Citation: K€ ammerle, J.-L., J. Coppes, S. Ciuti, R. Suchant, and I. Storch. 2017. Range loss of a threatened grouse species is related to the relative abundance of a mesopredator. Ecosphere 8(9):e01934. 10. 1002/ecs2.1934 Abstract. Mammalian generalist mesopredators can reach high densities in forest-farmland mosaic landscapes in the absence of top-down control. The abundance of generalist mesopredators is a potentially limiting factor for prey populations, especially ground breeding birds such as grouse. High mesopredator abundance has been associated with reduced reproductive success in grouse. There is little evidence, however, on how variation in mesopredator abundance affects grouse population trends while considering other environmental covariates. We make use of range maps spanning two decades (1993-2013) of a locally threatened capercaillie (Tetrao urogallus) population in the Black Forest, Germany, to assess whether range loss of grouse in forest-farmland mosaic landscapes can be explained by a gradient in red fox abundance, while accounting for other potential determinants of grouse range loss. We show that capercaillie range persistence was favored by increasing snow cover, decreasing index of red fox abundance, slightly increasing index for soil quality, and increasing population connectivity. Red fox abundance had the largest relative impact in areas already facing an elevated capercaillie extinction risk due to unsuitable site conditions, dense forests, or lack of connectivity, but the negative effect was compensated under otherwise optimal conditions. This indicates that the relative importance of predator abundance for prey population dynamics is mediated by environmental attributes, emphasizing the threat to remnant populations but also indicating potential for species conservation.

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As a non‐invasive and inexpensive method, the use of faecal glucocorticoid metabolites (FGM) analysis in wildlife research is increasing. Various environmental factors have been shown to influence FGM levels, or faecal corticosteroid metabolites (FCM) levels in birds, but most studies do not account for inter‐individual variance, which we hypothesized may substantially affect results. We combined FCM analysis with genetic analysis to identify the sex and individual's identity in samples collected in three consecutive winters; with repeated samples per individual, across the entire range of an endangered population of capercaillie Tetrao urogallus in south‐western Germany. Using generalized additive mixed models, we modelled FCM levels as a function of sex, season and environmental covariates at two spatial scales: location and home range. We compared two models: one including information on the individual animal and the other excluding this information (i.e. naïve model) to assess the influence of individual heterogeneity on the results obtained. Models accounting for inter‐individual differences explained 44.0% and 45.1% (at the location and home‐range scale respectively), while only very little (4.0% and 5.1%, respectively) was explained by the environmental predictors. When ignoring individual effects, the model results changed considerably with other, previously non‐informative predictors, becoming significant. In the full models, accounting for inter‐individual variance, weather conditions had no effect at either scale. FCM levels were negatively correlated with habitat quality at the sampling location, while human recreation at the home‐range scale led to elevated FCM levels. In the naïve models, two additional predictors appeared significant: one weather variable at the local scales and two at the home‐range scale. In all models, seasonal FCM patterns differed significantly between males and females. Synthesis and applications. By combining faecal corticosteroid metabolites (FCM) analysis with genetic individual assessment, we demonstrate that individual heterogeneity can explain most of the variance in faecal corticosteroid metabolites levels and that ignoring this information can lead to erroneous conclusions when testing for environmental stressors. We therefore stress the importance of identifying individuals when studying faecal corticosteroid metabolites in wildlife and recommend combining faecal corticosteroid metabolites analyses with genetic analyses to adequately address this issue.