Differential Seed Dispersal by Birds of the Tree Casearia nitida (Flacourtiaceae)

Data are presented which demonstrate (1) flight patterns of frugivorous birds are responsible for directional patterns of seed dispersal, (2) these patterns influence seed mortality, and (3) dispersal of seeds away from the parent tree is more likely to result from behaviors characteristic of obligate frugivores (Ramphastos sulfuratus and R. swainsonii) than frugivore/insectivores (Myiozetetes similts and M. granadensis). Investment by the parent tree in a seed carried by a frugivore is less likely to be wasted than investment in a seed carried by an frugivore because specialists are more likely to carry seeds away from areas of high seed mortality than opportunists. PLANTS HAVE A VARIETY OF CHARACTERISTICS which result in dispersal of their seeds by fruiteating animals. These involve the structure of the fruit (van der Pijl 1969) or other aspects of plant reproductive biology (Snow 1965; Smythe 1970; Snow 1971). Previous studies consider the effects of treatment by the digestive tract on the germination of seeds (Darwin 1859; Kerner 1897; Rick and Bowman 1961), the effects of long-distance dispersal on the biogeography of plants (Cruden 1966; Carlquist 1974), effects of dispersal on the survivorship of seeds which suffer high levels of animal predation (janzen 1971), and the expected influence of such predation on the local dispersion of plants (Janzen 1970). The variety and abundance of fruits in the tropics is correlated with a diverse bird fauna showing varying degrees of specialization for frugivory and seed dispersal (Wetmore 1914; Keast 1958; Orians 1969; Karr 1971; Morton 1973). Absent frcm previous studies are quantitative data showing the patterns of seed distribution produced by movements of different kinds of dispersers and the effects of this dispersal on the survivorship of seeds. Such information is particularly desirable in view of McKey's (1975) argument that the dispersal of seeds by specialized birds which are obligate frugivores should be qualitatively different from dispersal by opportunistic birds which facultatively feed on fruit or insects. If this point of view accurately reflects close coevolutionary relationships between obligate frugivores and the plants upon which they feed, we predict that specialists should also produce patterns of seed distribution and resulting seed mortality more beneficial to the plant than those produced by birds with less dependence on fruit. This paper offers a partial test of this prediction and suggests further research which will elucidate relationships between plants and their dispersal