Does pollinator composition and behaviour affect the reproductive success of a rare plant species (Arnica montana)?

Abstract

Through the tight link between plants and their pollinators, plant species often decline when their pollinators decline and vice versa. These nominal declines are well studied for many plant-pollinator networks. Less well known is the effect that the decline of a plant species has on the behaviour of its pollinators, and how potential changes in pollinator behaviour affect the reproductive success of the plant. This study investigates the reproductive success of the plant species Arnica montana (family Asteraceae), the pollinator composition at sites with A. montana, and the pollen loads carried by pollinators at varying population sizes of the plant. Pollinators were caught in Scania, Southern Sweden, and the pollen they carried was identified using a convolutional neural network. To assess plant reproductive success, filled seeds were counted at the end of the flowering season. The results suggest a special role of specific genera of hoverflies and dagger flies for the pollination of A. montana and show that the proportion of these pollinators as visitors increases with increasing population size of the plant. While the average proportion of A. montana pollen in a sample does not seem to increase with increasing population size, more pollen samples contained a specifically high proportion of A. montana pollen in larger populations, possibly suggesting increased floral constancy by the pollinators at larger A. montana population sizes. Both effects started to level out from A. montana population sizes of approximately 100 stems and upwards. Similarly, the reproductive success of A. montana was significantly lower in populations consisting of less than 100-200 stems. Hence, this study finds evidence for a negative impact of changed pollinator composition and behaviour on A. montana reproductive success in small populations and suggests a minimum population size for effective generative reproduction of the plant to be at least 100 stems.

Popular Abstract

Can pollinators make a plant die out faster?

The ongoing biodiversity crisis leads to a lot of species becoming rarer or going extinct completely. Plants and their pollinators depend on each other for their survival – the pollinators rely on the plants for their food, while the plants need the pollinators for their reproduction. But when a plant species becomes rarer, this can affect how its pollinators behave, which in turn may make the pollination of the plant less effective.

If there are fewer plants of a species, they might attract fewer pollinators overall. It might also not be worth it for specialized pollinators – who are especially good at pollinating that specific plant – to visit the plants, since there are not enough resources for them in that location. And, more generalized pollinators might even ignore the rare plants, since they have learned how to collect food effectively from other plants. This behavior is called flower constancy and describes how pollinators often focus on one or a few plant species at a time. So, a rare plant species might be pollinated less effectively, which in turn makes it rarer and therefore leads to a vicious cycle.

To find out more about this possibility, I counted the number of Arnica montana (Mountain Arnica, Slåttergubbe in Swedish) flowers at different locations in Scania. Mountain Arnica has become rare, and the number of flowers in one location varies a lot. At the sites with the plant, I caught pollinators, determined to which species they belonged, and took a sample of the pollen grains they were carrying. With the help of image recognition AI, I then identified to which plant species the pollen grains belonged, so that I could see which pollinators had visited which plants. I also counted how many seeds had formed at the different sites, so how effective pollination was.

What influences how well the plants are pollinated?
After analysing the data, I found out that specific pollinators, especially some hoverfly (Syrphidae) and dagger fly (Empis, Empididae; a genus of fly that looks a little bit like a big, hairy mosquito) species, are especially good at pollinating Mountain Arnica. They carried more of the plant’s pollen and preferred the plant to other plants in the area. I also found that there really were more of these effective pollinators in bigger plant populations, and that there were more pollinators that focused on Mountain Arnica the more plants there were. The plants also produced more seeds in the bigger populations. Most of these effects started when there were around 100 flowers in an area.

According to my findings, different species of pollinators are attracted to smaller and larger plant populations, and the pollinators might also behave differently depending on how many flowers of a plant species are around. It can therefore be very important for the conservation of both plants and pollinators to preserve big plant populations or to manually increase the number of plants at a site if we want to help the plant to survive on its own.

Master’s Degree Project in Biology, 45 credits, 2025
Department of Biology, Lund University

Advisors: Ola Olsson and Magne Friberg
Division of Biodiversity & Evolution

• author: Hecht, Sophie
• supervisor: Ola Olsson; Magne Friberg
• organization: Degree Projects in Biology
• course: BION02 20242
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9185408
• date added to LUP: 2025-02-18 12:12:06
• date last changed: 2025-02-18 12:12:06