In This Article Expand or collapse the "in this article" section Mutualisms and Symbioses

  • Introduction
  • General Overviews
  • Defining Mutualism and Symbiosis
  • Exploitation
  • Specialization and Generalization
  • Population Dynamics

Ecology Mutualisms and Symbioses
Judith Bronstein
  • LAST REVIEWED: 17 December 2021
  • LAST MODIFIED: 25 September 2019
  • DOI: 10.1093/obo/9780199830060-0006


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.

General Overviews

The study of mutualism as a unified phenomenon is very young, even though individual mutualisms have been studied for years and, in some cases, for centuries. The first focused attention to mutualism as a whole arose in the early 1980s. Boucher, et al. 1982, a review article, lays out most of the conceptual issues that still drive ecological research in this field. These issues were picked up in greater depth in Boucher 1985. The authors of chapters within this edited volume use both theoretical and empirical approaches, address both applied and basic questions, and focus both on symbiotic and nonsymbiotic interactions. Written a decade later, Bronstein 1994 synthesizes the literature and shows that despite a widespread conviction that mutualism was poorly understood, a great deal of information had accumulated by that point. However, that information was not directed toward identifying broad generalizations about mutualism biology. This paper proposes eight promising future research directions; many of these directions now orient the field as a whole. Douglas 2010 focuses largely on ecological and functional aspects of symbiotic mutualisms, although evolutionary topics are also addressed, and much of the discussion is relevant to nonsymbiotic mutualisms as well. The coverage in this article is expanded upon and updated in Douglas 2015, a chapter in an edited book (Bronstein 2015a) that provides extensive coverage of the ecology and evolution of mutualism.

  • Boucher, Douglas H., ed. 1985. The biology of mutualism: Ecology and evolution. New York: Oxford Univ. Press.

    An important and still-relevant edited volume covering a wide range of conceptually oriented ecological topics. Contributions are both theoretical and empirical and both basic and applied; most are ecologically oriented.

  • Boucher, Douglas H., Sam James, and Kathleen H. Keeler. 1982. The ecology of mutualism. Annual Review of Ecology and Systematics 13:315–347.

    DOI: 10.1146/

    A wide-ranging review of mutualism ecology, succinctly covering many of the same topics treated in more depth in Boucher 1985. Available online for purchase or by subscription.

  • Bronstein, Judith L. 1994. Our current understanding of mutualism. Quarterly Review of Biology 69.1: 31–51.

    DOI: 10.1086/418432

    An analysis of the status of mutualism knowledge as of 1994. The article also lays out promising future research directions, most of which have now become the leading topics of inquiry. Available online for purchase or by subscription.

  • Bronstein, Judith L., ed. 2015a. Mutualism. New York: Oxford Univ. Press.

    A conceptually oriented volume that provides comprehensive coverage of major themes in the study of mutualism today. Each chapter include boxed vignettes that go into greater depth on specific case studies associated with the chapter theme, and specify open questions in need of further study.

  • Douglas, Angela E. 2010. The symbiotic habit. Princeton, NJ: Princeton Univ. Press.

    A full treatment of mutualistic symbiosis, with a strong focus on structure and function. Nonsymbiotic mutualisms are also addressed to some extent.

  • Douglas, Angela E. 2015. The special case of symbioses: Mutualisms with persistent contact. In Mutualism. Edited by Judith L. Bronstein, 20–34. New York: Oxford Univ. Press.

    DOI: 10.1093/acprof:oso/9780199675654.003.0002

    A concise treatment of symbiosis and the unique questions it poses about mutualism as a whole.

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