Herbivory is a form of predation in which an organism, known as a herbivore, consumes principally autotrophs such as plants, algae and photosynthesizing bacteria. By that definition, many fungi, some bacteria, many animals, some protists and a small number of parasitic plants can be considered herbivores. However, herbivory is generally restricted to animals eating plants. Fungi, bacteria and protists that feed on living plants are usually termed plant pathogens. Microbes that feed on dead plants are saprotrophs. Flowering plants that obtain nutrition from other living plants are usually termed parasitic plants. More generally, organisms that feed on autotrophs in general are known as primary consumers.
In zoology, a herbivore is an animal that is adapted to eat primarily plant matter (rather than meat). Although such animals are sometimes referred to as being vegetarian, this term is more properly reserved for humans who choose not to eat meat as opposed to animals that are unable to make such choices. Herbivores can be further classified into various sub-groups, such as frugivores, which eat mainly fruit; folivores, which specialize in eating leaves; nectarivores, which feed on nectar; among herbivorous insects and other arthropods, the level of feeding specialization can be far more fine-tuned, including seed-eaters ("granivores"), pollen-eaters ("palynivores"), plant fluid-feeders ("mucivores"), and those specialized to feed on wood ("xylophages") or roots ("rhizophages"). In other animals, the degree of specialization is not so advanced, however, and many fruit- and leaf-eating animals also eat other parts of plants, notably roots and seeds. The diets of some herbivorous animals vary with the seasons, especially in the temperate zones, where different plant foods are most available at different times of year.
There is a misperception that if an animal is herbivorous, it represents less danger to humans than a carnivore (or, sometimes, no danger at all). This is not logically sound; few animals, even carnivores, will seek humans as a food source, but any animal will attack a human if necessary to defend itself. For example, in national parks such as the United States' Yellowstone Park, bison represent significantly more danger to humans than wolves, which are likely to avoid people. Of Africa's Big Five game (a term coined by hunters in Africa to refer to the five most dangerous animals to hunt: Rhinoceros, Leopard, Cape Buffalo, Elephant and Lion), three are herbivores.
Herbivores form an important link in the food chain as they transform the sun's energy stored in the plants to food that can be consumable by carnivores and omnivores up the food chain. As such, they are termed the primary consumers in the food chain.
Plant defense against herbivory include a range of adaptations evolved by plants to improve their survival and reproduction by reducing the impact of animals that eat them. Plants have evolved an enormous array of mechanical and chemical defenses against herbivores.
These defenses include mechanical protections on the surface of the plant, production of complex polymers that reduce plant digestibility to animals, and the production of toxins that kill or repel herbivores. Defenses can either be constitutive, always present in the plant, or induced, produced or translocated by the plant following damage or stress. The term host plant resistance is also used by plant breeders to refer to these mechanisms.
Plants have also evolved features that enhance the probability of attracting natural enemies to herbivores. Specifically, they emit semiochemicals, odors that attract natural enemies, and provide food and housing to maintain the natural enemies’ presence.
A given plant species often has many types of defensive mechanisms, mechanical or chemical, constitutive or induced, which additively serve to protect the plant, and allow it to escape from herbivores.
In some cases, herbivory is actually encouraged by plants to assist in reproduction. A notable example is the production of nectar to attract bees, which are necessary for pollination.
Herbivore adaptations to defenses
Herbivores are dependent on plants for food, and have coevolved mechanisms to obtain this food despite the evolution of a diverse arsenal of plant defenses against herbivory. Herbivores adaptations to plant defense have been likened to “offensive traits” and consist of those traits that allow for increased feeding and use of a host. Plants, on the other hand, protect their resources for use in growth and reproduction, by limiting the ability of herbivores to eat them. Relationships between herbivores and their host plants often results in reciprocal evolutionary change. When a herbivore eats a plant it selects for plants that can mount a defensive response, whether the response is incorporated biochemically or physically, or induced as a counterattack. In cases where this relationship demonstrates “specificity” (the evolution of each trait is due to the other), and “reciprocity” (both traits must evolve), the species are thought to have coevolved. The escape and radiation mechanisms for coevolution, presents the idea that adaptations in herbivores and their host plants, has been the driving force behind speciation.
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