Geographic and temporal associations between floral morphology and pollinators in Wachendorfia paniculata: a moving target

Geographic variation in floral dimensions has often been associated with co-variation in pollinator dimensions, giving rise to plant ecotypes that are finely matched to the morphology and sensory modalities of local pollinators. Wachendorfia paniculata is a geographically variable, handed flower with preliminary observations suggesting that differences in pollinator wingspans between populations might be driving differences in the spatial separation between floral reproductive structures across the Western Cape, South Africa. We investigated the extent of this geographic variation and its relationship to local pollinator types across 15 W. paniculata populations. The distances between reproductive parts varied significantly between populations, but we also found that the distances narrowed with time. Contrary to expectation, large pollinators appeared unimportant in the extant pollinator landscape. Even though the mean weighted wingspan of pollinators in a population did not predict the degree of reproductive separation, increasing pollinator visitation rates were associated with a narrowing of the distance between the lower anthers. This trend, which was driven primarily by honey bee visitation rates, suggests that frequent visits by small pollinators might select for a narrower distance between lower anthers. The narrowing of the horizontal distances between floral reproductive parts over time explained a significant proportion of the variation in the degree of reproductive separation when locality was included in the statistical model. We conclude that the variation in the degree of reproductive separation cannot be explained by one variable alone, but rather that various biotic and abiotic selective pressures are likely at play.