Herbivore
A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat non-vascular autotrophs such as mosses, algae and lichens, but do not include those feeding on decomposed plant matters or macrofungi.
As a result of their plant-based diet, herbivorous animals typically have mouth structures well adapted to mechanically break down plant materials, and their digestive systems have special enzymes to digest polysaccharides. Grazing herbivores such as horses and cattles have wide flat-crowned teeth that are better adapted for grinding grass, tree bark and other tougher lignin-containing materials, and many of them evolved rumination or cecotropic behaviors to better extract nutrients from plants. A large percentage of herbivores also have mutualistic gut flora made up of bacteria and protozoans that help to degrade the cellulose in plants, whose heavily cross-linking polymer structure makes it far more difficult to digest than the protein- and fat-rich animal tissues that carnivores eat.
Etymology
Herbivore is the anglicized form of a modern Latin coinage, herbivora, cited in Charles Lyell's 1830 Principles of Geology. Richard Owen employed the anglicized term in an 1854 work on fossil teeth and skeletons. Herbivora is derived from Latin herba 'small plant, herb' and vora, from vorare 'to eat, devour'.Definition and related terms
Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in general are known as primary consumers.Herbivory is usually limited to animals that eat plants. Insect herbivory can cause a variety of physical and metabolic alterations in the way the host plant interacts with itself and other surrounding biotic factors. Fungi, bacteria, and protists that feed on living plants are usually termed plant pathogens, while fungi and microbes that feed on dead plants are described as saprotrophs. Plants that obtain nutrition from other living plants are usually termed parasitic plants. There is, however, no single exclusive and definitive ecological classification of consumption patterns; each textbook has its own variations on the theme.
Evolution of herbivory
The understanding of herbivory in geological time comes from three sources: fossilized plants, which may preserve evidence of defence, or herbivory-related damage; the observation of plant debris in fossilised animal faeces; and the construction of herbivore mouthparts.Although herbivory was long thought to be a Mesozoic phenomenon, fossils have shown that plants were being consumed by arthropods within less than 20 million years after the first land plants evolved. Insects fed on the spores of early Devonian plants, and the Rhynie chert also provides evidence that organisms fed on plants using a "pierce and suck" technique.
During the next 75 million years, plants evolved a range of more complex organs, such as roots and seeds. There is no evidence of any organism being fed upon until the middle-late Mississippian,. There was a gap of 50 to 100 million years between the time each organ evolved and the time organisms evolved to feed upon them; this may be due to the low levels of oxygen during this period, which may have suppressed evolution. Further than their arthropod status, the identity of these early herbivores is uncertain. Hole feeding and skeletonization are recorded in the early Permian, with surface fluid feeding evolving by the end of that period.
Herbivory among four-limbed terrestrial vertebrates, the tetrapods, developed in the Late Carboniferous. The oldest known herbivorous tetrapod is Desmatodon hesperis, belonging to the clade Diadectidae. Early anamniotic tetrapods were feeding on other aquatic vertebrates and invertebrates, and some amphibians would also evolve into terrestrial carnivores. The first amniotes were insectivores due to their small size, but later forms were larger and carnivious, before some eventually adated to a herbivorous lifestyle. In addition to Diadectidae, four other tetrapod clades independently evolved adaptations towards herbivory from the Late Carboniferous to early Permian; the Captorhinidae, the parareptilian Bolosauridae, and the two synapsid clades Edaphosauridae and Caseidae. The entire dinosaur order ornithischia, which existed during Jurassic and Cretaceous, was composed of herbivorous dinosaurs. Carnivory was a natural transition from insectivory for medium and large tetrapods, requiring minimal adaptation. In contrast, a complex set of adaptations was necessary for feeding on highly fibrous plant materials.
Arthropods evolved herbivory in four phases, changing their approach to it in response to changing plant communities. Tetrapod herbivores made their first appearance in the fossil record of their jaws near the Permio-Carboniferous boundary, approximately 300 million years ago. The earliest evidence of their herbivory has been attributed to dental occlusion, the process in which teeth from the upper jaw come in contact with teeth in the lower jaw is present. The evolution of dental occlusion led to a drastic increase in plant food processing and provides evidence about feeding strategies based on tooth wear patterns. Examination of phylogenetic frameworks of tooth and jaw morphologies has revealed that dental occlusion developed independently in several lineages tetrapod herbivores. This suggests that evolution and spread occurred simultaneously within various lineages.
Food chain
Herbivores form an important link in the food chain because they consume plants to digest the carbohydrates photosynthetically produced by a plant. Carnivores in turn consume herbivores for the same reason, while omnivores can obtain their nutrients from either plants or animals. Due to a herbivore's ability to survive solely on tough and fibrous plant matter, they are termed the primary consumers in the food cycle. Herbivory, carnivory, and omnivory can be regarded as special cases of consumer–resource interactions.Feeding strategies
Two herbivore feeding strategies are grazing and browsing. For a terrestrial mammal to be called a grazer, at least 90% of the forage has to be grass, and for a browser at least 90% tree leaves and twigs. An intermediate feeding strategy is called "mixed-feeding". In their daily need to take up energy from forage, herbivores of different body mass may be selective in choosing their food. "Selective" means that herbivores may choose their forage source depending on, e.g., season or food availability, but also that they may choose high quality forage before lower quality. The latter especially is determined by the body mass of the herbivore, with small herbivores selecting for high-quality forage, and with increasing body mass animals are less selective. Several theories attempt to explain and quantify the relationship between animals and their food, such as Kleiber's law, Holling's disk equation and the marginal value theorem.Kleiber's law describes the relationship between an animal's size and its feeding strategy, saying that larger animals need to eat less food per unit weight than smaller animals. Kleiber's law states that the metabolic rate of an animal is the mass of the animal raised to the 3/4 power: q0=M3/4
Therefore, the mass of the animal increases at a faster rate than the metabolic rate.
Herbivores employ numerous types of feeding strategies. Many herbivores do not fall into one specific feeding strategy, but employ several strategies and eat a variety of plant parts.
| Feeding Strategy | Diet | Examples |
| Algivores | Algae | Krill, crabs, sea snail, sea urchin, parrotfish, surgeonfish, flamingo |
| Frugivores | Fruit | Ruffed lemurs, orangutans |
| Folivores | Leaves | Koalas, gorillas, red colobuses, many leaf beetles |
| Nectarivores | Nectar | Honey possums, hummingbirds |
| Granivores | Seeds | Hawaiian honeycreepers, bean weevils |
| Graminivores | Grass | Horses |
| Palynivores | Pollen | Bees |
| Mucivores | Plant fluids, i.e. sap | Aphids |
| Xylophages | Wood | Termites, longicorn beetles, ambrosia beetles |
Optimal foraging theory is a model for predicting animal behavior while looking for food or other resources, such as shelter or water. This model assesses both individual movement, such as animal behavior while looking for food, and distribution within a habitat, such as dynamics at the population and community level. For example, the model would be used to look at the browsing behavior of a deer while looking for food, as well as that deer's specific location and movement within the forested habitat and its interaction with other deer while in that habitat.
This model has been criticized as circular and untestable. Critics have pointed out that its proponents use examples that fit the theory, but do not use the model when it does not fit the reality. Other critics point out that animals do not have the ability to assess and maximize their potential gains, therefore the optimal foraging theory is irrelevant and derived to explain trends that do not exist in nature.
Holling's disk equation models the efficiency at which predators consume prey. The model predicts that as the number of prey increases, the amount of time predators spend handling prey also increases, and therefore the efficiency of the predator decreases. In 1959, S. Holling proposed an equation to model the rate of return for an optimal diet: Rate =Energy gained in foraging / + time handling )
Where s=cost of search per unit time f=rate of encounter with items, h=handling time, e=energy gained per encounter.
In effect, this would indicate that a herbivore in a dense forest would spend more time handling the vegetation because there was so much vegetation around than a herbivore in a sparse forest, who could easily browse through the forest vegetation. According to the Holling's disk equation, a herbivore in the sparse forest would be more efficient at eating than the herbivore in the dense forest.
The marginal value theorem describes the balance between eating all the food in a patch for immediate energy, or moving to a new patch and leaving the plants in the first patch to regenerate for future use. The theory predicts that absent complicating factors, an animal should leave a resource patch when the rate of payoff falls below the average rate of payoff for the entire area. According to this theory, an animal should move to a new patch of food when the patch they are currently feeding on requires more energy to obtain food than an average patch. Within this theory, two subsequent parameters emerge, the Giving Up Density and the Giving Up Time. The Giving Up Density quantifies the amount of food that remains in a patch when a forager moves to a new patch. The Giving Up Time is used when an animal continuously assesses the patch quality.