How has land-use and climate affected phenology, thorax-width and proportional abundance of Bombus terrestris and Bombus lucorum in southern Sweden?

Abstract

Bumblebees and other pollinators have recently experienced population and diversity declines linked to habitat loss and fragmentation due to land use change. A previous study in Sweden showed a big shift in bumblebee community composition, where the Bombus terrestris complex (B. terrestris, Bombus lucorum) highly increased in relative abundance, but the relative contribution of the two different species in this complex has not yet been assessed. Because species differ in life history traits, they are differently robust to land-use change. For example, a long foraging distance, which is often related to a large thorax width, can be suitable for bumblebees in intensively cultivated landscapes where forage resources are often scarce and unevenly distributed. In the near future, climate change is expected to become a major driver of change, affecting for example the phenology and distribution of bumblebee species and to some extent, these changes have already begun. Wild bees have become active earlier in the season, likely as a response to rising temperatures and earlier spring emergence, but the extent of this response may differ between species and regions. It was therefore interesting to assess the two dominant species of Bombus terrestris complex to see if their proportional abundances, phenology and thorax width have changed over time and whether this differs between landscape types, possibly because of land-use and climate change.

I studied males and queens of B. terrestris (1177 males, 183 queens) and Bombus lucorum (719 males, 42 queens) specimens from the Biological Museum at Lund University that cover the period from 1871 to 2009. I collected information from their labels and measured the distance between the wing-bases of bumblebees, which is called inter-tegular distance (ITD), to answer the following main questions: 1) has the phenology of these bumblebees changed over this time period? 2) has the thorax-width (as an estimate of maximum foraging distance) of these bumblebees changed over this time period? 3) has the proportional abundance of the two species changed? In addition, I tested if any of these potential changes differed between intensively cultivated landscapes of the southern and coastal parts of Scania (the plains) and mixed landscapes of central Scania.

The thorax-width of B. lucorum males was larger in intensively cultivated plains than in mixed landscapes, while the thorax width of B. terrestris males decreased over time, independently of landscape context. Finally, I found that males of both species emerges earlier by the time and that there was a change in proportional abundance affected by time, but it was not affected by landscape context. Due to the small sample size of the queens of B. terrestris and B. lucorum I couldn't draw definitive conclusions, but I did not find evidence of earlier emergence for queens of either species. There was no significant difference in proportional queen abundance between the two species across landscape contexts or time. In contrast to my prediction for thorax width, B. terrestris queens showed a decrease over time, while the queens of B. lucorum did not show any change. While I cannot draw any firm conclusions about differences between the castes due to the low number of queens, the data I collected on males suggests that there are important differences between how the population size and thorax-width co-vary with land-use intensity and time.

Popular Abstract

Bigger size better life and more chances

Everyone knows how important it is to have food to eat and a place two live, not only for humans but also for animals. Modern agriculture has made life easier for humans but at the same time, it made life much more difficult for some animals. Recently, bumblebees declined because of modern agriculture and humans are the loser in this case because the bumblebees have an important role in increasing plant fruit and seed. Where in the modern agricultural landscape, there are fewer flowers and the distances between suitable nest sites and flower-rich patches are often long.
Still, some species of bumblebee that have big thorax size doing well in the modern agricultural landscape. Their large thorax size means strong flight muscles which give them a better chance to find and reach flower patches that far away from their nests.
For this reason, I expected that the thorax size of my studied species, Bombus terrestris and Bombus lucorum, has become larger by the time to cope with the modern agricultural landscape.
My main question was: has the thorax size of B. terrestris and B. lucorum changed over the period from 1871 to 2009 and does this change differ between large-scale (super modern) and small scale (more traditional) agricultural landscapes. I also wanted to know which one of the two species doing better in the modern landscape.

Since global warming is expected to be one of the threats for the bumblebees in the future, I was also interested to see if B. terrestris and B. lucorum become active earlier in the season to cope with rising temperatures.
I studied 1177 males and 183 queens of B. terrestris and 719 males and 42 queens of B. lucorum specimens from the Biological Museum at Lund University. These specimens had all been collected in Scania in southern Sweden. The picture below shows the used tools (microscope and digital caliper) to measure the thorax size.

The result showed that males (but not queens, possibly due to data deficiency) of both species have become active earlier in the season over time. That could be because of climate change or the food deficiency force them to reproduce males earlier in the season. There were larger individuals of B. lucorum in modern landscapes than the less modern landscapes, but there was no difference for B. terrestris. Maybe because B. terrestris already has a big thorax size, so it doesn’t show a difference between modern and less modern landscape. Also, B. terrestris was more common than B. lucorum in modern landscapes but the thorax size of B. terrestris decreased over the study period and which could be caused by other factors that I did not cover in my study.
Accordingly, I suggest that if we enhance the flower resources in the modern landscape where we can take advantage of the bumblebees and the modern methods of agriculture. To do this, I suggest planting high-quality flowering plants in suitable distances where bumblebee reaches it easily and taking into account the diversity of used plants with different flowering periods to guarantee the food resource for the whole activity period of the bumblebee.

Master’s Degree Project in Biology/Conservation Biology/45credits 2019-2020
Department of Biology, Lund University

Supervisor: Lina Herbertson ([e-mail])
Co-supervisors: Anna Persson ([e-mail])
Karin Johnson ([e-mail])
Rune Bygebjerg ([e-mail])

• author: Khalaf, Reem
• supervisor: Lina Herbertsson; Anna Persson; Karin Johnson
• organization: Degree Projects in Biology
• course: BION02 20191
• year: 2020
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9002897
• date added to LUP: 2020-01-24 15:38:10
• date last changed: 2020-01-24 15:38:10