Pollinator response to within-patch spatial context determines reproductive output of a giant rosette plant

Abstract

The reproductive output of many plants depends on the interaction between plant spatial pattern and pollinator behaviour. Pollinators tend to concentrate their efforts on patches of flowers offering higher rewards. The spatial relationship of an individual plant to those around it (its spatial context) is also important for its reproductive output. This study examines the effect of patch size and spatial context on the reproductive output of Puya hamata, a hummingbird-pollinated, semelparous, giant rosette plant in the Andes. Hummingbird behavioural response to Puya patch size and the effect of plant density on flowering plant size were assessed. The reproductive output (flower, fruit and seed production, seed viability, germination rates) of plants in relation to patch size and spatial context was determined. Isolated Puya inflorescences were visited by a higher diversity of mostly trap-lining hummingbirds, while plants in aggregations were almost exclusively visited by one territorial species. Spatial context did not affect Puya size at flowering, or the numbers of flowers, fruits and seeds produced. However, with respect to seed viability and germination rates, reproductive output was highest in isolated plants, and plants on the edges of patches, but lowest in plants at the centre of large patches. The effect of spatial context on reproductive output in P. hamata depends on several key conditions being met: the patches are normally made up of closely related plants and the pollinators, by switching to territoriality, restrict gene flow into patches and lower the effective reproductive output of the plants within those patches. Other plant–pollinator relationships meeting these conditions are likely to produce similar outcomes. Such situations are worthy of attention, not just for their ecological interest, but also for their implications for the management of species and genetic diversity.