Our current understanding of mutualism

It is widely believed that mutualisms, interspecific interactions that benefit both species, have been grossly neglected relative to their true importance in nature. I have reviewed the recent primary literature in order to assess quantitatively the frequency of studies of mutualism, the types of questions they address, and their general scientific approach. All articles appearing from 1986 to 1990 in nine major journals that publish ecological and evolutionary research were examined. It is clear that mutualism research is not in fact rare. Studies of interspecific interactions made up about 22 % of the over 4500 articles published during this period; of these, about one-quarter investigated some form of mutualism. Over 90 % of them investigated plant-animal interactions, primarily pollination (52 %) and seed dispersal (31 %), a bias probably related in part to the particular journals examined. The diversity of questions addressed in these articles was surprisingly low. The majority (63 %) focused simply on identifying the mutualists of some species of interest. Furthermore, almost all studies were unilateral, that is, they focused on only one of the interacting species, plants being studied much more frequently than their animal partners. Mutualism studies do not appear to have focused on mutualism as a form of interaction in the same way as studies of competition and predation. Rather, researchers have treated mutualism primarily as a life history attribute of one of the two partners. Consequently, although an impressive amount of information has accumulated about these interactions, we are still far from achieving an overall picture that transcends the boundaries of particular taxa or combinations of taxa. Three other obstacles have prevented data on mutualisms from being brought together: the historical isolation of studies of different kinds of mutualism, a nearly total disconnection between mutualism theories and empirical studies, and the unilateral approach almost always used to study these bilateral interactions. I identify eight research questions whose answers have the potential to reveal broad-based generalizations about the evolution and ecology of mutualism.