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Results for “environmental variability”

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At least 325 records · Page 18Linked to original sources
PubMed1997

Precision agriculture: introduction to the spatial and temporal variability of environmental quality.

Precision agriculture aims at adjusting and fine-tuning land and crop management to the needs of plants within heterogeneous fields. Production aspects have to be balanced against environmental threshold values and modern information technology has made it possible to devise operational field systems. A reactive approach is described, using yield maps and sensors. A proactive approach uses simulation modelling of plant growth and solute fluxes to predict optimal timing of management practices. Precision agriculture, combining both approaches, is seen as making a major contribution towards the development of sustainable agricultural production systems.

Agriculture↗
PubMedDate not supplied

A method to measure the effective gas path length in the environmental or variable pressure scanning electron microscope.

A simple method is described to determine the effective gas path length when incident electrons scatter in the gas above the specimen. This method is based on the measurement of a characteristic x-ray line emitted from a region close to the incident beam. From various experimental measurements performed on various microscopes, it is shown that the effective gas path length may increase with the chamber pressure and that it is also often dependent of the type of x-ray bullet.

Journal Article↗
PubMed2003

Random walks in the history of life.

The simplest null hypothesis for evolutionary time series is that the observed data follow a random walk. We examined whether aspects of Sepkoski's compilation of marine generic diversity depart from a random walk by using statistical tests from econometrics. Throughout most of the Phanerozoic, the random-walk null hypothesis is not rejected for marine diversity, accumulated origination or accumulated extinction, suggesting that either these variables were correlated with environmental variables that follow a random walk or so many mechanisms were affecting these variables, in different ways, that the resultant trends appear random. The only deviation from this pattern involves rejection of the null hypothesis for roughly the last 75 million years for the diversity and accumulated origination time series.

Animals↗
PubMed2005

Employment participation following spinal cord injury: relation to selected participant demographic, injury and psychological characteristics.

PURPOSE: To determine the relationship between selected demographic, injury and psychological characteristics and the extent of employment undertaken following spinal cord injury (SCI). METHOD: Data were collected from 289 persons with SCI who found employment post-injury. A survey was administered to patients of a specialist spinal cord injury unit located in South-Eastern Australia. A variety of demographic, injury and psychological variables were assessed for their relationship to the extent of employment undertaken following SCI. Extent of employment, was defined as time spent employed as a proportion of the time available for employment, and is hereafter referred to as "workrate." RESULTS: On average, participants reported having been employed for almost two-thirds of their available time. Study variables explained 22% (18% adjusted) of the variance in post-injury workrate. Individually, demographic and psychological variables, but not injury variables, were found to relate to the outcome measure. CONCLUSION: Using a novel measure of employment outcome, this study yielded findings consistent with other research using alternate employment measures. Results suggest that demographic and psychological variables are more related to this measure of employment participation than are injury variables. The addition of environmental variables may allow for the explanation of more variance in outcome.

Adult↗
PubMed2006

Dietary proteins as environmental modifiers of type 1 diabetes mellitus.

Type 1 diabetes is an autoimmune disease in which the patient's immune system destroys the insulin-secreting beta-cells in the pancreatic islets of Langerhans. A majority of cases is thought to occur as a result of gene-environment interactions. The identity of the environmental factors remains unknown mainly because of the difficulty in linking past exposures with later disease development. Overall, the data suggest a model in which individuals develop diabetes by several different pathways, each influenced by numerous genetic and environmental variables. The most investigated environmental factors are diet and viruses. In this review, we examine the evidence that the source of dietary proteins can modify diabetes outcome, describe new approaches to identify candidate diabetes-related dietary agents, examine possible links with gut dysfunction, discuss some of the limitations, and propose a multifactorial model for dietary modification of diabetes. The key to diabetes pathogenesis, its prevention, and the ultimate success of beta-cell replacement therapies lies in understanding how the environment controls disease expression. Dietary proteins could be one of these keys.

Animals↗
PubMed2006

GIS-based niche modeling for mapping species' habitat.

Ecological "niche modeling" using presence-only locality data and large-scale environmental variables provides a powerful tool for identifying and mapping suitable habitat for species over large spatial extents. We describe a niche modeling approach that identifies a minimum (rather than an optimum) set of basic habitat requirements for a species, based on the assumption that constant environmental relationships in a species' distribution (i.e., variables that maintain a consistent value where the species occurs) are most likely to be associated with limiting factors. Environmental variables that take on a wide range of values where a species occurs are less informative because they do not limit a species' distribution, at least over the range of variation sampled. This approach is operationalized by partitioning Mahalanobis D2 (standardized difference between values of a set of environmental variables for any point and mean values for those same variables calculated from all points at which a species was detected) into independent components. The smallest of these components represents the linear combination of variables with minimum variance; increasingly larger components represent larger variances and are increasingly less limiting. We illustrate this approach using the California Gnatcatcher (Polioptila californica Brewster) and provide SAS code to implement it.

Animals↗
PubMed2005

Abundance of three bacterial populations in selected streams.

The population sizes of three bacterial species, Acinetobacter calcoaceticus, Burkholderia cepacia, and Pseudomonas putida, were examined in water and sediment from nine streams in different parts of the United States using fluorescent in situ hybridization (FISH). Population sizes were determined from three sites (upstream, midstream, and downstream) in each stream to compare differences in the occurrence and distribution of the species within each stream and among streams. Physical and chemical variables measured reflected differences in environmental conditions among the streams. In the water, B. cepacia numbers were highest in the agricultural, Iowa stream. P. putida numbers were highest in the southern coastal plain streams, Black Creek (GA) and Meyers Branch (SC). Compared to the other two species, the abundance of A. calcoaceticus was similar in all the streams. In sediment, the greatest abundance of all three species was found in the Iowa stream, while the lowest was in Hugh White Creek (NC). Detrended correspondence analysis (DCA) explained 95.8% and 83.9% of the total variation in bacterial numbers in water and sediment of the streams, respectively. In sediments and water, B. cepacia numbers were related to nitrate concentrations. A. calcoaceticus in water clustered with several environmental variables (i.e., SRP, pH, and conductivity) but benthic populations were less well correlated with these variables. This study reveals the potential influence of various environmental conditions on different bacterial populations in stream communities.

Acinetobacter↗
PubMed2001

Geomagnetic disturbance associated with decrease in heart rate variability in a subarctic area.

BACKGROUND: Physical environmental variables, such as the natural variation in the geomagnetic field in and around the earth, influence biological processes and human health. The effect of geomagnetic disturbances on heart rate variability (HRV) in healthy students in a subarctic area is studied herein. SUBJECTS AND METHODS: Seven-day records by Holter ECG were obtained from eight clinically healthy subjects in Alta, Norway (70 N). Frequency- and time-domain measures of HRV were compared between 24-hour spans of high geomagnetic disturbance versus quiet conditions. RESULTS: A 5.9% increase in the 24-hour average of HR (P = 0.020) and a 25.2% decrease in HRV (P = 0.002) were documented on days of high geomagnetic disturbance. The decrease in spectral power was found primarily at frequencies lower than 0.04 Hz and was not statistically significant around 3.6 sec. CONCLUSIONS: The physiological mechanism involved may be other than the parasympathetic, usually identified with spectral power centered around 3.6 sec, a spectral region wherein no statistically significant differences were found.

Adult↗
PubMed1991

Shiftwork. Consequences and considerations.

1. The shiftworker's inability to adapt to shiftwork schedules can lead to a loss of physical and psychological well being, and can produce negative safety and performance consequences. 2. The factors that contribute to the intolerance of working shiftwork schedules are complex and interrelated and include host variables (i.e., circadian rhythms) and environmental variables (i.e., type of shifts worked). 3. The most direct and consistent effect of working shiftwork is the impairment of the quality and quantity of sleep. 4. Interventions should include a program that monitors workers' tolerance to shiftwork and provides information and recommendations for employees to effectively manage a lifestyle that incorporates nighttime work schedules.

Adaptation, Physiological↗
PubMed2004

Mutators in space: the dynamics of high-mutability clones in a two-patch model.

Clones of bacteria possessing high-mutability rates (or mutators) are being observed in an increasing number of species. In a constant environment most mutations are deleterious, and hence the spontaneous mutation rate is generally low. However, mutators may play an important role in the adaptation of organisms to changing environments. To date, theoretical work has focused on temporal variability in the environment, implicitly assuming that environmental conditions are constant through space. Here, we develop a two-patch model to investigate how spatiotemporal environmental variability and dispersal might influence mutator dynamics. Environmental conditions in each patch fluctuate between two states; the rate of fluctuation varies in each patch at differing phase angles. We find that at low and intermediate rates of fluctuation, an increase in dispersal results in a decrease in the density of mutators. However, at high rates of environmental change, dispersal causes an increase in mutator density. For all frequencies of environmental fluctuation these trends are enhanced as the phase angle approaches 180 degrees. We argue that future work, both empirical and theoretical, is needed to improve our understanding of how spatiotemporal variability impacts on mutator densities and dynamics.

Alleles↗
PubMed2002

Characterizing man-made and natural modifications of microbial diversity and activity in coastal ecosystems.

The impacts of growing coastal pollution and habitat alteration accompanying human encroachment are of great concern at the microbial level, where much of the ocean's primary production and biogeochemical cycling takes place. Coastal ecosystems are also under the influence of natural perturbations such as major storwns and flooding. Distinguishing the impacts of natural and human stressors is essential for understanding environmentally-induced change in microbial diversity and function. The objective of this paper is to discuss the applications and merits of recently developed molecular, ecophysiological and analytical indicators and their utility in examining anthropogenic and climatic impacts on the structure and function of coastal microbial communities. The nitrogen-limited Neuse River Estuary and Pamlico Sound, North Carolina are used as examples of ecosystems experiencing both anthropogenic (i.e., accelerating eutrophication) and climatic stress (increasing frequencies of tropical storms and hurricanes). Additional examples are derived from a coastal monitoring site (LEO) on the Atlantic coast of New Jersey and Galveston Bay, on the Gulf of Mexico. In order to assess structure, function, and trophic state of these and other coastal ecosystems, molecular (DNA and RNA-based) characterizations of the microbial taxa involved in carbon, nitrogen and other nutrient transformations can be combined with diagnostic pigment-based indicators of primary producer groups. Application of these methods can reveal process-level microbial community responses to environmental variability over a range of scales. Experimental approaches combined with strategic monitoring utilizing these methods will facilitate: (a) understanding organismal and community responses to environmental change, and (b) synthesizing these responses in the context of ecosystem models that integrate physical, chemical and biotic variability with environmental controls.

Animals↗
PubMed1993

Environmental uncertainty and variable diapause.

We analyze a stage-structured model of a population that displays variable diapause in a randomly varying environment. The ruggedness of the environment is measured by the extent of random variation in per-capita reproductive success. We show how variable diapause and environmental characteristics affect the population's stochastic growth rate. In rugged unpredictable environments, phenotypes that show some tendency to diapause are found to have a higher growth rate than nondiapausing phenotypes. In harsh rugged environments, some tendency to diapause may be all that permits population persistence. Positive serial autocorrelation causes the optimal diapause fraction to decrease, while negative autocorrelation causes that fraction to increase. The structured model behaves very differently from a scalar model for large diapause fractions even in uncorrelated environments, and in many cases predicts a broad optimum. The difference between models is due to the extreme variability of stage structure in populations subject to even small variability when diapause tendency is high.

Animals↗
PubMed2026

Climate-Driven Niche Tracking and Genomic Resilience Shape Future Distribution of a Widespread Agricultural Weed.

Understanding how agriculturally important species respond to environmental change is critical for maintaining productivity, mitigating agroecosystem threats and sustaining resilience. While crops have traditionally been the focus in agroecosystems, agricultural weeds are integral components that often face even stronger selective pressures, making them powerful models for investigating ecological and evolutionary responses to climatic and human-mediated challenges. Insights from how weeds adapt rapidly under these pressures can inform strategies to improve agricultural outcomes, since both pests and crops evolve under the same multivariate selective pressures. Here, we integrate two centuries of distribution records with whole-genome sequencing from natural populations of the most damaging weed in Europe-Alopecurus myosuroides (blackgrass) - to examine its ecological and evolutionary responses in agroecosystems. Blackgrass largely maintained its historical climatic niche, expanding its range primarily by tracking environments analogous to those it historically occupied. Genome-wide analyses revealed a polygenic basis of environmental responses, with most loci linked to single environmental variables and a subset showing limited environmental pleiotropy, indicating modular adaptation to the complex selective pressures of managed agricultural landscapes. Coupling these genomic-environment relationships with projected climate change and genomic offset analyses indicated that most blackgrass populations will remain well aligned with future conditions. Our findings show that ecological niche tracking and polygenic adaptation allow agricultural weeds like blackgrass to persist under rapid environmental change, offering insights relevant not only for weed management but also for designing resilient cropping systems under future climates.

Plant Weeds↗
PubMedDate not supplied

Quitting smoking.

Four factors which influence smoking treatment outcome are identified: environmental variables, client characteristics, process variables, and specific treatment approaches. Important environmental factors are stress and social support. Of client characteristics, sex is the best predictor of treatment success. Men are more likely to quit and maintain abstinence than women. However, the majority of women alter their smoking habits during pregnancy. Low-income persons and ethnic minorities are underrepresented among subjects in treatment studies and have larger percentages of smokers in the population at large. Extraverted smokers are more likely to begin to smoke and have difficulty quitting. Also, the more anxious, poorly adjusted smoker has more trouble quitting than the less troubled smoker. The higher the client's sense of self-efficacy, the better the chance of that person entering treatment and doing well. Furthermore, smokers who take in lower levels of nicotine are more successful at quitting. Many process questions are suggested. Few have been approached empirically. The effectiveness of ex-smokers as therapists in smoking cessation programs has not been systematically investigated, even though the smoking history of therapists is a question frequently asked by clients. We suggest that the skill and empathy of group leaders is more important than smoking history. Smoking therapists should be aware of nonspecific treatment factors such as positive expectations, social reinforcement, and self-disclosure which may have a powerful influence on the efficacy of smoking treatment. Specific treatment approaches were classified into three categories: low-contact approaches, including educational, self-help, and minimal treatment approaches; psychological treatments; and pharmacological treatment. Education, self-help, and minimal treatment approaches are thought to be accretively effective when the large size of the audience is considered. Also, innovative treatments which address the influence of social support systems and physiological addition are promising treatments for individuals requiring a structured or intensive method of quitting.

Black or African American↗
PubMedDate not supplied

Teenage smoking: some problems with interpreting the evidence.

UNLABELLED: This paper attempts to address the methodological problem of disentangling complex interrelationships between teenage smoking and other variables when using survey data. As in any research based on survey data, it is difficult to distinguish systematic from random patterns due to other variables. METHOD: A pragmatic approach of classifying variables into three groups of objectively measured demographic, socio-environmental variables and subjectively measured sociopsychological variables was adopted. A secondary cross-sectional survey data source was used to fit and test three models. RESULTS: The results suggest socio-environmental and socio-psychological variables may effect teenage smoking through demographic and some other variables not measured in the survey. CONCLUSION: Standard statistical techniques are unable to distinguish between the different effects and their impact on the outcome. However, they can be used to re-examine and re-evaluate the results as potential evidence. A re-assessment of results suggests that effects of variables reported in the literature on teenage smoking may be over estimated. For example, self-esteem may not be as important and the role of peer pressure may not be as clear as it has been made out to be.

Adolescent↗