Jordano, P., C. García, J.A. Godoy, and J.L. García-Castaño. 2006. Differential contribution of frugivores to complex seed dispersal patterns. Submitted, August 2006.
Animal seed dispersers show ample variation in their contribution to fruit removal in natural plant populations. However, no direct evidences are available about their effects on two central aspects of dispersal: distance, and probability of seed dispersal among populations, involving long-distance dispersal events. We used DNA-based genotyping techniques on Prunus mahaleb seeds dispersed by frugivorous birds (small- and medium-sized passerines) and carnivorous mammals to infer the source trees, dispersal distances, and the probability for each seed to come from outside the study population. Small passerines contributed mainly to short-distance dispersal, with 71% of the dispersal events located <100 m from the source trees and a small fraction of the seeds coming from other populations. The long-distance dispersal events were disproportionately contributed by medium-sized birds (50\% of the seeds dispersed >110 m), and carnivorous mammals (50\% of the seeds dispersed >495 m). When the quantitative contribution to fruit removal was accounted for, mammals were responsible for 66.9% of the immigrant seeds, while birds accounted for 33.1%. Our results provide direct evidence that frugivores widely differ in their effects on long-distance dispersal patterns and to seed-mediated gene flow and connectivity in fragmented landscapes. Despite highly diverse coteries of mutualistic frugivores dispersing seeds, critical long-distance dispersal might rely on very few species. Population declines or extinction of key frugivore species can seriously impair seed-mediated gene flow in fragmented landscapes by truncating the long-distance dispersal events and collapsing seed arrival to a restricted subset of available microsites.
|Photo: Schematic dispersal of a Prunus mahaleb tree. From top, clockwise: woodpigeon Columba palumbus, mistle thrush Turdus viscivorus, stone marten Martes foina, black redstart Phoenicurus phoenicurus, red fox Vulpes vulpes, robin Erithacus rubecula, and blackbird Turdus merula.|