Nutritional Ecology at Landscape Scales

Linking Nutrition to Lamb Survival, Movement, and Disease Transmission in Bighorn Sheep

Disease (primarily pneumonia) has been linked to the episodic decline of bighorn sheep populations throughout their range, including in Idaho. Yet, a complete understanding of the factors that influence susceptibility of bighorn sheep to disease, or of the factors that influence the spread of disease within and among populations, remains elusive. Nutritional condition integrates a variety of complex responses of ungulates to their environment, and individuals in poor condition often have both a higher probability of contracting disease and a lower probability of survival once infected. Moreover, nutritional condition can strongly influence animal movements, which has important implications for the spread of disease. Some evidence suggests that because of their unique physiology and life history, bighorn sheep may be more sensitive to variation in the quality, abundance, or distribution of forage resources than other species of ungulates. The degree to which spatiotemporal variation in the nutritional landscape might modulate disease dynamics in bighorn sheep, however, has not been evaluated. This project is designed to address two broad categories of questions:

Lamb survival: Is early lamb survival modulated by nutrition in two bighorn sheep herds that occupy different nutritional landscapes in Idaho?

  1. Is late-winter body condition of ewes related to subsequent neonatal mortality from a variety of sources?

  2. How does late-winter body condition influence use of the nutritional landscape by ewes, and how do those behavioral decisions affect lamb survival?

  3. How does variation in the nutritional landscape across space and time influence movements of ewes, and what are the ramifications of those movements for lamb survival?

Study Location: Idaho, USA.

Timeline: This project was initiated in the fall of 2017, and is being conducted by Nikie Bilodeau for her MS degree.


Predicting Vital Rates of Elk

Pregnancy rates, which in ungulates are directly affected by nutritional condition in late summer, can strongly influence the finite rate of increase of a population (λ). As a result, information on pregnancy rates can be used by managers to predict population trends and make decisions ranging from allowable harvest to access restrictions. Reliably quantifying variation in nutritional condition and/or associated vital rates, however, has previously required the capture and handling of numerous individuals from each population of interest, making those data expensive, time-consuming, and risky to collect. Modern vegetation indices such as NDVI and EVI provide a mechanism for mapping the nutritional landscape available to ungulates, but thus far few researchers have attempted to link those indices with on-the-ground measures of forage quality and abundance, as well as behavior, condition, and vital rates of large herbivores. The primary objectives of this project are to: 1) model the relationship between remotely sensed vegetation indices and empirical estimates of spatiotemporal variation in forage quality and abundance for elk; and 2) combine the resulting forage model with data on elk habitat selection and pregnancy rates to model spatiotemporal variation in pregnancy rates as a function of habitat quality at a Statewide scale.

Study Location: Idaho, USA.

Timeline: This project was completed in the spring of 2019 by Sierra Robatcek for her MS degree. Read Sierra’s thesis here.


Fitness Consequences of Interference Competition

Over the past several decades, mule deer populations have declined precipitously throughout their historic range in western North America. At the same time, elk populations have remained stable or have increased. Accordingly, interference competition with elk has been hypothesized to be a leading cause of mule deer declines. Despite growing evidence that such competition occurs, however, the mechanisms by which interference competition might indirectly impact fitness of mule deer (a necessary prerequisite for causing population declines) have not been established. The goals of this project are to understand (a) whether elk displace mule deer from the most favorable parts of the nutritional landscape, (b) whether that displacement translates into effects on early winter body condition (and thus, ostensibly, fitness), and (c) whether the effects of interference competition are modulated by individual variation in mule deer behavior.

Study Location: Starkey Experimental Forest and Range, Oregon, USA.

Timeline: This project was completed in the spring of 2018 by Jennifer Merems for her MS degree. Read Jen's thesis here.