The effect of arable field margin composition on invertebrate biodiversity

Abstract A replicated field experiment designed to compare five types of field margin in terms of their invertebrate biodiversity was established in North Yorkshire, UK. Four replicate margins on arable land each contained five treatment plots 72 m long by 6 m wide. The treatments were: (1) cropped to the edge; (2) sown ‘tussocky’ grass mix; (3) sown ‘grass and wildflower’ mix; (4) Split margin (3 m ‘tussocky’ grass adjacent to hedge and 3 m ‘grass and wildflower’ next to crop); and (5) natural regeneration. Invertebrates were sampled by pitfall trapping, sweep netting, and butterfly and bumblebee transects, and identified to species level. The use of different flower species by foraging bumblebees was also examined. Despite all treatments containing a flush of annual weeds early in the establishment year, the five margin types were distinct in their vegetation composition by their first mid-summer. Where statistically significant results were obtained for invertebrates, avoidance of the ‘cropped’ treatment was by far the commonest observed response. This tendency could be clearly demonstrated amongst the carabids, spiders, butterflies, bumblebees, millipedes and harvestmen, with margins often containing double or more the number of invertebrates of similar areas cropped to the edge. Having avoided the crop, preferences for other margin types were mixed, but there was a marked tendency by many nectar and/or pollen-feeding, flying insects towards greater abundance on those margins containing sown ‘wildflowers’ (e.g. butterflies Meadow Brown and Ringlet, Bumblebees, Pollen Beetle Meligethes sp.) or flowers either sown or unsown (Soldier Beetle Rhagonycha fulva ). There was a less strong tendency for predatory species occurring in the vegetation canopy and depending on small, especially flying insects for food, to also prefer the flowery treatments (e.g. total spiders caught by sweeping, seven-spot ladybird). Harvestmen in autumn reject natural regeneration in favour of any sown treatment. Only one species, the carabid Nebria brevicollis , was trapped in higher numbers in the cropped treatment than on any sown margin, and then only in autumn. Of the six common bumblebee species, the two longest-tongued species showed different patterns of flower visitation from the remaining four. These results clearly demonstrate that sown field margins can rapidly produce substantial biodiversity benefits on arable land, with the resulting fauna influenced by the type of field margin created.