Effects of grass and browse consumption on the winter mass dynamics of elk

Abstract

Much of the research into herbivore ecology and evolution has focused on patterns and mechanisms of niche partitioning in the diets of specialized grazers and browsers, but the significance of the balance of grazing and browsing within intermediate feeders present unique questions that have received less attention. We explored the nutritional effects of varying the balance of grass and browse in the winter diets of elk (Cervus elaphus). We compiled published data from three similar studies that monitored the mass dynamics of captive elk fed diets of pure grass, pure browse or 1 of 14 mixed diets in winter feeding trials. Elk lost mass (up to 22% of initial body weight) in 29 of 33 feeding trials, similar to wild elk in winter. We used regression models of mass dynamics, considering the linear, quadratic or logarithmic effects of the proportion of the diet that was grass ( approximately 1 - proportion of the diet that was browse) and the additive and interactive effects of nitrogen intake. Diet composition had strong effects on mass dynamics, and all models explained > or =73% of the variation (adjusted r (2)) in mass dynamics. Nitrogen intake had uniformly positive effects on mass balance, and increasing grass intake caused improved maintenance of body mass up to a point, but further increases in the proportion of grass in the diet had neutral or negative effects on body mass. Overall, the data suggest that elk are adapted to consuming mixed diets. Nonetheless, data on the foraging behavior and diet selection of wild elk in many populations show that elk often consume grass-dominated diets and sometimes consume browse-dominated diets, but rarely consume mixed diets. Physiological adaptations to mixed diets may place unique spatio-temporal constraints on diet selection in intermediate feeders and impose large penalties for a sub-optimal balance of grazing and browsing.

Glucocorticoid stress hormones and the effect of predation risk on elk reproduction

“…Risk effects can also cascade to affect community composition and ecosystem function (2, 7, 54, 55). For elk, exposure to wolf predation is associated with changes in grouping patterns (56,57), habitat selection (34,35), vigilance (31-33), foraging (33), diet (39,58), nutrition (38)(39)(40)58), reproductive physiology (30), and demography (8,30). Elk populations in the GYE (and elsewhere e.g., Banff National Park) have decreased in size since wolf recolonization, while elk populations in neighboring areas with low wolf density grew steadily as a result of weak limitation by winter snowfall (43,45,46,59).…”

“…Elk strongly prefer grazing to browsing, and habitat type is a strong predictor of the balance of grazing and browsing in elk diets (37). Experiments with GYE elk show that changes in the balance of grazing and browsing affect the rate of mass loss over winter (38). For GYE elk in the Gallatin population, changes in habitat selection and feeding behavior were associated with significant changes in diet and nutrition (38,39), including a shift from grazing to browsing (39) and a reduction in estimated intake rates by 27% of maintenance requirements (40).…”

“…Experiments with GYE elk show that changes in the balance of grazing and browsing affect the rate of mass loss over winter (38). For GYE elk in the Gallatin population, changes in habitat selection and feeding behavior were associated with significant changes in diet and nutrition (38,39), including a shift from grazing to browsing (39) and a reduction in estimated intake rates by 27% of maintenance requirements (40). GYE elk steadily lose body mass and fat through winter (33,41), and nutritional condition affects pregnancy rates in elk (42).…”

Why should a grazer browse? Livestock impact on winter resource use by bharal Pseudois nayaur

“…Van Wieren (1996) subsequently demonstrated the robustness of this classiWcation by examining the graminoid content in the diet of 45 ruminant species. Intermediate feeders are expected to have adapted to a diet with speciWc proportions of graminoids and browse because of the broad diVerences in grass and browse (Christianson and Creel 2009). Yet, despite the specialization, many herbivores show considerable temporal variation in the contribution of graminoids and browse to their diets (Bodmer 1990;Brown and Doucet 1991).…”

Using NDVI and EVI to Map Spatiotemporal Variation in the Biomass and Quality of Forage for Migratory Elk in the Greater Yellowstone Ecosystem

“…Because digestibility declines with seasonal vegetation growth, elk are thought to preferentially graze on grasses and forbs (e.g., elk sedge, Idaho fescue, etc.) in early growth stages because they provide an optimal trade-off of biomass and quality [39][40][41][42]. In the GYE, migratory elk are known to follow variation in grassland phenology from their low-elevation winter range to their high-elevation summer range to maximize their access to forage in early growth stages with adequate levels of crude protein and digestibility [43,44].…”