Pollinator sharing and low pollen–ovule ratio diminish reproductive success in two sympatric species of Portulaca (Portulacaceae)

We studied the breeding systems, phenology, pollination, and reproductive success of Portulaca grandiflora and P. hirsutissima (Portulacaceae) at Parque Nacional do Catimbau in the Brazilian Caatinga. The species flowered synchronously, exhibited similar floral morphology, offered pollen as the sole floral resource and shared the same bee species as pollinators. They visited flowers of both Portulaca species at similar frequencies, and their visitation peaks occurred at the same time of day. The pollinators made no distinction between the species during foraging. Both plant species are self-compatible, but produce fewer fruits and seeds after self-pollination than after cross-pollination. Comparing the reproductive success in areas where both species co-occur to that of areas with only one of them, seed set of P. grandiflora decreased by 25% and of P. hirsutissima by 36%. This is likely due to diminished flow of conspecific pollen grains, as the pollen-ovule ratio was extraordinarily small.