Factors influencing productivity and recruitment of elk in northern New Mexico
Authors: Watkins Bruce E. , Bergman Eric J. , Dhaseleer Leslie C. , Bernal Lance J. 
Published: December 2022

Declining recruitment in elk (Cervus canadensis) populations is a common issue faced by managers in western North America. To better understand a decline in calf:female (≥1 yr) ratios in northern New Mexico, USA, we investigated the influence of bottom‐up factors on the condition and productivity of 1,885 adult (≥2 yr), female Rocky Mountain elk (C. c. nelsoni) harvested on the Vermejo Park Ranch during December and January, 2009–2016. We used ingesta‐free body fat (IFBF) estimated from kidney fat mass as a measure of condition. Based on maximum likelihood model selection, age, harvest date, hunt zone, pregnancy status, lactation status (as determined in Dec–Jan), June–August precipitation, and December–March mean temperature were important variables for predicting IFBF and field‐dressed mass (FDM). Age, IFBF, FDM, harvest date, and June–August precipitation were important variables for predicting conception date, pregnancy rate, and lactation rate. Pregnancy status and lactation status were also important for predicting lactation rate and pregnancy rate, respectively. Older females (≥12 yr) had progressively lower IFBF and FDM and later conception dates than prime females (3–11 yr) and their pregnancy rates declined an average of approximately 9%/year after age 11. The probability of pregnancy in prime females generally exceeded 0.95 when IFBF was ≥12% and FDM was ≥155 kg in late December and early January. Lactating females had lower IFBF, FDM, pregnancy rates, and later conception dates than nonlactating females. The mean IFBF of females harvested on 1 December was generally 2.3–2.7 percentage points higher than values of females harvested on 31 January within age and lactation categories. There was strong evidence that greater IFBF and FDM, higher pregnancy rates, and earlier conception dates in nonlactating females and all adult females were related to increased June–August precipitation (P < 0.01) during the conception year, but, with the exception of conception date, there was little evidence in lactating females. Greater conception year June–August precipitation (P = 0.04) and greater mean annual IFBF of nonlactating females (P < 0.01), but not conception year IFBF of lactating females (P = 0.94), were related to higher subsequent September calf:female ratios. There was also strong evidence that earlier mean conception dates and higher pregnancy rates of adult females (P < 0.01) were related to higher calf:female ratios. The only birth year variables at least moderately related to higher calf:female ratios were lower mean IFBF (P = 0.03) and FDM (P = 0.02) of adult females that likely reflected negative lactation effects. Based on our bivariate models, September calves/100 females increased 10.7 calves per 0.1 increase in the annual adult pregnancy rate and 10.9 calves per 10‐cm increase in June–August precipitation during the conception year. Our results indicated that bottom‐up factors related to summer precipitation the previous year and age structure of the adult female population had meaningful effects on September calf:female ratios at Vermejo during our study. We found strong evidence of a nexus among summer precipitation, IFBF, conception dates, pregnancy rates, and following year calf:female ratios in nonlactating females but not in lactating females even though probability of pregnancy was primarily determined by IFBF irrespective of lactation status.