Facilitation and competition shape a geographical mosaic of flower colour polymorphisms

Abstract

Abstract Flower colour differs dramatically between populations for some plant species; yet, we know little about what drives this variation. Such polymorphisms can be influenced by plant–pollinator interactions, but whether they are also influenced by pollinator‐mediated plant–plant interactions is unknown. We test whether flower colour polymorphisms can arise through convergence (facilitation) or divergence (competition) of flower phenotypes resulting from plant–plant interactions mediated by the shared, and only, pollinator (orange‐breasted sunbird) of 10 Erica communities in South Africa. Sunbird visitation rates to the less‐preferred Erica species in communities increased with colour similarity to the most‐visited species, suggesting that polymorphisms can be maintained by selection for colour convergence within communities, which promotes shared signals through rewarding mimicry (i.e. facilitation). Colour similarity was lowest when risk of reproductive interference was greatest: that is, when reproductive morphology (stigma–anther distance) and flowering phenology was most similar, and when floral density differed greatly between species. This implies that polymorphisms can also be maintained by selection for colour divergence, since this promotes assortative pollinator foraging (i.e. competition). Different populations of a species may experience opposite selective pressures, depending on their community context. This is the first evidence that a geographic mosaic of plant–plant pollination interactions could maintain colour polymorphisms in congeneric species sharing a single pollinator. A free Plain Language Summary can be found within the Supporting Information of this article.