Synchrony of pollinators and plants in the winter rainfall area of South Africa—observations from a drought year

The arid winter rainfall area of South Africa is a centre of bee diversity and endemism. Host plant synchrony is hypothesized as one possible mechanism for an elevated rate of speciation in desert bees, where rainfall is considered to initiate for the emergence especially of oligolectic (pollen specialist) bees. In Namaqualand in 2003, a drought year, the absence of early precipitation postponed the main flowering season by five weeks to early/mid September. Despite the lack of rain and flowers bee activity was already very high in August as recorded with colour trapping. Five highly specialised species of Rediviva (Melittidae) as well as many other bee species had emerged before the flowering season had started. The majority of bees were probably not nesting as none of the collected individuals carried pollen. Therefore failure in reproduction can be assumed. These observations show that bees in Namaqualand are not synchronised with their host plants and that rainfall might not be an abiotic cue for their emergence which contradicts current hypotheses. Usually, precipitation as well as flowering season in Namaqualand is fairly predictable. Thus it seems that evolutionary processes did not lead to adaptation of (oligolectic) bees to their host plants as one would expect in a xeric environment.