Mutualism may be facultative, when the species involved are capable of existence independent of one another, or obligate, when the relationship is imperative to the existence of one or both species.
Mutualism in plants is demonstrated in the association of fungi and algae to form lichens, of nitrogen-fixing bacteria with the roots of legumes, and of fungal mycorrhizae with the roots of many flowering plants. Further, there are many intimate relations between plants and animals.
Mutualism is evident in the presence of photosynthetic algal cells in the protective ectoderm of green hydra and those associated with turbellarians, molluscs, annelids, bryozoans rotifers, protozoan’s and the egg capsules of salamanders. The algae give off oxygen, benefiting the animals, which in turn supply CO2 and N2 to the plants.
Likewise, certain beetles, ants and termites cultivate fungi for food. Bacteria in the caeca and intestine of herbivorous birds and mammals aid in the digestion of cellulose. The cross pollination of flowers by the agency of insects and birds seeking nectar and pollen is of such great importance that many structural adaptations in both plants and animals fit the one to the other to ensure the success of the function.
Moreover, birds and mammals are of great importance as agents of plant distribution. Seeds, fruits, even entire plants become attached to feathers or fur or ingested seeds are eaten and eliminated unharmed with the faeces.
Interspecific mutualism is nicely demonstrated by the flagellate protozoan, Trichonyinpha an obligate anaerobe in the gut of several species of wood-eating termites where it digest cellulose. Trichonympha also occurs in the alimentary canal of wood-eating roach Cryptoccrus.
The termite and roach reduce the wood to small fragments, passing them through the alimentary canal to hind gut, where the protozoans digest the cellulose, changing it into sugar. The host benefits the protozoa by removing harmful metabolic waste products and maintaining anaerobic conditions in the intestine.