Partitioning of pollinators during flowering in an African Acacia community

Competition for pollination is an important factor structuring flowering in many plant communities. We examined mechanisms reducing interspecific poll en flow in a community of 10 Acacia species in a highly seasonal savannah h abitat in Tanzania. Partitioning is achieved, in part, through separation o f flowering in space and seasonal time, and through interspecific differenc es in pollinator guilds. Nevertheless, coflowering Acacia species shared se veral pollinators; this means that interspecific pollen transfer Is possibl e. We analyzed daily patterns of pollinator activity and pollen release in 10 Acacia assemblages containing a total of 10 Acacia species. Pollinator a ctivity was scored using counts at flowers over constant time intervals thr oughout the day. Fallen availability was assessed using a simple method whi ch allows quantification of pollen exposed on the surface of the Acacia inf lorescence. Sympatric co-flowering Acacia species each show high intraspeci fic synchrony but release their pollen at different rimes of day. Pollinato rs rapidly harvest available pollen and move from one Acacia species to the next, following the daily sequence of pollen release. The activity of shar ed pollinators is structured throughout the day as a result of temporal pat terns of pollen release across Acacia species. The observed temporal struct uring of pollen release is compatible with patterns predicted to result fro m competitive displacement. Additional support or a competition-based expla nation for this patterning comes from the observation that an Acacia specie s flowering without competitors shows no synchronized peak of pollen availa bility at any time of day.