Standardized butterfly surveys: comparing transect counts and area-time counts in insect monitoring

The observed insect decline, which threatens agricultural productivity and ecosystem stability, calls for comprehensive international insect monitoring. Monitoring implementation demands standardisation and the integration of new and innovative methods. Therefore, we compared two quantitative butterfly survey methods - the commonly applied transect counts (or 'Pollard walks') and more extensive area-time counts. We evaluated the influence of the two methods on the estimation of biodiversity variables such as species richness and species abundance to examine whether they could be applied alternatively for the calculation of butterfly trend indicators. During 576 surveys we conducted 5-minute transect counts and 25-minute area-time counts simultaneously at 144 different sites in Western Austria. The estimated relative butterfly abundance of the two methods for 119 observed species showed a strong linear relationship. While we found 2.4 times more species per site with the more extensive area-time counts than with the transect counts, we also observed a strong correlation between estimates of local abundance (Pearson's r = 0.85) and observed species richness (Pearson's r = 0.81) based on the two methods. Area-time counts provide thorough assessments on a given location, enabling a close connection to specific habitat types and facilitating comparability with other plot-based biodiversity assessments. They are more suitable than transect counts when aiming to analyse the drivers of temporal and spatial variability in butterfly occurrence. Furthermore, area-time counts can be used synergistically for the calculation of international butterfly abundance trends (e.g., European butterfly indicators), as we found strong linear relationships for all applied indicators with both methods.