EFFECTS OF DENSITY AND WEATHER ON SURVIVAL OF BIGHORN SHEEP LAMBS (OVIS-CANADENSIS)

Density-dependence in juvenile survival may be difficult to detect if survival is also affected by density-independent factors. We investiga ted the relationships among weather parameters, population density, an d lamb survival of bighorn sheep with long-term data from a marked pop ulation where we manipulated population density. We distinguished neon atal survival and winter survival. Density interacted with weather var iables to affect neonatal survival; spring and winter temperatures had a positive effect on neonatal survival only when population density w as high. Neonatal survival was positively affected by spring precipita tion independently of population density. Winter survival was positive ly correlated with temperature and precipitation during the previous s pring, negatively correlated with density, and independent of winter t emperature or snowfall. The effect of weather on lamb winter survival did not vary with density. Bighorn lambs are well adapted to harsh win ter weather, but spring weather influenced survival of lambs at birth and during the subsequent winter, possibly through its effects on fora ge availability. Our study clearly demonstrates density-dependence in lamb survival. Some of the effects of weather on lamb survival are den sity-independent, others are mediated by an interaction with populatio n density.