Symbiosis

Symbiosis is a close and usually long-term association between organisms of two different species. In the broad sense used in most textbooks, it covers three main kinds of relationship, defined by who gains and who loses. In mutualism both partners benefit, as in lichens, where a fungus and a photosynthetic alga or bacterium share nutrients and shelter. In commensalism one partner benefits while the other is barely affected, as with barnacles riding on whales. In parasitism one partner, the parasite, benefits at the expense of its host, which is usually weakened rather than killed quickly.

Symbioses shape ecosystems and bodies alike. Reef-building corals depend on algae living inside their tissues, and when warming water makes the corals expel these partners, the reefs bleach. Termites rely on protozoa in their gut to digest the cellulose in wood, and the human gut hosts a vast community of microbes. Over long periods, some partners become so dependent on each other that neither can survive alone.

Usage varies. Some biologists restrict the word to mutually beneficial partnerships, while others stretch it to cover competition or any interaction between unlike organisms, so it is worth checking how a textbook or exam defines it.