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

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.

Functional diversity of farmland bees across rural–urban landscapes in a tropical megacity

“…Furthermore, urban areas generally have high availability and diversity of floral resources in gardens and parks (e.g., including horticultural flowering plants; Banaszak‐Cibicka et al, 2018; Geslin, Oddie, et al, 2016). This could increase niche space availability (e.g., types of flowering resources, phenological diversity of flowering plants; see Prendergast et al, 2022; Tew et al, 2022) and, contrary to our expectations, result in higher functional diversity in these landscapes (Figure 3e).…”

Pollinator-flower interactions in gardens during the COVID-19 pandemic lockdown of 2020

“…However significant garden heterogeneity has been documented in other studies of garden pollinators and resources (e.g. Prendergast & Ollerton 2021;Tew et al 2022). There were also a number of surveyors who were isolating with parents or other relatives and therefore not conducting surveys in their own gardens.…”

“…Domestic and public gardens are increasingly recognised as potential synanthropic hotspots of pollinator diversity within the matrix of human-dominated landscapes that characterises many parts of the world, and as areas that deliver multiple ecosystem services, including pollination of fruit and vegetable crops (Matteson et al 2008;Davies et al 2009;Owen 2010;Erenler 2013;Norfolk et al 2013Norfolk et al , 2014Camps-Calvet et al 2016;Foster et al 2017;Bendifallah & Ortiz-Sánchez 2018;Baldock et al 2019;Levé et al 2019;Marín et al 2019;Majewska & Altizer 2020;Tew et al 2021;Prendergast 2021;Prendergast & Ollerton 2021). However, the effectiveness of gardens in supporting pollinators varies according to taxon, locality, garden management, and generalizationspecialization range of occurring interactions, especially in urban areas (Maruyama et al 2019;Theodorou et al 2020;Baldock 2020;Prendergast et al 2022;Tew et al 2022).…”

Pollinator nutrition and its role in merging the dual objectives of pollinator health and optimal crop production

“…Encroachment of grasses and other ruderal weeds can result in temporal declines in the value of these interventions for maintaining pollinator diversity and abundance [ 101 ], yet few studies have examined the effects of pollinator habitat enhancement on crop pollination beyond four years since establishment [ 9 ], largely precluding inference about the possible role of planted trees. Flowering trees can provide early and abundant floral resources within a small footprint [ 102 , 103 ], while also providing additional non-floral resources such as nesting sites, resins and shelter [ 7 ]. Incorporating trees and other more permanent habitat features into cropping systems is key to supporting future multifunctional landscapes, both in terms of the costs of establishment and maintenance and through their role in supporting multiple ecosystems services and biodiversity [ 104 ].…”