Investigating the Mechanisms Behind Moth Declines: Plants, Land-use and Climate

Moth populations have declined across large areas of north-western Europe since the mid-20th century, mirroring similar declines in other insect groups. The mechanisms behind these
declines are likely manifold, but it is believed that agricultural intensification is a key factor. There were two aims of this thesis: (1) to elucidate the mechanisms behind moth decline in the UK, and (2) to determine ways in which farmland habitats could be improved for moths. Counter to expectations, between 1968 and 2016, the declines in total moth abundance were least severe in the most agriculturally intensive areas and were most severe in semi-natural habitats, as well as in urban environments. Species richness, while remaining stable at the national level, declined in only one habitat type: broadleaf woodland. No evidence was found to support the hypotheses that habitat loss, shading of the understory by canopy-closure, or overgrazing by deer had driven these declines within broadleaf woodland. Evidence was found that warm winters negatively impacted moth abundance, but this effect was consistent across all habitats. Although declines were least severe in improved grassland and arable land, the declines in total abundance were significant and ongoing, despite widespread and increasing adoption of agri-environment schemes (AES) since the early 1990s. In this thesis, the role of nectar resources and larval hostplants were explored within AES field margins on arable land, with the aim of determining how these small areas of habitat could be best managed to enhance moth abundance and diversity. It was found that the diversity of moths was greatly increased, and abundance moderately so, when margins were sown with a wide range of wildflowers and grasses, in comparison to only grasses. The evidence suggested that this was due primarily to the provision of larval hostplants, with nectar resources playing a secondary role. Overall, this thesis demonstrates that, in order to improve the environment for moths, AES wildflower margins should be encouraged over and above plain grass margins. This thesis also demonstrates that while agricultural intensification is likely responsible for some of the observed declines in moths, there are other mechanisms, as yet unknown, at work in both semi-natural habitats and urban areas.