Pollinator facilitation between florally contrasting congeners scales up to regional co-occurrence patterns

Abstract

Plant–plant facilitation is pervasive in nature and can have community-wide consequences directly through amelioration of abiotic stress or indirectly through shared mutualists. Despite being widely demonstrated within local communities, indirect facilitation via pollinators varies in its impact on plant fitness and has rarely been tested for its role in structuring communities at larger spatial scales. We hypothesized that pollinator facilitation among closely related but florally contrasting plant species can enhance the probability of their regional co-occurrence. We tested for local pollinator facilitation with field observations and common garden experiments between two Bidens spp. (Asteraceae) with contrasting flowers. Whereas B. cernua has a showy sunflower-like inflorescence (radiate inflorescence), B. frondosa has evolutionarily lost its ray florets (discoid inflorescence) and is comparatively inconspicuous in visual display. We also broadly sampled (co-)occurrence patterns of these and other Bidens spp. across New York State (USA). The presence of showy B. cernua increased floral visitation to discoid B. frondosa in mixed natural populations and common garden experiments, but only increased seed set of B. frondosa in natural populations. We found little evidence for a cost of pollinator facilitation for the facilitating species B. cernua. Both species showed enhanced seed set from conspecific pollen supplementation but no cost of heterospecific pollen deposition, suggesting an overall benefit of increased pollinator visitation. Across New York State, B. cernua and B. frondosa co-occurred more often than expected by chance. Moreover, frequent co-occurrence between radiate and discoid Bidens was general across multiple species in the genus. Despite the overall pattern of Bidens spp. to spatially disaggregate, likely due to competition, radiate Bidens species strongly aggregated with discoid Bidens species more often than expected. Synthesis. Plants with a showy floral display conferred pollinator facilitation to a congener that lost that display, and display trait contrast predicted species co-occurrence across the landscape. Our study underscores the potential for positive interactions to reduce competitive displacement among close relatives and impact species distributions. We propose the novel hypothesis that congener facilitation may be a general ecological mechanism that serves to compensate for the evolutionary loss of an adaptive trait.