Impacts of mixed farming systems on nocturnal invertebrates and bats

Abstract

Intensive farming is a key driver of insect declines across the globe, via homogenisation of landscapes, use of synthetic fertilisers and insecticides, land use change and habitat loss. Declining insect abundance may have bottom-up effects on insectivores such as bats. Mixed farming, where crops and livestock are integrated into the crop rotation is thought to mitigate biodiversity declines via reduced synthetic inputs and increasing landscape heterogeneity. In addition, mixed farming could be beneficial for bats in the autumn, as pasture or livestock may provide greater insect resources than bare (i.e. harvested) arable fields at a time of year that is crucial for bats feeding up prior to hibernation. In chapters two to four I aimed to assess the direct and indirect effects (mediated by landscape attributes and local features) on bats and nocturnal insects. I collected data across 26 farms in North-East Scotland (13 mixed and 13 arable farms) on nocturnal insect abundance and diversity using light trapping during the summer months and bat activity during both the summer months and the autumn using acoustic methods. I found very little overall benefit of mixed compared with arable farming. Key findings were that Pipistrelle habitat selection was different between the two farm types, with Pipistrellus pygmaeus avoiding arable fields and selecting woodland where this was available. Pipistrellus pipistrellus preferred buildings on both farm types and avoided arable fields on mixed farms in favour of pasture. In addition, woodland edge density at the landscape scale, which was slightly higher on mixed farms, was positively associated with bat activity, moth abundance and moth species richness. Taller hedgerows, which were marginally lower on mixed farms benefitted both bats and a range of insect groups. Insect abundance and diversity and bat activity were similar over pasture than between crop fields, suggesting that pasture management may be preventing mixed farming from being beneficial. Thus, in chapter five I assessed the effect of a regenerative grazing practice, ‘mob-grazing’ on moths. Mob grazing aims to mimic wild megaherbivore grazing behaviour by increasing herd size whilst also increasing pasture rest time and moving livestock daily. I compared moth abundance and diversity in 13 fields mob grazed by cattle and 13 set-stock fields (where cattle are kept in the same field for several weeks). I found no direct benefit of mob grazing on moths. However, mob grazing reduced the negative effect of sheep grazing on caterpillar abundance. Grazing intensity also had a negative effect on caterpillar abundance and species richness. Findings from this thesis can help to inform both farmers seeking to mitigate biodiversity loss and government agricultural policies that aim to encourage wildlife-friendly farming practices.