Bee declines: what’s the stress all about?

This blog post is provided by Aoife Cantwell-Jones and tells the #StoryBehindThePaper for the paper “Signatures of increasing environmental stress in bumblebee wings over the past century: Insights from museum specimens”, which was recently published in the Journal of Animal Ecology. The authors looked at how bumblebee wing asymmetry has changed over the 20th century. They found that wing asymmetry started increasing around 1925 and that warmer and wetter years coincided with higher wing asymmetry.

Museums are special places. As a child I loved peering through display cases at rows of carefully preserved animals showcasing the diversity of life. These curated displays are windows into the past, providing a sneak-peek into Earth’s history. My favourites though were always the pinned insects – their intricate wings glinting with reflected light and little labels next to them describing where and when that insect had been found. I never imagined however how much went on behind the scenes at museums or how much biological data these specimens could contain.

I got my first glimpse of this during my master’s degree at Imperial College London, where I had an amazing opportunity to join a collaboration between my supervisors Drs Richard Gill and Andres Arce and a network of UK museums. This network included some of the UK “museum greats” – the Natural History Museum (London), National Museums Scotland (Edinburgh), Oxford University Museum of Natural History, Tullie House Museum and Art Gallery Trust (Carlisle), and World Museum (Liverpool). This collaboration focused on bumblebees – important insect pollinators – to better understand how human activities are driving their long-term trends.

Figure 1: Pinned bumblebee (Bombus muscorum) specimens from National Museums Scotland (photo: Andres Arce).

Bumblebees are facing a number of threats, and some species are showing evidence of declines. While a number of factors are prime suspects of driving declines, including land-use change, pesticide use, spread of invasive species and bee diseases, and climate change, how populations are responding is less well known. One challenge we face in disentangling and quantifying the cause(s) of declines is that a bee can be experiencing many of these potential stressors at the same time. Another problem is that the impact of these stressors on bee populations may not be immediately visible. For example, the effect of climate one year might not show until many years later.

One solution is to use some measure of stress that causes an immediate and permanent change to the individual experiencing it. Just like humans accumulate scars through hard times, bumblebee wings are thought to become asymmetrical if they experienced stressful conditions during development (also known as fluctuating asymmetry). We therefore speculated that we should see changes in bumblebee wing asymmetry over the 20th century, as humans have had an increasingly wide impact on the natural environment.

We therefore painstakingly photographed and measured the wings of bumblebee museum specimens from around the UK over the 20th century. This involved many hours of staring at computer screens trying to delimit bumblebee wing shapes or type up the information in the specimen labels. We chose to focus on four species, representing both relative “winner” and “loser” species, and we looked at how wing asymmetry changed between 1900 and 2000. Additionally, to start to investigate possible causes, we also looked at which climate conditions correlated with higher wing asymmetry.

Figure 2: Pinned bumblebee specimen (Bombus lapidarius) (photo: Andres Arce).

Whilst we found a lot of variation in the degree of symmetry across specimens, an overall pattern emerged. On average, bumblebees at the end of the 20th century had less symmetrical wings. Intriguingly, wing asymmetry did not seem to just increase linearly between 1900 and 2000, but rather the increase seemed to start around 1925. We also found that warmer and wetter years coincided with higher wing asymmetry.

While we could only look for correlations with our data, the trend of higher asymmetry under warmer years is disquieting, given these years should increase in frequency with climate change. These insights into past stress are however just the beginning for this project. Wing asymmetry is a potential proxy of stress, and more work is currently being done to look at morphological and genomic changes. Progress is also being made to associate other drivers with such responses. This should give a more mechanistic insight into why some bumblebee species have been more vulnerable to change over the past century.

Museums will always have a special place in my heart, not only for their role in inspiring young scientists through their beautiful collections, but also for the research they enable, including revealing the past century of stress for bumblebees.

Read the paper

Arce, A. N., Cantwell-Jones, A., Tansley, M., Barnes, I., Brace, S., Mullin, V. E., Notton, D., Ollerton, J., Eatough, E., Rhodes, M. W., Bian, X., Hogan, J., Hunter, T., Jackson, S., Whiffin, A., Blagoderov, V., Broad, G., Judd, S., Kokkini, P. … Gill, R. J. (2022). Signatures of increasing environmental stress in bumblebee wings over the past century: Insights from museum specimens. Journal of Animal Ecology, 00, 1–13. https://doi.org/10.1111/1365-2656.13788

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