Riparian field margins: their potential to enhance biodiversity in intensively managed grasslands

1. Riparian field margins (i.e. fenced off areas adjacent to watercourses) are becoming widespread in the UK as a means of mitigating diffuse pollution in intensive grasslands. By providing additional habitats for wildlife, they can potentially enhance farmland biodiversity. This study examined a range of riparian margins to determine their impact on invertebrates. 2. Over a 4-year period, key invertebrates were monitored by pitfall traps at 69 riparian sites in south-west Scotland. Riparian margins were classified as open margins (i.e. no fence between the field and watercourse), narrow margins (fences erected within 2.6 m of the watercourse) and wide margins (fences erected over 5 m from the watercourse). Pitfall transects were established at each site in the margin and field. 3. Generalised Linear Mixed Models (GLMMs) were used to model the influence of riparian management and vegetation height on invertebrate activity density. Riparian margins closed to livestock (i.e. wide and narrow margins) had the highest activity densities of Hemiptera, Opiliones, Symphyta larvae and Arionidae, and this could not solely be attributed to differences in vegetation height. For most invertebrate groups, activity density did not differ between wide and narrow margins. Only Linyphiidae, predatory coleopteran larvae and Lumbricidae had higher activity densities in the fields than adjacent riparian margins. 4. The erection of fences along watercourses creates habitat heterogeneity in intensively managed grasslands resulting in enhanced invertebrate populations and thus potentially more prey for foliage gleaning birds. Although margin width did not have a significant influence on invertebrate activity density, wider margins provide a greater area of semi-natural habitat.