Frugivory and seed dispersal: ecological and evolutionary aspects

Any scientific discipline needs a theoretical framework to guide its development and to sharpen the questions its researchers pursue. In biology, evolution is the grand theoretical framework, and an his­ torical perspective is necessary to understand present-day biological conditions. In its formative years, the modern study of the fruit-frugivore mutualism was guided by the ''specialist-generalist'' paradigm developed by D. Snow, D. McKey, and H. Howe. Howe reviews the current status of this evolution­ ary paradigm and points out that it has been dismissed by many workers before being adequately tested. This is because ecologists working with the tropical plants and frugivorous birds for which the paradigm was originally developed rarely measure the seed dispersal effectiveness of different disperser species. He indicates that this paradigm still has heuristic value and suggests that several additional ecological paradigms, including the concept ofkeystone species ofplants and frugivores and the role that frugivores play in density-dependent mortality in tropical trees, are worth studying. The concept of seed dispersal quality has been central to discussions of fruit-frugivore coevolution. Schupp thoroughly reviews data bearing on this concept, constructs a hierarchical framework for viewing disperser effectiveness, and points out that disperser effectiveness depends on both the quantity and quality of seed dispersal. Effectiveness, in turn, affects both evolutionary and ecological relationships between dispersers and their food plants.