Dispersal of dogwood seeds by avian frugivores: Ecological and evolutionary consequences

Seed dispersal activities of birds have probably selected for plant characteristics such as brightly colored and nutritious fleshy fruits through mutualistic interactions between birds and plants. The primary goals of this research were to better understand the evolution of the plant and disperser interaction and the current selective forces in this mutualism by documenting the effectiveness of avian frugivores as seed dispersers for gray dogwood (Cornus racemosa) and flowering dogwood (C. florida). Effective dispersal was quantified, in terms of plant reproductive success, as the number of seeds moved by a frugivore to sites where germination and survival to reproductive maturity are likely. In four years, 15 bird species removed fruits from gray dogwoods; however, four species consistently removed more than 80% of the fruits; only ten frugivores dispersed flowering dogwoods and only three consistently removed most of the fruits. The most effective gray dogwood seed dispersers were Brown Thrashers, Red-eyed Vireos and Swainson\u27s Thrushes, because they removed a large number of fruits and likely deposited the seeds in optimal edge habitat between open fields and woods. Two flowering dogwood frugivores (American Robin and Hermit Thrush) were highly effective dispersers. They removed most of the fruits. Simulation of frugivore foraging, superimposed on a 5.6ha map of dogwoods (DBH $\geq$ 3cm), indicated that these two birds deposited most seeds close ($\u3c$15m) but not directly under adult dogwoods. Germination and seedling survival experiments indicated that edge habitat was better gray dogwoods than open fields, woods or under thickets and that flowering dogwood seeds were more likely to attain reproductive size if they were deposited 5-10m from an adult than if they were under or further (15-20m or 25-30m) away from an adult. Coadaptations among birds and plants were investigated by comparing the dogwoods and interpreting their dispersal system in the continuum of small fruits with generalized frugivores to large and nutritious fruits with specialized frugivores. Demonstrated variation in effectiveness among surprisingly few major dogwood frugivores, and variation among dogwoods in their life histories, make this temperate genus an important focus for testing hypotheses concerning coevolution between plants and animals