New publication – Landscape context and plant population size affect morph frequencies in heterostylous Primula veris ‐ results of a nationwide citizen‐science campaign

Text and photos by Tsipe Aavik

Citizen scientists bring surprising insights into cowslip mating system

About half of the individuals of cowslip (Primula veris) have flowers with a short style, while the other half of individuals produce flowers with a long style. Recent discoveries have suggested that the loss and fragmentation of habitats may shake this optimal balance of morphologically different plants. This, in turn, decreases the reproductive success of plants and jeopardizes their future viability.

As the primary habitat, cowslips prefer semi-natural grasslands, which have experienced a dramatic area loss over the last hundred years throughout Europe. The study coordinated by the ecologists of the University of Tartu and the Estonian Fund for Nature aimed to examine whether this drastic landscape change has led to deviations in morph balance. To collect data across Estonia, they decided to implement a citizen science approach. A specifically designed web platform facilitated an easy upload of data. “In addition, the task and importance of the cowslip observation campaign were explained in detailed guidelines, numerous videos, social media, and other communication platforms,” commented Tsipe Aavik, the lead researcher of the heterostyly project.

Cowslip is a distylous species with two morphologically different types of flowers. Plants with short-styled or S-morphs (photo A; sometimes referred to as thrum plants) carry flowers with a short style and long anthers, while long-styled L-morphs (or pin plants; photo B) have a long style and short anthers. Generally, successful fertilization takes place only in case of reciprocal pollen flow between different morphs, while within-morph crosses lead to no progenies.

Novel insight into fundamental research

Nearly 1,700 observations obtained during the campaign led to unexpected discoveries, is now published in the Journal of Ecology. First, data revealed a systematic dominance of short-styled morphs over long-styled morphs. Second, morph frequencies were more likely to deviate in smaller populations and deviations increased in landscapes with higher human population density.

“We are very thankful to all participants who helped to collect heterostyly data at such an unprecedented scale,” commented Tsipe Aavik. “Although the first discoveries on heterostyly were made already by Darwin more than 150 years ago, data obtained in the citizen science campaign has helped us to add novel aspects into this fascinating topic with a long history of research.”

The findings brought novel insights into fundamental research: the systematic dominance of the short-styled morphs is an intriguing but previously undescribed aspect in this otherwise well-studied plant mating system. However, perhaps even more important are the implications of these findings for conservation because deviating morph frequencies are likely to jeopardize the viability of heterostylous plants. The findings thus describe another threat in the list of negative consequences of habitat loss.

Perhaps Europe could be looking for cowslips one day

The surprising findings of the study have seeded an idea to widen the geographic scope of the study to examine whether the observed patterns are confirmed in other European countries. Furthermore, the discovery about the role of human population density altering morph balance encourages to look at the patterns of heterostyly in landscapes with more intense human impact than Estonia with its relatively low human population density. “In the spring of 2020, when we repeated the campaign, Latvian citizen scientists warmly welcomed the opportunity to contribute to heterostyly observations. But perhaps one day we all can participate in a project ‘Europe is looking for cowslips’,” adds Aavik with a hope that there still are places in Europe where one can find this beautiful and intriguing plant, cowslip.

Citation: Aavik, T., Carmona, C. P., Träger, S., Kaldra, M., Reinula, I., Conti, E., … & Kaisel, M. Landscape context and plant population size affect morph frequencies in heterostylous Primula veris‐results of a nationwide citizen‐science campaign. Journal of Ecology,


  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.

This post was originally published in EurekAlert (link)

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