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The most common system responses attributed to microfloral grazers (protozoa, nematodes, microarthropods) in the literature are increased plant growth, increased N uptake by plants, decreased or increased bacterial populations, increased CO2 evolution, increased N and P mineralization, and increased substrate utilization. Based on this evidence in the literature, a conceptual model was proposed in which microfloral grazers were considered as separate state variables. To help evaluate the model, the effects of microbivorous nematodes on microbial growth, nutrient cycling, plant growth, and nutrient uptake were examined with reference to activities within and outside of the rhizosphere. Blue grama grass (Bouteloua...
We evaluated the effects of plant functional group richness on seasonal patterns of soil nitrogen and phosphorus cycling, using serpentine grassland in south San Jose, California. We established experimental plots with four functional types of plants: early-season annual forbs (E), late-season annual forbs (L), nitrogen-fixers (N), and perennial bunchgrasses (P). These groups differ in several traits relevant to nutrient cycling, including phenology, rooting depth, root:shoot ratio, size, and leaf C:N content. Two or three species of each group were planted in single functional group (SFG) treatments, and in two-, three-, and four-way combinations of functional groups. We analyzed available nutrient pool sizes,...
We conducted a field experiment to assess interrelationships between leaf-tissue secondary chemistry, avian predation, and the abundance and diversity of arthropods occurring on sagebrush (Artemisia tridentata) in central Oregon. Arthropods were removed from individual shrubs, some of which were then caged to exclude birds. Secondary chemistry and arthropods were sampled at intervals up to 56 wk following the defaunation/caging treatment. Recovery rates differed among arthropod taxa and functional groups. Several sap-sucking homopterans and hemipterans reached control levels within 2-4 wk of the treatment, whereas abundances of parasitoids and predators recovered to match control numbers only 6 wk after defaunation....
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A year's study was made of 13 species of mammals in a desert community in southeastern Arizona. Mammal density averaged 17.4/ha: 66% Dipodomys merriami and 10.5% Onychomys torridus. Average biomass was 1130 g/ha: 40% D. merriami and 40% Lepus californicus. Annual energy flow of mammals was 105,950 kcal/ha: 55% by a granivore (D. merriami), 22% by a browser (L. californicus) and 6.5% by an insectivore (0. torridus). 94.6% of the energy flow was spent in maintenance and 5.4% in growth. The secondary pro- ductivity of the dominant D. merriami was 1.2% of its energy flow; that of Peromyscus ere- micus, the resident species with the lowest and least stable density, was 1.7%. A dominant species may be more important in...
Some theories and experimental studies suggest that areas of low plant species richness may be invaded more easily than areas of high plant species richness. We gathered nested-scale vegetation data on plant species richness, foliar cover, and frequency from 200 1-m2 subplots (20 1000-m2 modified-Whittaker plots) in the Colorado Rockies (USA), and 160 1-m2 subplots (16 1000-m2 plots) in the Central Grasslands in Colorado, Wyoming, South Dakota, and Minnesota (USA) to test the generality of this paradigm. At the 1-m2 scale, the paradigm was supported in four prairie types in the Central Grasslands, where exotic species richness declined with increasing plant species richness and cover. At the 1-m2 scale, five forest...
Survival or extinction of an endangered species is inherently stochastic. We develop statistical methods for estimating quantities related to growth rates and extinction probabilities from time series data on the abundance of a single population. The statistical methods are based on a stochastic model of exponential growth arising from the biological theory of age or stagestructured populations. The model incorporates the socalled environmental type of stochastic fluctuations and yields a lognormal probability distribution of population abundance. Calculation of maximum likelihood estimates of the two unknown parameters in this model reduces to performing a simple linear regression. We describe techniques for rigorously...
We integrated experimental and natural gradient field methods to investigate effects of climate change and variability on flowering phenology of 11 subalpine meadow shrub, forb, and graminoid species in Gunnison County, Colorado (USA). At a subalpine meadow site, overhead electric radiant heaters advanced snowmelt date by 16 d and warmed and dried soil during the growing season. At three additional sites, a snow removal manipulation advanced snowmelt date by 7 d without altering growing season soil microclimate. We compared phenological responses to experimental climate change with responses to natural microclimate variability across spatial gradients at small and landscape scales, as well as across a temporal gradient...
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A stable colony of the desert tortoise, Gopherus agassizii, that lives on the Beaver Dam Slope in extreme southwestern Utah, where the temperature extremes of the desert environment transcend the lethal limits of cold-blooded vertebrates provided material for the studies upon which this report is based. This is the first of a series of papers planned to deal with reptiles and their adaptations that are shown chiefly by their behavior patterns in the arid regions of interior western North America. Three trips into the area between April, 1930 and August, 1935 introduced the authors to some of the summer activities of tortoises. Additional work was initiated on November 28, 1936 when, after two days of searching,...
The geomorphic effectiveness of extreme floods increases with aridity and decreasing watershed size. Therefore, in small dry watersheds extreme floods should control the age structure and spatial distribution of populations of disturbance-dependent riparian trees. We examined the influence of extreme floods on the bottomland morphology and forest of ephemeral streams in a semiarid region. Along six stream reaches on the Colorado Piedmont we examined channel changes by analyzing a rectified sequence of aerial photographs spanning 56 yr, and we investigated the spatial distribution of different-aged patches of forest by aging 189 randomly sampled cottonwood trees. Channel change in these ephemeral sand-bed streams...
In semiarid landscapes, the linkage between runoff and vegetation is a particularly close one. In this paper we report on the results of a long-term and multiple-scale study of interactions between runoff, erosion, and vegetation in a piñon–juniper woodland in New Mexico. We use our results to address three knowledge gaps: (1) the temporal scaling relationships between precipitation and runoff; (2) the effects of spatial scale on runoff and erosion, as influenced by vegetation; and (3) the influence of disturbance on these relationships. On the basis of our results, we tested three assumptions that represent current thinking in these areas (as evidenced, for example, by explicit or implicit assumptions embedded...
Increasing demands on arid and semiarid ecosystems, which comprise one-third of Earth's terrestrial environment, create an urgent need to understand their biodiversity, function, and mechanisms of change. Sagebrush (Artemisia) steppe, the largest semiarid vegetation type in North America, is endangered because of losses to agriculture, excessive grazing, and invasive species. Establishment in 1950 of what is now designated as the Idaho National Engineering and Environmental Laboratory (southeastern Idaho, USA) created the largest existing reserve of this extensive vegetation type. We used cover, density, and frequency data for vascular plants sampled on 79 permanent plots nine times during 45 years to (1) assess...
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In the arid southwest of North America, winter precipitation penetrates to deep soil layers, whereas summer "monsoon" precipitation generally wets only surface layers. Use of these spatially separated water sources was determined for three dominant tree species of the pinyon-juniper ecosystem at six sites along a gradient of increasing summer precipitation in Utah and Arizona. Mean summer precipitation ranged from 79 to 286 mm, or from 18% to 60% of the annual total across the gradient. We predicted that, along this summer rainfall gradient, populations of dominant tree species would exhibit a clinal off-on response for use of water from upper soil layers, responding at particular threshold levels of summer precipitation...
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The fine roots of trees are concentrated on lateral branches that arise from perennial roots. They are important in the acquisition of water and essential nutrients, and at the ecosystem level, they make a significant contribution to biogeochemical cycling. Fine roots have often been studied according to arbitrary size classes, e.g., all roots less than 1 or 2 mm in diameter. Because of the size class approach, the position of an individual root on the complex lateral branching system has often been ignored, and relationships between the form of the branching root system and its function are poorly understood. The fine roots of both gymnosperms and angiosperms, which formed ectomycorrhizae (EM) and arbuscular mycorrhizae...
We analyzed and radiocarbon-dated 205 fossil woodrat middens from 14 sites in central and northern Wyoming and adjacent Utah and Montana to document spatiotemporal patterns of Holocene invasion by Utah juniper (Juniperus osteosperma). Holocene migration into central and northern Wyoming and southern Montana from the south proceeded by a series of long-distance dispersal events, which were paced by climate variability and structured by the geographic distribution and connectivity of suitable habitats on the landscape. The migration of Utah juniper into the region involved multiple long-distance dispersal events, ranging from 30 to 135 km. One of the earliest established populations, on East Pryor Mountain in south...
Understanding the mechanisms by which nonnative species successfully in vade new regions and the consequences for native fauna is a pressing ecological issue, and one for which niche theory can play an important role. In this paper, we quantify a com prehensive suite of morphological, behavioral, physiological, trophic, and life-history traits for the entire fish species pool in the Colorado River Basin to explore a number of hypotheses regarding linkages between human-induced environmental change, the creation and mod ification of ecological niche opportunities, and subsequent invasion and extirpation of species over the past 150 years. Specifically, we use the fish life-history model of K. O. Winemiller and K....
Ecosystem susceptibility to invasion by nonnative species is poorly understood, but evidence is increasing that spatial and temporal variability in resources has large-scale effects. We conducted a study in Artemisia tridentata ecosystems at two Great Basin locations examining differences in resource availability and invasibility of Bromus tectorum over elevation gradients and in response to direct and interacting effects of removal of perennial herbaceous vegetation and fire. We monitored environmental conditions, soil variables, and B. tectorum establishment and reproduction over two years. Soil water (measured as the number of days soil matric potential was >?1.5 MPa) and nitrate availability (measured as micromoles...
The basic factors controlling competitive relationships between perennials and annuals on our ranges must be thoroughly understood if more than trial and error attempts at restoration are to be made. It is the objective of the present study to increase such understanding. Published in Ecological Monographs, volume 37, issue 2, on pages 89 - 111, in 1967.
    map background search result map search result map Energy relationships of the mammals of a desert shrub (Larrea tridentata) community Studies of the desert tortoise, Gopherus agassizii Fine root architecture of nine North American trees Intra- and Interspecific Variation for Summer Precipitation Use in Pinyon-Juniper Woodlands Energy relationships of the mammals of a desert shrub (Larrea tridentata) community Studies of the desert tortoise, Gopherus agassizii Intra- and Interspecific Variation for Summer Precipitation Use in Pinyon-Juniper Woodlands Fine root architecture of nine North American trees