A deer and two fawns feeding on some foliage

Herbivores

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

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 cycle(chain).

Feeding strategies

Herbivores differ in the extent, specificity and nature of their feeding.

They can be grouped according to which part of the plant they eat: 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 techniques used to get at the foodstuff are wide and varied, and include the "pierce and suck" technique, surface fluid feeding, hole feeding, margin feeding and skeletonisation.^[2]

Evolution of herbivory

A fossil Viburnum lesquereuxii leaf with evidence of insect herbivory; Dakota Sandstone (Cretaceous) of Ellsworth County, Kansas. Scale bar is 10 mm.

Long thought to be a Mesozoic phenomenon, evidence for herbivory is found almost as soon as fossils which could show it. Within under 20 million years of the first fossils of sporangia and stems towards the close of the Silurian, around , there is evidence that they were being consumed.^[3] Animals 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.^[2]

During the ensuing 75 million years, plants evolved a range of more complex organs - from roots to seeds. There is no evidence for these being fed upon until the middle-late Mississippian, . There was a gap of 50 to 100 million years between each organ evolving, and it being fed upon; this may be due to the low levels of O₂ during this period, which may have suppressed evolution.^[3] Further than their arthropod status, the identity of these early herbivores is uncertain.^[3] Hole feeding and skeletonisation are recorded in the early Permian, with surface fluid feeding evolving by the end of that period.^[2]

Arthropods have evolved herbivory in four phases, changing their approach to herbivory in response to changing plant communities.^[4]

Plant defenses

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 defences against herbivores.

These defences 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

Aphids are fluid feeders on plant sap.

See also

• List of food habits
• Grazing
• Browser
• Pollination
• Seed dispersal
• Seed predation
• Veganism

References

Further reading

• Bob Strauss, 2008, Herbivorous Dinosaurs, The New York Times
• Danell, K., R. Bergström, P. Duncan, J. Pastor (Editors)(2006) Large herbivore ecology, ecosystem dynamics and conservation Cambridge, UK : Cambridge University Press. 506 p. ISBN 0521830052
• Crawley, M. J. (1983) Herbivory : the dynamics of animal-plant interactions Oxford : Blackwell Scientific. 437 p. ISBN 0632008083
• Olff, H., V.K. Brown, R.H. Drent (editors) (1999) Herbivores : between plants and predators Oxford ; Malden, Ma. : Blackwell Science. 639 p. ISBN 0632051558

External links

• The herbivore defenses of ''Senecio viscusus''
• Herbivore defense in ''Lindera benzoin''

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