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.