FRUIT CHOICE AND SEED DISSEMINATION BY BIRDS AND MAMMALS IN THE EVERGREEN FORESTS OF UPLAND MALAWI

Françoise Dowsett-Lemaire

The study of interactions between fruit and their vertebrate consumers has generated a great deal of interest in recent decades. Most authors have investi¬ gated the habits of single bird or mammal species (e.g. Phillips, 1926, 1927, 1928 ; Frith, 1957 ; Snow, 1962; Gautier-Hion, 1971 ; Alexandre, 1978) or the assem¬ blages of species — mainly birds — attracted to the fruits of particular trees (e.g. Land, 1963; Willis, 1966; Leek, 1969; Howe, 1977, 1981; Bronstein & Hoffmann, 1987). Studies of the fruit-eating behaviour of phylogenetically-related groups of consumers are fewer (e.g. Snow & Snow, 1971 ; Crome, 1975 ; Gautier-Hion, 1980 ; Beehler, 1983) and those dealing with a whole class of vertebrates such as birds are evidently rare (Frost, 1980 — incompletely publis¬ hed ; Herrera, 1984 ; Wheelwright et al. 1984). Noteworthy is the work by Gautier-Hion et al., (1985a) who analysed fruit choice in a large community of mammals (bats excepted) and seven large canopy bird species in a Gabon rain forest.

Early models of plant-frugivore interactions have stressed the distinction between specialized, obligate frugivores that feed on, and disperse efficiently, high-quality fruits (with low and extended fruit production), and the unspeciali¬ zed, opportunist feeders that take less nutritious fruits (with scattered, shorter seasons) (McKey, 1975 ; Howe & Estabrook, 1977 ; Snow, 1981). However, subsequent field work has largely failed to confirm these original predictions on diet specialization and dispersal quality ; most studies of birds have, instead, emphasized the tremendous diversity of frugivores at fruiting trees, even with high-reward fruits (e.g. McDiamird et al, 1977 ; Frost, 1980 ; Wheelwright et al, 1984 ; other references in Wheelwright & Orians, 1982, p. 403). Non-obligate, «opportunist » fruit-eaters may be better seed disseminators than obligate frugivores (Pratt & Stiles, 1983). McKey’s (1975) theory on diet specialization and dispersal quality has also been criticized strongly on theoretical grounds (Wheelwright & Orians, 1982 ; Howe, 1984 ; Herrera, 1985 ; Wheelwright, in press) : plants have little or no control over the quality of dispersal of their seeds, and finely-tuned coevolved interactions between plants and their seed dispersers are expected to be at best very rare in nature (Wheelwright & Orians, 1982). Rather, diffuse coevolution between groups of species is more likely to occur (Janzen, 1980) and can explain the broad co-adaptations observed between fruits and their consumers (van der Pijl, 1969). Recent examples are the

Rev. Ecol. (Terre et Vie), vol. 43, 1988

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