The importance of gardens for urban nectar supply

Lead author Nick Tew discusses recent Journal of Ecology article: Quantifying nectar production by flowering plants in urban and rural landscapes. Find out more about the importance of residential gardens for nectar supply within urban areas of the UK. You can also read the press release for this article here.

Insect pollinators are faced with an impoverished supply of food across much of the UK’s countryside. Hay meadows have been lost, hedgerows cleared and herbicides have blotted out our iconic arable weeds. Against this backdrop, towns and cities have been growing larger, now housing 84% of the country’s population and covering 6% of its land (that’s five times the area of all the National Nature Reserves). These artificial urban landscapes can be remarkably good for some pollinators, with bees particularly tolerant of city living.

An ashy mining bee in central Bristol, UK. Cities can harbour surprisingly diverse pollinator communities. Photo: N. Tew

We wanted to know how much nectar is produced in urban landscapes to see how they compare with rural areas and investigate which urban land uses are the most important. Sugars from nectar are the main energy source for adult pollinators, vital for powering their flight muscles. We used existing flower count data, collected by quadrat sampling in two previous studies, but needed to find nectar data for 536 flowering plants. We used a combination of published sources and field measurements, then filled the gaps with predictive modelling. Nectar can be measured in a relatively straightforward fashion – extracted with a glass microcapillary tube and its sugar concentration quantified with a handheld refractometer.

Extracting nectar from Helleborus argutifolius using a glass microcapillary tube. Photo: N. Tew

We found that urban landscapes are hotspots of nectar source diversity, rather than total sugar quantity. This means that the nectar supply in towns and cities is produced by more plant species, most of which are not native to the UK, and it is not dominated by a few key nectar sources, as is common in the countryside. Within cities, residential gardens are vital, providing 81-88% of all the nectar sugar. Along with allotments they are the top land use for nectar source diversity and nectar sugar quantity per square metre, and they also cover around 30% of land in our cities.

Residential gardens produce the vast majority of a city’s nectar. Photo: N. Tew

However, there is a potential caveat to treasuring gardens as resource-rich oases for insect pollinators, and that relates to the nature of the plant communities they contain. These are unnatural assemblages of species, thrown together haphazardly by gardeners. Most are not native and even the natives are often highly modified horticultural cultivars. Whether or not this matters for pollinators is open to debate. The UK does not have endemic pollinators, so the species we do have are also found in continental Europe. Given that, there is no intrinsic reason to expect that a UK pollinator should prefer a native species, say oxeye daisy, to a European one, say lavender. Speaking more broadly, flowering plants tend to attract coarse taxonomic groupings of pollinators, not single specialised species, so again, there is no clear reason why non natives should be less valuable. Those pollinators which do specialise on certain plants may struggle to find appropriate forage in gardens and similarly, certain larval food sources for Diptera and Lepidoptera may be lacking. Nevertheless, it is clear that gardens are resource-rich refuges, valuable for many insect pollinator species. Given the extent to which intensive agriculture has devastated rural biodiversity and the uncertainties presented by climate change, studying urban landscapes could be useful, helping us to challenge our traditional views on the roles introduced species play in communities.

Nick Tew  University of Bristol, United Kingdom

You can read the full paper online: Quantifying nectar production by flowering plants in urban and rural landscapes

The press release for this paper is available on the BES News site: Pioneering research reveals gardens are secret powerhouse for pollinators

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