How accurate are single site transect data for monitoring butterfly trends?: Spatial and temporal issues identified in monitoring Lasiommata megera

Multiple transect counts following Butterfly Monitoring Scheme (UKBMS) guidelines and Jolly-Seber estimates of population size were used to monitor the abundance of second generation Lasiommata megera on a single site in southern England. The two methods resulted in different patterns of emergence being detected. The proportion of the population (estimated by Jolly-Seber) recorded with transect counts depended on the time of day and weather with afternoon transect counts best recording the trend in abundance over the flight period, but even then counts recorded a variable fraction of the population (6.2-51.3%). Increasing the frequency with which transect counts are carried out per week reduced variation and increased the fit of transect counts to Jolly-Seber generated population estimates. However, indices of abundance generated from randomly selected transect counts for L. megera within sampling weeks varied 4-fold and indices for other butterfly species were also highly variable. For L. megera, transect count variability is attributed to non-representative placement of the transect route and changes in the behaviour and spatial distribution in relation to population size and season. We suggest that transect counts need to be fully validated before the data are used to monitor changes of butterfly populations at individual sites.