Seed Disperser Effectiveness: The Quantity Component and Patterns of Seed Rain for Prunus mahaleb

In this paper we analyze the seed dispersal stage of the Prunus mahaleb-frugivorous bird interaction from fruit removal through seed delivery within the context of disperser effectiveness. The effectiveness of a frugivorous species as a seed disperser is the contribution it makes to plant fitness. Effectiveness depends on the quantity of seed dispersed ("quantity component") and the quality of dispersal provided each seed ("quality component"). For the main frugivores, we studied abundance, visitation rate, and feeding behavior, the major variables influencing the quantity component of effectiveness, and the postforaging microhabitat use and resultant seed shadows, which set the stage for postdispersal factors that will influence the quality component of effectiveness. Legitimate seed dispersers (SD) swallowed fruits whole and defecated or regurgitated intact seeds; pulp consumers (PC) pecked fruits to obtain pulp and dropped seeds to the ground, but some species occasionally dispersed seeds (PCSD species). Overall numbers of fruits removed (i.e., handled) by avian frugivores were similar in the two study years; however, the estimated percentage of seeds dispersed differed significantly, with lower relative dispersal success in the year with greater relative abundance of PC species. Similar numbers of seeds were dispersed in the two years despite nearly a fourfold difference in number of fruits produced. Fruit crop size explained >80% variance in the number of seeds dispersed per tree. A total of 38 species of birds were recorded during censuses, with frugivores representing 68.8% of them; the relative representation of SD, PC, and PCSD species was 42.2%, 17.2%, and 9.4%, respectively. Individual trees showed extensive variation in visitation rates, ranging from 0.3 to 41.6 visits/10 h in any year. The main visitors were the SD species Phoenicurus ochruros (10.8 visits/10 h), Turdus viscivorus (9.2 visits/10 h), Erithacus rubecula (3.5 visits/10 h), and Sylvia communis (2.6 visits/10 h); and the PC species Fringilla coelebs (16.7 visits/10 h) and Parus ater (4.7 visits/10 h). Species with large quantity components of effectiveness typically had either high visit or high feeding rates, combined with high probability of dispersing a handled seed. Variation among species in fruit-handling behavior, however, was the main factor influencing variation in the quantity component. Visit rate in turn was influenced largely by local abundance. No single frugivore tra