The Princeton Guide to Ecology

Mutualisms are interactions between two species that benefit both of them. Individuals that interact successfully with a mutualist experience greater success than those that do not. Behaving mutualistically is therefore of direct benefit to the individual itself. As Charles Darwin first pointed out in On the Origin of Species, mutualism does not require any special concern for the well-being of the partner. Although knowledge of mutualism lags behind that of other interspecific interactions, some important generalizations have emerged: nearly all mutualisms involve costs, not only benefits; outcomes of mutualisms are often context dependent; and mutualisms are often beset with cheaters that take advantage of rewards without conferring benefits in return. Mutualisms are increasingly recognized as fundamental to patterns and processes within ecological systems and are of growing concern in a conservation context. Persistence of individual species may frequently depend on preservation of the organisms, not only the habitats, on which they depend.

context dependency. Spatial and temporal variation in the strength and/or outcome of mutualism that can be attributed to the local environmental context; also referred to as conditionality

cooperation. Mutually beneficial interactions among individuals of the same species, often involving social interactions such as foraging or parental care

facilitation. Modification of some component of the abiotic or biotic environment by one organism that enhances colonization, recruitment, and establishment of another

facultative mutualism. A mutualism that increases an organism’s success but that is not absolutely required for its survival and/or reproduction

mutualism. A two-species interaction that confers survival and/or reproductive benefits to both partners

obligate mutualism. A mutualism without which an organism will fail to survive and/or reproduce

symbiosis. An interaction (positive, negative, or neutral) in which two species exist in intimate physical association for most or all of their lifetimes and are physiologically dependent on each other

Interactions between species influence ecological processes at the level of the population, the community, and the ecosystem. Virtually all species on Earth are involved in multiple interspecific interactions at any one time. For example, an individual plant may simultaneously interact with pollinators, seed dispersers, root symbionts, herbivores, seed predators, and plant competitors.

Interspecific interactions are most commonly classified according to their effects on the two species. The effect of any given interaction on a population attribute (usually either population growth or fitness) of a given species can be positive (+), negative (–), or neutral (0). Thus, there are six possible pairwise outcomes, commonly referred to as mutualism (+,+), competition (–,–), commensalism (+,0), neutralism (0,0), amensalism (–,0), and predation, parasitism, and herbivory (+,–). This classification is based on discrete (+, –, 0) effects on each of the interacting populations. As will be discussed in what follows, however, divisions among different forms of interspecific interactions are not nearly so black and white: effects actually range continuously from positive to negative in interesting and important ways.

Ecologists have given deep and prolonged attention to two interspecific interactions: predation and

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