Article: Theodorou, P, Radzevičiūtė, R, Lentendu, G et al. Urban areas as hotspots for bees and pollination but not a panacea for all insects. Nat Commun 11, 576 (2020). https://doi.org/10.1038/s41467-020-14496-6.
Flying from flowering plant to flowering plant, bees collect pollen and nectar for food. In doing so, they are also one of the most prolific pollinators in nature. Dozens of species of flowering plants depend on various bee species to reproduce, including many that are common food sources for humans around the world. Bees also use that collected pollen and nectar to produce honey, which is used by humans for food and medical purposes.
However, bees are susceptible to many different diseases, which in combination with another phenomenon known as colony collapse disorder has led to significant reduction in the bee population worldwide.
This is a problem because of bees’ crucial role in pollinating so many different plants that are part of the animal and human food chains. Without bees, these food chains would be in danger of collapsing, with eventual mass starvation being possible.
As is the case with many other animals, human activity plays a role in bee populations. Increased human activity, particularly urbanization and agriculture-related activities, generally lead to a loss of habitat – which impacts different animal species differently.
The impact on bees and several other common insect species is the focus of an article by Theodorou and colleagues recently published in Nature Communications.
Article Methods and Results
The authors focused on insect habitats in Germany (see Figure 1 below). They studied both urban and rural locations. Each urban location was paired with a nearby rural agricultural location to enable as close a comparison as possible.
They counted the numbers of different types of insects using traps to collect insect specimens. The results (shown below in Figure 2) are somewhat surprising.
Here, mean OTU richness is a measure of the numbers of the insects depicted in each graph. While insects in general (‘Insecta’ graphs a and b) are more numerous in rural areas than in urban areas, bees (graph i) are far more numerous in urban areas.
This is due to two major reasons.
First, the specific type of flowers and plants in urban areas versus those in rural areas play a role. In the rural areas under study in this article, most of the plants do not produce flowers. This is true of many human food crops like wheat. Since bees can only typically feed on flower pollen and nectar, non-flowering plants do not offer any attraction for bees.
Unlike in nature, where flowering and non-flowering plants are interspersed together, agricultural land typically sees dozens or even hundreds of acres planted with the same plant or grain crop– and very often, it is a non-flowering variety. This means that all those acres are completely empty of any kind of plant that bees can use for food. Thus, that area is no longer functional as bee habitat.
In urban areas, however, there is not nearly enough open land to plant many acres of crops. Thus, the most common plant arrangements in these areas are personal backyard gardens, window planters, porch plant pots or other home garden. A far smaller percentage of these types of plantings are non-flowering plants. In a typical home garden in an urban areas, any non-flowering plants are typically mixed in with or near to flowering plants. This means that in urban areas, there are very few large areas of plants that are inhospitable to bees.
The second is a concept called edge density, which is a measure of the number of edges a given area has. To illustrate this, I made the below figure.
First, let’s look at the black squares. Each of the sixteen black squares in the rightmost figure is the same size as each of the sixteen black squares in the center figure. And the sixteen squares together are equal in area to the big black square in the leftmost figure. As you can see, the center figure is simply the leftmost figure divided into sixteen equal squares.
Second, let’s look at the small blue and red solid boxes. The red boxes are close to at least one of the edges of the big black square (leftmost figure). The blue boxes, however, are not. To get to a blue box from outside the big square, it’s necessary to cross or pass over at least one square that has a red box (center figure). However, if each small square is separated from its neighboring squares (rightmost figure), then one can get directly to a blue box without having to go over a square with a red box.
Thought equal in area, the rightmost figure has a much greater edge density than the leftmost figure.
Urban gardens are much like the rightmost figure: smaller individual areas, with much greater edge density. Rural agricultural areas are more like the leftmost figures: large areas of crops. This is efficient for farming, but not so for bees’ access to the few flowering plants that may exist in these areas.
Though urban areas are generally less habitable for most animals than rural areas, recent research shows bees populations are more robust in urban areas. This is due largely to the types and architecture of gardens and other vegetation found in these locales. Overall though, other insect species and insects as a whole are far more numerous in rural areas. That said, as their populations continue to fall worldwide, urban areas may prove to be unexpected refuges for bees. To help this cause, everyone – especially city dwellers – can plant and raise flowering plants.