Bird-mediated selection on fruit display traits in Celtis ehrenbergiana (Cannabaceae)

Abstract

Background: In mutualistic interactions of fleshy-fruited plants and seed-dispersing birds, dispersers act as natural selection agents on fruit display traits. Bird-mediated phenotypic selection on maternal and seed level plant traits has been postulated to be uncoupled. However, this key step in the understanding of the co-evolutionary processes has seldom been explored. Goals: To study the pattern and strength of phenotypic selection exerted by birds on two different plant life stages: maternal (mean and within-plant variation of fruit traits) and offspring (individual seed size) levels. Organism: The one-seeded fleshy-fruited tree, Celtis ehrenbergiana (Klotzsch) Liebm. Field site: Natural forests of the Biosphere Reserve ‘Parque Costero del Sur’, near the shore of the Río de La Plata, Buenos Aires, Argentina. Methods: We sampled 24 randomly selected trees. For each focal tree, we recorded bird visits and behaviour, and fruit-related traits. Dispersed seeds were collected at the same location. Results: At the maternal level, birds exerted positive directional selection on mean sugar concentration. Sub-individual variation in fruit traits was not observed to be a target of bird-mediated selection. At the individual seed level, birds exerted positive directional selection on seed size. Conclusions: While birds exert selection pressures on reward-related traits, plants obtain an advantage through a larger seed size.