Range loss of a threatened grouse species is related to the relative abundance of a mesopredator

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.