Linking nutritional condition to population performance in a shifting climate: Approaching the holy grail
The mechanism by which climate change affects population performance of wildlife species often is not well understood. In ungulates, we recognize that nutrition is the driving factor that affects population change. By altered precipitation, temperature, and snow cover regimes, climate change is expected to affect quantity, quality, and accessibility of forage plants for alpine mammals. Bighorn sheep in the Sierra Nevada, an endangered ungulate, are alpine specialists and spend much or all of the year above tree-line. Quantifying nutritional effects by measuring nutrients in forages and approximating their uptake by animals at the landscape scale is daunting. Alternatively, we developed a unique approach to quantify nutritional status in free-ranging ungulates that establishes a direct link between populations and their habitats. Body fat integrates caloric gain and loss as determined by factors such as forage supply, competition for food among conspecifics, winter severity, and reproductive costs. Consequently, body fat in free-ranging animals is an integrated measure of nutritional status and represents how animals balance the energetics of their environment. Energy from forage and fat, in conjunction with protein, is the currency that ultimately underpins growth and reproduction in animals. We present data from bighorn sheep that couples life-history traits with nutrition. We sampled populations throughout the southern and central Sierra Nevada, California. We used ultrasonography and a palpation score to estimate body fat. We then analyzed relationships between body condition and vital rates estimated from radio-collared individuals. We also estimated population size and the finite rate of population growth (λ).
The physiological limits of total body fat varied between 0.5 and 26%. Mean pre-winter body fat among 8 herds was 8.2 – 15.1% and 14.8 – 23.7% for lactating and non-lactating females, respectively. Mean body fat of females in late winter ranged from 10.1 – 13.6%. Nutritional condition of adult females was positively related to survival and pregnancy. Population growth rate varied between 0.6 and 1.3 and was positive for populations that exhibited mean body fat for lactating females >10.4% in October. The relationship between body fat and population growth rate supports the notion that nutritional status can represent the proximity of a population to animal-indicated nutritional carrying capacity (NCC) and therein, provides a tractable alternative to attempting to quantify total available forage across a landscape. Measuring animal-indicated NCC provides an important advancement to measure the ability of alpine landscapes to support populations under a changing climate.