Determinants of Seed Disperser Effectiveness: the Quantity

— In this paper we analyze the seed dispersal stage of the Prunus mahaleb1 frugivorous bird interaction from fruit removal through seed delivery within the context of 2 disperser effectiveness. The effectiveness of a frugivorous species as a seed disperser is the 3 contribution it makes to plant fitness. Effectiveness depends on the quantity of seed dispersed 4 and the quality of dispersal provided each seed. For the main frugivores, we studied 5 abundance, visitation rate and feeding behavior, the major variables influencing the quantity 6 component of effectiveness, and the post-foraging microhabitat use and resultant seed 7 shadows which set the stage for postdispersal factors that will influence the quality component 8 of effectiveness. 9 Legitimate seed dispersers (SD) swallowed fruits whole and defecated or regurgitated 10 intact seeds; pulp consumers (PC) pecked fruits to obtain pulp and dropped seeds to the 11 ground, but some species occasionally dispersed seeds (PCSD species). Overall numbers of 12 fruits removed (i.e., handled) by avian frugivores were similar in the two study years; 13 however, the estimated percentage of seeds dispersed differed significantly, with lower 14 relative dispersal success in the year with greater relative abundance of PC species. Similar 15 numbers of seeds were dispersed in the two years despite a near four-fold difference in 16 number of fruits produced. Fruit crop size explained >80% variance in the number of seeds 17 dispersed/tree. 18 A total of 38 species of birds were recorded during censuses, with frugivores 19 representing 68.8 % of them; the relative representation of SD, PC, and PCSD species was 20 42.2, 17.2, and 9.4 %, respectively. There were no significant trends in overall frugivore 21 abundance between the two study years, but 10 of 16 SD species tended to be more abundant 22 in 1989 and 4 of 6 PC species tended to be less abundant, resulting in a greater relative and 23 absolute representation of SD species among frugivores in 1989, the year with greater relative 24 seed dispersal success. Individual trees showed extensive variation in visitation rates ranging 25 from 0.3 to 41.6 visits/10 h in any year. The main visitors were the SD species Phoenicurus 26 ochruros, 10.8 visits/10h; Turdus viscivorus, 9.2 visits/10h; Erithacus rubecula, 3.5 visits/10h; 27 Jordano and Schupp 3 and Sylvia communis, 2.6 visits/10h and the PC species Fringilla coelebs, 16.7 visits/10h and 1 Parus ater, 4.7 visits/10h. 2 Species with large quantity components of effectiveness typically had either high visit 3 or high feeding rates, combined with high probability of dispersing a handled seed. Variation 4 among species in fruit handling behavior, however, was the main factor influencing variation 5 in the quantity component. Visit rate in turn was influenced largely by local abundance. No 6 single frugivore trait, however, can adequately estimate the quantity component of disperser 7 effectiveness. A ‘gulper’/’masher’ dichotomy helps explain differences in fruit handling 8 among major frugivore types and shows many correlates with other aspects of frugivore 9 activity that ultimately influence the quantity component. 10 Most species showed marked preferences for microhabitats with plant cover, especially 11 P. mahaleb, mid-height shrubs, and Pinus (86.1 % of the departure flights) and avoided open 12 microhabitats. Most flights were over short distances (77.5 % to perches located within 30 m). 13 Among the main frugivores, 40.3 % of the exit flights were to perches >15 m away from the 14 feeding tree, but only 18.5 % of these flights were to perches >15 m from any P. mahaleb. 15 Covered microhabitats received significantly more seeds (39.3 ± 5.0 seeds dispersed/m, 1988; 16 31.7 ± 5.9 seeds dispersed/m, 1989) than open microhabitats (2.8 ± 0.7 seeds dispersed/m, 17 1988; 1.8 ± 0.4 seeds dispersed/m, 1989). 18 The potential contribution of each bird species to the seed rain in each microhabitat 19 was estimated from the number of visits recorded, the mean number of seeds dispersed/visit, 20 and the proportion of exit flights to each microhabitat. Microhabitats differed strongly in the 21 proportions of seeds delivered by the main frugivores, and bird species also differed in the 22 proportions of seeds delivered to a given microhabitat. The seed rain to covered microhabitats 23 was delivered by a more heterogeneous assortment of species than the seed rain to open sites. 24 The resulting seed shadow was a complex result of the interaction between movement patterns 25 of a suite of bird species differing in both the quantity of seed dispersed and microhabitat 26 preferences, and the landscape distribution of these microhabitat patches. This seed shadow 27 Jordano and Schupp 4 was extremely non-random due to both a strong overall preference by most of the birds for the 1 relatively scarce covered microhabitats, and to species-specific preferences for particular types 2 of covered microhabitats. Different microhabitat types not only received variable amounts of 3 dispersed seed, but also differed in the number and identity of disperser species contributing to 4 that seed rain. 5 6