Aherbivoreis ananimalanatomically and physiologicallyevolvedto feed onplants,especially uponvascular tissuessuch asfoliage,fruitsorseeds,as the main component of itsdiet.These more broadly also encompass animals that eatnon-vascularautotrophssuch asmosses,algaeandlichens,but do not include those feeding ondecomposedplant matters(i.e.detritivores) ormacrofungi(i.e.fungivores).

Adeerand two fawns feeding on foliage
Asawflylarva feeding on a leaf
Tracks made by terrestrial gastropods with theirradulas,scrapinggreen algaefrom a surface inside a greenhouse

As a result of their plant-based diet, herbivorous animals typically havemouthstructures (jawsormouthparts) welladaptedtomechanically break downplant materials, and theirdigestive systemshave specialenzymes(e.g.amylaseandcellulase) to digestpolysaccharides.Grazingherbivores such ashorsesandcattleshave wide flat-crownedteeththat are better adapted for grindinggrass,tree barkand other tougherlignin-containing materials, and many of them evolvedruminationorcecotropicbehaviors to better extract nutrients from plants. A large percentage of herbivores also have mutualisticgut floramade up ofbacteriaandprotozoansthat help todegradethecellulosein plants,[1]whose heavilycross-linkingpolymerstructure makes it far more difficult to digest than theprotein- andfat-rich animaltissuesthatcarnivoreseat.[2]

Etymology

Herbivore is the anglicized form of a modern Latin coinage,herbivora,cited inCharles Lyell's 1830Principles of Geology.[3]Richard Owenemployed the anglicized term in an 1854 work on fossil teeth and skeletons.[3]Herbivorais derived from Latinherba'small plant, herb'[4]andvora,fromvorare'to eat, devour'.[5]

Herbivory is a form ofconsumptionin which anorganismprincipallyeatsautotrophs[6]such asplants,algaeand photosynthesizingbacteria.More generally, organisms that feed onautotrophsin general are known asprimary 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.[7][8]Fungi, bacteria, and protists that feed on living plants are usually termedplant pathogens(plant diseases), while fungi andmicrobesthat feed on dead plants are described assaprotrophs.Flowering plants that obtain nutrition from other living plants are usually termedparasitic plants.There is, however, no single exclusive and definitive ecological classification of consumption patterns; each textbook has its own variations on the theme.[9][10][11]

Evolution of herbivory

AfossilViburnum lesquereuxiileaf with evidence of insect herbivory; Dakota Sandstone (Cretaceous) of Ellsworth County, Kansas. Scale bar is 10 mm.

The understanding of herbivory in geological time comes from three sources: fossilized plants, which may preserve evidence of defence (such as spines), or herbivory-related damage; the observation of plant debris in fossilisedanimal faeces;and the construction of herbivore mouthparts.[12]

Although herbivory was long thought to be aMesozoicphenomenon, fossils have shown that plants were being consumed by arthropods within less than 20 million years after the first land plants evolved.[13]Insects fed on the spores of early Devonian plants, and theRhynie chertalso provides evidence that organisms fed on plants using a "pierce and suck" technique.[12]

During the next 75 million years[citation needed],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-lateMississippian,330.9million years ago.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.[13]Further than their arthropod status, the identity of these early herbivores is uncertain.[13]Hole feeding and skeletonization are recorded in the earlyPermian,with surface fluid feeding evolving by the end of that period.[12]

Herbivory among four-limbed terrestrial vertebrates, thetetrapods,developed in theLate Carboniferous(307–299 million years ago).[14]The oldest known example beingDesmatodonhesperis.[15]Early tetrapods were large amphibiouspiscivores.While amphibians continued to feed on fish and insects, some reptiles began exploring two new food types, tetrapods (carnivory) and plants (herbivory). The entiredinosaurorderornithischiawas composed of herbivorous dinosaurs.[14]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.[14]

Arthropodsevolved herbivory in four phases, changing their approach to it in response to changing plant communities.[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 earliest evidence of their herbivory has been attributed todental 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 ofphylogeneticframeworks of tooth and jaw morphologes has revealed that dental occlusion developed independently in several lineages tetrapod herbivores. This suggests that evolution and spread occurred simultaneously within various lineages.[17]

Food chain

Leaf minersfeed on leaf tissue between the epidermal layers, leaving visible trails

Herbivores form an important link in the food chain because they consume plants to digest the carbohydratesphotosyntheticallyproduced by a plant.Carnivoresin turn consume herbivores for the same reason, whileomnivorescan 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). Herbivory, carnivory, and omnivory can be regarded as special cases ofconsumer–resource interactions.[18]

Feeding strategies

Two herbivore feeding strategies aregrazing(e.g. cows) andbrowsing(e.g. moose). 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".[19]In their daily need to take up energy from forage, herbivores of different body mass may be selective in choosing their food.[20]"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 (and consequently highly nutritious) 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.[20]Several theories attempt to explain and quantify the relationship between animals and their food, such asKleiber's law,Holling's disk equation and the marginal value theorem (see below).

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.[21]Kleiber's law states that the metabolic rate (q0) of an animal is the mass of the animal (M) raised to the 3/4 power: q0=M3/4

Therefore, the mass of the animal increases at a faster rate than the metabolic rate.[21]

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.

Types of feeding strategies
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,manyleaf 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 theoryis 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.[22]

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.[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.[25][26]

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.[27][page needed]In 1959, S. Holling proposed an equation to model the rate of return for an optimal diet: Rate (R )=Energy gained in foraging (Ef)/(time searching (Ts) + time handling (Th))

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 (eating) 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.

Themarginal value theoremdescribes 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 (amount of food) falls below the average rate of payoff for the entire area.[28]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 (GUD) and the Giving Up Time (GUT). The Giving Up Density (GUD) quantifies the amount of food that remains in a patch when a forager moves to a new patch.[29]The Giving Up Time (GUT) is used when an animal continuously assesses the patch quality.[30]

Plant-herbivore interactions

Interactions between plants and herbivores can play a prevalent role inecosystemdynamics suchcommunitystructure and functional processes.[31][32]Plantdiversityanddistributionis often driven by herbivory, and it is likely that trade-offs between plantcompetitivenessanddefensiveness,and between colonization and mortality allow for coexistence between species in the presence of herbivores.[33][34][35][36]However, the effects of herbivory on plant diversity and richness is variable. For example, increased abundance of herbivores such as deer decrease plant diversity andspecies richness,[37]while other large mammalian herbivores like bison control dominant species which allows other species to flourish.[38]Plant-herbivore interactions can also operate so that plant communities mediate herbivore communities.[39]Plant communities that are more diverse typically sustain greater herbivore richness by providing a greater and more diverse set of resources.[40]

Coevolutionandphylogeneticcorrelation between herbivores and plants are important aspects of the influence of herbivore and plant interactions on communities and ecosystem functioning, especially in regard to herbivorous insects.[32][39][41]This is apparent in theadaptationsplants develop totolerateand/ordefendfrom insect herbivory and theresponses of herbivoresto overcome these adaptations. The evolution of antagonistic and mutualistic plant-herbivore interactions are not mutually exclusive and may co-occur.[42]Plant phylogeny has been found to facilitate the colonization and community assembly of herbivores, and there is evidence of phylogenetic linkage between plantbeta diversityand phylogeneticbeta diversityof insectcladessuch asbutterflies.[39]These types of eco-evolutionary feedbacks between plants and herbivores are likely the main driving force behind plant and herbivore diversity.[39][43]

Abiotic factors such asclimateandbiogeographical featuresalso impact plant-herbivore communities and interactions. For example, in temperate freshwater wetlands herbivorous waterfowl communities change according to season, with species that eat above-ground vegetation being abundant during summer, and species that forage below-ground being present in winter months.[31][36]These seasonal herbivore communities differ in both their assemblage and functions within thewetland ecosystem.[36]Such differences in herbivore modalities can potentially lead to trade-offs that influence species traits and may lead to additive effects oncommunitycomposition and ecosystem functioning.[31][36]Seasonal changes and environmental gradients such as elevation andlatitudeoften affect thepalatabilityof 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 increasedtanninlevels which results in a decline of arthropodspecies richness,[45]and increased palatability of plant communities at higher elevations wheregrasshoppersabundances are lower.[32]Climatic stressors such asocean acidificationcan lead to responses in plant-herbivore interactions in relation to palatability as well.[46]

Herbivore offense

Aphidsare fluid feeders onplant sap.

The myriad defenses displayed by plants means that their herbivores need a variety of skills to overcome these defenses and obtain food. These allow herbivores to increase their feeding and use of a host plant. Herbivores have three primary strategies for dealing with plant defenses: choice, herbivore modification, and plant modification.

Feeding choice involves which plants a herbivore chooses to consume. 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. This involves a tradeoff however, between foraging on many plant species to avoid toxins or specializing on one type of plant that can be detoxified.[47]

Herbivore modification is when various adaptations to body or digestive systems of the herbivore allow them to overcome plant defenses. This might include detoxifyingsecondary metabolites,[48]sequestering toxins unaltered,[49]or avoiding toxins, such as through the production of large amounts ofsalivato reduce effectiveness of defenses. Herbivores may also utilize symbionts to evade plant defenses. For example, some aphids use bacteria in their gut to provide essentialamino acidslacking in their sap diet.[50]

Plant modification occurs when herbivores manipulate their plant prey to increase feeding. For example, some caterpillars roll leaves to reduce the effectiveness of plant defenses activated by sunlight.[51]

Plant defense

A plant defense is a trait that increases plant fitness when faced with herbivory. This is measured relative to another plant that lacks the defensive trait. Plant defenses increase survival and/or reproduction (fitness) of plants under pressure of predation from herbivores.

Defense can be divided into two main categories, tolerance and resistance. Tolerance is the ability of a plant to withstand damage without a reduction in fitness.[52]This can occur by diverting herbivory to non-essential plant parts, resource allocation, compensatory growth, or by rapid regrowth and recovery from herbivory.[53]Resistance refers to the ability of a plant to reduce the amount of damage it receives from herbivores.[52]This can occur via avoidance in space or time,[54]physical defenses, or chemical defenses. Defenses can either be constitutive, always present in the plant, or induced, produced or translocated by the plant following damage or stress.[55]

Physical, or mechanical, defenses are barriers or structures designed to deter herbivores or reduce intake rates, lowering overall herbivory.Thornssuch as those found on roses or acacia trees are one example, as are the spines on a cactus. Smaller hairs known astrichomesmay cover leaves or stems and are especially effective against invertebrate herbivores.[56]In addition, some plants havewaxesorresinsthat alter their texture, making them difficult to eat. Also the incorporation ofsilicainto cell walls is analogous to that of the role ofligninin that it is a compression-resistant structural component of cell walls; so that plants with their cell walls impregnated with silica are thereby afforded a measure of protection against herbivory.[57]

Chemical defenses aresecondary metabolitesproduced by the plant that deter herbivory. There are a wide variety of these in nature and a single plant can have hundreds of different chemical defenses. Chemical defenses can be divided into two main groups, carbon-based defenses and nitrogen-based defenses.[58]

  1. Carbon-based defenses includeterpenesandphenolics.Terpenes are derived from 5-carbon isoprene units and comprise essential oils, carotenoids, resins, and latex. They can have several functions that disrupt herbivores such as inhibitingadenosine triphosphate (ATP)formation, moltinghormones,or the nervous system.[59]Phenolics combine an aromatic carbon ring with a hydroxyl group. There are several different phenolics such as lignins, which are found in cell walls and are very indigestible except for specialized microorganisms;tannins,which have a bitter taste and bind to proteins making them indigestible; and furanocumerins, which produce free radicals disrupting DNA, protein, and lipids, and can cause skin irritation.
  2. Nitrogen-based defenses are synthesized from amino acids and primarily come in the form ofalkaloidsand cyanogens. Alkaloids include commonly recognized substances such ascaffeine,nicotine,andmorphine.These compounds are often bitter and can inhibit DNA or RNA synthesis or block nervous system signal transmission. Cyanogens get their name from thecyanidestored within their tissues. This is released when the plant is damaged and inhibits cellular respiration and electron transport.[citation needed]

Plants have also changed features that enhance the probability of attracting natural enemies to herbivores. Some emit semiochemicals, odors that attract natural enemies, while others provide food and housing to maintain the natural enemies' presence, e.g.antsthat reduce herbivory.[60]A given plant species often has many types of defensive mechanisms, mechanical or chemical, constitutive or induced, which allow it to escape from herbivores.[61]

Predator–prey theory

According to the theory ofpredator–prey interactions, the relationship between herbivores and plants is cyclic.[62]When prey (plants) are numerous their predators (herbivores) increase in numbers, reducing the prey population, which in turn causes predator number to decline.[62]The prey population eventually recovers, starting a new cycle. This suggests that the population of the herbivore fluctuates around the carrying capacity of the food source, in this case, the plant.

Several factors play into these fluctuating populations and help stabilize predator-prey dynamics. For example, spatial heterogeneity is maintained, which means there will always be pockets of plants not found by herbivores. This stabilizing dynamic plays an especially important role for specialist herbivores that feed on one species of plant and prevents these specialists from wiping out their food source.[63]Prey defenses also help stabilize predator-prey dynamics, and for more information on these relationships see the section on Plant Defenses. Eating a second prey type helps herbivores' populations stabilize.[63]Alternating between two or more plant types provides population stability for the herbivore, while the populations of the plants oscillate.[62]This plays an important role for generalist herbivores that eat a variety of plants. Keystone herbivores keep vegetation populations in check and allow for a greater diversity of both herbivores and plants.[63]When an invasive herbivore or plant enters the system, the balance is thrown off and the diversity can collapse to a monotaxon system.[63]

The back and forth relationship of plant defense and herbivore offense drivescoevolutionbetween plants and herbivores, resulting in a "coevolutionary arms race".[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 behindspeciation.[65][66]

Mutualism

While much of the interaction of herbivory and plant defense is negative, with one individual reducing the fitness of the other, some is beneficial. This beneficial herbivory takes the form ofmutualismsin which both partners benefit in some way from the interaction.Seed dispersalby herbivores andpollinationare two forms of mutualistic herbivory in which the herbivore receives a food resource and the plant is aided in reproduction.[67]Plants can also be indirectly affected by herbivores throughnutrient recycling,with plants benefiting from herbivores when nutrients are recycled very efficiently.[42]Another form of plant-herbivore mutualism is physical changes to the environment and/or plant community structure by herbivores which serve asecosystem engineers,such aswallowingby bison.[68]Swans form a mutual relationship with the plant species that theyforageby digging and disturbing the sediment which removes competing plants and subsequently allows colonization of other plant species.[31][36]

Impacts

Mixed feeding shoal ofherbivorous fishon acoral reef

Trophic cascades and environmental degradation

When herbivores are affected bytrophic cascades,plant communities can be indirectly affected.[69]Often these effects are felt when predator populations decline and herbivore populations are no longer limited, which leads to intense herbivore foraging which can suppress plant communities.[70]With the size of herbivores having an effect on the amount of energy intake that is needed, larger herbivores need to forage on higher quality or more plants to gain the optimal amount of nutrients and energy compared to smaller herbivores.[71]Environmental degradationfromwhite-tailed deer(Odocoileus virginianus) in the US alone has the potential to both change vegetative communities[72]through over-browsing and costforest restorationprojects upwards of $750 million annually. Another example of a trophic cascade involved plant-herbivore interactions arecoral reefecosystems.Herbivorous fishand marine animals are importantalgaeand seaweed grazers, and in the absence of plant-eating fish, corals are outcompeted and seaweeds deprive corals of sunlight.[73]

Economic impacts

Agricultural crop damage by the same species totals approximately $100 million every year. Insect crop damages also contribute largely to annual crop losses in the U.S.[74]Herbivores also affect economics through the revenue generated by hunting and ecotourism. For example, the hunting of herbivorous game species such as white-tailed deer, cottontail rabbits, antelope, and elk in the U.S. contributes greatly to the billion-dollar annually, hunting industry.[citation needed]Ecotourismis 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.[citation needed]

See also

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Further reading

  • Bob Strauss, 2008,Herbivorous DinosaursArchived18 November 2012 at theWayback Machine,The New York Times
  • Danell, K., R. Bergström, P. Duncan, J. Pastor (Editors)(2006)Large herbivore ecology, ecosystem dynamics and conservationCambridge, UK: Cambridge University Press. 506 p.ISBN0-521-83005-2
  • Crawley, M. J. (1983)Herbivory: the dynamics of animal-plant interactionsOxford: Blackwell Scientific. 437 p.ISBN0-632-00808-3
  • Olff, H., V.K. Brown, R.H. Drent (editors) (1999)Herbivores: between plants and predatorsOxford; Malden, Ma.: Blackwell Science. 639 p.ISBN0-632-05155-8