Supporting wild pollinators in agricultural landscapes through targeted legume mixtures

Abstract

Insect-pollinated legumes are rich in plant-based proteins making them a vital constituent of sustainable healthy diets for people and livestock. Furthermore, they deliver or support a range of ecosystem services that underpin agricultural production and their prevalence in agricultural landscapes is likely to increase. Under typical implementation and management, the value of legumes to pollinators has, however, been questioned. Through exploring a range of legume crops, grown as monocultures and mixtures, this study aims to identify multifunctional legume cropping systems that optimise forage availability for a diversity of wild pollinators whilst delivering a wide range of agronomic and environmental benefits. This study innovatively explores legume mixtures concurrently with monocultures of the component species using replicated small-plot field trials established in two geographical locations. Observational plots assessed the richness and abundance of floral resources, and wild pollinators (i.e. bumblebees and hoverflies) throughout the peak flowering period. Densely flowering, highly profitable legumes (e.g. Trifolium incarnatum and Trifolium mixes) supported abundant and rich pollinator assemblages. The functional makeup of floral visitors was strongly influenced by flower structure and hoverflies, with their shorter proboscises, were largely constrained to legumes with shallower corolla and open weed species. Floral richness was not a key driver of pollinator assemblages; however, clear intra-specific differences were observed in flowering phenology. Combining functionally distinct legumes with respect to flower structure and phenology, will support a wider suite of pollinating insects and help stabilise the temporal availability of forage. For highly competitive legumes (e.g. Vicia faba and Vicia sativa), planting in discrete patches is recommended to reduce the risk of less competitive species failing in mixtures. Legumes can provide valuable forage for pollinators; however, they fail to meet all resource requirements. They should therefore be used in combination with agri-environmental measures targeted to promote early-season forage (e.g. hedgerows and farm woodlands), open flowers for hoverflies, saprophytic hoverfly larval resources (e.g. ditches and ponds) and nesting habitats (e.g. undisturbed field margins).

Keywords: Agri-environment schemes; Bumblebee; Functional diversity; Hoverfly; Legumes; Nitrogen-fixing crops; Pollinators; Sustainable agriculture.

Graphical abstract

Fig. 1

Heatmap illustrating temporal trends in the interaction frequency between pollinator and plant species. Data is summarised irrespective of geographical location and year. To ease interpretation pollinators observed only once during the study are omitted as are plant species where only one interaction was observed.

Fig. 2

Heatmap illustrating the relative abundance of each pollinator functional group observed foraging on simple and complex weeds and the seven legume species. Graphs are based on percentage data summarised for each legume treatment irrespective of year, region, and month.

Fig. 3

Impact of legume treatment on abundance and richness for flowers (top), bumblebees (middle) and hoverflies (bottom). Graphs are based on the raw data. For each taxa, first the mean value per plot per year (i.e. averaging over survey periods) was calculated. Then for each treatment the mean value ( ± SEM) was calculated, irrespective of region and year. Fixed effect parameter estimates are provided in Table S4 for flowers and S5 for pollinators.

Fig. 4

Impact of season on mean floral density, irrespective of region and year. Each plot shows the mean floral abundance for a specific mixture (solid line), alongside floral abundance in legume monocultures present in the mixture in question (broken lines). This enables direct comparison between flowering phenology when species are planted as a mixture verses a monoculture.