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An interaction between two species that benefits both. Individualsthat interact with mutualists experience higher sucess than those that do not.Hence, behaving mutualistically is advantageous to the individual, and it doesnot require any concern for the well-being of the partner. At one time,mutualisms were thought to be rare curiosities primarily of interest to naturalhistorians. However, it is now believed that every species is involved in oneor more mutualisms. Mutualisms are thought to lie at the root of phenomena asdiverse as the origin of the eukaryotic cell, the diversification of floweringplants, and the pattern of elevated species diversity in tropical forests.
Mutualisms generally involve an exchange of substances or services thatorganisms would find difficult or impossible to obtain for themselves. Forinstance, Rhizobium bacteria found in nodules on the roots of manylegume (bean) species fix atmospheric nitrogen into a form (NH3)that can be taken up by plants. The plant provides the bacteria with carbon inthe form of dicarboxylic acids. The carbon is utilized by the bacteria asenergy for nitrogen fixation. Consequently, leguminous plants often thrive innitrogen-poor environments where other plants cannot persist. Anotherwell-known example is lichens, in which fungi take up carbon fixed duringphotosynthesis of their algae associates.
A second benefit offered within some mutualisms is transportation. Prominentamong these mutualisms is biotic pollination, in which certain animals visitflowers to obtain resources and return a benefit by transporting pollen betweenthe flowers they visit. A final benefit is protection from one's enemies.For example, ants attack the predators and parasites of certain aphids inexchange for access to the aphids' carbohydrate-rich excretions(honeydew).
Another consideration about mutualisms is whether they are symbiotic. Twospecies found in intimate physical association for most or all of theirlifetimes are considered to be in symbiosis. Not all symbioses are mutualistic;symbioses may benefit both, one, or neither of the partners.
Mutualisms can also be characterized as obligate or facultative (dependingon whether or not the partners can survive without each other), and asspecialized or generalized (depending on how many species can confer thebenefit in question).
Two features are common to most mutualisms. First, mutualisms are highlyvariable in time and space. Second, mutualisms are susceptible to cheating.Cheaters can be individuals of the mutualist species that profit from theirpartners' actions without offering anything in return, or else otherspecies that invade the mutualism for their own gain.
Mutualism has considerable practical significance. Certain mutualisms playcentral roles in humans' ability to feed the growing population. It hasbeen estimated that half the food consumed is the product of bioticpollination. See Ecology, Plant pathology