Herbivory (pronounced her-BIV-or–ee) is the process whereby an animal eats a plant or a plant-like organism such as a seaweed or phytoplankton. Plants and plant-like organisms such as seaweeds and phytoplankton are called primary producers, and are the bedrock of most of Earth’s ecosystems as they use the sun’s energy to make organic material. Herbivores are important as they are the first link in connecting the energy locked within plants to the vast number of animals within ecosystems. Herbivory is important in determining the population abundance and dynamics of individual plant species as herbivores often eat or kill whole plants or affect how much a plant can reproduce. Additionally, herbivores often regulate the species composition of plant communities, the total amount of plant biomass in the ecosystems, and the magnitude of ecosystem functional processes such as primary production, decomposition, and transfer of energy and materials up the food chain. Plants are not passive participants, however, as they have evolved a variety of special physical and chemical defenses to protect themselves from herbivores.
Herbivory, Resources, and Plant Species Richness
Plant growth can be limited by two primary factors: (1) what eats them (herbivores) and (2) what they need to grow (resources such as nutrients, water, and sunlight). Herbivory, like other types of disturbance, can help maintain species diversity in communities by removing plant species that compete intensively for resources, which allows other competitively inferior species to coexist. Without herbivores, the best competitors in many communities would win out and many plant species would thus be eliminated from the community. This process, however, often depends on the amount of resources available for plant growth. In areas that have low resource levels, plants grow slowly, which means that intense feeding by herbivores will remove many of the plants from the community and reduce the number of species that can survive there. But, when herbivores are removed from these areas, those plant species that are susceptible to herbivores may recover, which would increase the number of plant species in the area. In the opposite way, for areas with high levels of resources, plants grow quickly, meaning that in the absence of herbivores the fastest growing plant species will come to dominate the community and force other plant species out. Conversely, when herbivores are present, they often feed on these fastest growing, very competitive plant species, allowing other plant species to coexist alongside the dominant plant species.
Herbivores: Big vs. Small
Not all herbivores are created equal. In terrestrial systems, herbivores can range from small termites that weigh less than 1g up to 5000kg elephants. The largest herbivores, such as rhinos and elephants in Africa - often called megaherbivores - have very important impacts on ecosystems. These large herbivores often modify ecosystems by knocking down trees or creating vast areas of heavily grazed grasses, which in turn opens up the vegetation to light penetration and facilitates the presence of many other smaller herbivores such as zebra, wildebeest, and many other antelope that feed on grasses and herbs. Reductions in populations of these megaherbivores because of disease or poaching by humans can drastically change these ecosystems by turning open grasslands into closed woodlands; without herbivores constantly removing the vegetation, the ecological process of succession from grasslands to wooded landscapes can occur. On coral reefs, large herbivorous fishes continuously graze plant-like seaweeds. These seaweeds are very fast growing and can negatively impact the corals that grow on coral reefs by reducing their growth and preventing the establishment of new, juvenile corals. When these large herbivores are removed from coral reefs by fishing, reefs can become dominated by seaweeds instead of corals.
Bigger size does not necessarily mean stronger impacts on plant communities. Small herbivores, especially those species prone to periodic population explosions, can have dramatic and long-lasting effects on plant communities. Outbreaks of caterpillars, locusts, and other small herbivores can defoliate whole groves of trees and wreak havoc in many agricultural systems. In tropical forests, leaf-cutter [[ant|ants] can strip trees of leaves that the ants then use to grow fungal gardens within their colonies. Sometimes the smallest herbivores are the most important. In lakes and ocean ecosystems, the smallest herbivores -- small crustaceans such as copepods and krill -- eat phytoplankton, or single-celled algae that are the base of these food webs and are, in turn, important prey for larger animals. When these small herbivores are present in larger numbers, they prevent large blooms of phytoplankton, which can otherwise consume huge amounts of dissolved oxygen in the waters of lakes and coastal oceans, resulting in “dead zones” where no other organisms can survive.
Herbivory and Plant Defenses
In a world full of herbivores, how do plants survive? One contributing factor is that predators often control populations of herbivores so that herbivores do not dominate the landscape and cannot eliminate all the plants (See Predation; Trophic cascade). Another factor is that many plants have evolved defenses to fight back against their attackers. Many plants have physical defenses such as spines and thorns that reduce a herbivore’s physical ability to eat them and chemical defenses that make the plants taste bad or that can be toxic to the herbivores consuming them. Spines and thorns are typically defenses against larger herbivores; for example, the thorns of Acacia trees in Africa defend against large herbivores such as elephant and giraffe. Chemical defenses, however, work against many types of herbivores, although the same chemical may not work against all herbivore species. Many smaller invertebrate herbivores actually use these chemical defenses to their benefit – they live on and eat plants that have chemical defenses against larger herbivores because these plants are less likely to be eaten by large herbivores that might also eat the smaller herbivores by mistake. However, herbivores can go on the offense as well. Many herbivores use special digestive enzymes to counteract the chemical warfare that plants use as well as housing microbial symbionts in their guts that help to detoxify nasty chemicals.
Often a wide variety of plant defenses means that plants will not be vulnerable to all herbivores, and certain herbivores will have evolved special traits that allow them to eat some plants and not others. Recently, scientists have shown that having many different species of herbivores in a community may be better for the health of ecosystems than having fewer herbivore species. Different herbivore species often have different diets (they eat different species of plants), and having many different herbivore species present would mean that most of the plant species would be grazed and that no one plant species could take over the ecosystem.
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