Habitat fragmentation has transformed landscapes globally, leaving remnants embedded within a complex matrix that is rapidly becoming more developed. For many plant populations, the associated factors of decreased size and intensification of land-use surrounding them are expected to increase pollen limitation, unless autonomous self-pollination provides reproductive assurance. Decreased pollinator visitation is often assumed to drive these patterns, but other, less studied mechanisms might include increased heterospecific pollen transfer or decreased conspecific pollen availability via florivory. I investigated how pollen limitation, reproductive assurance, and their potential drivers vary with population size and land-use intensity surrounding populations of the biennial Sabatia angularis (Gentianaceae) across a fragmented landscape. In wild S. angularis populations and in controlled growth conditions, I estimated the relative capacity for seed production via autonomous self-pollination (‘autofertility’). In 22 wild populations over two years, I performed pollen supplementation and emasculation experiments measuring pollen limitation and reproductive assurance and quantified visitation rates of potential pollinators and a pollen consumer, conspecific pollen loads, and rates of heterospecific pollen deposition.
Results/Conclusions
The potential for reproductive assurance via autonomous self-pollination, i.e. autofertility, was 93% under natural, pollen-limited conditions, but only 51.6% relative to controlled (maximal) conditions. Pollen limitation and reproductive assurance were significant on average across populations in the first year of study. Variation in reproductive assurance across populations was significantly influenced by the interaction between population size and land-use intensity, which in turn rendered pollen limitation independent of these factors. Visitation and heterospecific pollen deposition rates were greatest in small populations and declined with population size, while conspecific pollen loads were greatest in intermediate sized populations. Ultimately, this study demonstrates that increased reliance on reproductive assurance is predicted where intense development surrounds small S. angularis populations, which can explain elevated selfing rates in fragmented populations of plant species more generally. Results from this study further point toward forces such as heterospecific pollen transfer, self-pollen limitation, and resource availability influencing the need and ability to rely on reproductive assurance.