A central theme of ecology is understanding how species coexist in an environment. A traditional explanation is that species partition the available resources by space or time, a process called niche partitioning. Different bees, for example, specialise on foraging on specific flower species, and some are physically adapted to do so. However, this specialisation does not provide a complete explanation, as coexisting bee species often overlap in their choice of flowers. Suggestions of complementary or alternative partitioning mechanisms include the selection of microhabitats based on temperature, light and wind conditions. Without a more thorough explanation of the mechanisms that underpin local coexistence, we may struggle to understand the factors behind the ongoing loss of bee diversity. In work recently published in Proceedings of the Royal Society B, researchers from Lund University, Stockholm University and Queen's University Belfast investigated whether differences in vision allow bumblebee species to forage in different light conditions. Their results show that bumblebees with a higher sensitivity to light forage in dimmer conditions. This study demonstrates the importance of including sensory traits in studies of pollinator habitats.
In a natural ecosystem, multiple species coexist. One of the ways in which they can reduce competition is to use the environment differently, a process called niche partitioning. Different species may focus on different food sources, for example, or nest in distinct parts of trees. In the case of bees, niche partitioning would suggest that multiple bee species living in the same area would forage from different flowers. However, bumblebee species have overlapping diets, so how do they avoid intense competition for food resources?
One possible explanation is that the bees select spatial or temporal microhabitats depending on temperature, light and wind conditions based on their physical traits. According to the microhabitat hypothesis, bees could avoid competition by exploiting flower resources at different times of day, for example. There are even some species of bees known to forage at night.
This type of visual niche partitioning is seen in several insect groups, including tropical butterflies, damselflies and fruit flies, and previous research has demonstrated that it is associated with visual traits. As bumblebees mainly use vision to find flowers and control their flights, it is possible that visual niche partitioning could help to explain the coexistence of different species. In this study, a team of researchers tested the hypothesis that bumblebee species demonstrate light-related niche separation and that this separation was related to differences in their visual traits (for example, bees that foraged in dim light had eyes with higher light sensitivity than those that forage in brighter light).
A Swedish forest in springtime gave them the ideal environment to test their hypothesis, as it offered bumblebees a single food source - bilberry flowers - in a location with varying light levels. The bilberry (Vaccinium myrtillus) is a primary food source for multiple bumblebee species. The structure of the forest creates sharp differences in light conditions, which also vary over the course of the day.
The researchers analysed the community composition of bumblebees in the forest, recording light intensity and temperature for each bee observation. They then linked the observations to key physical traits for each species, including body size (which influences thermoregulation and eye size).
The key to exploring whether bumblebee vision affects their foraging patterns lies in the eye parameter, which reflects the trade-off between resolution and light sensitivity associated with compound eyes. Eyes adapted for high resolution have a low eye parameter, while a high value suggests improved light sensitivity and better vision in low light conditions. Previous research has shown a connection between the eye parameter for bumblebee species and their habitat preferences.
At the Diamond-Manchester Imaging Beamline I13-2, the researchers used X-ray micro-CT to scan sample bumblebee eyes, performing volumetric and computational analyses. By scanning at I13-2, the researchers were able to not only rapidly acquire data from many specimens but the images they obtained enabled them to reconstruct 3D volumes of the bee eyes in the high-resolution detail required for their analyses.
As lead author Dr Océane Bartholomée explains:
These visual traits are hard to measure, and using micro-CT at Diamond allowed us to carry out a much deeper study than we would have been able to do if we could only examine more basic traits, such as body size.
By combining community composition analyses and trait measurements, the team showed that bumblebee species niche partition according to the light conditions. As light intensity increased, so did the abundance of bumblebee species with a low eye parameter (that is, eyes that are better adapted to bright light). In contrast, the abundance of bumblebee species with a high eye parameter (eyes that are well adapted to dim light vision) peaked at intermediate light levels or even decreased with light.
The results of this study are the first to show that sensory traits are important in understanding how bees use their environment, and that microhabitat niche partitioning related to sensory trait expression may well be one of the mechanisms underpinning species coexistence.
Dr Bartholomée says:
This technique of using sensory traits is quite new, and until recently, we have focused on animal size and diet, and the number of offspring produced each year, to gauge how they might respond to global changes. But we're starting to see more studies on how noise pollution affects animals, and light pollution, and a more thorough understanding of how sensory traits affect animal behaviour can only aid conservation efforts.
To find out more about the I13-2 beamline or discuss potential applications, please contact Principal Beamline Scientist Christoph Rau: [email protected].
Océane Bartholomée, Ciara Dwyer, Pierre Tichit, Paul Caplat, Emily Baird† and Henrik G. Smith, Shining a light on species coexistence: visual traits drive bumblebee communities, Royal Society (2023) DOI: 10.1098/rspb.2022.2548
Read more about bee eye research at Diamond:
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