Estimating the population size of specialised solitary bees

1. Reliable methods for quantifying population size are crucial for strategies to conserve endangered wild-bee species. Estimates of population size obtained through survey walks were compared with estimates obtained through mark-recapture studies in 10 populations of the red-listed solitary bee Andrena hattorfiana in southern Sweden. 2. The mean number of bees observed during survey walks was strongly correlated with estimates of population size obtained with mark-recapture. It was found that 5.5-23.4% (mean 13.4%) of the total population was observed during an average survey walk. 3. One component in mark-recapture analysis is the measure of survival of individuals. In the largest bee population, females of A. hattorfiana that emerged in early season were found to forage for pollen on average 18.4 days. 4. The findings suggest that during large-scale surveys, for example re-inventories for red-listed species, the population size of solitary bees can be quantified reliably and effectively by performing survey walks in a two-step process. The first step consists of survey walks to establish the relationship between number of bee observations per survey walk and mark-recapture population size for a small set of populations. In the second, simple observation survey walks can be performed for a large set of populations. In each population of A. hattorfiana, it is recommended that at least six survey walks are performed.