Small and surrounded: population size and land use intensity interact to determine reliance on autonomous selfing in a monocarpic plant.

Abstract

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 ('PL'), unless autonomous self-pollination provides reproductive assurance ('RA'). 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 investigate how PL and RA and their potential underlying mechanisms vary with population size and land use intensity surrounding populations in the biennial Sabatia angularis (Gentianaceae). I estimated the capacity for seed production via autonomous self-pollination (i.e. autofertility). Over 2 years in 22 S. angularis populations across a fragmented landscape, I performed emasculation and pollen supplementation experiments measuring RA and PL, and quantified visitation rates of potential pollinators and a pollen consumer, conspecific pollen loads and rates of heterospecific pollen deposition. Autofertility based on fruit mass was 93 % under PL but only 51.6 % relative to maximal conditions. PL and RA were significant on average across populations in the first year of study. Variation in RA was significantly influenced by the interaction between population size and land use intensity, which in turn rendered PL 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. Increased reliance on RA is predicted in small S. angularis populations surrounded by intense development, which can explain elevated selfing rates in fragmented populations of plant species more generally. Results from this study point toward forces such as heterospecific pollen transfer, self-pollen limitation or resource availability influencing the need and ability to rely on RA.