Observations that we can now associate with mutualism date back many centuries to natural history descriptions of species interactions by Herodotus, Aristotle, Cicero, Pliny, and others. Yet, the term mutualism was not used in a biological context until 1873, when Belgian zoologist Pierre van Beneden coined the term, stating that ''there is mutual aid in many species, with services being repaid with good behavior or in kind.'' Even though much of his focus was on competition and struggle within and among species, Charles Darwin was probably one of the first to give substantial attention to mutualism, especially pollination. Darwin pointed out that mutualistic interactions presented a significant challenge to his theory: individual organisms would not be expected to provide services or rewards for the sole benefit of individuals of another species, for such traits could not have evolved through natural selection. To resolve this dilemma, Darwin showed how plant traits that benefit animals, such as fruit and nectar, function first to increase a plant's own reproductive fitness. He pointed to cases in which the interests of mutualists could come into conflict, leading to cheating behaviors such as nectar robbing (the collection of nectar by flower-visitors that do not pick up or deposit pollen). In showing that mutualisms could emerge in nature strictly by selfish actions, limited by costs and driven by conflicts of interest between partners, Darwin laid much of the groundwork for current studies of the evolutionary ecology of mutualistic interactions.
Natural history lore about mutualisms may have accumulated for centuries, but a deeper ecological understanding of these interactions has lagged behind that of predation and competition. First, much more attention has been paid to mutualism from evolutionary biologists than from population and community ecolo-gists. As identified in Darwin's work, mutualism does present many interesting evolutionary issues, and mutua-listic species do indeed exhibit many uniquely evolved morphological and behavioral traits resulting from their interactions. A second reason for the lag in understanding relates to historical development of theory for the population dynamics and ecological stability of mutualism, compared with that of predation and competition. During the first two decades of the twentieth century, scientists such as A. J. Lotka and V. Volterra began theoretical investigations into how competition and predation influenced the stability and dynamics of interacting populations. Even though there are many recognized limitations to early results and mathematical approaches, they nevertheless provided ecologists of the time with a foundation upon which to build further theory and empirical research.
The history of theoretical research on the ecological dynamics of mutualism is quite different, however. Mutualism did not begin to be investigated theoretically until the mid-1930s. These models indicated that mutualism was either unstable, leading to unbounded population growth, or when stable, a very weak interaction having little influence on the dynamics of mutualistic populations. In the 1960s and 1970s, more theoretical investigations of mutualism began to emerge. However, these models largely replicated the results of earlier studies, and indicated that mutualisms were unstable, leading either to extinction or never-ending positive feedback and unbounded population growth. Since the 1980s, however, there has been much growth in the study of mutualism. Biologically realistic theory has accumulated that indicates that mutualism is not inherently unstable. The primary generalization to emerge from these models is that for mutualism to be stable, some factor must limit the positive feedback of the interaction on a population's growth rate. Factors incorporated into theory that have generated some stability include intra- and interspecific competition, predation, frequency dependence, spatial structure, and benefit and cost functional responses, most all of which involve some form of negative density dependence. Along with these theoretical studies, empirical studies ofmutualism have continued to increase in the past 20 years; these studies clearly indicate that mutualisms are common in nature and their populations do not in fact grow unbounded. Nonetheless, one of the most fundamental questions continuing to be asked about mutualism is what biological mechanisms prevent the inherent positive feedback of mutualism from leading to unbounded population growth.
Although there has been much growth in theoretical and empirical research on mutualism in recent years, there are still few generalizations and little conceptual unification across mutualistic interactions, which are indeed highly diverse and differ greatly in natural history (Table 1). As one indication of this, in current ecology textbooks, over 85% of the pages devoted to mutualism present natural history examples rather than concepts or theory. Despite such shortcomings, progress is being made on the ecology of mutualism. A few generalizations have emerged, including that nearly all mutualisms involve both benefits and costs; benefits and costs are themselves often density dependent, exhibiting functional responses; the outcome of interactions are often context dependent; and mutualisms often inherently entail conflicts of interests. Each of these generalizations is discussed below.
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