A herbivore is an animal that obtains its energy and nutrients by feeding on plants. Different types of herbivores eat different plant parts. For example, folivores feed on leaves, frugivores feed on fruits, granivores feed on seeds, pollinivores feed on pollen, and nectarivores feed on nectar. Herbivores can vary greatly in size, ranging from the largest terrestrial animals (elephants) and large marine mammals such as manatees and dugongs to small insects, nematodes and thrips. Herbivores are primary consumers (they receive their energy by consuming primary producers), so they play an important trophic role in ecological communities and food webs.
The Koala (Fascolarctos cinereus) is an Australian arboreal folivore. Photos by August, Hsueh-Cheng Ho. Australian Koala Foundation Because mature leaves are low in nutrients, and difficult to digest because of their high cellulose content, animals use many different strategies to eat leaves. Animals that feed on grass leaves are generally known as grazers. Grazers are typically not too choosy and eat all parts of leaves. Many grazers have mutualistic relationship with microbes that help them digest cellulose found in leaves. Buds and young leaves are often more nutritious and less-well defended, so browsers selectively choose to feed on high quality plant parts. Some insects actually feed inside of leaves (leaf mining). Many folivores spend most of their time feeding in trees (e.g., koalas, sloths, and folivorous monkeys).
Because leaves are low in nutrients, folivores must consume a significant biomass of leaves to meet their daily needs. Because of the heavy weight of leaves in their guts, there are very few flying herbivorous mammals or birds.
Mammals such as muskrats and moose are common folivores in North American marshes. Interstingly, the hippopotamus, the largest aquatic herbivorous mammal, obtains most of its food while foraging on land.
Large animals such as manatees, dugongs, and green turtles feed on vascular "sea grasses" that grow in shallow waters near the coast. Many invertebrates, such as sea urchins and abalone, reptiles such as the marine iquana, and many fishes feed on algae. Marine fishes us a variety of strategies to feed on algae (see Coral reef fish feeding behavior in the Caribbean). For example, parrotfishes, common tropical herbivorous fishes, scrape algae off of the rocks with their large beak-like teeth. Some damselfishes "farm" algae in small territories that they defend from other fishes,
Because eating a seed kills a potential plant, granivores are considered to be seed predators. Some granivores remove seeds directly from the plant whereas other granivores forage for seeds that lie dormant in the soil seed bank.
Some granivores play a role in seed dispersal, when they are unable to re-locate seeds that they have cached, and these seeds are thus able to germinate and grow.
The Ruby-throated hummingbird feeding on nectar. Photo by Ken Thomas. enature.com Many nectarivores form mutualistic relationships with plants because they carry pollen from flower to flower so that pollination can occur. Other nectarivores only feed on nectar without transfering pollen and are there fore known as nectar robbers. Coevolution between plant and pollinator often results in animals with phenotypic adaptations that allows them to effectively remove nectar from the flower and successfully carry pollen from one flower to another.
Pollinivores are animals that feed on pollen.There are pollinivorous invertebrates (e.g., mites, spiders, springtails, and insects) and vertebrates (e.g., marsupials, rodents, bats, and birds). Some pollinivores successfully transfer pollen from one flower to another so they act as pollinators whereas other pollinivores act only as "pollen predators". Most pollinivores remove pollen directly from the flower, but Neotropical canopy-dwelling ants in the genus Cephalotes forage for the pollen of wind-dispersed plants that has settled on leaves in the canopy.
Aphids feeding on ploem. Note the drop of honeydew secreted by the aphid. Photo from Cheadle Center of Biodiversity and Ecological Restoration. UCSB. Some bugs in the order Hemiptera feed on phloem by inserting their elongated mouthparts into the plant's vascular tissues. Because phloem is rich in sugar, but low in nutrients, some phloem-feeding insects have symbiotic micro-organisms that proved them with amino acids and others convert excess sugar into long-chain oligosaccharides that are extruded as "honeydew". Interestingly, some animals, e.g., some ants, indirectly feed on phloem by consuming honeydew. Some bird, such as sapsuckers, drill holes into the bark so that they can consume the phloem juices that are released.
Animals that feed on wood are known as xylovores. Such species act as herbivores while consuming wood from living plants, and as decomposers when feeding on dead wood. Some arthropods that feed on wood have symbiotic protists or bacteria living in their gut to aid in the digestion of cellulose whereas termites produce their own wood-digesting enzymes.
The cambium is the layer of actively dividing cells between the xylem and ploem tissues of plants and make up a small ring beneath the bark of woody plants. Becasue the living tissues of the cambium layer are more nutritious than the dead xylem tissues that make up wood, some animals feed selectively on the cambium. Some wood boring insects eat their way through the cambium.
Bark makes up the outer layer of the branches of woody plants. Some animals such as elephants, beavers, and squirrels strip the bark off of branches and eat them whereas insects such as bark beetles feed by burrowing through the bark tissue.
Some insects, both larvae and adults, nematodes, and mammals such as gophers feed on roots. Some animals will burrow into the ground to feed on tubers.
Herbivores in ecological communities
Herbivores are considered to be primary consumers because they feed on plants (primary producers). Thus, herbivores feeding on plants form the first link in almost all food chains. The abundance of herbivores can control the biomass of plants and and the abundance of secondary consumers that form the next higher trophic level.
The removal of the herbivore trophic level can allow plant growth to get out of control. For example the removal of herbaceous fishes by fishing and the loss of the herbaceus sea urchin, Diameda antillarum, has led to an increase in the standing crop of algae on many coral reefs in the Caribbean Sea. Similarly, the increase in the population size of herbivore, often caused by an decrease in the population size of their predators, can greatly reduce the biomass of plants. The invasion of introduced herbivores can have harmful effects on the native plant community.
Pollination by nectarivores and pollinivores and seed dispersal by frugivores and granivores have an important influence on the distribution and abundance of plants in ecological communities.
Plants that feed on other plants
Although some plants receive energy and nutrients by "feeding" on other plants, these plants are referred to as "parasitic plants" rather than herbivorous plants.
References and further reading
- Cautious climbing and folivory: a model of hominoid differentation E. E. Sarmiento in Human Evolution Volume 10, Number 4, August, 1995
- Frugivore Wikipedia
- Nectarivore Wikipedia
- Xylophagy Wikipedia
- Phloem-sap feeding by animals: problems and solutions. A.E. Douglas
- Insects: Bee San Diego Zoo
- Pollenivory in ants (Hymenoptera:Formicidae) seems to be much more common that it was thought. W. Czechowksi, B. Mark, K. Er?s, and E. Csata.