Quantifying nectar production by flowering plants in urban and rural landscapes

Nicholas E Tew,Stephanie Bird,Ian P Vaughan,Jane Memmott,Katherine C R Baldock,Simon G Potts,Graham N Stone,Ignasi Bartomeus

doi:10.1111/1365-2745.13598

Abstract

Abstract Floral resources (nectar and pollen) provide food for insect pollinators but have declined in the countryside due to land use change. Given widespread pollinator loss, it is important that we quantify their food supply to help develop conservation actions. While nectar resources have been measured in rural landscapes, equivalent data are lacking for urban areas, an important knowledge gap as towns and cities often host diverse pollinator populations. We quantified the nectar supply of urban areas, farmland and nature reserves in the UK by combining floral abundance and nectar sugar production data for 536 flowering plant taxa, allowing us to compare landscape types and assess the spatial distribution of nectar sugar among land uses within cities. The magnitude of nectar sugar production did not differ significantly among the three landscapes. In urban areas the nectar supply was more diverse in origin and predominantly delivered by non‐native flowering plants. Within cities, urban land uses varied greatly in nectar sugar production. Gardens provided the most nectar sugar per unit area and 85% of all nectar at a city scale, while gardens and allotments produced the most diverse supplies of nectar sugar. Floral abundance, commonly used as a proxy for pollinators’ food supply, correlated strongly with nectar resources, but left a substantial proportion of the variation in nectar supply unexplained. Synthesis. We show that urban areas are hotspots of floral resource diversity rather than quantity and their nectar supply is underpinned by the contribution of residential gardens. Individual gardeners have an important role to play in pollinator conservation as ornamental plants, usually non‐native in origin, are a key source of nectar in towns and cities.

Highlights

Large-scale changes to land use and management intensity have resulted in the landscape-level depletion of floral resources, which provide food for insect pollinators (Baude et al, 2016; Carvell et al, 2006)
We show that urban areas are hotspots of floral resource diversity rather than quantity and their nectar supply is underpinned by the contribution of residential gardens
We found no significant difference in the magnitude of nectar sugar production in urban, farmland and nature reserve landscapes

Summary

Introduction

Large-scale changes to land use and management intensity have resulted in the landscape-level depletion of floral resources, which provide food for insect pollinators (Baude et al, 2016; Carvell et al, 2006). The comparative success of bees in urban areas is likely to be influenced by the availability of flower-rich green spaces, such as parks and gardens (Baldock et al, 2019; Hülsmann et al, 2015), which can be important sources of nectar and pollen This is further supported by experiments which recorded bumblebee colonies growing larger in urban and suburban habitats versus agricultural areas (Goulson et al, 2002; Samuelson et al, 2018; but see Milano et al, 2019 for a counter example)

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