Symbiosis is a beneficial relationship between two or more living organisms of different species. Symbiotic relationships may be long-term, short-term, physical, biochemical, parasitic or mutually beneficial. Motivating factors that move organisms into symbiotic interactions are generally ones of nutrition, shelter, and defense. These reasons may be facultative (the organisms choose to co-exist though they may survive independently) or obligatory (the relationship must exist for the survival of both or one of the species). Various types of symbiotic associations exist including mutualism, commensalism, parasitism, and competition. Physical symbiotic relationships include endosymbiosis and ectosymbiosis. Endosymbiosis occurs when one organism attaches itself to the tissues of another. Rhizobium bacteria have an endosymbiotic association with leguminous plants attaching to the root nodules from which the bacteria fixes nitrogen. Ectosymbiosis occurs when the organism lives on the host’s body.
In mutualism, organisms provide benefits to one another. These associations are the most common and exist in nature for a lifetime. Mutualistic interactions may be facultative or obligatory for the different species involved. Often, the species interact physically and biochemically. Nectar collecting birds and insects have mutually symbiotic relations with flowering plants because they need nectar from the flowers. After collecting nectar from the flowers, the birds and insects aid in pollination by transferring pollen from one flower to another. Sea anemone and clownfish depend on each other for protection from their predators.
Commensal relationships exist between species that provide benefits to one organism without significantly harming or helping the other organism. Hermit crabs for example use the shells of dead animals and other empty seashells for protection. In such a case, the organism which created the shell achieves no gain or loss. Cattle egrets that feed on disturbed insects during foraging do not benefit or harm the cattle. In commensal relationships, the organism may also depend on the other for shelter and transportation.
Parasitism is an antipathetic symbiotic relationship where one member in the relationship is harmed while the other benefits. Parasites live within (endoparasites) or outside (ectoparasites) the host’s body. The parasites acquire nutrients and benefit the host in no way, sometimes causing infections and death. Most living organisms are known to host one or more parasites during their lifetime. Parasites such as ticks suck the blood of their hosts causing infections to the organisms. Humans are hosts to various parasites such as intestinal worms and fleas which may be spread through contact with the parasites when feeding, cleaning or coming into contact with contaminated areas.
Completion occurs when organisms fight to acquire a certain resource. The stronger and more adapted individual often deprives the weaker organisms of the resource. Competition exists in all ecosystems as food, water, and shelter resources are few and inadequate for all. Charles Darwin cited the importance of competition as a natural process of balancing the ecosystem.
Symbiosis in a broader sense
Symbiosis is cited as one of the important relationships that must exist between living organisms. While the term initially referred to mutually beneficial relationships, scientists have broadened it to encompass all relations within species in an ecosystem. Biologists such as Lynn Margulis have argued the significant role played by symbiosis in evolution and co-evolution where species develop certain adaptations depends on the relationships with other organisms.