Effect of archer density on elk pregnancy rates and conception dates

Abstract

Archery hunting in Oregon has increased dramatically over the past 2 decades. At the same time, spring juvenile to adult female ratios of Rocky Mountain elk (Cervus elaphus) have been declining. This has raised concern that archery seasons may be disrupting elk breeding and contributing to the decline in recruitment. Two mechanisms could contribute to reduced juvenile:female ratios: 1) reduced pregnancy rates, and 2) delayed conception dates because of human disturbance during the rut. We varied the number of archery hunters at the Starkey Experimental Forest and Range over 13 years to evaluate effects of archer density on reproduction of elk. Archer densities were maintained at high densities during 4 years ðx ¼ 1:09 tags sold=km 2 Þ, low densities during 3 years ðx ¼ 0:53 tags sold=km 2 Þ, and no archers during 6 years. We determined pregnancy status, age, kidney fat index (KFI), lactation status, and fetus conception dates for 622 female elk harvested in December. We found pregnancy rate differences of 0.105, 0.080, and 0.021 between high and no archer density years (P ¼ 0.004), high and low archer density years (P ¼ 0.054), and low and no archer density years (P ¼ 0.616), respectively. Conception dates were 4 days later for high archer density compared to low archer density (P ¼ 0.006), but did not differ between high and no archer years (2 days; P ¼ 0.136) or between low and no archer years (2 days; P ¼ 0.108). We compared generalized linear model estimates of pregnancy rates and determined pregnancy rates for 28% of the lactating female elk to be affected by high archer density, whereas archer densities had no significant affect on pregnancy rate estimates for non-lactating females. We found no difference in conception dates among archer densities when comparing model estimates. Our results suggest that archer density and its interaction with nutritional condition of elk influence pregnancy rates of lactating females with low KFI levels; however, the effect of archer density alone does not explain the magnitude of decline in juvenile to female ratios observed in Oregon. ß 2012 The Wildlife Society.

Behavioral changes and nutritional consequences to elk (Cervus canadensis) avoiding perceived risk from human hunters

“…Although security areas for hunted species have typically been evaluated with respect to male vulnerability, risk effects on females during hunting seasons also have the potential to indirectly influence population productivity (Davidson et al. ). Following the risk allocation hypothesis, the demographic impacts of non‐consumptive effects likely depend on a population's nutritional baseline.…”

Roles of maternal condition and predation in survival of juvenile Elk in Oregon

“…Perhaps more telling, we found an inverse relation between IFBF autumn and birth date the following spring. Even when pregnancy rates are high, if nutrition is limiting, ovulation may be delayed; the higher the nutritional limitation, the later the breeding date (Trainer , Albon et al , Cook et al , Davidson et al ). This effect of inadequate summer and autumn nutrition is significant because timing and synchrony of birth date may be important for survival by allowing juveniles to increase body mass and be able to better withstand extreme winter events (Keech et al ), predator swamping to reduce vulnerability to predation (Geist ), timing of birth to coincide with optimal foraging for pre‐parturient and lactating females (Linnell and Andersen , Testa ), and lifetime fitness (Plard et al ).…”

Nutritional ecology of elk during summer and autumn in the Pacific Northwest

“…A second alternative hypothesis is that nutritional condition and pregnancy rates may be depressed by human disturbance (Davidson et al ) and predators (Creel et al ), which may increase daily movements, displace elk from nutritionally superior habitats, and disrupt normal feeding patterns. Much has been written regarding the deleterious effects of fear on elk (Creel et al , , ) largely based on research in Yellowstone National Park, but subsequent work in that ecosystem has failed to confirm those inferences of predator effects on nutritional condition, pregnancy rate, movements, and habitat use (White et al , Kauffman et al , White et al , Boonstra , Middleton et al 2013 a , b ).…”

“…However, they typically cannot identify causes of observed levels of condition. Animals may be thin because of inadequate nutrition, increased daily movements due to disturbance by human activities (Davidson et al ) and predators (Creel et al ), or because fear and displacement disrupt normal feeding patterns or force use of habitats that offer poor nutrition (Creel et al , but see White et al , Middleton et al 2013 b ). Further, it is not clear the extent to which performance responses are a result of classic density‐dependent mechanisms or are due to density‐independent ecological and successional influences on abundance, chemistry, and species composition in plant communities (DeYoung et al , Cook et al ).…”