Exclusive frugivory and seed dispersal of Rhamnus alaternus in the bird breeding season

We studied avian frugivory and seed dispersal in a dioecious shrub, Rhamnus alaternus, focusing on the quantitative and qualitative components of effectiveness. The study took place at three locations in the northeast of the Iberian Peninsula, and examined bird behaviour, intensity of feeding, and the consequences for seedling emergence. The coincidence between the bird breeding season and fruit ripening of R. alaternus in the absence of other ripe fruit, generates a monospecific interaction. The extant frugivorous species were mainly legitimate seed dispersers and their abundance was low. Sylvia melanocephala and S. undata were the most important at one site whereas S. atricapilla, Erithacus rubecula and Turdus merula predominated at the other two sites. Fruit handling took place directly on the branches. Bird species used microhabitats differently as first post-feeding perch, which usually was a short distance away. The low density of frugivorous birds in all localities, among others factors, resulted in satiation of the disperser community and many mature fruits unconsumed. Both adults and juveniles feed upon the plants and their foraging patterns are similar. Adults of S. melanocephala were observed to feed fruit to nestlings and consequently a second phase of dispersal potentially arises from the transport of fecal sacs. Pulp removal and passage through the digestive tract increased the probability of seedling emergence. This plant-dispersal interaction has important consequences, both positive and negative for the plant. Positively, the fruiting of R. alaternus at a time when other ripe fruits are not available avoids interspecific competition for seed dispersers. In addition, a low density of seed rain may reduce intraspecific competition. Negatively, the low density and small size of the breeding frugivorous bird community limit fruit handling and removal away from the parent plants, while the territorial behaviour of birds at that time of the year reduces the potential distances of seed dispersal.