By Cara Burke
Many gardening shows and magazines advocate for the planting of flowers that attract bees and pollinators. But can the simple choice of flowers make a difference to the survival of bees? It can be hard to believe that our gardening choices could make a real difference to a species’ decline, especially considering that the issues behind the decline of pollinator species are complex and varied. However, studies have found that the flowers planted over a given land area do make a real difference in year-to-year bee survival.
The decline in bee populations is a grave and pressing issue. Around 87.5% of flowering plants rely on pollinators (Weiner et al., 2014). Three out of four crops which produce fruits or seeds for human consumption depend on pollinators, and the volume of crops which depend on pollinators has increased by 300% in the last 50 years (Graziano da Silva, 2019). But bee species are in decline in Europe and Northern America. Every year for the past ten years, over a quarter of honeybee colonies in the United States are predicted to have died. Land management, agricultural industrialisation, pesticide use, diseases from introduced insect species and urbanisation all contribute to declining populations.
It is important to know that it is very difficult to measure the survival of bees year to year. They cannot be easily tagged. They are a superorganism. Their life cycle makes it hard to define their year-to-year survival. To save bees, we need an understanding of how changing landscapes change their population size (Carvell et al., 2017).
In 2017, a study by Claire Carvell et al. used a new method. A combination of habitat manipulation, land-use and habitat surveys, molecular genetics and demographic and spatial modelling. They non-lethally sampled the DNA of bees in Southern England. They found three bumblebee species and for all three, as the habitat quality increased, their year-to-year survival increased significantly. But they found that flower cover itself was not the biggest factor in bee survival, but rather the type of flowers grown (Carvell et al., 2017).
Does this mean that within cities, in people’s gardens, planting certain flowers will protect bee species? We still find many bees in urban environments. Species richness in the United Kingdom does not differ significantly between urban, farmland, and natural reserve environments. However, the numbers of bee colonies in urban environments are significantly lower (Baldock et al., 2015). Because urbanisation is a high cause of insect decline, it is important to look at urban solutions to the issue.
So what can individuals do? In the United Kingdom, 7% of land area is covered by cities and towns. Around 15 out of 25 million dwellings in cities studied had gardens. The green space of major cities can range from 21.8% to 26.8% (Loram et al., 2007). Given the evidence that bee species are significantly impacted by flower species, individual gardeners can have a significant impact on bee survival.
Most research so far has been conducted in the United Kingdom and the United States. Whilst they point towards flower cover and flower species having an impact on bee survival, the general environment and space between gardens could have quite an impact on how effective simply planting flowers might be. The environments across countries may also differ greatly, which may affect the efficiency of certain methods.
It is important to note that bees are not the only important pollinators. Insects other than bees provide 39% of visits to crop flowers, and these species can range from flies, wasps, beetles, moths, ants, birds and bats and more. They perform 25-50% of the total number of flower visits in 17 major crops, and their populations have a range of different needs to remain sustainable (Rader et al., 2016). A lot more research is needed to determine the best pollinator conservation methods. These methods will not rely solely on individual gardeners. Whilst the general population can help, solutions will require changes to agricultural land management, pesticide use, and urban and other environmental layouts.
Weiner, C. N., Werner, M., Linsenmair, K. E. & Blüthgen, N. (2014) Land-use impacts on plant—pollinator networks: interaction strength and specialization predict pollinator declines. Ecology. 95 (2), 466-474. Available from: http://www.jstor.org/stable/43494359.
Graziano da Silva, J. (2019) Why bees matter – The importance of bees and other pollinators for food and agriculture. Food and Agriculture Organization of the United States.
Carvell, C., Bourke, A. F. G., Dreier, S., Freeman, S. N., Hulmes, S., Jordan, W. C., Redhead, J. W., Sumner, S., Wang, J. & Heard, M. S. (2017) Bumblebee family lineage survival is enhanced in high-quality landscapes. Nature. 543 (7646), 547-549. Available from: https://doi.org/10.1038/nature21709. Available from: doi: 10.1038/nature21709.
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