Weather, Disease, and Bighorn Lamb Survival During 23 Years in Canyonlands National Park
Wildlife Society Bulletin
The Island-in-The-Sky desert bighorn herd (Ovis canadensis nelsoni), the largest in Canyonlands National Park, Utah, is used extensively by Utah Division of Wildlife Resources to re-establish herds in former habitat throughout the state. This study was undertaken because park managers became concerned about delayed herd recovery following 4 years of transplant removals and suspended trapping operations. This concern led to my study of effects of weather on yearly lamb:ewe ratios for the park's 23-year database. I used weather variables regulating the annual growing season, soil-moisture regime, and body condition of ewes to develop multiple-regression models. Regression models, developed from 10 and 16 years of data, explained 91% and 61% of the variation in lamb:ewe ratios, respectively; I estimated lamb:ewe ratios for remaining years of data and compared them with observed values. Maximum ambient temperatures in May of the same year lambs were born and precipitation from January through March 2 years before lambs were born (thought to relate to body condition of ewes) were variables correlated most strongly with lamb:ewe ratios. Peak of lambing occurs in April and May. May is the pivotal month, which, on average, has maximum temperatures that promote negative soil-moisture storage and a decrease in forage nutrients. Disease (contagious ecthyma) was first noticed in 1986 and apparently reduced lamb survival through 1992. I used data from disease years to estimate lamb survival, but not for model development.
Douglas, C. L., "Weather, Disease, and Bighorn Lamb Survival During 23 Years in Canyonlands National Park" (2001). Canyonlands Research Bibliography. Paper 142.