts of food. This is why a cow spends nearly all its waking hours grazing.

Besides cows and sheep, other herbivores include caterpillars, fishes, birds, and many other mammals. Many eat seeds and fruit instead of grass or leaves, and because of this specialized diet, have specialized tools, like a certain type of beak or bill. Certain finches have beaks that allow them to eat a certain type of seed. Other birds like the toucan have long, sharp bills to pluck berries or chop larger fruit into bite-size pieces. Herbivores are critically important to carnivores, since without them, a meat-eater would have no way of obtaining the life-giving energy first captured by green plants.

Herbivore

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.

See also Carnivore; Ecological productivity; Food chain/web; Heterotroph; Omnivore.

Bill Freedman

herbivore A heterotroph that obtains energy by feeding on primary producers, usually green plants. Compare carnivore; detritivore; omnivore. See also food-chain.

her·biv·ore / ˈ(h)ərbəˌvôr/ • n. an animal that feeds on plants.DERIVATIVES: her·biv·o·rous / (h)ərˈbiv(ə)rəs/ adj.