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 (i.e. detritivores) or macrofungi (i.e. fungivores).
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,[1] whose heavily cross-linking polymer structure makes it far more difficult to digest than the protein- and fat-rich animal tissues that carnivores eat.
[2] Herbivore is the anglicized form of a modern Latin coinage, herbivora, cited in Charles Lyell's 1830 Principles of Geology.
[5] Herbivory is a form of consumption in which an organism principally eats autotrophs[6] such as plants, algae and photosynthesizing bacteria.
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.
[12] During the next 75 million years[citation needed], plants evolved a range of more complex organs, such as roots and seeds.
[12] Herbivory among four-limbed terrestrial vertebrates, the tetrapods, developed in the Late Carboniferous (307–299 million years ago).
While amphibians continued to feed on fish and insects, some reptiles began exploring two new food types, tetrapods (carnivory) and plants (herbivory).
[16] Tetrapod herbivores made their first appearance in the fossil record of their jaws near the Permio-Carboniferous boundary, approximately 300 million years ago.
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 morphologes has revealed that dental occlusion developed independently in several lineages tetrapod herbivores.
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 (chain).
[19] In their daily need to take up energy from forage, herbivores of different body mass may be selective in choosing their food.
[20] 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 (see below).
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.
[23][24] 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.
[30] Interactions between plants and herbivores can play a prevalent role in ecosystem dynamics such community structure and functional processes.
[31][36] Seasonal changes and environmental gradients such as elevation and latitude often affect the palatability of plants which in turn influences herbivore community assemblages and vice versa.
[32][44] Examples include a decrease in abundance of leaf-chewing larvae in the fall when hardwood leaf palatability decreases due to increased tannin levels which results in a decline of arthropod species richness,[45] and increased palatability of plant communities at higher elevations where grasshoppers abundances are lower.
It has been suggested that many herbivores feed on a variety of plants to balance their nutrient uptake and to avoid consuming too much of any one type of defensive chemical.
For example, some aphids use bacteria in their gut to provide essential amino acids lacking in their sap diet.
[55] Physical, or mechanical, defenses are barriers or structures designed to deter herbivores or reduce intake rates, lowering overall herbivory.
[48][64] 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.
[42] Another form of plant-herbivore mutualism is physical changes to the environment and/or plant community structure by herbivores which serve as ecosystem engineers, such as wallowing by bison.
[71] Environmental degradation from white-tailed deer (Odocoileus virginianus) in the US alone has the potential to both change vegetative communities[72] through over-browsing and cost forest restoration projects upwards of $750 million annually.
[citation needed] Ecotourism is a major source of revenue, particularly in Africa, where many large mammalian herbivores such as elephants, zebras, and giraffes help to bring in the equivalent of millions of US dollars to various nations annually.
