Interacting effect of wolves and climate on recruitment in a northern mountain caribou population

There is limited research on the influence of Pacific-based climate in large herbivore populations. Additionally, much of our understanding on the effect of large-scale climate on ungulate population dynamics has occurred on forage-limited rather than predator-limited populations. We compared the influence of the Pacific Decadal Oscillation (PDO), North Pacific Index, and local weather on recruitment in a predator-limited mountain-dwelling caribou Rangifer tarandus caribou population in the Yukon Territory, Canada, across a range of wolf Canis lupus densities. A large-scale wolf removal program allowed us to examine the role of Pacific climate and weather when wolves were reduced to similar to 15% of their pre-removal levels. Recruitment was best explained by the interaction of wolf density and April-PDO, with wolf density explaining the most deviance. Predicted recruitment during good springs was 0.45 (SE = 0.04) during wolf removal and 0.29 (SE = 0.03) with no wolf removal. During poor springs (low PDO, increased snow depth) predicted recruitment was 0.55 (SE = 0.10) during wolf removal and 0.12 (SE = 0.03) with no wolf removal. With non-altered wolf densities, there was a positive relationship between April-PDO and recruitment due to reduced snow depth at calving, allowing parturient females to disperse up in elevation away from predators. When wolf densities were substantially reduced there was a slight negative relationship between April-PDO and recruitment, possibly due to a more rapid vegetation green-up reducing the temporal availability of highly nutritious forage necessary for lactation and subsequent calf growth. Attempts to find general relationships between climate and ungulate population dynamics have proven difficult due to different ecological mechanisms by which climate affects individuals across populations. Temporally varying factors, such as predator density, may also play an important role in uncovering the mechanistic relationship between climate and population dynamics.