Field surveys of capercaillie (Tetrao urogallus) in the Swiss Alps underestimated local abundance of the species as revealed by genetic analyses of non-invasive samples

Abstract

An increasing number of species are becoming threatened by habitat loss and fragmentation. Therefore, solid estimates of the species’ abundance in the remaining populations are required to develop suitable conservation measures and to monitor their effectiveness. The capercaillie (Tetrao urogallus L.) has experienced a dramatic decline in central Europe and has disappeared from large areas of its former natural range. In Switzerland, the species’ distribution, habitat requirements and demographic status were studied and evaluated in an attempt to support appropriate management decisions to conserve the species. National surveys of the capercaillie in Switzerland have traditionally been obtained from male counts at leks. However, individual attendance to the lek is sex- and age-specific. Thus, male counts at leks may provide a biased estimate of local population sizes. In the present study, we compared two alternative indirect methods to estimate the sizes of local populations at eight study sites situated in the Alps and Prealps of Switzerland. We first assessed the sizes of local populations from the observed density and distribution of direct and indirect evidence of the species’ presence during field surveys. Feather and faeces samples collected during field surveys were genotyped at twelve nuclear microsatellite loci and a sex-specific nuclear gene fragment. Individual genotypes were used as genetic tags to estimate the sizes of the eight local populations using an urn model developed for small populations. The index of local population sizes assessed from field surveys was lower than the number of unique genotypes at each study site, which itself underestimated the abundances of populations in most cases. Based on our results, the genetic tagging method appeared to be less biased than the field survey method. However, an alternative faeces sampling scheme, resulting in 2–3 genotypings per individual, could further improve the accuracy of the size estimates of local populations. Our study confirms that genetic tagging methods are a valuable tool to estimate the sizes of local populations and to monitor the response of rare and elusive species to management actions.