Pollinator-dependence and specialization in three sunbird-pollinated, fire-stimulated flowering species

Fires dramatically reduce both floral resources and pollinator abundance, potentially compromising flowering plant reproduction in post-fire landscapes through low pollination rates. Yet, some plant species flower shortly after fires (fire-stimulated flowering (FSF)), which seems a risky strategy given low pollinator abundance after fire. We investigated whether bird-pollinated plants with FSF have low pollinator dependence and generalized pollination systems to ensure reproduction in post-fire landscapes. This study was conducted in three nature reserves in fynbos within the Cape Floristic Region that experienced fires between 6 and 15 months previously. One bird-pollinated species (Watsonia pillansii, Satyrium carneum, or W. fourcadei) dominated at each site, where flower visitor counts were conducted in 15 plots (20 × 20 m), floral traits measured, and pollinator specialization for each species verified using hand pollination and exclusion experiments. The exclusion of bird pollinators from flowers highly reduced fruit set (>70%) for all three species, as well as seed set for the two Watsonia species. Although W. pillansii and S. carneum demonstrated partial self-compatibility, the fruit and seed set of self-pollinated inflorescences was reduced compared to cross- and naturally pollinated inflorescences, highlighting the importance of pollinators in the reproductive success of these species. Furthermore, all three species lack autogamy. Nevertheless, our results suggest that bird pollination, because of reduced competition for pollinator attention, remains the primary reproductive strategy. While these plants demonstrate partial self-compatibility and non-bird pollination, these mechanisms appear to complement rather than replace bird pollination, and may explain the persistence of these mutualisms despite recurring fire disturbances.