Generalisation and specialisation in hoverfly (Syrphidae) grassland pollen transport networks revealed by DNA metabarcoding.

Andrew Lucas,Dan W Forman,Matthew Hegarty,Natasha De Vere,Carolyn Greig,Penelope J Neyland,Col R Ford,Owen Bodger,Berry J Brosi,Nathan Sanders

doi:10.1111/1365-2656.12828

Abstract

Pollination by insects is a key ecosystem service and important to wider ecosystem function. Most species‐level pollination networks studied have a generalised structure, with plants having several potential pollinators, and pollinators in turn visiting a number of different plant species. This is in apparent contrast to a plant's need for efficient conspecific pollen transfer.The aim of this study was to investigate the structure of pollen transport networks at three levels of biological hierarchy: community, species and individual. We did this using hoverflies in the genus Eristalis, a key group of non‐Hymenopteran pollinators.We constructed pollen transport networks using DNA metabarcoding to identify pollen. We captured hoverflies in conservation grasslands in west Wales, UK, removed external pollen loads, sequenced the pollen DNA on the Illumina MiSeq platform using the standard plant barcode rbcL, and matched sequences using a pre‐existing plant DNA barcode reference library.We found that Eristalis hoverflies transport pollen from 65 plant taxa, more than previously appreciated. Networks were generalised at the site and species level, suggesting some degree of functional redundancy, and were more generalised in late summer compared to early summer. In contrast, pollen transport at the individual level showed some degree of specialisation. Hoverflies defined as “single‐plant visitors” varied from 40% of those captured in early summer to 24% in late summer. Individual hoverflies became more generalised in late summer, possibly in response to an increase in floral resources. Rubus fruticosus agg. and Succisa pratensis were key plant species for hoverflies at our sitesOur results contribute to resolving the apparent paradox of how generalised pollinator networks can provide efficient pollination to plant species. Generalised hoverfly pollen transport networks may result from a varied range of short‐term specialised feeding bouts by individual insects. The generalisation and functional redundancy of Eristalis pollen transport networks may increase the stability of the pollination service they deliver.

Highlights

The structure and function of pollination networks have been the subject of considerable research interest (Jordano, 2016; Nicolson & Wright, 2017; Petanidou, Kallimanis, Tzanopoulos, Sgardelis, & Pantis, 2008)
Networks were generalised at the site and species level, suggesting some degree of functional redundancy, and were more generalised in late summer compared to early summer
Our results demonstrate that pollination transport networks amongst Eristalis hoverflies are generalised, but that this generalisation may be a consequence of short-term specialisation by individuals on particular plant species

Summary

Introduction

The structure and function of pollination networks have been the subject of considerable research interest (Jordano, 2016; Nicolson & Wright, 2017; Petanidou, Kallimanis, Tzanopoulos, Sgardelis, & Pantis, 2008). It has been suggested that such networks can be both generalised and specialised at different levels of biological hierarchy, with individual insects engaging in short- term specialised feeding bouts, and efficiently moving pollen between plant conspecifics, whilst networks at the species and community level remain generalised (Armbruster, 2016; Brosi, 2016; Ollerton, 1996) Addressing this issue requires the investigation of individual pollinator behaviour, but is constrained by the limitations of existing techniques, such as following insects in the field (Ambrosino, Luna, Jepson, & Wratten, 2006; Brosi & Briggs, 2013), or morphologically identifying pollen grains carried by insects (Golding & Edmunds, 2003). The accurate visual identification of pollen requires considerable skill (Bruni et al, 2015; Hawkins et al, 2015) with some plant species groups being difficult to distinguish, even by experts (Galimberti et al, 2014)

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Conclusion