Mutualisms and Symbioses

Mutualisms, interactions between two species that benefit both of them, have long captured the public imagination. Humans are undeniably attracted by the idea of cooperation in nature. For thousands of years we have been seeking explanations for its occurrence in other organisms, often imposing our own motivations and mores in an effort to explain what we see. However, the importance of mutualisms lies much deeper than simply providing material for philosophical treatises and natural history documentaries. The influence of mutualisms transcends levels of biological organization from cells to populations, communities, and ecosystems. Mutualisms were key to the origin of eukaryotic cells and perhaps to the invasion of the land. Mutualisms occur in every aquatic and terrestrial habitat; indeed, ecologists now believe that almost every species on Earth is involved directly or indirectly in one or more of these interactions. Mutualisms are crucial to the reproduction and survival of many plants and animals and to nutrient cycles in ecosystems. Moreover, the ecosystem services mutualists provide (e.g., seed dispersal; pollination; carbon, nitrogen, and phosphorus cycles resulting from plant-microbe interactions) are leading these interactions increasingly to be considered conservation priorities, while acute risks to their ecological and evolutionary persistence are being identified. It is important to clarify the relationship between mutualism and symbiosis, because these concepts overlap and are often confounded. The term mutualism refers to all mutually beneficial, interspecific interactions, regardless of their specificity, intimacy, or evolutionary history. The term was first used in a biological context by Pierre-Joseph van Beneden, a Belgian zoologist, in 1873 (“There is mutual aid in many species, with services being repaid with good behaviour or in kind, and mutualism can well take its place beside commensalism” [van Beneden, 1873, “Un mot sur la vie sociale des animaux inférieurs,” Bulletin de l’Academie Royale de Belgique, series 2, 36, p. 785]). Albert Bernhard Frank and Heinrich Anton de Bary independently coined the term symbiosis a few years later in an attempt to group physiologically intimate interactions independent of their parasitic, commensal, or mutualistic outcome. Thus, some mutualisms are symbiotic (e.g., interactions between algae and fungi that form lichens), whereas others are not (e.g., plant-pollinator interactions). Conversely, some symbioses are mutualistic (e.g., lichens), whereas others are not (e.g., parasitic fungi that inhabit plant roots). It is appropriate to refer to all mutually beneficial, interspecific interactions as mutualisms, whether or not their physiological intimacy justifies referring to them as symbioses as well.