FRUGIVORE-MEDIATED SELECTION ON FRUIT AND SEED SIZE - BIRDS AND ST LUCIES CHERRY, PRUNUS MAHALEB

Authors
Citation
P. Jordano, FRUGIVORE-MEDIATED SELECTION ON FRUIT AND SEED SIZE - BIRDS AND ST LUCIES CHERRY, PRUNUS MAHALEB, Ecology, 76(8), 1995, pp. 2627-2639
Citations number
75
Categorie Soggetti
Ecology
Journal title
ISSN journal
00129658
Volume
76
Issue
8
Year of publication
1995
Pages
2627 - 2639
Database
ISI
SICI code
0012-9658(1995)76:8<2627:FSOFAS>2.0.ZU;2-T
Abstract
Frugivorous birds consumed >75% of the ripe fruits of a Prunus mahaleb population in southeastern Spain, but only half of the seed crop was successfully removed from parent plants by legitimate seed dispersers. For two consecutive years, I studied the sign and magnitude of phenot ypic selection exerted by frugivorous birds on fruit size and seed mas s, two key traits in this mutualistic plant-seed disperser interaction . Individual plants showed extensive phenotypic variation in these tra its, but among-individual variation accounted for <30% of total trait variance. Selection patterns were assessed at two levels by separating the effects of selection acting on the parent tree (among-crop select ion; comparing fruit removal and seed dispersal efficiency among indiv idual plants) and selection acting at the individual seed level (compa ring seed mass variation before and after dispersal by frugivorous bir ds). Dispersal efficiency (percentage of the seed crop dispersed) corr elated negatively with crop size, fruit size, and seed mass. However, only crop size was significantly, positively, correlated with the abso lute number of seeds dispersed relative to the population mean, used a s the estimator for relative fitness. Greater visitation by dispersers to smaller plants compensated for their lower fecundity but, for plan ts with larger crops, a greater number of seeds was dispersed despite lower dispersal efficiency. Directional and stabilizing/disruptive sel ection gradients on fruit traits were not significant or, at best, onl y marginally significant, indicative of weak and inconsistent selectio n effects on maternal phenotypes. In contrast, selection on individual seed phenotypes was significant. Seeds on the ground, after successfu l dispersal by frugivorous birds, were significantly smaller than seed s 'available' at the start of the fruiting season. Observed selection differentials on individual seed mass were -0.12 (1992) and -0.13 (199 3), suggesting that frugivores might exert strong selection on individ ual seed phenotypes irrespective of the maternal phenotype. This selec tion regime, with far-reaching demographic consequences but low potent ial for inducing evolutionary change in fruit traits, is expected on t he basis of known hierarchical selection cues used by foraging frugivo res. Fruit phenotypic variation might be irrelevant as a cue used by b irds for discrimination among fruit crops, but, given extensive within -crop variation, frugivores might strongly select among seed phenotype s in a process not related consistently to among-crop selection on mat ernal phenotypes.