Parasites and forage as determinants of body condition and population size in an imperiled ungulate

Rapid environmental changes are resulting in widespread changes in population size, health, and physiology of wildlife, especially at northern latitudes where the impacts of climate change are more pronounced. Barren-ground caribou ( Rangifer tarandus groenlandicus ) have declined across much of their range in recent decades, and while the ultimate causes are unknown, western science and local Indigenous Knowledge both point to changes in body condition and diseases as contributing factors. We used a bioenergetic integral projection model that mechanistically links caribou population dynamics to forage quality, parasitic nematodes, and flying parasitic and biting insects through their effects on body condition. We investigated (i) the effects of parasites on caribou populations and available forage biomass, (ii) the sensitivity of model dynamics to changes in key demographic and physiological parameters, and (iii) modeled how changes in insect harassment, nematode abundance, and forage quality are likely to impact caribou populations. We found that parasites and insects can have substantial impacts on nutritional condition and population dynamics of caribou, and that these impacts can cascade through the ecosystem. In the absence of parasites, caribou numbers were limited by forage resulting in population sizes reaching a stable carrying capacity. Reductions in foraging rate and increased energetic costs associated with parasites lowered caribou population size and increased biomass of available forage. Our study provides a critical contribution toward better understanding of the cumulative effects of multiple stressors on wild populations and outlines a framework for mechanistically evaluating the impacts of energetics, parasites, and disease on wildlife.