Floral enhancement of field margins in agricultural landscapes: from pollinator assemblages to hoverfly feeding ecology

Blackmore, Lorna (2023) Floral enhancement of field margins in agricultural landscapes: from pollinator assemblages to hoverfly feeding ecology. Doctoral thesis, University of East Anglia.

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Abstract

Field margins are a key policy response implemented via agri-environment schemes to address pollinator population declines in the UK, through the provision of these additional areas of seminatural habitat and the supplemental floral resource they provide. In this work I investigated two agri-environment scheme field margin options -sown wildflower or grass margins - and how these impacted farmland pollinator communities across Norfolk and Suffolk. This was achieved through analysing the data from insect specimens caught using trap sampling and floral transect surveys. My work confirms that sown flower margins attract larger numbers and diversities of bee species in comparison to grass margins in landscapes with low percentages of seminatural landcover, suggesting that field margins can provide valuable forage resource for bees in this context. Both bees and hoverflies were found to be more abundant in margins with higher floral densities. Although floral margins were found to consistently have higher floral densities, hoverflies, unlike bees, were not more abundant or diverse between field margin treatment types. To better understand the use of these margins by hoverflies I investigated key environmental factors likely to impact their visitation, namely the prevalence of surrounding seminatural habitat, margin treatment type and floral density. These variables were used as predictors of hoverfly community composition. Namely, the species, larval functional group affiliation, migratory status and morphological trait composition. I found that floral margins attracted a consistent suite of migratory detritivorous and zoophagous hoverflies, in comparison to the more variable composition of grass margins. Sampling in additional non-margin farmland habitats - crop and hedgerow - resulted in the finding that margin habitats supported comparatively greater abundances of non-migratory species. Grass margins were an important source of heterogeneity in the farmland landscape and contained the greatest abundance of nonmigratory hoverflies. Whereas floral margin communities consisted of comparatively more nectar specialists, larger species and migratory species. The use of morphological and ecological functional trait data revealed ecological relationships between hoverfly community-level mean traits, such as body size and degree of nectar-pollen specialism, and field management options that would not have been possible using measures of taxonomic abundance and diversity alone. Hoverfly community mean body size and mean nectar-pollen specialism score were also impacted by the amount of nearby seminatural habitat in the landscape. Nectar specialists were found to be relatively more abundant in floral margins, whereas grass margins had significantly more pollen specialists. Larger, less mobile hoverflies increased in abundance in response to an increase in seminatural habitat, suggesting the latter is a source (or corridor) of such species and that higher % of seminatural habitat increase landscape permeability. Ultimately, these results suggest, although most hoverfly species visit both floral and grass margin types, that the 2 frequency of different hoverfly species in each margin type resulted in a different trait and functional composition. Both grass and floristically enhanced field-margins support migratory and non-migratory hoverflies. However, floral margins support a higher proportion of migratory and zoophagous individuals, suggesting that these margins may primarily support ecosystem services such as pollination (mobile ranging, migratory species) and pest control services to crops (aphid eating zoophagous species) and less to conservation aims for declining hoverfly groups. Whereas, grass margins would appear to have the opposite effect, with the make-up of those communities containing more non-migrant individuals, which have been found to be suffering population declines. As floral margins are intended to provide an additional source of nutrition to pollinators, I also investigated the pollen diet of hoverflies from both field margin types. My findings support that a diversity of seminatural habitat is important for adult and larval hoverfly niches in agricultural landscapes. DNA metabarcoding revealed that pollen of un-sown plant species comprised the majority of the diets of six common hoverfly species on field margins with ~40% of hoverfly diet pollen originating from unsown herbaceous species, with sown margin flora pollen consisting of ~12% of the diet. I found that grass pollen is currently under-valued as a source of dietary pollen for hoverflies, as grass pollen accounted for ~21% of the diet. Although the abundance of grass flowers was not measured in this study, the proportionally higher prevalence of grass in the non-floristically enhanced margins may also account for the relatively high number of pollen specialists and non-migrant species – with the latter group perhaps being less reliant on the high energy nectar resources necessary for more mobile species. Ultimately, I found that floristically-enhanced and grass margins provide differing, yet complementary, uses to the farmland hoverfly community. The protection, and increasing area of seminatural habitat in farmland landscapes, as well as the increased provision of flowering grasses and morphologically accessible wildflowers, is encouraged for both the conservation of declining hoverfly populations, and for the maintenance of the ecosystems services both migratory and non-migratory species provide.

Item Type:	Thesis (Doctoral)
Faculty \ School:	Faculty of Science > School of Biological Sciences
Depositing User:	Jennifer Whitaker
Date Deposited:	28 Mar 2025 11:31
Last Modified:	28 Mar 2025 11:31
URI:	https://ueaeprints.uea.ac.uk/id/eprint/98900
DOI:

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