The Fruiting Ecology of Osyris Quadripartita: Individual Variation and Evolutionary Potential

Abstract

The fruiting ecology of Osyris quadripartita (Santalaceae), a hemiparasitic evergreen shrub, was studied over a 4—yr period in a Mediterranean scrubland of southern Spain. In addition to describing the natural history of seed dispersal in this species, objectives of the study were (1) to document patterns of individual and annual variation in seed—dispersal—related traits (fruit characteristics, crop size, fruiting phenology); (2) to determine the consequences, in terms of dispersal success, of individual variability; and (3) to examine the significance, in terms of differential reproductive output, of individual variability in dispersal—related attributes, in an attempt to evaluate the evolutionary potential for modification of these tratis. In this species during the 4—yr period considered, dispersal—related traits, although relevant to dispersal rates, were unrelated to individual differences in fitness because dispersal rates were irrelevant to fitness. O. quadripartita plants produce ripe fruits almost continuously, and most individuals have fruiting periods virtually encompassing the entire year. Significant individual variations exists in average fruiting schedule, crop size, and fruit characteristics. The peak of the fruiting period of individual plants may occur in almost any month of the year. Phenology, crop size, and fruit characteristics differ in the degree to which differences between individuals remain consistent from year to year in the face of population variation in these variables. Individual differences in crop size remain invariant among years, while differences in phenology and fruit characteristics do not. Individual variation in crop size is related to flower number, while annual differences are explained by variation in summer rainfall through influences on latent ovary abortion rates. There is within—plant seasonal variation in fruit characteristics, and there are also significant interplant and interannual differences in the way fruit characteristics change seasonally. Avian frugivores are present in the habitat throughout the year, but seasonal changes in abundance and amount of frugivory delimit distinct periods of high and low demand for O. quadripartita fruits. The probability of damage to ripe fruits depends on time of year (although there is considerable annual variation), crop size, and fruit size. Large fruits in small crops predictably face the highest probabilities of damage, and small fruits in large crops the lowest. The relation between the probability of damage and crop size, however, varied among plants and seasonally. Individual differences in fruit characteristics and crop size, but not in phenology, are related to individual variation in seed dispersal rates (proportion of ripe fruits dispersed). Nevertheless, dispersal rate accounts for only 0.2% of individual variation in relative contribution to the population's realized reproductive output, while estimated total number of fruits ripened accounts for 96%. This pattern is consistent from year to year. Components of interindividual variation that influence the dispersal prospects of fruits are eventually unimportant in accounting for individual variation in relative reproductive contribution.