Floral density and the facilitation of pollination – an empirical study

Foraging pollinators can be expected to choose patches with high densities of floral resources. As a result, plants in flower-dense patches may receive higher visitation rates and pollination, hence, the pollination of one plant may be facilitated by the presence of neighbour plants. Facilitation of pollination has been predicted by theoretical models and demonstrated in some empirical studies, all dealing with systems containing only two or very few species. The occurrence of facilitation in species-rich natural habitats remains more or less hypothetical, and recent theoretical models have suggested that narrow ground rules may restrict the occurrence of such facilitation in nature. We studied empirically how pollinator abundance, pollinator activity, and visitation rate responded to flower density in species rich semi-natural grasslands, in which the floral resources were manipulated by regulating the timing of grazing. Pollen limitation of fruit set was studied experimentally in two species, Lotus corniculatus and Primula veris, and the pollination effect on fruit production was compared with direct effects of grazing of reproductive organs. Both pollinator abundance and pollinator activity increased with patch flower density, whereas the visitation rate per flower showed a neutral or competitional response to flower density. Hence, no facilitation of pollination was detected in response to the overall flower density. Some species showed, however, a tendency to autofacilitation, i.e. increased visitation rate as a response to con-specific flower density. The fruit set of two experimental species was not pollen limited in any of the grazing regimes, indicating full pollination independent of visitation rate. Fruit production was, however, strongly affected by grazing of reproductive parts, which by far outweighed indirect effects on plant reproduction such as pollination. Based on the results, we discuss prerequisites of the facilitation of pollination and possible environments in nature in which to expect the phenomenon to occur.