Impact of spatial and temporal variation in calf survival on the growth of elk populations

The realized impact of a vital rate on population growth (X) is determined by both the relative influence of the vital rate on X (elasticity) and its magnitude of variability. We estimated mean survival and reproductive rates in elk (Cervus elaphus) and spatial and temporal variation in these rates from 37 sources located primarily across the Rocky Mountain region and northwestern United States. We removed sampling variance from estimates of process variance both within and across vital-rate data sets using the variance discounting method developed by White (2000). Deterministic elasticities calculated from a population matrix model parameterized with these mean vital rates ranked adult female survival (e(Scow) = 0.869) much higher than calf survival (e(Scalf)= 0.131). However, process variance in calf survival ((sigma) over cap2(Scalf)= 0.039) was > 11 times greater than process variance in female survival ((sigma) over cap (2)(Scow) = 0.003) across data sets and 10 times greater on average (sigma) over cap (2)(Scalf) = 0.020; (sigma) over cap (2)(Scow) = 0.002) within studies. We conducted Life-Stagc Simulation Analysis to incorporate both vital-rate elasticity patterns and empirical estimates of variability to identify those vital rates most influential in elk population dynamics. The overwhelming magnitude of variation in calf survival explained 75% of the variation in the population growth rates generated from 1,000 matrix replicates, compared to just 16% of the variation in lambda explained by variation in female survival. Variation in calf survival greatly impacts elk population growth and calls into question the utility of classical elasticity analysis alone for guiding elk management. These results also suggest that the majority of interannual variability that wildlife managers document in late-winter and spring elk surveys is attributable to variation in calf survival over the previous year and less influenced by variation in the harvest of females during the preceding autumn. To meet elk population size objectives, managers should consider the inherent variation in calf survival, and its apparent sensitivity to management, in addition to female harvest.