Landscape context and plant population size affect morph frequencies in heterostylous Primula veris—Results of a nationwide citizen-science campaign

Abstract

Heterostyly is a genetically determined floral polymorphism of style length promoting outcrossing between individuals of different morphs, which usually coexist within populations at equal frequencies. Loss in the area and connectivity of suitable habitats may cause deviations from the expected equal morph frequencies. However, there is a need to evaluate the generality of this pattern at larger spatial extents and to identify possible underlying mechanisms. A citizen-science approach was used to study morph frequencies in populations of the heterostylous grassland plant Primula veris across Estonia. We developed an online platform to facilitate an easy upload of the data. We examined the effect of the following variables in the surroundings of the study populations reflecting the landscape context on the deviation of morph ratios: (a) semi-natural grasslands, (b) forests and shrubs, (c) human population density and (d) a proxy for plant population size. The citizen-science approach provided unprecedented density of data from 1,700 localities. Nearly half of these observations, which were maintained for further analysis after data filtering, included over 62,000 short-styled morphs and about 54,000 long-styled morphs. Small populations were characterized by higher overall deviation of morph ratios from isoplethy (equal morph ratio). Deviation increased in semi-natural grasslands located in regions with high human population density. The significant effect of human population density and plant population size on deviations of morph frequencies suggests the role of stochastic demographic effects of habitat fragmentation on morph ratios. Overall lower proportion of long-styled morphs indicates that partial intra-morph compatibility shown in long-styled morphs may lead to higher inbreeding and related decline in fitness and abundance. Synthesis. Citizen-science data about the morph type of Primula veris across Estonia obtained with the help of thousands of people demonstrates that in addition to plant population size, landscape context may affect plant reproductive traits, such as heterostyly. Larger population size of P. veris can help to buffer against random fluctuations in this trait. Increasing impact of human activities may have a negative impact on both small and large populations. The exact underlying mechanisms of the prevalence of one morph over the other, however, pose novel questions for further research.