Floral neighbourhood effects on pollination success in red clover are scale-dependent

Plant-pollinator interactions are highly important because of its direct link to plant fitness and because such interactions involve species at different trophic levels. Our understanding of competitive and facilitative interactions among plants for pollination is of large importance to govern conservation of species and management of crop production. Still we know little about the scale dependency of plant-plant interactions for pollinator visitation which is central to both basic and applied ecology. Here, I tested whether plant-plant interactions for bumblebee visitation and subsequent reproduction were scale-dependent by studying the ecologically and economically important red clover (Trifolium pratense) and its floral neighbourhood. I expected that high floral densities at small scale could trigger facilitative interactions due to concentration effects of pollinators. This was expected to change into competitive interactions when floral densities became higher at coarser scales because of dilution effects. I also expected that increased bumblebee visitation would increase fruit production in red clover. The interaction for pollination success in red clover appeared to be scale-dependent in the landscape under study. At plot scale, the bumblebee visitation rate to red clover inflorescences increased with both intra- and interspecific floral density, indicating facilitative interaction for pollinator visitation among red clover flowers and its nearest floral neighbourhood. Increased visitation of bumblebees led to subsequent increased fruit set, but there was a saturating effect as fruit set did not increase when visitation rates exceeded a certain level. As the floral neighbourhood became denser at a coarser scale, the interactions for pollinator visitation changed towards competition. This study shows that key ecological processes such as plant-pollinator interactions may indeed be scale-dependent and that the relative importance of competition and facilitation among plants for attracting pollinators may change with scale. The density of the floral neighbourhood of a focal species (e.g. key-, rare-, crop species) may thus be of large importance for its pollination and fruit production. My findings suggest that increasing floral densities at fine scales may have facilitative effects on pollinator visitation to red clover, whereas pollinators may be drawn away when floral resources become denser at coarser scale.