Habitat Alteration and Plant Intra-Specific Competition for Seed Dispersers. An Example with Olea europaea var. sylvestris

Abstract

This paper examines the hypothesis that intra-specific competition for avian dispersers among wild olive trees (Olea europaea var. sylvestris, Oleaceae) might limit successful dispersal in disturbed habitats. To this aim, we studied the avian fruit removal of wild olive in a disturbed scrubland plot for two consecutive seasons. We found very low overall fruit removal, low abundance of avian seed dispersers, and low levels of fruit damage by biotic or abiotic agents. There was also a high interindividual variation in fruit removal success as well as a high interindividual heterogeneity in plant traits (fruit width, pulp/seed ratio, crop size and ripening phenology) in both seasons. Thus, we conclude that the low abundance of frugivorous birds severely limits the total amount of seeds dispersed from each tree of the population. We analysed the relationship between fruit removal and both plant traits and habitat characteristics. Three different estimates of fruit removal were calculated: number of seeds dispersed, dispersal efficiency (percentage of the crop consumed by avian dispersers) and dispersal success (relative contribution of each individual tree to the amount of seeds dispersed in the population). The number of seeds dispersed was positively affected mainly by crop size, but also by fruit-design traits (fruit width), and negatively by the ripening phenology. Fruit removal efficiency was positively related to fruit-design traits (fruit width and pulp/seed ratio) and ripening phenology (early ripening trees being favoured). Finally, fruit removal success was only related to fecundity (crop size and plant size). The low mean fruit removal levels and the strong interindividual variability in fruit traits produced very asymmetric results (thus enabling some individuals to have many of their seeds dispersed) and, consequently, led to an unbalanced individual contribution to the pool of seeds dispersed by the population. These results support the hypothesis that there is a high interindividual competition among plants for avian dispersers in disturbed habitats. Our results also suggest that a few individuals could probably inundate such habitats with their descendants, thus exerting a long-term influence on the subsequent genetic structure of the population.