Symbiosis is an evolutionary adaption in which two species interact in ways that are often related to the survival of one or both participants. The three basic types of are mutualism, commensalism and parasitism.
Mutualism Mutualism is the relationship most commonly associated with symbiosis. In mutualistic symbiosis, both parties benefit from the association. At times, the interdependency is so strong that neither species can survive without the other. This is called obligate codependency. Termites and the protozoa living in their digestive tracts have such a relationship. The protozoa provide the enzymes needed by the termites to digest cellulose. The protozoa cannot survive outside the termites and without them, the termites would starve. Mutualism can be further broken down into three main subtypes. These are resource-resource relationships, resource-service relationships and, more rarely, service-service relationships.
Resource-resource mutualism centers on the trading of one resource for another between the members of the association. A favorite example occurs in the plant world. The roots of many plants are colonized by a type of fungus. The fungus aids the plant by increasing its absorption of the water and minerals in the soil. In return, the fungus receives carbohydrates, sucrose and glucose from the host plant.
Service-resource mutualism is readily observed in the animal kingdom. There are many species of birds and fish that remove parasites from other animals. Oxpeckers clean elephants and other large mammals of lice and similar vermin. Cleaner wrasses perform an analogous service for larger fish. The client animals get improved health and the groomers a meal.
Service-service mutualism is rarer than the previous types. In this form of symbiosis, each partner provides the other with a service, such as shelter or protection. The relationship between anemones and clownfish is a commonly cited instance of service-service mutualism. The anemone gets protection from predators and the clownfish a safe place to live and breed.
Commensalism Commensalism is a form of symbiosis where one partner benefits without changing the status of the other. Cattle egrets and cows exemplify this type of symbiosis. As the cattle graze, moving through the grass, they stir up insects which the egrets eat. The cattle receive no benefit, nor are they harmed. The egrets get a meal.
Parasitism Parasites too are symbionts, even though they harm their partners to enrich themselves. There are two familiar forms of parasitism, known commonly as "food" and "brood." Food parasites live by consuming the blood and tissues of the host organism. Blood-sucking symbionts can introduce a secondary parasite into the host's further debilitating it. Fleas introducing tapeworm eggs into their host is a common instance of this.
Brood parasites have a different approach. In this case, the female parasite will lay her eggs in the host's nest. The host, unaware of the addition will raise the interloper, expending time and energy to benefit another's offspring. This has been seen among fish, insects and birds. The cuckoo may be the best-known example. The mother cuckoo lays her egg in a nest of a bird with similar eggs. Often the cuckoo is larger and hatches first, pushing its foster siblings from the nest and monopolizing the resources of the parent birds.
Symbiosis is a common approach to survival. Further study and careful observation reveal it in virtually all parts of the animal and plant kingdoms. It is a fascinating subject worthy of further exploration.