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

We collected and chemically analyzed urine in snow (snow-urine) and simulat ed physiology of elk (Cervus elaphus) and bison (Bison bison) at the popula tion level to assess changes in nutritional restriction, physical condition (i.e., fat reserves), and mortality rates on the Northern and Madison-Fire hole ranges and at Pelican Valley in Yellowstone National Park during winte rs 1987-88 (1988) to 1989-90 (1990). On the Northern and Madison-Firehole r anges, elk diets varied over the 3 winters with lowest consumption of grass es (P less than or equal to 0.006) and highest consumption of conifers (P l ess than or equal to 0.044) during the first winter post-fire (1989), which was also the most severe winter with respect to snow depth. Sedge use by N orthern range elk also varied (P less than or equal to 0.002) with lowest u se during pre-fire winter 1988 and greatest use during winter 1990; sedge u se was stable at the more thermally-influenced Madison-Firehole range. Decr easing (F less than or equal to 0.014) mean urinary potassium:creatinine (K :C) ratios reflected progressive nutritional restriction on all sampling ar eas during all 3 winters. Mean urinary urea nitrogen:creatinine (UN:C) rati os 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 i ndicated nutritional restriction was greatest during winter 1989, and restr iction was increasingly severe from the lower to upper elevations of the No rthern range and most severe at Madison-Firehole, where snow depths were gr eatest. On the Northern and Madison-Firehole ranges, bison use of grasses decreased (F less than or equal to 0.006) and sedges increased (P less than or equal to 0.005), while conifer use remained low and stable throughout the study. In Pelican Valley, bison food habits were unaltered during the study. Biso n 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 less than or equal to 0.014) throughout all w inters on all sampling areas. Mean UN:C ratios increased (P less than or eq ual to 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 tempo ral trends indicated that nutritional restriction of bison progressively in creased each winter and was most severe during winter 1989. Measured UN:C r atios 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, th ere were spatial and temporal differences in fat reserves and mortality rat es of elk and bison. The physiological model predicted that Madison-Firehol e elk and bison were more severely nutritionally restricted and had lower f at reserves during winter 1989 than elk and bison on the Northern range. Lo wer 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 amplifie d during the more severe winter.