Estimating sampling efficiency of diurnal Lepidoptera in farmland

Setting up effective survey strategies for biodiversity monitoring in agro-ecosystems is a major task in order to detect adverse effects on biodiversity before negative changes will manifest. Here, we studied the relative costs required for the monitoring of butterflies and selected diurnal moths (Papilionoidea et Hesperioidea; Zygaenoidea: Zygaenidae) in farmland. Analysing data from a well-established Lepidoptera monitoring system in Switzerland, we assessed the influence of inspection periods, inspection frequency and transect length on counts of diurnal Lepidoptera. Furthermore, we estimated the number of transects in relation to sampling effort necessary to detect changes of a given effect size for recorded species number (and abundance). Reducing the counting frequency from seven to four inspections per season still yielded 80–90 % of the species, as long as peak abundances in summer months were included. The variation in observed species number was mostly independent of inspection frequency, but strongly increased when transect length was reduced to less than 1 km. Sedentary Lepidoptera species are especially valuable indicators as their occurrences are directly linked to local effects on biodiversity, and the proportion of recorded sedentary species was not substantially affected by reduced inspection frequency. Transects of 1–1.5 km length were generally the most cost-efficient to detect an effect on total species number of diurnal Lepidoptera in arable landscapes, given that travelling distances between transects were short. Studying effects on rare species or selected species groups would involve higher sampling intensity and costs. Surveying schemes with reduced inspection frequency and transect lengths can detect changes in species richness and total abundance of diurnal Lepidoptera cost-effectively. Facing expected changes in agricultural policy and management, the results and recommendations presented here will help to implement and improve cost-efficient Lepidoptera schemes to monitor changes in arable landscapes.