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II.11 Mutualism and Symbiosis Judith L. Bronstein OUTLINE 1. Interspecific interactions and mutualism 2. Types of mutualism 3. Major ecological features of mutualisms 4. Conservation of mutualisms Mutualisms are interactions between two species that bene fit 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. GLOSSARY 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 1. INTERSPECIFIC INTERACTIONS AND MUTUALISM 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 classi fied 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 (þ), negative (), or neutral (0). Thus, there are six possible pairwise outcomes, commonly referred to as mutualism (þ,þ), competition (,), commensalism (þ,0), neutralism (0,0), amensalism (,0), and predation, parasitism, and herbivory (þ,). This classification is based on discrete [þ, , 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 competition, relationships that are negative for either one or both of the participants. Mutualisms, i.e., mutually positive interactions, are more poorly understood . However, they are increasingly recognized to be fundamental to patterns and processes within ecological systems. Mutualisms occur in habitats throughout the world, and ecologists now recognize that almost every species on earth is involved directly or indirectly in one or more of these interactions. In tropical rainforests , for example, the large majority of plants depend on animals for pollination and seed dispersal. Over 80% of all flowering plants are involved in mutualisms with beneficial fungi (mycorrhizae) that live on and in their roots. In the ocean, both coral reef communities and deep-sea vents are exceptionally rich with mutualisms. In fact, corals themselves obligately depend on the photosynthetic algae that inhabit them. Influences of mutualism transcend levels of biological organization from cells to populations, communities, and ecosystems. They are now thought to have been key to the origin of eukaryotic cells, as both chloroplasts and mitochondria were once free-living microbes. Mutualisms are crucial to the reproduction and survival of many organisms as well as to nutrient cycles in ecosystems . Moreover, the ecosystem services that mutualists provide (e.g., seed dispersal, pollination, and carbon, nitrogen, and phosphorus cycles resulting from...


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