The importance of trees for bumblebees in semi-natural grasslands

Abstract

Changes in the agricultural landscape have caused declines in bumblebee species richness and abundance. Semi-natural grasslands are key habitats for bumblebees in agricultural landscapes, and therefore it is important to understand which habitat characteristics promote species-rich bumblebee assemblages in these habitats. In this study, I investigate the importance of trees within semi-natural grasslands for providing a high-quality habitat for bumblebees. I calculated the species richness and abundance of bumblebees using a previously collected dataset in 34 semi-natural grasslands in southern Sweden. In addition, I quantified the total tree cover in these grasslands and determined the relevance of food resources provided by trees. I found that the species richness of bumblebees was positively affected by an increasing tree cover, whereas bumblebee abundance was negatively affected by tree cover. Furthermore, common species decreased in abundance with increasing tree cover. The provision of food resources by trees explained the significant effects on species richness, and the cover of non-resource-providing trees explained the variation in bumblebee abundance. In addition, trees had a stronger positive effect on species richness later in the season. Based on my results, I suggest that trees can promote biodiversity within semi-natural grasslands. These findings also suggest that trees provide complementary food resources for certain species, and also underline the importance of semi-natural grasslands in late season. Trees, when implemented correctly, improve habitat quality and benefit bumblebee presence in agricultural habitats.

Popular Abstract

Could trees, besides flowers, also be important for bumblebees?

Bumblebees are important for our society because they pollinate flowers and crops that we depend on. However, these important insects are declining. This means we need to take action to reverse this negative trend, but what? It is well-known that bumblebees live from nectar and pollen that they get from flowering herbs. But at the same time, there are many additional aspects related to the life cycle of bumblebees that is not as well-known. For example, maybe bumblebees do also depend on other food resources than just flowering herbs. In this study I investigated how important trees are as potential food resources for bumblebees in semi-natural grasslands.

As bumblebees and farmland are dependent on each other, both the causes and consequences of the population declines are most visible in this an agricultural landscape. The ongoing loss of habitats and intensification of farming is limiting the recovery of bumblebee populations. One example of a habitat that has not been much affected by these changes, are so-called semi-natural grasslands. Because of extensive land-use, these grasslands are rich in flowers and may therefore be one of the solutions that can improve the bumblebees’ habitat within farmland. To optimally manage these grasslands for the good of bumblebees, it is important to know which food resources are providing the best habitat quality for them.

I explored how much the presence of bumblebees in semi-natural grasslands is affected by flower- and tree-resources. To study this, I surveyed 34 of these semi-natural grasslands in Skåne, the southernmost province of Sweden. Based on data collected over five visits in 2017, I studied which species and how many individuals of bumblebees were present and how flower-rich the grasslands were. In 2019 I observed how many trees were standing in and around the grasslands.

Trees do good and bad for bumblebees
My results showed that tree species which contain pollen- or nectar-resources had a positive effect on the species richness of bumblebees, especially late in the cropping season (early August). However, trees without food resources had a negative effect on the total number of bumblebees present in the grasslands, especially in the middle season (mid-June). Common species of bumblebees did not benefit from tree resources. Flowering herbs had a positive effect on both species richness and abundance of bumblebees in semi-natural grasslands. Therefore, it is possible that trees, in combination with flowering herbs, provide a more diverse and nutrient-rich diet for bumblebee species that have more specific habitat requirements. At the same time, I found that common species were more dependent on resources from flowering herbs, which in turn may be negatively affected by trees that are shading the herbaceous flowers.

This study shows that trees are important for bumblebees. However, I note that the positive effects of trees vary, because also flower cover is important for bumblebees. Although I have only studied the importance of trees in semi-natural grasslands, the resources from trees may also be relevant in other parts of the agricultural landscape and contribute to a higher diversity of bumblebees. This could mean that trees help improve future conservation efforts for pollinators in farmland.

Master’s Degree Project in Biology, 30 credits, 2019, Department of Biology, Lund University

Advisors: Romain Carrié & Johan Ekroos, Centre for Environmental and Climate Research (CEC)

• author: Vervoort, Maarten
• supervisor: Romain Carrié; Johan Ekroos
• organization: Degree Projects in Biology
• course: BIOM02 20191
• year: 2019
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 8997856
• date added to LUP: 2019-11-18 10:47:34
• date last changed: 2019-11-18 10:47:34