Fruit size in wild olives: implications for avian seed dispersal

Frugivorous birds in the Mediterranean basin may disperse seeds using two different feeding behaviours: swallowing the fruit whole or pecking pieces of pulp. Swallowing is the common behaviour of frugivorous seed dispersers in the Mediterranean basin (Herrera 1984a) and most of the fleshy-fruited plants in the region rely on this bird feeding behaviour to disperse their seeds (Herrera 1984b, 1995). This mutualistic system can only occur if, to some extent, fruit size and gape width adjust each other (Herrera 1984a; Wheelwright 1985; Debussche & Isenmann 1989). Seed dispersal may become difficult for plant species showing such a high variation in fruit size that some of their fruits cannot be swallowed by some bird species (Rey 1992; Wheelwright 1993). This variation may occur among and/or within individual plants (Jordano 1984; Wheelwright 1993; Herrera et al. 1994; Obeso & Herrera 1994; Jordano 1995). Feeding on large Mediterranean fruits, such as those dispersed by mammals (Debussche & Isenmann 1989; Herrera 1989), is not a problem for small frugivorous birds accustomed to peck on fruits (gulping chunks of pulp instead of swallowing the fruit whole). Pulp pecking may allow the dispersal of small seeds from large multiseeded fruits such as Arbutus unedo, Ficus carica, Punica granatum, Fragaria vesca and several species of Morus, Rubus and Opuntia (Thomas 1979; Jordano 1981, 1982, 1984; Herrera 1984b; Bronstein & Hoffmann 1987). However, seed dispersal may be seriously limited if Functional Ecology 1997 11, 611–618