Urban Gardens Provide Stable Nectar Supply for Pollinators

Source Article: Tew, N. E., Baldock, K. C. R., Vaughan, I. P., Bird, S., & Memmott, J. (2022). Turnover in floral composition explains species diversity and temporal stability in the nectar supply of urban residential gardens. Journal of Applied Ecology, 59, 801–811. https://doi.org/10.1111/1365-2664.14094   

Featured Image:Hover flies like this one are important pollinators for many plant species. Image Source: Avesgaspar, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons

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Insect pollinators are in trouble as a result of habitat loss, pesticides, diseases, and climate change. Their population sizes are decreasing, and many plants on farms or in the wild need them to be able to grow fruits and reproduce. Insects like bees and hover flies visit flowers, sip sweet nectar, and get dusted with pollen. In the process, they move pollen around from flower to flower. This allows for plants to reproduce sexually, with pollen fertilizing eggs that grow into seeds. With so much at stake for plants around the world and the humans who depend on them, how can we stop the decline of insect pollinator populations?

Urban Gardening Can Be Pretty Sweet 

Urban areas may have a role to play in supporting insect pollinators, and such areas are expanding. While urban expansion can drive habitat loss for many plants, animals, and insects, gardens in urban areas commonly include flowering plants where pollinators can find nectar, their most nutritious, energy-rich food. The combined land area of gardens in cities can also be high, up to 36% of the city’s land. In comparison, rural areas have been providing less and less nectar to pollinators, making urban garden nectar availability extra important.

Even if urban gardens do provide substantial nectar to pollinators, timing could be an issue. If every garden blooms around the same time, nectar will be highly available, but only for a short time. Pollinators require a stable supply of nectar from spring through fall. So, do urban gardens provide pollinators with a stable supply of nectar? Can we grow better gardens to support pollinators? Researchers in the United Kingdom designed a study to find out.

Putting a Number on Nectar

The study was conducted in Bristol, UK, at 59 gardens of various locations, neighborhood incomes, sizes, and styles. Once a month between March and October of 2019, researchers visited each garden. During each visit, all plants in flower were identified and counted. Then, researchers determined the amount of nectar sugar produced per day. They did this in a few ways. To directly measure the nectar sugar production, flowers were covered with a mesh bag for 24 hours to keep insects out. Then, the nectar was rinsed out of the flower with a small amount of water. A refractometer then measured the amount of sugar in the water. For flowers that were too uncommon to measure directly, the nectar sugar production was estimated using prior research or through predictive modeling based on the flower type, flower size, and plant family.

Using this data, researchers were able to estimate how much nectar was available at any given time at each garden from March through October. They did this using a model, which also investigated whether neighborhood income or garden size could explain patterns in the nectar availability data over time. Then, they ran simulations of how nectar availability for a pollinator is affected when visiting multiple gardens within a 100 m (about a football field length) radius which, on average, includes 93 gardens in Bristol. They also investigated the relationship between the number of flowering species in a garden and how stable the nectar supply was over time. Finally, they studied the diversity of flowering species from garden to garden.

It’s What’s Inside That Counts

The amount of nectar produced by different gardens varied tremendously, with the most productive garden making more than 700 times as much nectar as the least productive garden. However, larger gardens did not have consistently higher nectar availability than smaller gardens. The differences in total nectar production were instead driven by management choices of the gardeners, for example, planting ornamental flower borders around the edges of the garden.

The timing of nectar availability also differed from garden to garden, with at least one garden producing its peak amount of nectar in each month from March to October. Because pollinators can visit multiple gardens, this means that pollinators in Bristol have access to nectar whenever they need it. The more gardens the pollinators can access, the more stable their nectar supply is over time. Like total nectar production, the variation in timing was driven by differences in gardeners’ choices, specifically their choices of plant species. Gardens planted with more species had more stable nectar production over time.

Additionally, different gardens had very different collections of plant species, so the fact that gardens are planted independently by different gardeners means that urban areas can end up with high species diversity. Different species flower at different times, meaning that in a diverse garden (or an urban area with many gardens) there may always be something in flower for pollinators to enjoy! This is an example of the portfolio effect, the stabilizing effect of biodiversity. By having many types of gardens with many plant species, urban gardens provide a stable supply of nectar for pollinators. While there was a far more stable nectar supply over time in Bristol than in the typical rural area, there was still much less nectar available to pollinators in the early and late growing season than in midsummer. In late summer, most nectar was produced by flowers that not all pollinators could utilize, which could leave some pollinators vulnerable. For example, Fuchsia magellanica produced up to half of the available nectar in late summer and fall, but its flower shape made the nectar inaccessible to many insects. Pieris produced around a third of the available nectar in March and April, the most of any plant, but its nectar was also inaccessible to many insects due to its flower shape.

So, How Can I Support Pollinators?

The timing of peak nectar availability likely differs from place to place, but there are some general lessons gardeners anywhere can learn from this study:

• Consider planting species that flower at different times from each other.
• Plant your garden with many different flowering species. 
• Grow flowers that have an open structure that is accessible to short-tongued insects, like some bees.
• Plant flowering trees, shrubs, and vines, which produced most of the nectar in this study. They can grow flowers in three dimensions!
• Utilize small spaces. Bigger gardens do not always have more nectar; even if your space is limited, your gardening choices can make it a valuable resource to pollinators!

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