Fruit variation and competition for dispersers in natural populations of Smilax aspera [Liliaceae, woody climber, stomach content, passerine birds, Andalusia, Spain]

The berries of Smilax aspera L. (Liliaceae), a woody climber of mediterranean scrub, may have either one, two or three seeds. One-seed fruits have the largest, and three-seed the smallest, pulp/seed ratio, so differently-seeded fruits have a differential feeding value to birds feeding on them. In Andalusia, southern Spain, the overall proportions of fruits with different seed number was found to vary between populations as well as between individuals of the same population. S. aspera fruiting season overlaps extensively with the fruiting period of many other species and all of them share the same few species of bird dispersers. It was then hypothesized that interpopulation variation in fruit seed numbers was related to interspecific competition among plants for dispersers. To test this hypothesis, twelve popululations of S. aspera were sampled for ripe fruits. For each sample, the proportions of one-, two- and three-seed fruits were determined, and a set of environmental variables was recorded at each sampling locality. Multiple stepwise regression analysis shows that the best single predictor of the mean number of seeds per fruit is the number of plant species bearing fruits simultaneously and equally or more abundant than S. aspera. Assuming that the intensity of competition among plant species for dispersers must be roughly proportional to the number and population sizes of competitor species, these results suggest that inter-population variation in the number of seeds per fruit is actually related to variation in competitive pressures, provided that (1) dispersers are in short supply, (2) dispersers select to some extent the most profitable fruits, and (3) number of seeds has some genetic component. Evidence supporting these three assumptions is presented, thus suggesting that observed variation in fruit seed numbers must be attributed to interspecific competition for dispersers among plants.