The bigger the better? Comparing urban and rural wild bees

Abstract

Insect pollinators, such as bees, provides us with important ecosystem services. Unfortunately, a decline in key pollinators has been seen globally. Habitat loss and fragmentation, e.g, caused by urbanization, have been pointed out as important driving factors of pollinator declines. However, studies have also shown that species richness of pollinators can be greater in urban than in rural habitats. A previous study done in Skåne, Sweden, showed that adult individuals of bumblebees were larger in more complex semi-natural landscapes, with a higher flower availability. In this study the body size of the bumblebee Bombus terrestris and the solitary bee Lasioglossum morio collected in urban areas and in rural farmland was measured via the inter-tegular distance (ITD), hypothesising that areas with larger food availability would generate larger individuals of bees. The results showed that individuals of B. terrestris were significantly larger in urban areas, and largest in the densest urban areas, while there was no difference in size between urban and rural L. morio. However, the results showed either no or a negative correlation between ITD and species richness of flowering plants in the gardens where the bees where captured. Furthermore, there was no correlation between vegetation cover of the surrounding area and ITD. The effects on body size is likely due to B. terrestris having a large foraging range than L. morio, in combination with the urban areas being less permeable leading to only larger individuals being able to utilize beneficial patches, such as backyards, in the city. This study is not able to answer the interesting question of whether this difference in ITD between rural and urban B. terrestris is a genetic response to the landscape or a result of phenotypic plasticity. The results show that even though an urbanized landscape can be highly fragmented and hard to access for bees, it can provide good quality habitats for certain pollinators. As our urban areas grow larger and denser to accommodate a growing urban human population, creating pollinator-friendly cities with easily accessible good quality habitats should be included in future strategies to conserve urban pollinators.

Popular Abstract

In the last 60 years, humans have changed the environment around us drastically, because we have drastically changed the way we live. From a landscape filled with small scale farms, each with its own agricultural fields and pastures, to a landscape of cities surrounded by large agricultural fields growing just a single crop. In 2018 the urban population of the world reached 4,2 billion people, and that number is projected to increase to 6,7 billion people by 2050. To accommodate this growing urban population, cities need to expand. This means more housing, more roads and more people in one small area. Urbanization has long been thought to have negative effects on animals and plants. When building our houses, we destroy their natural habitats. We take away both their food and nesting sites. When building roads, we make obstacle’s which animals have difficulty crossing, making their habitats smaller and smaller. But what is we could figure out a way to expand our cities and benefit biological life at the same time? What if we could build cities where both humans and animals can thrive?

Pollinators such as butterflies, bees, moths and flies, are not only beautiful too look at, or perhaps sometimes annoying when buzzing around your room at night. They are extremely important in providing pollination services. Without them, we would not have all of the food that we have today. But pollinators, just as many other animal groups, are disappearing all over the world. Their decrees in numbers are often associated with destruction of their habitats, for instance urbanization. At the same time, studies have shown that many pollinators thrive in some cities. With their private gardens and public parks, cities have the potential to provide pollinators with both nesting places and food. I wanted to investigate how well pollinators are doing in the city of Malmö. Just as with humans, the size of a bee is correlated with the amount of food that they get. The more food available, the larger the bee. In Skåne, the farmland is often very simple, with large agricultural fields growing only one type of crop that will flower only once during the season. This leads to a landscape poor in food for many pollinators. In the city, houses, and gardens are located closer together. And between man-made structures such as houses and roads there are parks with a diversity of different flowers, that are flowering throughout the entire season. Could it be that the city of Malmö could be better suited for pollinators than the surrounding rural area?

To answer this, I measured the size of almost 400 bees, the larger bumblebee Bombus terrestris and the much smaller solitary bee Lassioglossum morio, from gardens both within the city and gardens in the surrounding farmland. What I found was that bumblebees were in fact larger in the city. But interestingly, and contradictory to what I believed, they were not bigger because of more food being available in the city. In fact, there was actually a negative correlation between food availability and bee size. Meaning that bees were largest in the densest populated areas of the city. Throwing the original hypothesis out the door, I came to the conclusion that bees are not doing better in the city, but rather that the food available for bees in the city is difficult to reach. Think of the city as a maze if you will. In the middle is a large table with all the food you can imagine. But to get to the food you must first make it through the maze. The larger you are, the more energy you have on reserve to make the trip towards the middle. If you are really big, perhaps you can even jump over the hedges to go straight to the food. It’s the same way for bumblebees in the city. Only the biggest individuals are able to fly through the maze which is the city and find the gardens and parks in which their food is located. However, this study shows that it is possible for bees to survive in the city. Meaning that we do not have only two choices: Expanding our cities or saving pollinators. We can do both. Nested within tall buildings and large roads are gardens and parks perfect for pollinators. The problem is only how they are going to get there. We need to think ahead. Planning our urban development so that our cities are pollinator-friendly, making the maze of houses and roads less difficult for them to navigate. In doing so cities could become urban refuges for pollinators as the surrounding farmland intensifies.

Master’s Degree Project in Biology 30 credits 2018
Department of Biology, Lund University

Advisor: Anna Persson, Centre of Environmental and Climate Research (CEC), Lund University
Co-advisor: Karin Johnson, Biological Museum, Lund University

• author: Thelin, Sara
• supervisor: Anna Persson; Karin Johnson
• organization: Degree Projects in Biology
• course: BIOM02 20182
• year: 2019
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 8972424
• date added to LUP: 2019-03-05 09:18:10
• date last changed: 2019-03-05 09:18:10