How pollinator decline affect plant-plant interactions for pollinator is studied in the Early View article ‘Experimental reduction of pollinator visitation modifies plant-plant interactions for pollination’ by Amparo Lázaro and co-workers.
Several studies have indicated a widespread pollinator decline, caused mainly by land-use changes, degradation of natural habitats, fragmentation and habitat loss. Since the majority of plant species are dependent on animal pollination for reproduction, pollinator decline may influence plant reproduction and the persistence of plant populations. However, a pollinator decline may also affect the way plants interact for pollination because these interactions depend on the abundance of plants and pollinators in the community.
To simulate a pollinator decline we set up a novel experiment to reduce pollinator visitation in two communities (one lowland and one alpine) in Southern Norway (see also Lundgren et al. 2013). In the experiment we compared control plots with plots where pollinator visitation had been reduced by means of dome-shaped cages constructed by bending two PVC-tubes diagonally and covering them with fishnet. The fishnet was totally transparent, so flowers were fully visible from outside the net. In order to allow flower visitors inside cages to exit easily, we left an opening between the mesh and the ground, and another opening in the top of the dome. This experiment effectively reduced pollinator visitation without modifying the composition or behaviour of pollinators, or other important biotic and abiotic variables.
Lázaro et al. (2014) shows that the reduction in pollinators modified plant-plant interactions for pollination in all the six species studied; although for two of them these interactions did not affect seed set. Pollen limitation and seed set data showed that the reduction of pollinator visits most frequently resulted in novel and/or stronger interactions between plants in the experimental plots that did not occur in the controls. Although the responses were species-specific, there was a tendency for increasing facilitative interactions with conspecific neighbours in experimental plots where pollinator availability was reduced. Heterospecifics only influenced pollination and fecundity in species in the alpine community and in the experimental plots, where they competed with the focal species for pollination. The patterns observed for visitation rates differed from those for fecundity, with more significant interactions between plants in the controls in both communities. This study warns against the exclusive use of visitation data to interpret plant-plant interactions for pollination, and helps to understand how plant aggregations may buffer or intensify the effects of a pollinator loss on plant fitness.