Novel grid-based population estimates correlate with actual population sizes of the marsh fritillary (Euphydryas aurinia), while transect and larvae counts are less reliable

1. Established butterfly survey methods that are useful for monitoring species that are widely distributed and numerous may be less accurate for more rare species. We therefore need new monitoring approaches.2. We describe a plot-based survey method, where butterflies or larvae nests are counted within 1-ha grid cells. The aim was to compare this grid method with more traditional transect counts and evaluate both methods in relation to high-quality capture-mark-release (CMR) population estimates (reflecting the true population). We do this using data from a large population of the marsh fritillary butterfly in Sweden. Moreover, we followed the overall population trend from 2017 to 2021 for both adult butterflies and larvae.3. Results showed a higher detection probability using the grid method compared with transect counts, which for adult butterflies seem to be explained by time effort. Moreover, grid surveys of adult butterflies showed a clear significant relationship with the estimated true population size from CMR, while transect counts did not. For larvae, both methods showed significant relationships with the estimated adult population size, but the grid method found 5.7 times more larvae. The overall popu-lation fluctuated significantly across years. In years with low densities, the transect method largely failed to detect the species.4. The grid method seems more reliable for detecting the marsh fritillary and for estimating its population size, and thus, tracking the population trend. We propose this novel method to be integrated into surveys and monitoring of biodiversity, especially when focusing on rare habitat specialists that are normally underrepre-sented in monitoring based on volunteer counts.
Funding Agencies|Cementa AB; Svenska Forskningsradet Formas