As summer approaches in the northern hemisphere, gardeners will be turning the soil and planting seeds in the hope of getting something edible in the autumn. Urban gardens are an odd space for pollinators to exploit. The variety of plants in gardens supports diversity while paving, gravel and concrete remove habitable space. Robert McDougall and colleagues at the Universities of New England, Armidale and Sydney examined fifteen food-producing gardens in Sydney and Wollongong to see how urban features affected seed production and the total yield of a model plant, pak choi (Brassica rapa). They found that while honeybees made over half the visits to the plants, pollination factors were did not have a big role in improving yield compared to plant diversity.
Understanding how pollinators behave in urban environments is important. Apart from being interesting in their own right, McDougall and colleagues point out that around 75% of food crops benefit from insect pollinators. It’s not clear if urban gardens help or hinder pollinators. People are happy for pollinators to visit flowers, but nesting sites can be harder to find. There are also other stresses in an urban environment. Soil contamination is common, and the air can be laced with pollution from cars. Urban heat islands can change the growing season of plants.
McDougall and colleagues set out to examine both how an urban environment affects pollination and how the same environment affects yields of urban agriculture more generally. To do this, they set up 15 gardens in New South Wales in the spring of 2016. They focussed their study on observing potted Brassica rapa subsp. chinensis (pak choi) plants. The team chose these plants as they’re known to be sensitive to insect pollination, so the plants could be expected to magnify the environment’s effects on pollinators.
The team found that pollinator richness had a positive relationship with floral density but a negative relationship with floral richness, the variety of flowers in a garden. This seems to be a puzzle. Why would increasing the variety of flowers available reduce the apparent variety of pollinators?
McDougall and colleagues have a potential solution. In their article, they write: “The negative relationship observed between pollinator richness and floral species richness may be a result of competition between model plants and plants present within the gardens… As we only observed pollinators visiting model plants, rather than surveying gardens more generally, a greater proportion of the flower visiting species present in more florally diverse gardens may have been missed due to a preference for flowers other than those on model plants.”
The most significant influences on yield per square metre were local canopy cover and garden scale species richness, and this accounted for two-thirds of the variability between plots. McDougall and colleagues write: “One possible explanation for this is the ‘urban heat island’ a phenomenon whereby cities are often warmer than surrounding rural or natural environments due to higher levels of heat-retaining surfaces, increased atmospheric CO2 and reduced evapotranspiration… Increased temperatures resulting from this can cause heat stress in plants…, potentially reducing yields. Increased canopy cover can help mitigate the urban heat island effect by shading heat-retaining surfaces and increasing evapotranspiration …, which could potentially explain the positive relationship between canopy cover and yield.”
The benefit of garden species richness is more speculative. One reason the botanists consider is pest control. Increased diversity of plants provides more varied habitats for insects looking for pests to eat. However, they also point out that managing these more diverse gardens needs a lot of work, so it could be a sign of gardeners who are more intensively managing their plots.
The authors conclude: “While environmental features had a minor impact on pollination of model plants, they had a much larger impact on food plant output when yield was summed across all crops grown in the study sites, including those not dependent on insect pollinators. This implies that assessments of ecosystem services in urban environments, and management decisions related to these, may be better focused on outcomes from systems of diverse species communities, rather than on the provision of single services or the success of a single plant species.”
McDougall, R., Kristiansen, P., Latty, T., Jones, J. and Rader, R. (2022) “Pollination service delivery is complex: Urban garden crop yields are best explained by local canopy cover and garden scale plant species richness,” The Journal of Applied Ecology. https://doi.org/10.1111/1365-2664.14136