Factors influencing elk recruitment across ecotypes in the Western United States

Abstract

Ungulates are key components in ecosystems and economically important for sport and subsistence harvest. Yet the relative importance of the effects of weather conditions, forage productivity, and carnivores on ungulates are not well understood. We examined changes in elk (Cervus canadensis) recruitment (indexed as age ratios) across 7 states and 3 ecotypes in the northwestern United States during 1989-2010, while considering the effects of predator richness, forage productivity, and precipitation. We found a broad-scale, long-term decrease in elk recruitment of 0.48 juveniles/100 adult females/year. Weather conditions (indexed as summer and winter precipitation) showed small, but measurable, influences on recruitment. Forage productivity on summer and winter ranges (indexed by normalized difference vegetation index [NDVI] metrics) had the strongest effect on elk recruitment relative to other factors. Relationships between forage productivity and recruitment varied seasonally and regionally. The productivity of winter habitat was more important in southern parts of the study area, whereas annual variation in productivity of summer habitat had more influence on recruitment in northern areas. Elk recruitment varied by up to 15 juveniles/100 adult females across the range of variation in forage productivity. Areas with more species of large carnivores had relatively low elk recruitment, presumably because of increased predation. Wolves (Canis lupus) were associated with a decrease of 5 juveniles/100 adult females, whereas grizzly bears (Ursus arctos) were associated with an additional decrease of 7 juveniles/100 adult females. Carnivore species can have a critical influence on ungulate recruitment because their influence rivals large ranges of variation in environmental conditions. A more pressing concern, however, stems from persistent broad-scale decreases in recruitment across the distribution of elk in the northwestern United States, irrespective of carnivore richness. Our results suggest that wildlife managers interested in improving recruitment of elk consider the combined effects of habitat and predators. Efforts to manage summer and winter ranges to increase forage productivity may have a positive effect on recruitment. Ó

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

“…Throughout their range, elk occupy vastly different habitats where seasonal and regional differences in quality and quantity of nutritional resources vary as evident by autumn body fat of lactating females (Cook et al ). Additionally, assemblages and densities of carnivores vary throughout the range of elk, leading to variation in juvenile and adult survival (Griffin et al , Brodie et al , Lukacs et al ). These studies highlight the importance of understanding factors affecting population dynamics of elk at local and regional scales before embarking on management actions to improve productivity.…”

“…Therefore, simultaneous investigations of climate and bottom‐up and top‐down forcing on prey populations are of greater utility than if examined separately. Increasingly more studies are attempting to assess the synergistic effects of these factors (Vucetich et al , Garrott et al , Melis et al , Johnson et al , Lukacs et al ).…”

NDVI exhibits mixed success in predicting spatiotemporal variation in caribou summer forage quality and quantity

“…Investigators have demonstrated associations between NDVI and plant primary productivity and biomass (Tucker and Sellers 1986, Reed et al 1994, Thoma et al 2002, but less is known about the utility of NDVI for predicting the nutritional quality of ungulate forages, a key factor driving patterns of ungulate behavior and demography (Fryxell 1991, Tollefson et al 2011. Also, NDVI has been negatively associated with ground plant biomass in some habitat types (Borowik et al 2013), and summer NDVI has been negatively correlated with ungulate recruitment (Lukacs et al 2018) and abundance (Fauchald et al 2017). For example, higher NDVI is commonly assumed to reflect better forage conditions, but investigators have found that animals select for intermediate rather than high values of NDVI (Mueller et al 2008, Singh et al 2010.…”

Effects of wolf pack size and winter conditions on elk mortality

“…Additionally, elk are managed for their social importance (McCorquodale ), economic value (Fried et al , Schorr et al ), and diverse ecological effects (Kauffman et al , Mech ). Over the past several decades there has been a broad‐scale decline in elk recruitment across the western United States (Lukacs et al ). Concomitantly, wolves have become well established in many areas of the western United States since their reintroduction in 1995 (Jimenez et al ) and there is pressure for state wildlife management agencies to manage wolves to benefit ungulate populations.…”