Brian L. Dick

Niche partitioning among mule deer, elk, and cattle: Do stable isotopes reflect dietary niche?

Abstract We examined dietary niches of mule deer (Odocoileus hemionus), North American elk (Cervus elaphus), and free-ranging cattle (Bos taurus) that frequently co-occur in western North America. We tested the hypothesis that those three species would exhibit little overlap in diet and that mule deer, the smallest in body size of the three species, would forage more selectively than either elk or cattle. We determined diet composition from microhistological analysis and used principal components analysis to assess dietary niches. In addition to those conventional methods, we also assessed whether dietary overlap among those three ruminants would be reflected in stable isotope ratios (δ13C and δ15N) from fecal pellets. Principal component 1 represented a foraging axis based on plant classes, whereas principal component 2 represented a continuum from grazing to browsing, which revealed complete separation among those three large herbivores. Similarly, δ13C and δ15N differed significantly among species and indicated differences in moisture regimes within habitats and types of forages used by those three ruminants. Mule deer had the greatest variability in diet and foraged on more xeric forages than did either elk or cattle. Stable isotopes elucidated differences in dietary niche among the three ruminants that were not evident from dietary analysis alone.

Herbivore Optimization by North American Elk: Consequences for Theory and Management

Abstract Understanding herbivore optimization has implications for theories underpinning ecosystem processes, management of large herbivores, and the landscapes they inhabit. We designed an experiment to examine interactions related to density dependence of North American elk (Cervus elaphus) and resulting plant responses to herbivory in the Blue Mountains of Oregon, USA, from 1999 to 2001. We experimentally created high (20.1 elk/km2) and low (4.1 elk/km2) population densities of elk and built exclosures to examine effects of herbivory on productivity and species composition of plants. We hypothesized that if herbivore optimization occurred with increasing density of elk, there should be a concordant increase in plant production, followed by a decline in productivity as grazing intensity continued to increase (i.e., herbivore optimization). Net aboveground primary productivity (NAPP) increased from no herbivory to herbivory by elk at moderate density and then declined as herbivory by elk continued to increase in areas with high NAPP (mesic and logged forests) but not in areas with low NAPP (xeric forests and grasslands). Herbivore optimization occurred across all functional groups of plants, including graminoids, forbs, and shrubs for high-NAPP areas. Herbivore optimization may be difficult to detect in woody plants compared with graminoids because of their differing structure and growth forms. Although herbivore optimization previously has been reported in grasslands, our study documents this phenomenon in woody plant communities. We hypothesize that such subtle changes in NAPP from herbivory might be more common than previously thought; carefully designed experiments are required to detect those responses to herbivory by large herbivores. Apparent offtake of plants followed a similar pattern to NAPP and was greatest at intermediate levels of herbivory by elk, and then declined as NAPP approached zero. Quality of plants, as indexed by percent nitrogen (N), also exhibited a parabolic function with increasing density of elk. Nonetheless, we observed no changes in species composition or diversity of plants with our density manipulations of elk, probably because of the extensive history of grazing by native and domestic herbivores in the Blue Mountains, the resilience of the remaining plants to herbivory, and the short 3-year duration of our study. Likewise, we observed no increases in rates of nutrient cycling with changes in densities of elk, perhaps because areas where large amounts of elk urine and feces were concentrated (e.g., grazing lawns) did not occur in this ecosystem. Herbivore optimization could have ramifications for population dynamics of ungulates. We argue that other processes, such as migration, predation, or harvest, might be necessary to maintain areas of increased NAPP because of strong density-dependent feedbacks and negative effects of ungulates on their food supply would not allow equilibrium between herbivores and their food supply at the low densities where we documented herbivore optimization. We also hypothesize that increased NAPP at low to moderate population densities of herbivores could lead to rapid population growth of these herbivores and help reduce density-dependent feedbacks and promote overshoots of carrying capacity. Our data do not support stocking large densities of ungulates on western rangelands to obtain peak production of NAPP. We recommend maintaining low to moderate densities of large herbivores in ecosystems if goals are to maximize NAPP and forage quality or to maximize body condition and reproduction of ungulates.

Effects of Male Age and Female Nutritional Condition on Elk Reproduction

Spring calf:cow ratios in some Rocky Mountain elk (Cervus elaphus) populations of northeast Oregon have declined by almost 80% over the last 40 years. Studies have identified the age of breeding males and the nutritional condition of females as potentially contributing factors. We conducted a study in 2 trials, from 1989 to 1993 and from 1995 to 1999, to assess the effects of male age on conception dates and pregnancy rates of female elk in northeast Oregon. Results of the first trial, reported previously, showed a significant influence of male age on conception dates but not on pregnancy rates. The second trial, reported here, was intended to validate findings of the first. trial and to evaluate the interaction of male age and female nutritional condition. We managed an elk population within a 78-km2 enclosure to allow a single cohort of males to function as herd sires as they matured from 1.5 to 5.5 years of age. From animals killed in December, we estimated pregnancy rates, age, nutritional condition (kidney fat index, [MFI]), and lactation status of females and the conception dates of their fetuses. Mean conception dates occurred 1 week earlier as male age increased and were related to KFI in females. Nutritional condition of female elk was 67% higher in 1995 when breeding was by yearling males than in years when breeding was by 4-year-old or 5-year-old males. Pregnancy rates did not differ among ages of males. We used analysis of covariance with female nutritional condition as the covariate to evaluate the interactive effects of male age and female nutritional condition on conception dates of females bred by males of different ages across 2 trials. Mean conception dates (adjusted for female nutritional condition) pooled by age of males decreased from 4 October with yearling male sires to 21 September with 5-year-old male sires. Mean KFI of pregnant, lactating female elk was 118 during the validation and 148 during the initial trial. Pregnancy rates did not differ by male age between trials. We stress the importance of understanding the interactions between age of males and nutritional condition of females prior to interpreting the results of management strategies designed to retain older males because of the many factors that affect calf elk survival.

Population density of North American elk: effects on plant diversity

Large, herbivorous mammals have profound effects on ecosystem structure and function and often act as keystone species in ecosystems they inhabit. Density-dependent processes associated with population structure of large mammals may interact with ecosystem functioning to increase or decrease biodiversity, depending on the relationship of herbivore populations relative to the carrying capacity (K) of the ecosystem. We tested for indirect effects of population density of large herbivores on plant species richness and diversity in a montane ecosystem, where increased net aboveground primary productivity (NAPP) in response to low levels of herbivory has been reported. We documented a positive, linear relationship between plant-species diversity and richness with NAPP. Structural equation modeling revealed significant indirect relationships between population density of herbivores, NAPP, and species diversity. We observed an indirect effect of density-dependent processes in large, herbivorous mammals and species diversity of plants through changes in NAPP in this montane ecosystem. Changes in species diversity of plants in response to herbivory may be more indirect in ecosystems with long histories of herbivory. Those subtle or indirect effects of herbivory may have strong effects on ecosystem functioning, but may be overlooked in plant communities that are relatively resilient to herbivory.

Effects of density dependence on diet composition of North American elk Cervus elaphus and mule deer Odocoileus hemionus: an experimental manipulation

Abstract We examined interactions related to resource partitioning and competition with density-dependent processes among mule deer Odocoileus hemionus and North American elk Cervus elaphus at two different population densities of elk. We used an experimental approach to examine changes in diet diversity, selection of diets, dietary importance, niche breadth and overlap among sympatric species of large herbivores with changes in population density of one species. We hypothesized that diets of both species would change with changing population density and dietary niche would be expanded to include forages of lower quality in the areas with increased competition for resources. We used microhistological analysis corrected for differential digestibility of forages to estimate diets of mule deer and elk from faeces in two study areas with high and low population densities of elk. For both mule deer and elk, dietary niche was expanded in the high-density area compared with the low-density area, and included forages of lower quality and palatability. Our results indicate that negative feedbacks associated with density-dependent processes interact with competitive interactions among sympatric species of large herbivores.

Investment in Constitutive Immune Function by North American Elk Experimentally Maintained at Two Different Population Densities

Natural selection favors individuals that respond with effective and appropriate immune responses to macro or microparasites. Animals living in populations close to ecological carrying capacity experience increased intraspecific competition, and as a result are often in poor nutritional condition. Nutritional condition, in turn, affects the amount of endogenous resources that are available for investment in immune function. Our objective was to understand the relationship between immune function and density dependence mediated by trade-offs between immune function, nutritional condition, and reproduction. To determine how immune function relates to density-dependent processes, we quantified bacteria killing ability, hemolytic-complement activity, and nutritional condition of North American elk (Cervus elaphus) from populations maintained at experimentally high- and low-population densities. When compared with elk from the low-density population, those from the high-density population had higher bacteria killing ability and hemolytic-complement activity despite their lower nutritional condition. Similarly, when compared with adults, yearlings had higher bacteria killing ability, higher hemolytic-complement activity, and lower nutritional condition. Pregnancy status and lactational status did not change either measure of constitutive immunity. Density-dependent processes affected both nutritional condition and investment in constitutive immune function. Although the mechanism for how density affects immunity is ambiguous, we hypothesize two possibilities: (i) individuals in higher population densities and in poorer nutritional condition invested more into constitutive immune defenses, or (ii) had higher parasite loads causing higher induced immune responses. Those explanations are not mutually exclusive, and might be synergistic, but overall our results provide stronger support for the hypothesis that animals in poorer nutritional condition invest more in constitutive immune defenses then animals in better nutritional condition. This intriguing hypothesis should be investigated further within the larger framework of the cost and benefit structure of immune responses.

Sexual segregation in North American elk: the role of density dependence

We investigated how density-dependent processes and subsequent variation in nutritional condition of individuals influenced both timing and duration of sexual segregation and selection of resources. During 1999–2001, we experimentally created two population densities of North American elk (Cervus elaphus), a high-density population at 20 elk/km2, and a low-density population at 4 elk/km2 to test hypotheses relative to timing and duration of sexual segregation and variation in selection of resources. We used multi-response permutation procedures to investigate patterns of sexual segregation, and resource selection functions to document differences in selection of resources by individuals in high- and low-density populations during sexual segregation and aggregation. The duration of sexual segregation was 2 months longer in the high-density population and likely was influenced by individuals in poorer nutritional condition, which corresponded with later conception and parturition, than at low density. Males and females in the high-density population overlapped in selection of resources to a greater extent than in the low-density population, probably resulting from density-dependent effects of increased intraspecific competition and lower availability of resources.

Linking habitat selection to fitness-related traits in herbivores: the role of the energy landscape

Animals may partially overcome environmental constraints on fitness by behaviorally adjusting their exposure to costs and supplies of energy. Few studies, however, have linked spatiotemporal variation in the energy landscape to behaviorally mediated measures of performance that ostensibly influence individual fitness. We hypothesized that strength of selection by North American elk (Cervus elaphus) for areas that reduced costs of thermoregulation and activity, and increased access to high-quality forage, would influence four energetically mediated traits related to fitness: birth mass of young, nutritional condition of adult females at the onset of winter, change in nutritional condition of females between spring and winter, and neonatal survival. We used a biophysical model to map spatiotemporally explicit costs of thermoregulation and activity experienced by elk in a heterogeneous landscape. We then combined model predictions with data on forage characteristics, animal locations, nutritional condition, and mass and survival of young to evaluate behaviorally mediated effects of the energy landscape on fitness-related traits. During spring, when high-quality forage was abundant, female elk that consistently selected low-cost areas before parturition gave birth to larger young than less-selective individuals, and birth mass had a strong, positive influence on probability of survival. As forage quality declined during autumn, however, lactating females that consistently selected the highest quality forage available accrued more fat and entered winter in better condition than less-selective individuals. Results of our study highlight the importance of understanding the dynamic nature of energy landscapes experienced by free-ranging animals.

Reproduction in North American elk Cervus elaphus: paternity of calves sired by males of mixed age classes

Our objective was to examine effects of groups of mixed numbers and ages of male North American elk Cervus elaphus on the reproductive performance of females. We conducted research at the Starkey Experimental Forest and Range in northeastern Oregon, USA, during 1993-2000. Each spring in late March, we released 40 female elk, eight yearling (9-month old) male elk and 2-8 branch-antlered elk (i.e. ≥ 2 years of age during rut the following autumn) into a 622-ha fenced pasture. Elk were gathered during autumn and early winter, and were brought to winter feeding grounds where blood samples were drawn to determine pregnancy status. The following spring, females were released into an 80-ha pasture prior to parturition. We searched for and captured newborn calves and obtained ear-punch samples for genetic analysis. We used 18 microsatellite loci to establish paternity of each calf. We varied the ratio of mature males (i.e. ≥ 3 years old) to female ratio from 0.03 to 0.21. As expected, mature males (older and heavier) were more successful in siring calves than were younger males. Within age classes, however, body mass in spring did not accurately predict mating success in autumn. Reproductive rates were not affected by season of grazing by cattle, yearling male to female ratio or mature male to female ratio. Sire age had no effect on mean dates of calf births or on calf weights. Neither sire age nor season of grazing by cattle had significant effects on calf weights; however, mean date of birth was significantly earlier when cattle grazing occurred during the previous autumn than when cattle grazed during the preceding spring. Furthermore, the number of calves sired by yearling males was greater when cattle grazing occurred during autumn, than when grazing occurred during spring. In the years with disruptive cattle grazing during rut, females mated not only with yearling males, in general, but often with those who were lighter in body mass during the previous spring than others in the same cohort. The extent to which those yearling males are untested in combat with older, dominant herd bulls may have genetic consequences leading to differences in fitness and subsequent reductions in calf survival. Please note that the supplementary information, including Appendix SI mentioned in this article, is available in the online version of this article, which can be viewed at www.wildlifebiology.com

Stocking Rate and Fuels Reduction Effects on Beef Cattle Diet Composition and Quality

Abstract An experiment was conducted to evaluate the influence of forest fuels reduction on diet quality, botanical composition, relative preference, and foraging efficiency of beef cattle grazing at different stocking rates. A split plot factorial design was used, with whole plots (3 ha) being fuel reduced or no treatment (control), and split plots (1 ha) within whole plots were grazed to three levels of forage utilization; (low) 3 heifers · ha−1, (moderate) 6 heifers · ha−1, (high) 9 heifers · ha−1, with a 48-h grazing duration. Grazing treatments were applied in August of 2005 and 2006. Cattle diet composition and masticate samples were collected during 20-min grazing bouts using six ruminally cannulated cows in each experimental unit. Relative preference indices indicated a strong preference for grass regardless of treatment and stocking rate. Grass consumption was lower in control pastures (P < 0.05) and tended (P < 0.095) to decrease with increased stocking rates. Shrub use was higher in control pastures displaying a quadratic effect (P < 0.05) due to stocking, whereas shrub use increased with stocking rate across all treatments. Cattle grazing control pastures consumed diets higher in crude protein compared to cattle grazing treated pastures (P < 0.05). In vitro dry matter digestibility values were lower (P < 0.05) in control sites and tended (P = 0.10) to decrease with increased stocking rates. In both control and treated pastures, bites per minute and grams consumed per minute declined (P = 0.003) with increased stocking, indicating foraging efficiency of cattle decreases with increased stocking rates. Our data indicated cattle grazing late season grand fir habitat types have a strong preference for grasses regardless of treatment or stocking rate. However, as stocking rate increased in both control and treated pastures, grass consumption decreased, shrub consumption increased, and foraging efficiency decreased.