No abstract

The contribution of small mammals to nitrogen cycling could have repercussions for the producer community in the maintaining or perhaps magnifying of nitrogen availability. Our objective was to model nitrogen outputs (deposition of feces and urine) of small mammals in an old-field ecosystem and estimate the amount of fecal and urinary nitrogen deposited annually. To address this objective, we used models from laboratory studies and combined these with data from field studies to estimate dietary nitrogen and monthly and annual nitrogen outputs from fecal and urine deposition of five rodent species. The models accounted for monthly fluctuations in density and biomass of small-mammal populations. We estimated that the minimal amount of nitrogen deposited by rodents was 1.0 (0.9-1.1) and 2.7 (2.6-2.9) kg Nha(-1) year(-1) from feces and urine, respectively, for a total contribution of 3.7 (3.5-4.0) kg Nha(-1) year(-1). Hispid cotton rats (Sigmodon hispidus) accounted for >75% of the total nitrogen output by small mammals. Our estimates of annual fecal and urinary nitrogen deposited by rodents were comparable to nitrogen deposits by larger herbivores and other nitrogen fluxes in grassland ecosystems and should be considered when assessing the potential effects of herbivory on terrestrial nitrogen cycles.

Endangered giant pandas (Ailuropoda melanoleuca) are bears (Family Ursidae), within the order Carnivora. They specialize on an herbivorous diet of bamboo yet retain a gastrointestinal tract typical of their carnivorous ancestry. The evolutionary constraints of their digestive tract result in a low extraction efficiency from bamboo (<40% in reported studies). The goal of this study was to determine the energy digestibility of bamboo by giant pandas used in digestibility trials and through subsequent analyses with bomb calorimetry. Seven digestibility trials were conducted (three with bamboo-only diets and four with supplemental diets). Energy digestibilities ranged from 7.5-38.9% for mixed diets and 9.2-34.0% for bamboo-only diets. The bamboo-only trials summarized here represent, to our knowledge, the first empirical data available for energy digestibility on a bamboo diet for giant pandas.

No abstract

Nitrogen (N) enrichment of terrestrial ecosystems dramatically changes ecosystem diversity and structure of plant communities. Research designed to elucidate effects of nitrogen addition on mammalian assemblages is rare. We investigated nitrogen requirements of hispid cotton rats (Sigmodon hispidus) and fulvous harvest mice (Reithrodontomys fulvescens), small mammals native to the tallgrass prairie of the southern Great Plains, USA, to better understand population responses of these species to nitrogen enrichment. We studied reproductive requirements by measuring growth of offspring under varying levels of dietary nitrogen. We predicted that dietary niche would dictate nitrogen requirements, such that the larger herbivore (S. hispidus) would have a lower dietary need for nitrogen per unit mass than the small omnivore/granivore (R. fulvescens). Reproductive output (measured as mass gain of litters and offspring) was responsive to varying nitrogen in cotton rats but not in harvest mice. Nitrogen intake that supported 50% survival of juvenile harvest mice (1.34% dietary nitrogen) also was adequate for maximum growth (1.29%). Cotton rats potentially drew on maternal nutrient stores to support litter growth at low levels of dietary nitrogen (as low as 1.08%). Overall, nitrogen requirements for maximum reproduction were greater (2.31% dietary nitrogen) for cotton rats. We conclude that life history characteristics and body size constraints rather than dietary niche explain the differential species response to variation in dietary nitrogen. Our results imply that nitrogen enrichment in old‐field succession in the southern Great Plains may lead to dominance by cotton rats and a reduction in diversity of the small‐mammal assemblage. Consumers with similar abilities to take advantage of increased environmental nitrogen may likewise dominate other ecosystems.