Ecologists now recognize that one of the few generalizations that can be made about mutualisms is that nearly all of them involve both benefits and costs for each interacting species (Table 1). Mutualistic outcomes arise when the benefits of an interaction outweigh costs for both interacting species, such that the net effects of the interaction equal benefits minus costs. Currencies used as measures of benefits and costs often vary among mutualisms, but commonly include physiological or behavioral responses to various direct and indirect measures of growth, survival, and reproduction. Whatever currency is used to measure benefits and costs, they both are implicitly understood to ultimately affect reproduction and/or survival, or possibly some energetic currency, as these are the fundamental units for ecological and evolutionary processes.
Benefits are goods and services that a mutualistic species cannot obtain affordably, or at all, in the absence ofits partner(s). Three general classes of benefits occur among mutualisms: transportation, protection, and food/nutritional resources (Table 1 ). Transportation involves the movement of oneself or one's gametes, including, for example, pollen dispersal by pollinators and seed dispersal by frugivores. Benefits of protection involve the defense, guarding, or shelter of a mutualist from natural enemies (e.g., predators, herbivores, parasites, parasitoids) or the abiotic environment. Examples include ant protection of plants from herbivores and of certain other insects from predators and parasitoids. Benefits may also include nutritional resources, ranging from nutrient and carbon exchanges in plant/mycorrhizal interactions to food substances provided by plants in return for protection by ants.
Although most of the benefits that mutualists provide one another have long been known, it has only recently been recognized that mutualistic interactions also involve costs. Costs of mutualism arise as a consequence of the provision of resources and services to partner(s). Costs include investments in structures and substances to reward mutualists (e.g., nectar) and the energy and time spent obtaining those rewards (Table 1 ). In most cases, there is interspecific exchange of benefits and costs, such that the benefits accruing to one mutualist translate into the costs experienced by its partner and vice versa. For instance, the plant invests in the production of nectar at some cost to itself; that nectar is the benefit received from a floral visit by the pollinator. The pollinator also experiences a cost, in terms of time and energy spent obtaining that nectar. Time and energy costs can be difficult to measure; also, they are only incurred in cases where the interaction actually takes place. In contrast, other costs are incurred whether or not an individual does in fact interact with its mutualistic partner(s). For example, nectar is generally produced by plants regardless of whether pollinators actually visit a flower.
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