Space Invaders? Exotic Bees in the Urban Landscape
Original Paper: Fitch, G., Wilson, C.J., Glaum, P., Vaidya, C., Simao, M.C. and M.A. Jamieson. 2019. Does urbanization favour exotic bee species? Implications for the conservation of native bees in cities. Biology Letters, 15(12), p.20190574. https://royalsocietypublishing.org/doi/pdf/10.1098/rsbl.2019.0574
Featured Image Source: Façade Outside of “Burt’s Bees.” 2017. Credit: tombarta, Flickr.
Picture a bouquet of flowers you might bring to a wedding or a funeral, to a loved one on a happy day, or to someone feeling blue. Now think about some of your favorite summer crops, from blueberries to broccoli, apples to asparagus. If you have a fondness for fruits and veggies, honey in your tea, or a vase of wildflowers on your dining table, you have some very small workers to thank – bees.
Why Bees?
Bees are expert pollinators, and the vast majority of the plants we eat rely on pollination. At least 30% of the world’s crops and 90% of all plants require some sort of cross pollination, which is what bees do best. Pollinators like bees play a key role in one out of every three bites of food we eat. Though they are not the only pollinators in the animal kingdom, bees are responsible for pollinating millions of agricultural crops each year accounting for about 84% of crops grown for human consumption. Their efforts lead to the production of many seeds, nuts, berries, and fruits that animals use as a food source. Honey isn’t just for humans either; animals like racoons, possums, and insects love to eat this delicious and nutritious snack. If we had no more bees, many plants that humans and wildlife use as food sources would die off, leaving our plates empty and animals in peril.
Trouble For Bees
Reports of declines in bee populations across the globe have been widely circulated over the past few decades. Bee populations are in decline for reasons including the changing climate, a loss of habitat and food sources, invasive species, and exposure to pesticides like neonicotinoids.
Nevertheless, different bees face different threats. There are 20,000 species of bees, one of which is a common, globally distributed domesticated animal – the honeybee. While honeybees are not yet considered threatened or endangered, their populations are monitored due to the fear of potential losses for the agriculture industry, for which honeybees are a major crop pollinator. As a result, much attention has been focused on honeybee health and a number of organizations dedicated to “saving the bees” have surfaced over the past decade.
However, native and wild bees are also very important to our food industry and for preserving ecosystem function. In some cases, native bees are actually better pollinators than honeybees, as certain flowers only release their pollen when vibrated at specific frequencies. Bees can also be specialized for the plant they are pollinating. Honeybees are generalists, meaning they are good at pollinating a bunch of different plants, but they may not do as good of a job as a bee that is specialized for pollinating one plant species. Honeybees cannot, for example, successfully pollinate tomatoes, as they are too small – instead, bumble bees do this job. For this reason, wild bees are crucial pollinators for many of our crops, and in some cases their efforts yield more “fruitful” results than those of generalist honeybees. Despite their importance to society and the environment, there has been little effort focused on conserving native and wild bee populations, even though a number of these bee species are endangered. As a result, we don’t know very much about their population dynamics and how they are responding to environmental threats.
The Urbanization of Bees
One up and coming target for bee conservation has been the urban landscape, as some recent findings have indicated that cities can maintain diverse bee communities. Though on the surface these findings seem promising for bee conservation, many of these studies do not actually address whether this is a positive thing for native bees. One group of scientists decided to delve further into this topic by looking at the effects of urbanization on bee species. Specifically, they wanted to find out whether exotic bees, including the European honeybee, were found more abundantly in cities and other urban areas than in rural communities and how their presence affected native bees.
The study took place in Michigan, where researchers sampled 41 farms and gardens around the southeastern section of the state. They collected bees, conducted surveys of flowering plants, noted the bees’ nesting strategies, and monitored temperatures monthly at each site during the summers of 2014 and 2017. The study looked at both native bees and non-native bees, but it also considered honeybees and non-honeybee exotic species to be in different categories when conducting analyses because honeybees live mainly in human-managed colonies in the area, so their abundance data may not reflect that of other wild exotic bee species.
The scientists found that, overall, the number of exotic bee species and the number of individual exotic bees increased with urbanization. Each of the non-honeybee exotic species showed a similar trend, indicating that the correlation was not due to one over-represented species. However, the abundances of both native bees and honeybees did not significantly change from rural to urban areas, which was surprising.
The study also determined that there were more exotic bees in urban areas than rural ones due to outside factors that led to increases in exotic bee abundance and not because of a decrease in the number of native bees in urban areas. Exotic plant abundance, proximity to international ports of entry, and urban warming had no effect on the association between urbanization and overall exotic bee abundance. However, there were more exotic bees where there were more flowering plants available, and the closer sites were in proximity to international ports of entry, the fewer non-honeybee exotics. These findings contradicted other studies that found that exotic bees preferentially go to exotic plants, indicating that the increased abundance of exotic plants does not influence the success of exotic bees in cities.
Exotic bee abundance did not affect native bee abundance except when bees overall were at high abundance. In these cases, native bee abundance decreased as non-honeybee exotic abundance increased, but this relationship was not due to any measured factor of native bee abundance. This suggests that there is a density-dependent effect of exotic bees on native bees, meaning the more total bees in the area, the better exotic bees fare in comparison to native bees. This correlation could be due to competition for food, nest sites, or other resources when many bees are present. Certain factors, such as availability of types of nesting sites, could favor exotic bees because more exotic bee species use cavity-nesting spots.
Can Cities Really Be New Sites For Native Bee Conservation?
Promoting bee-friendly management of urban land is important, though it is vital to take into account the type of bee communities supported by cities and to be aware of the effects of urbanization on native bees. Urban areas can support a different suite of species than rural areas, though this does not always lead to a positive outcome. Native species may be negatively affected by changes caused by urbanization, which could increase the prevalence of a small number of exotic species that live near and benefit from association with humans. With this in mind, mitigating natural bee habitat loss is crucial for the survival of native bees.
There is good news: some agricultural companies send out pollinator-friendly plants to those who request them, and bee-safe management tools are being put in place to mitigate pollinator loss. You can even help out your native bees at home by creating forage opportunities, such as planting native plants and letting a few weeds grow and flower. Monitoring your use of pesticides is also important for bees and other wildlife as these chemicals may cause them harm. So go plant some flowers and “bee nice” to your friendly neighborhood pollinators!
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