Kelley M. Stewart

Temporospatial Distributions of Elk, Mule Deer, and Cattle: Resource Partitioning and Competitive Displacement

Abstract Rocky Mountain elk (Cervus elaphus nelsoni), Rocky Mountain mule deer (Odocoileus hemionus hemionus), and cattle frequently co-occur on landscapes in the northwestern United States. We hypothesized that niche overlap would be greatest between introduced cattle with either of the 2 native herbivores because coevolution between native elk and mule deer should have resulted in strong patterns of resource partitioning. We observed strong differences among species in use of space, especially elevation, steepness of slope, and use of logged forests. We used 2 temporal windows to examine both immediate (6 h) and long-term (7 days) effects of competition. We noted strong avoidance over a 6-h period among the 3 ungulates. That effect was weaker for the previous 7 days. Cattle were generalists with respect to habitat selection; the 2 native herbivores avoided areas used by cattle. Mule deer and elk avoided one another during the short temporal window (6 h), although spatial differences in habitat use often were not maintained over 7 days. Elk used lower elevations when cattle were absent and moved to higher elevations when cattle were present, indicating shifts in niche breadth and competitive displacement of elk by cattle. We demonstrated strong partitioning of resources among these 3 species, and presented evidence that competition likely has resulted in spatial displacement.

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.

Identification of source-sink dynamics in mountain lions of the Great Basin

Natural and anthropogenic boundaries have been shown to affect population dynamics and population structure for many species with movement patterns at the landscape level. Understanding population boundaries and movement rates in the field for species that are cryptic and occur at low densities is often extremely difficult and logistically prohibitive; however genetic techniques may offer insights that have previously been unattainable. We analysed thirteen microsatellite loci for 739 mountain lions (Puma concolor) using muscle tissue samples from individuals in the Great Basin throughout Nevada and the Sierra Nevada mountain range to test the hypothesis that heterogeneous hunting pressure results in source‐sink dynamics at the landscape scale. We used a combination of non‐spatial and spatial model‐based Bayesian clustering methods to identify genetic populations. We then used a recently developed Bayesian multilocus genotyping method to estimate asymmetrical rates of contemporary movement between those subpopulations and to identify source and sink populations. We identified two populations at the highest level of genetic structuring with a total of five subpopulations in the Great Basin of Nevada and the Sierra Nevada range. Our results suggest that source‐sink dynamics occur at landscape scales for wide‐ranging species, such as mountain lions, and that source populations may be those that are under relatively less hunting pressure and that occupy refugia.

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.

Assessing sexual segregation in deer

Sexual segregation in temperate and arctic ruminants is defined as the differential use of space by the sexes outside the mating season. This phenomenon is widespread among taxa, and is especially prevalent among sexually dimorphic deer (Cervidae). Understanding how different genders are distributed across the landscape and how to assess these spatial patterns is of theoretical and applied importance. We developed a simple model to evaluate effects of landscape grain (i.e., patch characteristics), sampling scale, and population density of deer on detection of sexual segregation. We created landscape maps of 2 areas in which landscape grain was changed while other landscape metrics (e.g., area, shape, and diversity) were held relatively constant. We created a high-density population of deer to emulate conditions near ecological carrying capacity (K), and a low-density population at <K/2. Sexes of deer were assigned to 4 habitats based on differences in habitat selection derived from an ideal-free distribution, which created spatial separation of the sexes similar to observations in empirical studies. We sampled this pattern of sexual segregation for both areas at large and small scales of measurement using quadrats arranged systematically. We also compared the degree of sexual segregation for the coarse-grained landscape, where the sexes used habitats differently, with a null model in which habitat preferences were identical for each gender of deer. The null model emulated conditions during rut when the sexes were aggregated. Sexual segregation was significantly greater where habitat use differed between sexes, indicating that our model was correct and that the degree of spatial segregation was not an artifact of patch configuration, sampling scale, or population density. Logistic regression revealed that population density and the size of the sample unit significantly affected our ability to assess differences in the spatial distributions of male and female deer where differential use of habitats occurred. Variation in landscape grain, however, did not influence the detection of sexual segregation. Results from our model emphasize the importance of evaluating effects of population density and especially sampling scale on assessing spatial separation of the sexes. Failure to do so may result in not recognizing patterns of sexual segregation on the landscape, or in misinterpreting that phenomenon, which clearly holds consequences for those managing large herbivores or their habitats.

FLUCTUATING ASYMMETRY IN ANTLERS OF ALASKAN MOOSE: SIZE MATTERS

Abstract We studied characteristics of paired antlers, including types of asymmetry, from 1,501 Alaskan moose (Alces alces gigas). We observed no evidence of antisymmetry in moose antlers, but number of tines was greater on left than right antlers, indicating directional asymmetry. Absolute and relative fluctuating asymmetry (FA) occurred for palm characteristics but not for beam circumference. Relative FA varied inversely with the overall size of antlers for attributes of the palm, which was expected for a secondary sexual characteristic. Smaller-antlered males exhibited greater FA than did larger-antlered moose in palm characteristics. Because large-antlered males, which mate most often among moose and other polygynous cervids, expressed the least relative FA, we hypothesize that this metric indicates quality of individual moose. Whether symmetry of antlers is related to antler breakage or honest advertisement or whether females select mates based on FA is unknown and deserves additional study.

White-tailed deer use of clearings relative to forage availability

Use of woodland clearings by white-tailed deer (Odocoileus virginianus) may be in response to increased availability of forage within open patches or increased ability of deer to locate predators. We tested predictions, based upon the forage-maturation hypothesis, that white-tailed deer used areas with the greatest availability of high-quality forage, and that habitat use changed seasonally depending upon availability and quality of major types of forage in their diet. We tested those predictions in subtropical thorn woodland in South Texas, United States. Treatments included (1) areas with continuous woody cover as controls, (2) clearings with high availability of forbs and shrub sprouts, (3) clearings with low availability of shrub sprouts, (4) clearings with low availability of forbs, and (5) clearings with low availability of forbs and shrub sprouts. Intensity of use by deer during summer and autumn increased with increasing biomass of shrub sprouts and then declined with increasing shrub biomass as areas became dominated by mature shrubs with less accessible, usable forage. During spring, intensity of deer use increased in clearings with increases in forage availability and quality, indicated by an index to carrying capacity, then declined as vegetation matured. Responses of white-tailed deer to clearings supported the forage-maturation hypothesis in which herbivore responses to clearings resulted, in part, from the presence of shrub sprouts of high nutritional quality, particularly during summer and autumn when forage availability was low. Maintenance of clearings that are interspersed in a woodland matrix and maintaining high levels of immature shrub sprouts may alter the spatial distribution of white-tailed deer on the landscape.

Mammal Community Dynamics: Ungulates in western coniferous forests: habitat relationships, population dynamics, and ecosystem processes

Wild ungulates play important roles in coniferous forest throughout western North America. Their biology is well known compared with that of other species of wildlife. They have sufficiently large home ranges to integrate spatial patterns across landscapes. Finally, they are often migratory (Wallmo 1981, Nicholson et al. 1997). Their life-history characteristics require consideration of entire landscapes rather than isolated patches of habitat for purposes of conservation and management (Hanley 1996, Kie et al. 2002). Ungulates require temporally and spatially diverse habitat components such as food and cover. These mammals can have significant effects on vegetation composition and basic ecosystem processes such as nutrient cycling, thereby acting as keystone species (Molvar et al. 1993, Wallis de Vries 1995, Hanley 1996, Hobbs 1996, Nicholson et al. 1997, Simberloff 1998, Kie et al. 2002). Ungulates have economic value to society as well. Most species provide recreational hunting opportunities and also can have non-consumptive, aesthetic values (Loomis et al. 1989). Conversely, ungulates can cause damage to gardens and other landscaping (Conover 1997), to agricultural crops, (Austin and Urness 1993), and to new tree seedlings (Bandy and Taber 1974). Damage from deer–vehicle collisions also can be substantial (Romin and Bissonette 1996). Four species of ungulates, all members of the family Cervidae, commonly occur in coniferous forests in western North America: elk (Cervus elaphus), moose (Alces alces), mule and black-tailed deer (Odocoileus

Spatial Distributions of Mule Deer and North American Elk: Resource Partitioning in a Sage-Steppe Environment

Abstract We examined spatial distributions and resource partitioning among female mule deer (Odocoileus hemionus) and female North American elk (Cervus elaphus) during summer and winter in southeastern Idaho, USA. Our objective was to understand differences in distributions and habitat selection by these two species of large herbivores in a sagebrush-steppe ecosystem. We used multi-response permutation procedures to examine seasonal distributions of mule deer and elk. We compared animal locations with random locations on a GIS-based habitat map to examine habitat selection and importance. Both species were more widely distributed during summer than winter, when distributions of both species were more restricted. During winter, habitat selection differed among mule deer and elk in use of aspect; east facing slopes were selected by elk and west-facing slopes by mule deer. Mule deer also were located closer to roads than random locations, which may have resulted from avoidance of elk. We observed no differences in habitat selection between mule deer and elk during summer, although these large herbivores showed resource separation by strongly partitioning use of habitats. Conversely, both species used the shrub-steppe ecosystem during winter, but partitioned habitat mostly via use of aspect and distance to roads. Our research provides insights into niche partitioning between these large mammals.

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.