GEOGRAPHICAL ECOLOGY AND VARIATION OF PLANT-SEED DISPERSER INTERACTIONS - SOUTHERN SPANISH JUNIPERS AND FRUGIVOROUS THRUSHES

Spatial and temporal predictability in the mutual selective pressures of plants and frugivorous birds is a prerequisite for coevolution to o ccur. I examine the interaction patterns of strongly frugivorous thrus hes (Turdus spp.) and their major winter food plants (Juniperus spp., Cupressaceae) and how they vary in space and time. Spatial congruency, rarely considered in seed dispersal studies, is studied at three spat ial scales: 1) the total species range; 2) regional distribution; and 3) local abundance and its variation between seasons. Southern Spanish frugivorous thrushes and junipers show very low congruence,in distrib ution patterns at each of these scales. Most juniper species show geog raphic distributions that are nested within the geographic ranges of t hrush species. Bird species showed greater habitat breadth values than plants and were found in a greater percentage of localities. The loca l bird abundance was strongly correlated across years and sites with t he local availability of juniper cones. Cone production varied markedl y between years, but the rankings for different species in different y ears were statistically concordant at mid-elevation and lowland sites. Both bird abundance and cone production showed greater temporal than spatial variability. Variation of cone productions at both temporal an d spatial scales was greater than variability in bird abundance. Speci es with strong interactions of mutual dependence showed very low value s of biogeographic congruence, caused by differences in geographic ran ge and habitat specificity. This obviously limits the possibilities fo r pairwise, specific coevolution to occur. However, mutual effects of species groups are possible to the extent that the component species a re ecologically 'interchangeable' in their selective effects and other constraints on coevolution are not operating. The approach used here to examine the patterns of species interactions at different biogeogra phic scales might prove useful in comparative studies of plant-animal interactions.