Symbiosis is defined as an ecological interrelationship between two (or more) species in which the effects of that relationship are often expressed as being harmful or beneficial. In some discussions of ecology, symbiosis is often known as consortism. Two species that are found in intimate physical association for most or all of their lifetimes are considered to be in symbiosis. See also: Ecological communities; Ecology; Species concept

The interactions between symbiotic organisms can fall into a number of classic categories, depending on whether the symbiosis is mandatory for the survival of one or both species and on which partners benefit. These categories include mutualism (in which the symbiosis is mutually obligatory), cooperation (mutually beneficial but not obligatory), commensalism (beneficial but not obligatory for one partner and neither helpful nor harmful for the other), parasitism (obligatory for one partner and harmful but not immediately fatal for the other), amensalism (neutral for one species and harmful to the other), and predation (one species kills and eats the other). Interspecific competition, in which individuals of different species depend on limited supplies of a common environmental resource, is a symbiotic relationship in which neither species is seen to benefit. Neutralism describes interactions that have no evident effect on either partner species. See also: Ecological competition; Mutualism; Parasitology; Predator-prey interactions

Symbiotic relationships occur throughout the spectrum of biodiversity. For example, pollination is a mutualistic (reciprocally beneficial) plant–animal interaction: a host plant receives the service of insect pollination, and it rewards its insect pollinator in some way—typically with nectar or pollen, but occasionally with a mating site, resin for nest construction, floral aromas, or even plant-generated heat. Ectomycorrhizal symbiosis is a very important obligate association formed between the roots of many plant species, especially trees, and a diverse range of soil fungi. Microbes also engage in a range of symbiotic partnerships with each other and with larger organisms. For example, aphids contain large populations of the bacterium Buchnera aphidicola. The Buchnera bacteria provide their hosts with amino acids, particularly tryptophan, and the insects will die if they are treated with antibacterial antibiotics. Symbioses are seen even at the cellular level. For example, the eukaryotic cell is believed to have originated as a symbiosis between a primitive cell and a bacterium ancestral to modern-day mitochondria. See also: Biodiversity; Ectomycorrhizal symbiosis; Microbial interactions; Mitochondria; Plant-animal interactions; Pollination

Unfortunately, symbiotic relationships are quite sensitive to human-induced environmental changes. Habitat fragmentation, atmospheric pollution, and the introduction of nonnative species (invasive species) can have drastic consequences for many organisms, endangering their very existence. Thus, rather than focusing solely on species in isolation, conservationists and ecologists must increasingly attempt to preserve the critical symbioses in which endangered organisms are involved. See also: Air pollution; Endangered species; Invasion ecology; Invasive species and their effects on native species