Winter nutritional restriction and simulated body condition of yellowstone elk and bison before and after the fires of 1988

We collected and chemically analyzed urine in snow (snow-urine) and simulated physiology of elk (Cervus elaphus) and bison (Bison bison) at the population level to assess changes in nutritional restriction, physical condition (i.e., fat reserves), and mortality rates on the Northern and Madison-Firehole ranges and at Pelican Valley in Yellowstone National Park during winters 1987-88 (1988) to 1989-90 (1990). On the Northern and Madison-Firehole ranges, elk diets varied over the 3 winters with lowest consumption of grasses (P ≤ 0.006) and highest consumption of conifers (P ≤ 0.044) during the first winter post-fire (1989), which was also the most severe winter with respect to snow depth. Sedge use by Northern range elk also varied (P ≤ 0.002) with lowest use during pre-fire winter 1988 and greatest use during winter 1990; sedge use was stable at the more thermally-influenced Madison-Firehole range. Decreasing (P ≤ 0.014) mean urinary potassium:creatinine (K:C) ratios reflected progressive nutritional restriction on all sampling areas during all 3 winters. Mean urinary urea nitrogen:creatinine (UN:C) ratios of elk varied (P < 0.001) on the lower, middle, and upper Northern range and at Madison-Firehole during all 3 winters, except on the lower Northern range during winter 1989. Values of mean UN:C ratios and temporal trends indicated nutritional restriction was greatest during winter 1989, and restriction was increasingly severe from the lower to upper elevations of the Northern range and most severe at Madison-Firehole, where snow depths were greatest. On the Northern and Madison-Firehole ranges, bison use of grasses decreased (P ≤ 0.006) and sedges increased (P ≤ 0.005), while conifer use remained low and stable throughout the study. In Pelican Valley, bison food habits were unaltered during the study. Bison and elk diets were more similar at Madison-Firehole than on the Northern range, suggesting greater competition for food at Madison-Firehole. Mean K:C ratios of bison declined (P ≤ 0.014) throughout all winters on all sampling areas. Mean UN:C ratios increased (P ≤ 0.030) on the Northern, Madison-Firehole, and Pelican Valley ranges in each winter. Mean UN:C ratios were lower in bison than in elk, but temporal trends indicated that nutritional restriction of bison progressively increased each winter and was most severe during winter 1989. Measured UN:C ratios and herd composition were used by the physiological model to predict DEI on each range for bison and elk. At the predicted DEI on each range, there were spatial and temporal differences in fat reserves and mortality rates of elk and bison. The physiological model predicted that Madison-Firehole elk and bison were more severely nutritionally restricted and had lower fat reserves during winter 1989 than elk and bison on the Northern range. Lower fat reserves and higher winter mortality rates of elk and bison calves compared to cows predicted by the physiological model were consistent with the differential pressure exerted by density-dependent factors and amplified during the more severe winter. The temporal and spatial dimensions of our physiological assessments and the variation of winter severity enabled us to demonstrate how the snow-urine/UN:C technique in combination with physiological modeling can be used to document the ecological context of the animals' physiological responses at the population level.