Lepidoptera communities across an agricultural gradient: how important are habitat area and habitat diversity in supporting high diversity?

Agricultural expansion and intensification have been linked with losses of biodiversity and disruption of key ecosystem services in farmed landscapes. A number of mitigation and adaptation strategies e.g. agri-environment schemes, have been implemented to counter these declines but their effectiveness has been questioned by conservationists as well as policy makers. A key concern is the lack of knowledge about how conservation efforts might best be directed; especially in terms of aligning the scale and type of implementation in different landscapes with the niche requirements and dynamics of different species and taxa. Here we focus on how the landscape context of farmed systems at different spatial scales determines the abundance of habitat generalist and specialist butterfly and moth species in southern England. We used butterfly data from 20 UK Butterfly Monitoring Scheme sites, supplemented with moth data from a light trap survey of 11 sites, in southern England where the predominant habitat type sampled is lowland calcareous grassland. Our results show that larger areas of semi-natural grassland generally support larger numbers and a greater species richness of butterflies and moths, but that the composition of the Lepidoptera fauna changes with habitat size depending on the diversity of habitats in the landscape, particularly at the larger spatial scale. Larger areas of grassland in less diverse landscapes result in a proportionally greater number and diversity of habitat specialists, whilst habitat diversity is important in maintaining numbers and diversity of more generalist wider countryside species. Large areas of single habitat types may only be economically maintained at the expense of habitat diversity. Whilst these sites may be important in promoting abundance and diversity of selected specialist species, they may be prone to lower overall species richness. This has implications for improving the design and implementation of agri-environment schemes aimed at providing suitable habitat in order to promote the abundance and diversity of Lepidoptera and other taxa.