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 pollen flow in a community of 10 Acacia species in a highly seasonal savannah habitat in Tanzania. Partitioning is achieved, in part, through separation of flowering in space and seasonal time, and through interspecific differences in pollinator guilds. Nevertheless, coflowering Acacia species shared several pollinators; this means that interspecific pollen transfer Is possible. We analyzed daily patterns of pollinator activity and pollen release in 10 Acacia assemblages containing a total of 10 Acacia species. Pollinator activity was scored using counts at flowers over constant time intervals throughout the day. Fallen availability was assessed using a simple method which allows quantification of pollen exposed on the surface of the Acacia inflorescence. Sympatric co-flowering Acacia species each show high intraspecific synchrony but release their pollen at different rimes of day. Pollinators rapidly harvest available pollen and move from one Acacia species to the next, following the daily sequence of pollen release. The activity of shared pollinators is structured throughout the day as a result of temporal patterns of pollen release across Acacia species. The observed temporal structuring of pollen release is compatible with patterns predicted to result from competitive displacement. Additional support or a competition-based explanation for this patterning comes from the observation that an Acacia species flowering without competitors shows no synchronized peak of pollen availability at any time of day.