Temporal and Spatial Variation of Nitrogen in Wyoming Big Sagebrush

Sagebrush steppe systems represent one of North America’s greatest conservation challenges. Shrinking habitat and declining animal populations have prompted researchers to fill gaps in our understanding of the ecology of this system, particularly at a scale relevant to individual animals. What animals eat and why are fundamental questions linked to habitat quality and use. We investigated the temporal and spatial heterogeneity of the dietary quality of food to better understand habitat use by a sagebrush specialist, the pygmy rabbit (Brachylagus idahoensis). Within a foraging patch, pygmy rabbits selectively browse on individual Wyoming big sagebrush (Artemisia tridentata wyomingensis) that are high in nitrogen. Therefore, we hypothesized that variation in nitrogen would influence habitat use by pygmy rabbits at the patch scale. As a first step to test this hypothesis, we investigated the temporal and spatial variation in nitrogen content of patches of sagebrush within a study site in southcentral Idaho. Nitrogen concentration was determined using the Kjeldahl method. We used multivariate analysis of variance to test for differences in nitrogen content among three months within the winter season (November, January, and March) and between patches of sagebrush on mounds with deeper soils where pygmy rabbits burrow (on-mound) versus patches of sagebrush in shallower soils adjacent to mounds (off-mound). We found that nitrogen content of sagebrush was temporally and spatially dynamic. For both on- and off-mound plants, nitrogen content was significantly higher in March than November. Regardless of season, nitrogen content was significantly higher in plants on mounds compared to off-mound plants. Understanding the phenology and spatial heterogeneity of nitrogen content will help ecologists better assess diet quality and habitat quality within and among landscapes and seasons. Moreover, effective management and restoration efforts of sagebrush depend upon understanding patterns in nutrient availability to pygmy rabbits and other sagebrush specialists

Seasonal temperature acclimatization in a semi-fossorial mammal and the role of burrows as thermal refuges.

Small mammals in habitats with strong seasonal variation in the thermal environment often exhibit physiological and behavioral adaptations for coping with thermal extremes and reducing thermoregulatory costs. Burrows are especially important for providing thermal refuge when above-ground temperatures require high regulatory costs (e.g., water or energy) or exceed the physiological tolerances of an organism. Our objective was to explore the role of burrows as thermal refuges for a small endotherm, the pygmy rabbit (Brachylagus idahoensis), during the summer and winter by quantifying energetic costs associated with resting above and below ground. We used indirect calorimetry to determine the relationship between energy expenditure and ambient temperature over a range of temperatures that pygmy rabbits experience in their natural habitat. We also measured the temperature of above- and below-ground rest sites used by pygmy rabbits in eastern Idaho, USA, during summer and winter and estimated the seasonal thermoregulatory costs of resting in the two microsites. Although pygmy rabbits demonstrated seasonal physiological acclimatization, the burrow was an important thermal refuge, especially in winter. Thermoregulatory costs were lower inside the burrow than in above-ground rest sites for more than 50% of the winter season. In contrast, thermal heterogeneity provided by above-ground rest sites during summer reduced the role of burrows as a thermal refuge during all but the hottest periods of the afternoon. Our findings contribute to an understanding of the ecology of small mammals in seasonal environments and demonstrate the importance of burrows as thermal refuge for pygmy rabbits

Occupancy and Abundance of American Badgers and Piute Ground Squirrels in the Sagebrush-Steppe: Implications of the Fire-Cheatgrass Cycle

Sagebrush-steppe is experiencing vast changes due to biological invasions and changing fire characteristics. Understanding how these changes influence functionally important animals is essential for ecosystem management. American Badgers (Taxidea taxus) are an apex predator and ecosystem engineer within sagebrush ecosystems. Piute Ground Squirrels (Urocitellus mollis) are also an ecosystem engineer as well as an essential prey source for many predators.  Our objective was to evaluate the relative importance of large-scale changes, abiotic processes, and biotic processes on badgers and ground squirrels. We samples 163 1-ha plots across a gradient of burn histories within a 1,962 km2 area in Southern Idaho, USA. At each plot, we characterized ground squirrel and badger occupancy, ground squirrel relative abundance, and many environmental variables. We used information-theoretic approaches to evaluate competing hypotheses concerning occupancy of ground squirrels and badgers, and ground squirrel relative abundance. Results suggest that ground squirrel occupancy was positively associated with abiotic characteristics (e.g., higher precipitation and finer textured soil). Badger occupancy was positively associated with ground squirrel occupancy and agriculture. Relative abundance of ground squirrels was positively associated with finer textured soils, but negatively associated with cheatgrass (Bromus tectorum), fire frequency, agriculture and shrubs. Managers can focus restoration efforts on areas with high cheatgrass and shrub cover, if ground squirrels are a management objective. These results support previous hypotheses suggesting abiotic processes are important for herbivore occupancy. However, we provide support that a combination of abiotic, biotic and disturbance processes are important for mesocarnivore occupancy and herbivore abundance

Dietary Partitioning of Toxic Leaves and Fibrous Stems Differs Between Sympatric Specialist and Generalist Mammalian Herbivores

Dietary specialists often reside in habitats that provide a high and predictable abundance of their primary food, which is usually difficult for other herbivores to consume because of high levels of plant toxins or structural impediments. Therefore, sympatric specialist and generalist herbivores may partition food resources within and among plants. We compared how a dietary specialist (pygmy rabbit, Brachylagus idahoensis) and generalist (mountain cottontail, Sylvilagus nuttallii) used sagebrush as a food resource during winter across 3 field sites in Idaho, USA, and in controlled feeding trials with captive rabbits. The proportion of sagebrush consumed by both rabbit species varied among sites, indicating that characteristics of sagebrush plants and the surrounding plant community influenced use of sagebrush. In addition, free-ranging and captive pygmy rabbits consumed a greater proportion of sagebrush and cropped smaller stem diameters with a greater proportion of sagebrush leaves (high monoterpenes, low fiber) relative to stems (low monoterpenes, high fiber) than did cottontails. Cottontails frequently discarded the leafy tips of sagebrush branches. Cottontails are more tolerant of fiber and less tolerant of sagebrush toxins than pygmy rabbits. Cottontails consumed large diameter stems, which diluted toxins in sagebrush but increased fiber intake and reduced digestible nitrogen intake. Pygmy rabbits are less tolerant of fiber but more able to detoxify and eliminate sagebrush toxins than cottontails. Pygmy rabbits consumed small diameter stems, which reduced fiber intake, but increased intake of toxins from sagebrush leaves. Although partitioning of stems and leaves within sagebrush plants may provide a mechanism for coexistence of specialist and generalist rabbits, higher than expected dietary overlap between both free-ranging and captive rabbits in winter might create resource competition in areas with high-density sympatric populations or low availability of sagebrush. In addition, these contrasting foraging strategies have the potential to influence dynamics of sagebrush communities over time

Antioxidant Capacity of Wyoming Big Sagebrush (\u3cem\u3eArtemisia tridentata\u3c/em\u3e SSP. \u3cem\u3eWyomingensis\u3c/em\u3e) Varies Spatially and is Not Related to the Presence of a Sagebrush Dietary Specialist

Sagebrush (Artemisia spp.) in North America is an abundant native plant species that is ecologically and evolutionarily adapted to have a diverse array of biologically active chemicals. Several of these chemicals, specifically polyphenols, have antioxidant activity that may act as biomarkers of biotic or abiotic stress. This study investigated the spatial variation of antioxidant capacity, as well as the relationship between a mammalian herbivore and antioxidant capacity in Wyoming big sagebrush (Artemisia tridentata wyomingensis). We quantified and compared total polyphenols and antioxidant capacity of leaf extracts from sagebrush plants from different spatial scales and at different levels of browsing by a specialist mammalian herbivore, the pygmy rabbit (Brachylagus idahoensis). We found that antioxidant capacity of sagebrush extracts was positively correlated with total polyphenol content. Antioxidant capacity varied spatially within and among plants. Antioxidant capacity in sagebrush was not related to either browsing intensity or duration of association with rabbits. We propose that the patterns of antioxidant capacity observed in sagebrush may be a result of spatial variation in abiotic stress experienced by sagebrush. Antioxidants could therefore provide a biomarker of environmental stress for sagebrush that could aid in management and conservation of this plant in the threatened sagebrush steppe

Fearscapes: Mapping Functional Properties of Cover for Prey with Terrestrial LiDAR

Heterogeneous vegetation structure can create a variable landscape of predation risk—a fearscape—that influences the use and selection of habitat by animals. Mapping the functional properties of vegetation that influence predation risk (e.g., concealment and visibility) across landscapes can be challenging. Traditional ground-based measures of predation risk are location specific and limited in spatial resolution. We demonstrate the benefits of terrestrial laser scanning (TLS) to map the properties of vegetation structure that shape fearscapes. We used TLS data to estimate the concealment of prey from multiple vantage points, representing predator sightlines, as well as the visibility of potential predators from the locations of prey. TLS provides a comprehensive data set that allows an exploration of how habitat changes may affect prey and predators. Together with other remotely sensed imagery, TLS could facilitate the scaling up of fearscape analyses to promote the management and restoration of landscapes

Understanding Tradeoffs Between Food and Predation Risks in a Specialist Mammalian Herbivore

Understanding habitat use by animals requires understanding the simultaneous tradeoffs between food and predation risk within a landscape. Quantifying the synergy between patches that provide quality food and those that are safe from predators at a scale relevant to a foraging animal could better reveal the parameters that influence habitat selection. To understand more thoroughly how animals select habitat components, we investigated tradeoffs between diet quality and predation risk in a species endemic to sagebrush Artemisia spp. communities in North America, the pygmy rabbitBrachylagus idahoensis. This species is a rare example of a specialist herbivore that relies almost entirely on sagebrush for food and cover. We hypothesized that pygmy rabbits would forage in areas with low food risk (free of plant secondary metabolites, PSMs) and low predation risk (high concealment). However, because of relatively high tolerance to PSMs in sagebrush by pygmy rabbits, we hypothesized that they would trade off the risk of PSM-containing food to select lower predation risk when risks co-occurred. We compared food intake of pygmy rabbits during three double-choice trials designed to examine tradeoffs by offering animals two levels of food risk (1,8-cineole, a PSM) and predation risk (concealment cover). Rabbits ate more food at feeding stations with PSM-free food and high concealment cover. However, interactions between PSMs and cover suggested that the value of PSM-free food could be reduced if concealment is low and the value of high concealment can decrease if food contains PSMs. Furthermore, foraging decisions by individual rabbits suggested variation in tolerance of food or predation risks

Modeling Trade-Offs Between Plant Fiber and Toxins: A Framework for Quantifying Risks Perceived by Foraging Herbivores

When selecting habitats, herbivores must weigh multiple risks, such as predation, starvation, toxicity, and thermal stress, forcing them to make fitness trade-offs. Here, we applied the method of paired comparisons (PC) to investigate how herbivores make trade-offs between habitat features that influence selection of food patches. The method of PC measures utility and the inverse of utility, relative risk, and makes trade-offs and indifferences explicit by forcing animals to make choices between two patches with different types of risks. Using a series of paired-choice experiments to titrate the equivalence curve and find the marginal rate of substitution for one risk over the other, we evaluated how toxin-tolerant (pygmy rabbit Brachylagus idahoensis) and fiber-tolerant (mountain cottontail rabbit Sylviagus nuttallii) herbivores differed in their hypothesized perceived risk of fiber and toxins in food. Pygmy rabbits were willing to consume nearly five times more of the toxin 1,8-cineole in their diets to avoid consuming higher levels of fiber than were mountain cottontails. Fiber posed a greater relative risk for pygmy rabbits than cottontails and cineole a greater risk for cottontails than pygmy rabbits. Our flexible modeling approach can be used to (1) quantify how animals evaluate and trade off multiple habitat attributes when the benefits and risks are difficult to quantify, and (2) integrate diverse risks that influence fitness and habitat selection into a single index of habitat value. This index potentially could be applied to landscapes to predict habitat selection across several scales

Spectral Fingerprints Predict Functional Chemistry of Native Plants Across Sagebrush-Steppe Landscapes

Landscapes are changing and under threat from anthropogenic activities, decreasing land cover, contaminated air and water quality, and climate change. These changes impact native communities and their functions at all spatial scales. A major functional trait being affected across these communities is nitrogen. Nitrogen supports plant nutrient cycling and growth, serves as an indicator for crude protein and productivity, and offers quality forage for wild and domestic herbivores. We need better ways to monitor nitrogen across space and time. Current monitoring is elaborate, time-consuming, and expensive. We propose drawing from agricultural methodologies to incorporate near-infrared spectroscopy as a technique in detecting and monitoring nitrogen concentrations across a threatened shrub-steppe ecosystem. We are currently developing calibration equations for nitrogen in sagebrush across four species (Artemisia tridentata wyomingensis, A. tripartita, A. arbuscula, A. nova), three study sites and two seasons. Preliminary results suggest that nitrogen can be accurately predicted across all sites, species, and seasons, explaining 75-90% of the variation in nitrogen. These results indicate that near infrared spectroscopy offers a rapid, noninvasive diagnostic tool for assessing nitrogen in wild systems. This advancing technology is important because it economizes the collection of ecological data in rapidly changing landscapes and provides land managers and researchers with valuable information about the health and sustainability of their lands