The effects of flower supplementation on pollinators and pollination along an urbanisation gradient

Societal Impact Statement

Enhancing urban greenspaces for pollinator communities by planting flower patches is increasingly common, but their efficacy for different groups of insects (bees, hoverflies and moths) is unclear. Our city-scale experiment demonstrated that the effect of adding flower patches on pollinators is complex, and direct benefits to specific insects are difficult to detect. However, adding flower patches increased pollination services, with an increase in seed set in a model horticultural crop, particularly in more urban areas. Flower patches are likely to benefit pollinator communities and, in turn, humans through pollination services, but further research needs to disentangle the multi-level mechanisms driving variation in pollination and pollinators to improve the ecological and societal benefit provided by this management intervention.

Summary

The addition of flower patches in human-modified ecosystems is a common practice to mitigate pollinator declines and boost pollination. However, the benefits of these additions for both pollinator communities and pollination services are rarely tested together, especially in urban environments.

In a city-scale experiment, we added floral resources to urban allotments and monitored the effects on communities of bees, hoverflies and moths, and tested for improved seed set in a model crop (tomato, Solanum lycopersicum).

The addition of flower patches had no detectable effect on allotment pollinator communities but led to a 25.3% increase in tomato seed set, providing evidence that flower patch augmentation can improve urban pollination. Seed set was relatively higher in more urban sites, suggesting an “oasis effect” where pollinating insects are concentrated when greenspace resources are limited. This highlights the precarity of pollination services in highly urban areas.

Our results suggest that planting flower patches can positively affect pollination services in urban areas; however, the mechanisms remain unclear. Although we did not detect strong effects on pollinator communities when we added flower patches, differences in visitation networks between major pollinator taxa suggest that flower patch addition is likely to benefit some, but not all, insects, and further research is needed to assess the suitability of current flower seed mixes for non-bee taxa. Overall, adding floral resources to urban systems may play an important role in supporting pollination but highlights the complexity of identifying the specific drivers and taxa underpinning this ecosystem service.