Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “environmental variability”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 721 records · Page 40Linked to original sources
PubMed2005

Human whole-body reaching in normal gravity and microgravity reveals a strong temporal coordination between postural and focal task components.

Previous experiments by our group in normal gravity (1 G) have revealed spatial relationships between postural and focal components of whole-body reaching and pointing movements. We suggested that these relationships could be explained partly through the use of gravity to displace the CoM and attain the object or target position. In this study we compared human whole-body reaching in 1 G and microgravity (0 G) in order to more fully investigate how gravity contributes to strategies adopted for task execution and to determine possible invariant temporal relationships between multiple segments. Whole-body reaching movements made from the standing position in two experimental conditions of execution speed (naturally paced and as fast as possible) were recorded during periods of 1 G and 0 G in parabolic flight. Overall, at each speed of reaching, movement times were significantly slower when performed in 0 G than in 1 G for two of the three subjects, but all subjects were able to produce significantly faster movements in 0 G than in 1 G. Despite similar general trends across subjects observed in 1 G, angular displacements of reaching movements performed in 0 G differed greatly between subjects. There were changes at all joints, but above all at the shoulder and the ankle. However, despite a high intersubject and intratrial variability in 0 G, in both gravity conditions all subjects demonstrated times to peak curvilinear velocity for the finger (end effector) and the whole-body centre of mass (CoM) that coincided, regardless of the speed of execution. Moreover, cross-correlations between multiple segment curvilinear velocities and those of the CoM revealed tight, highly correlated temporal relationships between segments proximal to the CoM (which was expected). However, for more distal segments, the correlations were weaker, and the movements lagged behind movements of the CoM. The major and most interesting finding of this study was that although the finger was the most distal within the segment chain, with respect to the CoM, it was highly correlated with the CoM (0.99--0.98, all conditions) and with no time lag. Despite the large intersubject and inter-environmental variability recorded in this study, temporal relationships between postural task components (CoM displacements) and those of the focal movement (end-effector trajectory) were consistently conserved.

Acclimatization↗
PubMed2005

Medical considerations in triathlon competition: recommendations for triathlon organisers, competitors and coaches.

Competitors in triathlons experience a range of environmental conditions and physiological demands in excess of that found in individual sport events of comparable duration. Consequently, there is a broad range of possible medical problems and complications that must be taken into account when preparing for such races. For most competitors, an Olympic-distance triathlon typically takes between 2-4 hours to complete. This race begins with a swimming segment of 1500 m. Given the wide variety of race venues found around the world, these swims occur in an assortment of water temperatures (from warm to cold) and conditions (from ocean surf to lake calm). Swimmers often exit the water in a state of moderate dehydration and hypothermia and then immediately start the 40 km cycling leg. Many do so in their swimming attire. A wide variety of road surfaces, technically challenging topography, variable environmental conditions and dramatically changing velocities can be encountered on the cycle course. The race concludes with a 10 km running leg. Since it is the final leg, it is often completed in higher ambient temperatures than those encountered at the start, with the athlete possibly running in a significant state of dehydration and fatigue. Other medical problems commonly encountered in triathlon include: muscle cramping, heat illness, postural hypotension, excessive exposure to ultraviolet radiation, musculoskeletal injuries and trauma, gastrointestinal problems as well as post-race bacterial infection, immunosuppression, sympathetic nervous system and psychological exhaustion, and haemolysis. The rate of occurrence of such events and the severity of their potentially negative outcomes is a function of the methods used by both the race organisers and the competitors to prevent or respond to the conditions imposed by the race. Triathletes also commonly compete in both shorter 'sprint distance' events (in the range of a 0.75 km swim, 20 km cycle and 5 km run) and longer events including both one-half and full Ironman distances (2.5 and 3.8 km swim, 80 and 180 km cycle, 20 and 42 km run, respectively), as well as ultra-distance events that exceed the Ironman distance. In the longer events, the previously mentioned medical considerations are further magnified and additional considerations such as hyponatraemia can also occur. Reducing risk associated with these concerns is accomplished by: taking into account weather and water temperature/conditions data prior to event scheduling; effective swim, cycle and run course organisation and management; environmental monitoring prior to and during the event; the implementation of a water safety plan; provision of appropriate fluid replacement throughout the course; implementation of helmet use and non-drafting regulations in the cycling leg; and competitor knowledge regarding fluid replacement, biomechanical technique, physical preparation, safe equipment and course familiarity. Despite these concerns, triathlon participation appears to relatively safe for persons of all ages, assuming that high-risk adults undertake health screening.

Adult↗
PubMed2000

Effect of wind and solar radiation on metabolic heat production in a small desert rodent, Spermophilus tereticaudus.

To understand better how complex interactions between environmental variables affect the energy balance of small diurnal animals, we studied the effects of the absence and presence of 950 W m(-)(2) simulated solar radiation combined with wind speeds ranging from 0. 25 to 1.00 m s(-)(1) on the metabolic rate and body temperature of the round-tailed ground squirrel Spermophilus tereticaudus. As wind speed increased from 0.25 to 1.00 m s(-)(1), metabolic heat production increased by 0.94 W in the absence of solar radiation and by 0.98 W in the presence of 950 W m(-)(2) simulated solar radiation. Exposure to simulated solar radiation reduced metabolic heat production by 0.68 W at a wind speed of 0.25 m s(-)(1), by 0.64 W at 0.50 m s(-)(1) and by 0.64 W at 1.00 m s(-)(1). Body temperature was significantly affected by environmental conditions, ranging from 32. 5 degrees C at a wind speed of 1.0 m s(-)(1) and no irradiance to 35. 6 degrees C at a wind speed of 0.50 m s(-)(1) with 950 W m(-)(2 )short-wave irradiance. In addition, several unusual findings resulted from this study. The coat of S. tereticaudus is very sparse, and the observed heat transfer of 5.68+/-0.37 W m(-)(2 ) degrees C(-)(1) (mean +/- s.e.m., N=11) is much higher than expected from either allometric equations or comparative studies with other rodents of similar mass. Solar heat gain was remarkably low, equalling only 10 % of intercepted radiation and suggesting a remarkably high regional thermal resistance at the tissue level. Animals remained normally active and alert at body temperatures as low as 32.5 degrees C. These findings suggest a unique combination of adaptations that allow S. tereticaudus to exploit a harsh desert environment.

Animals↗
PubMed1988

Paediatric campylobacter diarrhoea from household exposure to live chickens in Lima, Peru.

Although Campylobacter jejuni is a frequent enteropathogen in cases of paediatric diarrhoea in developing countries, its route of transmission is not well understood. An age-matched, case-control study of children with C. jejuni diarrhoea was therefore carried out in Lima, Peru, from January 1983 to April 1986 to identify the risk factors and vehicles of transmissions. As cases, 104 children less than 3 years of age were selected and compared with controls of the same age with non-gastrointestinal illnesses. Household exposure to live chickens was an important risk factor (odds ratio, 11; after adjusting for socioeconomic and environmental variables). Subjects in index households had a higher frequency of infection than those in control households, and infected young children were more likely to be ill than older children or adults, suggesting that immunity may be acquired from natural infection. The risk factors identified suggest that direct contact with the faeces of C. jejuni-infected chickens in the household environment was largely responsible for transmission of the organism to susceptible infants.

Animals↗
PubMed2005

Developing an optimal river typology for biological elements within the Water Framework Directive.

The Water Framework Directive requires a river classification based on environmental variables (a typology) to be created as a structure for reporting ecological status. A single permutation procedure, utilising the same variables repeatedly but with different categorical divisions, enabled both the choice of variables and the boundary divisions for these variables to be optimised simultaneously in the development of the typology. This, in addition to a data set which appropriately combined different biological elements, enabled a typology to be developed which was far more effective than a System A, CCA-derived or expert opinion-based typology in segregating communities. This optimal typology could be used to improve the performance of ecological quality assessment methods.

Animals↗
PubMed1995

Bioaccumulation of polycyclic aromatic hydrocarbons by marine organisms.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the marine environment, occurring at their highest environmental concentrations around urban centers. While they can occur naturally, the highest concentrations are mainly from human activities, and the primary sources are combustion products and petroleum. Two factors, lipid and organic carbon, control to a large extent the partitioning behavior of PAHs in sediment, water, and tissue; the more hydrophobic a compound, the greater the partitioning to these phases. These two factors, along with the octanol-water partition coefficient, are the best predictors of this partitioning and can be used to determine PAH behavior and its bioavailability in the environment. It is well known that the lipid of organisms contains the highest levels of hydrophobic compounds such as PAHs, and that organic carbon associated with sediment or dissolved in water can have the greatest influence on PAH bioavailability. Partitioning of combustion-derived PAHs between water and sediment may be much less than predicted, possibly because associations with particles are much stronger than expected. This reduced partitioning may produce erroneous results in predicting bioaccumulation where uptake from water is important. Accumulation of PAHs occurs in all marine organisms; however, there is a wide range in tissue concentrations from variable environmental concentrations, level and time of exposure, and species ability to metabolize these compounds. PAHs generally partition into lipid-rich tissues, and their metabolites can be found in most tissues. In fish, liver and bile accumulate the highest levels of parent PAH and metabolites; hence, these are the best tissues to analyze when determining PAH exposure. In invertebrates, the highest concentrations can be found in the internal organs, such as the hepatopancreas, and tissue concentrations appear to follow seasonal cycles, which may be related to variations in lipid content or spawning cycles. The major route of uptake for PAHs has been debated for years. For the more water-soluble PAHs, it is believed that the main route of uptake is through ventilated water and that the more hydrophobic compounds are taken in mainly through ingestion of food or sediment. There are many variables, such as chemical hydrophobicity, uptake efficiency, feeding rate, and ventilatory volume, which may affect the outcome. The route of uptake may be an important issue for short-term events; however, under long-term exposure and equilibrium conditions between water, prey, and sediment, the route of uptake may be immaterial because the same tissue burdens will be achieved regardless of uptake routes.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2003

Long-term trends in water quality and their impact on macroinvertebrate assemblages in eutrophic lowland rivers.

Long-term trends in water quality in eutrophic lowland rivers in eastern England were investigated and their impact on macroinvertebrate assemblages studied. Dissolved oxygen (DO) declined significantly in eight rivers in Essex and Suffolk over 40 years to 1998. Chloride concentrations significantly increased during the same period in most rivers. Total oxidized nitrogen and soluble reactive phosphorus increased until the 1980s, then began to decline. Biotic scores (Lincoln Quality Index) generally increased over 14 years to 1998 and there were significant positive relationships between biotic scores and several nutrients. Invertebrate families and environmental variables sampled over the eight rivers in a dry year (1997) and a wet year (1998) were subjected to multivariate analysis. River stretches were grouped according to substrate requirements of indicator invertebrates. In the dry year, those river stretches behind mills or immediately downstream of sewage treatment works (STW) were grouped. In the wet year, there was only one separate group, comprising sites downstream of STWs. Nutrients, DO and low flows have a much greater influence on water quality, and hence invertebrate assemblages, during drought years than during wet years.

Animals↗
PubMed1983

[Environmental and occupational factors in the development of congenital malformations].

Teratology has grown from a little known discipline to a discipline embracing a vast accumulation of literature on experimental studies in many animal forms. Emphasis has shifted from preoccupation with descriptions of anatomical defects to concern about subtle and interacting causative factors. The aim of this work is to assemble a source of facts, concepts, methods and references within the broad scope of teratology. Our review summarizes reports of teratogenic effects in mammals. The major intent of this work, however, has been to introduce into a field of teratology. Special attention has been paid to the relation of animal studies to the problem in man and to the interactions between teratogens and environmental variables. We present a number of illustrative examples of various kinds of interactions that may occur and their possible significance. Striking degrees of potentiative interactions have been demonstrated in laboratory animals when two or more compounds were used simultaneously at doses at or below the threshold level for teratogenicity of the same compound when applied singly. We propose the name coteratogenesis for the agents which are not teratogenic but which aggravate teratogenicity of the other factors known as teratogens.

Abnormalities, Drug-Induced↗
PubMed2006

The use of a D-optimal design to model the effects of temperature, NaCl, type and acid concentration on Lactobacillus pentosus IGLAC01.

AIMS: To study the effects of temperature, NaCl and acid (HCl, citric, acetic and lactic) concentrations on the specific growth rate (mu), lag phase (lambda), and h0 of Lactobacillus pentosus IGLAC01. METHODS AND RESULTS: Response surface (RS) methodology (D-optimal design) was used with a dummy variable, to account for the different types of acids. The variable ranges were: 16-30 degrees C, 0-70 g l-1 NaCl, and 0-5 g l-1 acid (or 0-2.5 g l-1 HCl). Time to detection from optical density data was used to deduce mu and lambda. The RS models for log2mu and log2lambda, according to acid types, were estimated and the effects of variables were quantified by their z-generalized values. A relationship between ln h0 with temperature was also found. CONCLUSIONS: The mu of L. pentosus IGLAC01 can be doubled by increasing temperature by 10.3 degrees C or by decreasing NaCl by 48 g l-1 (harmonic, averaged, z values, Z); citric was the least inhibitory acid (zmu=-96.2) and lactic the strongest (zmu=-5.7), according to their generalized z values, z. A twofold lambda increase was achieved from a decrease of 3.1 degrees C (HCl), or 4.27 degrees C (citric) or 36 g l-1 NaCl increase (both acids) (expressed as zlambda ); the same effect was obtained from a decrease of 4.37 degrees C, 54 g l-1 NaCl increase, or 10 g l-1 acetic or lactic acid additions (expressed as Zlambda values). SIGNIFICANCE AND IMPACT OF THE STUDY: Valuable information on the effects of environmental variables on the biological parameters of L. pentosus IGLAC01, which could be used for the optimization of olive, cucumber or other vegetable fermentations, is obtained.

Acetic Acid↗
PubMed1998

Short stature at school entry--an index of social deprivation? (The Wessex Growth Study).

This study was carried out to examine the biological and environmental variables associated with non-organic short stature. We observed an unselected population of very short normal children (SN) and their age- and sex-matched controls (C) within the community. All 14,346 children in two health districts entering school during 2 consecutive years were screened for short stature, and those whose height lay below the 3rd centile, according to Tanner and Whitehouse standards (n = 180) were identified. Excluding 32 with pathology, five from ethnic minorities and three who refused to take part, the remaining SN children (mean height SDS-2.26) were matched with 140 age- and sex-matched controls (C) of average height (mean height SDs 0.14). Birth weight, target height and predicted adult height (based on parental height and bone age respectively), medical and social background (obtained from parental interviews), and school performance (assessed by class teachers) were the main outcome measures. Mean birth weight of the SN children was significantly lower than C (SN = 2845 g, C = 3337 g, P < 0.001). Mean mid-parental target height was also very different (SN = 162.0 cm, C = 170.9 cm, P < 0.001). Thirty-five per cent of SN children (C = 6%) had height SD scores below parental target range, though only 10% had predicted heights below target range (mean delay in bone age 0.68 years). There was a significant difference between SN children and C in the number of children in the household (SN = 2.8, C = 2.4 (P = 0.007) and in socio-economic status (P < 0.002). Many more SN children were in social classes IV and V (SN = 31%, C = 13%, P < 0.002), and had an unemployed father (SN = 22%, C = 10%, P < 0.010), highlighting the importance of environmental influences on growth. One in four SN children was judged to have serious psychosocial problems. However, the lower the socio-economic class, the less likely the SN children were to be inappropriately short for parents. Significantly more SN children were reported to have asthma (SN = 18%, C = 7%, P < 0.007) and eczema (SN = 19%, C = 5%, P < 0.001), though only the latter was significantly associated with stature below target height for both SN and C groups. Biological variables are often insufficient to explain short stature. No child, whatever the parental height, should be dismissed as normal without careful evaluation, as poor growth in the early years may be an important pointer to an adverse but potentially remediable environment.

Adult↗
PubMed2004

Using human disease outbreaks as a guide to multilevel ecosystem interventions.

Human health often depends on environmental variables and is generally subject to widespread and comprehensive surveillance. Compared with other available measures of ecosystem health, human disease incidence may be one of the most useful and practical bioindicators for the often elusive gauge of ecologic well-being. We argue that many subtle ecosystem disruptions are often identified only as a result of detailed epidemiologic investigations after an anomalous increase in human disease incidence detected by routine surveillance mechanisms. Incidence rates for vector-mediated diseases (e.g., arboviral illnesses) and direct zoonoses (e.g., hantaviruses) are particularly appropriate as bioindicators to identify underlying ecosystem disturbances. Outbreak data not only have the potential to act as a pivotal warning system for ecosystem disruption, but may also be used to identify interventions for the preservation of ecologic health. With this approach, appropriate ecologically based strategies for remediation can be introduced at an earlier stage than would be possible based solely on environmental monitoring, thereby reducing the level of "ecosystem distress" as well as resultant disease burden in humans. This concept is discussed using local, regional, and global examples, thereby introducing the concept of multilevel ecosystem interventions. Key words: bioindicators, disease control, disease outbreaks, ecologic management, ecosystem health, surveillance.

Disease Outbreaks↗
PubMed2005

[Risk factors for atopic dermatitis and familial prevalence in Spain (ELIHO study)].

INTRODUCTION: Atopic dermatitis (AD) is a cutaneous disease of unknown etiology. It shows a clear genetic predisposition with probable environmental modulation. This study evaluated risk factors associated with diagnosis and flares of AD in Spanish children. PATIENTS AND METHODS: We performed an observational, multicenter, retrospective case-control study that included 4243 children aged less than 14 years old with AD and 978 controls matched for age and sex. Family history of disease and environmental variables were collected in both groups and clinical history of AD was recorded in the case group. RESULTS: Significant risk factors for AD were: a family history of the disease and concomitant cutaneous infections. The prevalence of AD in first degree relatives was 39 % and that in second degree relatives was 19 % (higher in maternal than paternal lines). The mean age of children with AD was 4.2 (SD 3.4) years and the mean age at diagnosis was 1.5 (SD 2.2) years, with a mean of 2.9 (SD 2.6) flares during the previous year. Cold weather (Cantabrian and Continental Iberian Peninsula areas) was related to a greater number of flares. Children with AD had a greater number of concomitant cutaneous diseases and infections than children in the control group. CONCLUSIONS: AD is mainly a genetic disease, with climatic factors involved in severity modulation, and with important immunological alterations. In contrast, this study found no domestic environmental factors that were associated with disease onset.

Adolescent↗
PubMed2005

Geographic and environmental sources of variation in lake bacterial community composition.

This study used a genetic fingerprinting technique (automated ribosomal intergenic spacer analysis [ARISA]) to characterize microbial communities from a culture-independent perspective and to identify those environmental factors that influence the diversity of bacterial assemblages in Wisconsin lakes. The relationships between bacterial community composition and 11 environmental variables for a suite of 30 lakes from northern and southern Wisconsin were explored by canonical correspondence analysis (CCA). In addition, the study assessed the influences of ARISA fragment detection threshold (sensitivity) and the quantitative, semiquantitative, and binary (presence-absence) use of ARISA data. It was determined that the sensitivity of ARISA was influential only when presence-absence-transformed data were used. The outcomes of analyses depended somewhat on the data transformation applied to ARISA data, but there were some features common to all of the CCA models. These commonalities indicated that differences in bacterial communities were best explained by regional (i.e., northern versus southern Wisconsin lakes) and landscape level (i.e., seepage lakes versus drainage lakes) factors. ARISA profiles from May samples were consistently different from those collected in other months. In addition, communities varied along gradients of pH and water clarity (Secchi depth) both within and among regions. The results demonstrate that environmental, temporal, regional, and landscape level features interact to determine the makeup of bacterial assemblages in northern temperate lakes.

Bacteria↗
PubMed2005

Relationship of temporal and spatial variabilities of ammonia-oxidizing bacteria to nitrification rates in Monterey Bay, California.

Temporal and spatial dynamics of ammonia-oxidizing bacteria (AOB) were examined using genes encoding 16S rRNA and ammonia monooxygenase subunit A (AmoA) in Monterey Bay, Calif. Samples were collected from three depths in the water column on four dates at one mid-bay station. Diversity estimators for the two genes showed a strong positive correlation, indicating that overlapping bacterial populations had been sampled by both sets of clone libraries. Some samples that were separated by only 15 m in depth had less genetic similarity than samples that were collected from the same depth months apart. Clone libraries from the Monterey Bay AOB community were dominated by Nitrosospira-like sequences and clearly differentiated from the adjacent AOB community in Elkhorn Slough. Many Monterey Bay clones clustered with previously identified 16S rRNA and amoA groups composed entirely of marine sequences, supporting the hypothesis that these groups are specific to the marine environment and are dominant marine AOB. In addition, novel, phylogenetically distinct groups of AOB sequences were identified and compared to sequences in the database. Only one cluster of gammaproteobacterial AOB was detected using 16S rRNA genes. Although significant genetic variation was detected in AOB populations from both vertical and temporal samples, no significant correlation was detected between diversity and environmental variables or the rate of nitrification.

Ammonia↗
PubMed2002

Development of the permeability/performance reference compound approach for in situ calibration of semipermeable membrane devices.

Permeability/performance reference compounds (PRCs) are analytically noninterfering organic compounds with moderate to high fugacity from semipermeable membrane devices (SPMDs) that are added to the lipid prior to membrane enclosure. Assuming that isotropic exchange kinetics (IEK) apply and that SPMD-water partition coefficients are known, measurement of PRC dissipation rate constants during SPMD field exposures and laboratory calibration studies permits the calculation of an exposure adjustment factor (EAF). In theory, PRC-derived EAF ratios reflect changes in SPMD sampling rates (relative to laboratory data) due to differences in exposure temperature, membrane biofouling, and flow velocity-turbulence at the membrane surface. Thus, the PRC approach should allow for more accurate estimates of target solute/vapor concentrations in an exposure medium. Undersome exposure conditions, the impact of environmental variables on SPMD sampling rates may approach an order of magnitude. The results of this study suggest that most of the effects of temperature, facial velocity-turbulence, and biofouling on the uptake rates of analytes with a wide range of hydrophobicities can be deduced from PRCs with a much narrower range of hydrophobicities. Finally, our findings indicate that the use of PRCs permits prediction of in situ SPMD sampling rates within 2-fold of directly measured values.

Calibration↗
PubMed2001

Impact of preadjusting a quantitative phenotype prior to sib-pair linkage analysis when gene x environment interaction exists.

The investigation of potential gene x environment (G x E) interactions is an important facet in the study of complex diseases. When G x E interaction exists, linkage analyses of the interacting gene must treat the environmental factor appropriately. Specifically, the common approach of regressing out an environmental factor prior to linkage analysis may be inappropriate if that factor has an interaction with the gene. This is explored here in the Genetic Analysis Workshop 12 simulated data set using the G x E interaction between major gene four (MG4) and environmental factor two (E2). The analysis shows that preadjusting the quantitative trait three (Q3) phenotype for the main effects of several environmental variables, including one (E2) that interacts with MG4, affects the results of a Haseman-Elston linkage analysis. In particular, the agreement in detecting linkage between preadjusting versus not preadjusting was only 78% and 66% using alpha levels of 0.05 and 0.10, respectively. For both approaches, incorporating an interaction term in the regression models enabled linkage to be detected where the evidence was either minimal or not present in an identical-by-descent main effects-only model. Furthermore, preadjustment for E2 did not appear to account for the major discrepancies between the approaches.

Chromosome Mapping↗
PubMed2001

Environment-sensitive hydrogels for drug delivery.

Environmentally sensitive hydrogels have enormous potential in various applications. Some environmental variables, such as low pH and elevated temperatures, are found in the body. For this reason, either pH-sensitive and/or temperature-sensitive hydrogels can be used for site-specific controlled drug delivery. Hydrogels that are responsive to specific molecules, such as glucose or antigens, can be used as biosensors as well as drug delivery systems. Light-sensitive, pressure-responsive and electro-sensitive hydrogels also have the potential to be used in drug delivery and bioseparation. While the concepts of these environment-sensitive hydrogels are sound, the practical applications require significant improvements in the hydrogel properties. The most significant weakness of all these external stimuli-sensitive hydrogels is that their response time is too slow. Thus, fast-acting hydrogels are necessary, and the easiest way of achieving that goal is to make thinner and smaller hydrogels. This usually makes the hydrogel systems too fragile and they do not have mechanical strength necessary in many applications. Environmentally sensitive hydrogels for drug delivery applications also require biocompatibility. Synthesis of new polymers and crosslinkers with more biocompatibility and better biodegradability would be essential for successful applications. Development of environmentally sensitive hydrogels with such properties is a formidable challenge. If the achievements of the past can be extrapolated into the future, however, it is highly likely that responsive hydrogels with a wide array of desirable properties can be made.

Animals↗
PubMed1999

Extinction risk and the 1/f family of noise models.

In order to predict extinction risk in the presence of reddened, or correlated, environmental variability, fluctuating parameters may be represented by the family of 1/f noises, a series of stochastic models with different levels of variation acting on different timescales. We compare the process of parameter estimation for three 1/f models (white, pink and brown noise) with each other, and with autoregressive noise models (which are not 1/f noises), using data from a model time-series (length, T) of population. We then calculate the expected increase in variance and the expected extinction risk for each model, and we use these to explore the implication of assuming an incorrect noise model. When parameterising these models, it is necessary to do so in terms of the measured ("sample") parameters rather than fundamental ("population") parameters. This is because these models are non-stationary: their parameters need not stabilize on measurement over long periods of time and are uniquely defined only over a specified "window" of timescales defined by a measurement process. We find that extinction forecasts can differ greatly between models, depending on the length, T, and the coefficient of variability, CV, of the time series used to parameterise the models, and on the length of time into the future which is to be projected. For the simplest possible models, ones with population itself the 1/f noise process, it is possible to predict the extinction risk based on CV of the observed time series. Our predictions, based on explicit formulae and on simulations, indicate that (a) for very short projection times relative to T, brown and pink noise models are usually optimistic relative to equivalent white noise model; (b) for projection timescales equal to and substantially greater than T, an equivalent brown or pink noise model usually predicts a greater extinction risk, unless CV is very large; and (c) except for very small values of CV, for timescales very much greater than T, the brown and pink models present a more optimistic picture than the white noise model. In most cases, a pink noise is intermediate between white and brown models. Thus, while reddening of environmental noise may increase the long-term extinction probability for stationary processes, this is not generally true for non-stationary processes, such as pink or brown noises.

Animals↗