Mutualism

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Mutualism is the association between organisms of two different species, in which each is benefited. Mutualistic arrangements develop between organisms that have different living requirements. Protocooperation is the first form of mutualism. It involves interaction which benefits both species.

An example of protocooperation occurs between soil bacteria or fungi, and higher plants growing in the soil. Neither species is dependent on the association, but all microflora, higher plants, and soil fauna participate in determining soil composition and fertility. Soil bacteria and fungi interact with each other, forming nutrients required by plants. They obtain nutrients from root nodules and decaying organic matter. Plants benefit by getting necessary mineral nutrients and carbon dioxide.

A further example of protocooperation is the relationship between ants and aphids. The ant forages for food on trees and shrubs infested with honeydew-secreting species such as aphids, mealybugs, and some scales. The ant collects the sugary material and transports it to its nest as food for the developing young. In some cases, the ant actually stimulates the aphid to secrete honeydew directly into its mouth. Some ant species even protect the honeydew producers from natural enemies. This means that ant attended trees usually bear much heavier infestations of aphids.

Plants whose flowers are pollinated by insects and birds benefit from protocooperation. The plants, especially those with colourful flowers bearing nectar glands, undergo cross pollination because of the insect’s behaviour. The insect benefits from the food supply of pollen and nectar.

Cleaning symbiosis occurs mostly in birds and fish. The Egyptian plover picks insect pests from the backs of buffalo, antelope, giraffes, rhinoceroses, and even leeches from the open mouths of crocodiles. The cattle egret in America, performs the same function. Certain fish function as cleaners of other fish, removing ectoparasites, cleaning wounded tissue, and removing dead flesh. Even predatory fish rely on cleaning symbionts, and remain passive while they are cleaned. Such fish cleaners are often concentrated in fixed sites, called cleaning stations, where other fish come to be cleaned.

OBLIGATIVE MUTUALISM

In many communities, the most stable and interdependent associations between species are those based on obligative mutualism. Obligative mutualism is necessary for the survival of each species, and coexistence is needed. To ensure that mutualism continues from one generation to the next, the species have to evolve. This may involve adaptations in structure or behaviour. Other mutualistic relationships are maintained by mutual encounters. This is achieved by having a large population density in one or both species.

A number of bacteria-protozoan associations occur in water. For example, endosymbiotic bacteria, which exist in the cytoplasm of protozoan flagellates, are able to digest cellulose in quantities to provide for themselves and their hosts. Mutualism also occurs between fungi and single-celled algae including lichens. The fungus penetrates the algal cells with feeding tubes. The species benefit from nutrient exchange, maintenance of water and mineral balance, and resistance to drying or to extreme temperatures. Whether the association is truly mutualistic is difficult to resolve. Some authorities state that the fungi parasitize the algae. Ecologically, however, the lichen is far better able to cope with its environment. They are able to invade many more habitats than can either species living alone.

Single-celled algae are also symbiotic in marine habitats, where photosynthesis is restricted. Algae are symbiotic with protozoans, sponges, coelenterates, rotifers, flatworms, molluscs, echinoderms, and tunicates. The algal cell provides oxygen and manufactures food for its host. The cell receives physical support, water, minerals, and a proper environment.

Mutualism between algae and motile coral polyps result in behavioural changes. The larvae of corals that contain yellow-green algae move toward light, but planulae, which lack algae are unresponsive to light. Corals with algae grow more rapidly, take on different shapes, and are much denser than corals without algae.

An important association is nitrifying bacteria with the roots of leguminous plants. Nitrogen-fixation occurs after the bacteria have invaded the plant roots and stimulated the host. Nodules are formed that encapsulate the bacteria. This association is closely related to parasitism. The bacterium benefits from the nutrients. The plant has benefited by the improved fertility of the soil.

Many insect species contain microscopic endosymbionts, including bacteria, fungi, yeast and protozoans. These organisms provide nutrients for the host, including vitamins, digestive enzymes and glucose sugar. The host provides the symbiont with a protected microhabitat containing food, water, minerals, and a proper chemical environment. The endosymbionts are rarely found in a free living state, and their hosts are unable to survive in their absence. They may be extracellular, living in the mouth, gut, rectum, blood spaces or excretory tubes. They may be intracellular, and live in the cytoplasm of various cells of the host. Because of the association of the symbionts, and the dependant nature of the mutualism, intricate mechanisms have evolved for the transmission of the endosymbiont from one generation of the host to the next.

A further mutualistic association occurs between wood-eating insects and cellulose-digesting protozoans. At an early age, the wood roach ‘Cryptocecus punctulatus’, acquires the intestinal cellulose-digesting symbionts. It retains them for life. The termite, also a wood-eater, during its immature stages, loses its symbionts at each moult. The termites, however, re-infect themselves. The newly moulted nymphs ingest anal secretions from non-moulting nymphs and obtain the intestinal organisms.

Many insects have adapted to the use of fungi as a source of food. These include the wood-boring insects, e.g. ambrosia beetles (family Platypdidae) and bark beetles (family Scolytidae). The adult beetles introduce the fungus into the nests they make when they invade a tree. Adult sawflies (family Tenthredinoidea) and timber borers lay their eggs on or beneath the surface of the bark of a tree. They introduce the fungus with the eggs. The fungi then grow in the nests formed in the wood, and provide food for the developing insect larvae. Termites and ants also cultivate fungal areas in their colonies.

Some tropical termites (subfamily Macroterminae) are fungus-raising termites. They use the fungi to control the climate of the nest. The humidity and temperature are maintained by constant, high, favourable levels as a result of the metabolic activities of the fungi. Fungus culturing ants, gain nutrients from the fungus. Leaf-cutting ants (Atta) clip off and carry pieces of fresh leaves to their nests, on which the fungi grow. The fungal areas are established in large well-ventilated underground cavities. They are attended to by a class of workers, who remove unwanted fungi and bacteria. All members of the colony feed on strands of the cultured fungi.

Certain insects are obligatory mutualists of the plants they pollinate. The California desert yucca (Yucca), can be pollinated only by the yucca moth. The moth is dependent on the yucca flower ovary. It is a place to deposit its egg and develop its young. The common Smyrna fig (Fiscus carica) can fruit only after pollination by the fig wasp (Blastophaga psenes).

Insects also enter into protective relationships with plants. An example is the acacia ant (Pseudomyrmes ferruginea) which lives on and gets its food from the bull-horn acacia (Acacia cornigera). The ant protects the acacia from intruding vines, competing plants, and herbivorous insects. The ant depends on the acacia and the acacia cannot survive without the ant. A disadvantage of this mutualism is that this acacia has lost its chemical defences against insect defoliators.

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