Turnover in floral composition explains species diversity and temporal stability in the nectar supply of urban residential gardens

Nicholas E Tew,Ian P Vaughan,Jane Memmott,Katherine C R Baldock,Stephanie Bird

doi:10.1111/1365-2664.14094

Abstract

Abstract Residential gardens are a valuable habitat for insect pollinators worldwide, but differences in individual gardening practices substantially affect their floral composition. It is important to understand how the floral resource supply of gardens varies in both space and time so we can develop evidence‐based management recommendations to support pollinator conservation in towns and cities. We surveyed 59 residential gardens in the city of Bristol, UK, at monthly intervals from March to October. For each of 472 garden surveys, we combined floral abundances with nectar sugar data to quantify the nectar production of each garden, investigating the magnitude, temporal stability, and diversity and composition of garden nectar supplies. We found that individual gardens differ markedly in the quantity of nectar sugar they supply (from 2 to 1,662 g), and nectar production is higher in more affluent neighbourhoods, but not in larger gardens. Nectar supply peaks in July (mid‐summer), when more plant taxa are in flower, but temporal patterns vary among individual gardens. At larger spatial scales, temporal variability averages out through the portfolio effect, meaning insect pollinators foraging across many gardens in urban landscapes have access to a relatively stable and continuous supply of nectar through the year. Turnover in species composition among gardens leads to an extremely high overall plant richness, with 636 taxa recorded flowering. The nectar supply is dominated by non‐natives, which provide 91% of all nectar sugar, while shrubs are the main plant life form contributing to nectar production (58%). Two‐thirds of nectar sugar is only available to relatively specialised pollinators, leaving just one‐third that is accessible to all. Synthesis and applications. By measuring nectar supply in residential gardens, our study demonstrates that pollinator‐friendly management, affecting garden quality, is more important than the size of a garden, giving every gardener an opportunity to contribute to pollinator conservation in urban areas. For gardeners interested in increasing the value of their land to foraging pollinators, we recommend planting nectar‐rich shrubs with complementary flowering periods and prioritising flowers with an open structure in late summer and autumn.

Highlights

Flower-visiting insects including bees and hoverflies are crucial pollinators of many wild plants and agricultural crops (Klein et al, 2007; Ollerton et al, 2011)
We investigate for the first time how the nectar supply of residential gardens varies in space and time and use our results to develop evidence-based management recommendations for pollinator conservation in urban areas
We focus on three characteristics of the nectar supply in gardens. (a) Magnitude: we predict substantial variation in nectar sugar production among gardens and an overall peak in summer when we expect more plants to be in flower. (b) Temporal stability: we predict that individual gardens will vary in their seasonal patterns of nectar sugar production such that not all gardens will peak at the same time of year

Summary

Introduction

Flower-visiting insects including bees and hoverflies are crucial pollinators of many wild plants and agricultural crops (Klein et al, 2007; Ollerton et al, 2011). Diverse pollinator communities can be found in gardens throughout the world (Baldock et al, 2019; Fetridge et al, 2008; Marín et al, 2020; Martins et al, 2017; Staab et al, 2020) Despite their small individual size, residential gardens collectively cover 16%–36% of cities in different countries (Baldock et al, 2019; Colding et al, 2006; Loram et al, 2007; Mathieu et al, 2007; Ossola et al, 2021) and provide an estimated 85% of nectar in urban areas in the UK (Tew et al, 2021). Gardens offer a unique opportunity for pollinator conservation where the combined action of many individuals can have a major impact on foraging resources at a landscape scale (Goddard et al, 2010)

Results

Discussion

Conclusion