The consequences of specialization for pollination in a rare South African shrub,Ixianthes retzioides (Scrophulariaceae)

Ixianthes retzioides is a rare South African shrub with large yellow oil-secreting flowers. Populations studied previously revealed low levels of reproductive success and the absence of a specialized oil-collecting pollinator. We report the discovery of a population in which the predicted specialist pollinator,Rediviva gigas, is present. The presence of this oil-collecting bee resulted in a mean fruit set per plant of 97.2 ± 8.3% (N = 9) and a mean seed set of 331.6 ± 162.6 (N = 52). This represents a 7.4-fold increase in fruit set and a 7.8-fold increase in seed set over a previously studied population lacking the specialist pollinator. The contrast between populations with and without the specialist pollinator dramatically illustrates the potential benefits and costs of specialization for pollination, respectively.