Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Carrying Capacity”

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 415 records · Page 23Linked to original sources
PubMed1996

A control problem in a polluted environment.

Due to the importance of remediating polluted environments, many mathematical models have been developed to describe population-toxicant interactions. This article considers the situation where the amount of pollution in the environment is restricted to lie below a certain value M. A model is presented and viewed as a control problem in which the toxicant input into the environment is the controllable variable. An explicit formula is computed for the input function that meets certain restrictions. By analytically studying the dynamics of the population, it is possible to provide conditions on how to set M in order to guarantee survival of the population without a significant reduction of its carrying capacity. Numerical and experimental data are described that support these results.

Animals↗
PubMed1996

Bionomic equilibrium of two-species system. I.

The concept of bionomic equilibrium is used to obtain limits of cost per unit effort and maximum value of effort for the joint harvesting of three types of two-species system: (1) a logistic growth model of two ecologically independent species, (2) a logistic growth model of two species that have competitive interactions, and (3) a Lotka-Volterra model of one prey and one predator. In each case, the existence of feasible bionomic equilibrium points and that of partially feasible bionomic equilibrium points are considered separately. First it is shown that cost per unit effort has both upper and lower threshold values if feasible bionomic equilibrium points occur, whereas it has only the upper threshold values if partially feasible bionomic equilibrium points occur. Further, the threshold values depend solely on the catchability coefficients, the selling prices of unit biomasses, and the parameters of the given model, namely the biotic potentials, carrying capacities, etc. Next, it is proven that for the first type of feasible point, if the cost per unit effort is kept under proper threshold values, then corresponding to each such value, the maximum value of the efforts exerted can be calculated in terms of the given parameters only. Moreover, the set of all such maximum values of the efforts for different choices of cost per unit effort is always bounded and the supremum of these maximum efforts is independent of the choice of cost per unit effort.

Animals↗
PubMed1996

Spatially explicit models for the population dynamics of a species colonizing an island.

We construct reaction-diffusion models for the population dynamics of a species colonizing an island from a source population on a continent. We view the source population as inducing a density or flux of immigrants onto the island and interpret colonization as succeeding if the population on the island is predicted to persist even when immigration from the continent is stopped. To capture the observation that a sufficiently large population or density must be attained for colonization to succeed, we assume Allee (i.e., bistable) dynamics rather than logistic dynamics for the colonizing population. We consider the cases of colonization in both the absence and presence of a competitor. We use reaction-diffusion theory, especially comparison methods and sub- and supersolutions, to determine how parameters such as the distance from the continent to the island and the dispersal, birth and mortality rates, carrying capacity, and minimum viable population density of the colonizing species affect the outcome of the attempted colonization. In the case of colonization in the presence of a competitor we consider a number of scenarios involving different types and strengths of competition. Our analysis permits us to draw conclusions about the characteristics of a species that make it a good colonizer.

Animals↗
PubMed1992

Elevation of blood vitamin D2 levels does not impede the release of vitamin D3 from the skin.

The mechanism for the transfer of fat-soluble vitamin D3 from the avascular basal cellular layers of the epidermis to dermal capillaries and peripheral circulation is unknown, although vitamin D-binding protein (DBP) is thought to mediate this process. To evaluate the effect of increased occupancy of vitamin D carrier(s) on vitamin D3 removal from the skin, serial serum vitamin D2 and D3 concentrations were determined in three groups of six healthy volunteers given combinations of an oral dose of vitamin D2 (50,000 IU) and a fixed dose of UVB radiation (27 mJ/cm2). Serum vitamin D3 levels increased significantly following UVB (time effect, P < .01 by ANOVA), but the response remained unchanged after pretreatment with vitamin D2, increasing from 3 +/- 1 to 14 +/- 5 ng/mL (mean +/- SEM), versus UVB alone, 5 +/- 1 to 16 +/- 5 ng/mL. Elevation of serum vitamin D2 levels was also similar in the groups given vitamin D2 alone (< 1 to 64 +/- 8 ng/mL) and vitamin D2 + UVB (< 1 to 45 +/- 8 ng/mL). There was no time or treatment effect for changes in serum levels of 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, or (DBP) levels (P > .1). We conclude that vitamin D3 egress from the skin is not affected by elevated circulating vitamin D concentrations; thus, the cutaneous release of vitamin D is probably mediated by a protein such as DBP with a high carrying capacity for the vitamin.

Administration, Oral↗
PubMed1987

Model analysis of the enhancement of tissue oxygenation by hemodilution due to increased microvascular flow velocity.

The effect of hemodilution on oxygen delivery to the tissue was investigated analytically by taking into consideration the oxygen loss that occurs along the arterial microvasculature due to diffusion into the tissue and shunting to parallel running venules. The theoretical findings were related to experimental data on microvascular oxygen distribution and the blood flow weighted by the oxygen-carrying capacity of plasma and red blood cells (Q) during hemodilution. It was found that at 30-33% hematocrit, the amount of oxygen brought to the tissues is increased by 5% when diffusion is the only mechanism of oxygen loss, and by 15% when the loss is due to arteriole-venule (A-V) shunting. These increases are relative to the conditions of normal hematocrit, and are in addition to the 10% increase due to the enhancement of Q caused by hemodilution. The analysis was extended to conditions of low oxygen tension and flow condition, characteristic of ischemia. In the case of severe ischemia the total increase in oxygen delivery at hematocrit 30-33% was 38% for tissues with diffusion losses only, and 66% when shunting losses are predominant. These results suggest that hemodilution is particularly effective in increasing oxygenation in ischemic tissue, while it has a comparatively small effect in normal conditions.

Arterioles↗
PubMedDate not supplied

Cardiovascular adjustments to exercise: hemodynamics and mechanisms.

The integrated response to severe exercise involves fourfold to fivefold increases in cardiac output, which are due primarily to increases in cardiac rate and to a lesser extent to augmentation of stroke volume. The increase in stroke volume is partly due to an increase in end-diastolic cardiac size (Frank-Starling mechanism) and secondarily due to a reduction in end-systolic cardiac size. The full role of the Frank-Starling mechanism is masked by the concomitant tachycardia. The reduction in end-systolic dimensions can be related to increased contractility, mediated by beta adrenergic stimulation. Beta adrenergic blockade prevents the inotropic response, the decrease in end-systolic dimensions, and approximately 50% of the tachycardia of exercise. The enhanced cardiac output is distributed preferentially to the exercising muscles including the heart. Blood flow to the heart increases fourfold to fivefold as well, mainly reflecting the augmented metabolic requirements of the myocardium due to near maximal increases in cardiac rate and contractility. Blood flow to the inactive viscera (e.g., kidney and gastrointestinal tract) is maintained during severe exercise in the normal dog. It is suggested that local autoregulatory mechanisms are responsible for maintained visceral flow in the face of neural and hormonal autonomic drive, which acts to constrict renal and mesenteric vessels and to reduce blood flow. However, in the presence of circulatory impairment, where oxygen delivery to the exercising muscles is impaired as occurs to complete heart block where normal heart rate increases during exercise are prevented, or in congestive right heart failure, where normal stroke volume increases during exercise are impaired, or in the presence of severe anemia, where oxygen-carrying capacity of the blood is limited, visceral blood flows are reduced drastically and blood is diverted to the exercising musculature. Thus,, visceral flow is normally maintained during severe exercise as long as all other compensatory mechanisms remain intact. However, when any other compensatory mechanism is disrupted (even the elimination of splenic reserve in the dog), reduction and diversion of visceral flow occur.

Adaptation, Physiological↗
PubMed1989

The role of catecholamines in regulating arterial oxygen content during acute hypercapnic acidosis in rainbow trout (Salmo gairdneri).

The involvement of catecholamines in regulating arterial oxygen content (CaO2) during hypercapnic acidosis in rainbow trout (Salmo gairdneri) was investigated by comparing physiological responses during acute normoxic hypercapnia (a condition in which plasma epinephrine is elevated) and acute hyperoxic hypercapnia (a condition in which plasma epinephrine is not elevated). Red blood cell pH (rbc pH) was maintained significantly higher in the normoxic hypercapnic fish despite similar reductions in whole blood pH (pHe) in both groups. Elevation of rbc pH in the normoxic hypercapnic fish was abolished by pre-treatment with the beta-adrenoceptor antagonist, propranolol, whereas injection of epinephrine into hyperoxic hypercapnic fish significantly raised rbc pH. Arterial blood oxygen carrying capacity increased only in the normoxic hypercapnic fish due to significant increases in blood haemoglobin (Hb) levels. The ability of trout to elevate blood Hb was abolished by pre-treatment with the alpha-adrenoceptor antagonist, phentolamine. Injections of epinephrine into normocapnic fish caused increases in blood Hb and concomitant decreases in spleen wet weight and Hb content. Adrenergic elevation of blood Hb was not observed in splenectomized fish. CaO2, although depressed during normoxic hypercapnia, was indeed regulated when compared to CaO2 values predicted from the in vitro Root effect. Pre-treatment with phentolamine, but not propranolol, abolished the ability of trout to regulate CaO2.

Acidosis↗
PubMed1993

The ecological challenges to population growth.

Human societies have developed by overcoming the constraints of nature. The result is population growth that will exceed, and probably has already exceeded, the carrying capacity--it is the most dramatic period in the history of the planet. It is not only growth, but existing numbers and their demands, that destroy the ecological basis. The obligation on humanity now is to add to this success over nature a negative instead of a positive feedback system. There can be no simple technical answer; solutions must come from the co-ordinated attention to environment, population, poverty and equity. Most political social and economic institutions militate against the changes needed. Two approaches are outlined: a tool for thought for communities to determine their aims in terms of fundamental human needs, and a resource modelling system to assess whether such aims are physically possible.

Conservation of Natural Resources↗
PubMed1993

Demographic entrapment.

Demographic entrapment is a situation in which a population exceeds the carrying capacity of its own ecosystem and its 'connectedness' to other ecosystems. It can be looked upon as a disorder of the demographic transition which presents serious ethical problems in that there are occasions on which there is a conflict between the interests of the child and the community. Reasons are given for the aid agencies' recognizing the existence of entrapment, which presently they do not. Since there are at present considerable obstacles to their doing this, we must accept a period of 'legitimate double-think' meanwhile.

Africa↗
PubMed1985

Biochemical and functional disturbances in red blood cells of herring gulls ingesting Prudhoe Bay crude oil.

Heinz body hemolytic anemia developed in Herring Gull (Larus argentatus) nestlings given oral doses of 10 ml of Prudhoe Bay crude oil per kilogram of body weight per day for 5 days. Associated disturbances in red blood cells were increased amounts of reduced glutathione (GSH), peroxidation of membrane lipids, an increase in membrane permeability, and a decrease in the oxygen-carrying capacity of cyanomethemoglobin-convertible hemoglobin. Among groups of gulls given different cumulative doses of oil over a 6-day period, significant covariance with dose and dependence on dose was demonstrated for packed cell volume, hemoglobin, and red cell GSH. Rapid defecation of oil by gulls indicated that the effective dose was substantially less than the administered dose. Pronounced damage to red cells occurred in some birds administered oil for only 2 days. These data imply that the toxic effects of ingested oil may contribute significantly to the morbidity and mortality of oil-contaminated birds.

Animals↗
PubMed1989

Diamphidia toxin, the bushmen's arrow poison: possible mechanism of prey-killing.

The effects of a 60,000 mol. wt protein from the pupae of the beetle, Diamphidia nigro-ornata have been studied. In concentrations as high as 50 micrograms/ml, the toxin has little effect on the propagated compound action potential of isolated nerve trunks, or on the voltage-gated sodium and potassium channels of voltage-clamped single skeletal muscle fibers. In the anesthetized cat, the toxin has no specific effect on the neuro-muscular or the cardiovascular systems. It has a markedly hemolytic effect, and could reduce hemoglobin levels by as much as 75%. Plasma hemoglobin is increased, with resultant extensive hemoglobinuria and associated histopathological changes in the kidneys. Blood pressure, heart rate, PO2, PCO2, and oxygen-saturation remain essentially normal until the terminal stages of intoxication. Contrary to previous conclusions, we find no support for any particular neurotoxicity of the poison. The complex systemic effects, and possibly the prey-killing, can probably be attributed to the extensive hemolysis, reduced oxygen-carrying capacity, and generalized tissue hypoxia.

Action Potentials↗
PubMed1994

Organochlorine and metal contaminants in baleen whales: a review and evaluation of conservation implications.

The literature on organochlorine and metal contaminants in tissues of baleen whales includes data for approximately 1000 individuals in 10 species from various oceans of the world. Concentrations of these contaminants in tissues of baleen whales are low. Sources of variation in organochlorine concentrations in whales include age, sex, region of exposure, and feeding habits. Concentrations of sigma DDT and PCBs in baleen whales appear higher in the northern hemisphere than in the southern oceans, perhaps due to greater contamination of northern ecosystems and a higher incidence of fish in the diet. However, maximum concentrations are generally lower in tissues of baleen whales than in other marine mammal species, both on global and local scales. This is predictable based on the general distribution, habitats and trophic levels of baleen whales. We reviewed laboratory studies on the effects of selected organochlorine contaminants on direct mortality and impaired reproduction in other mammals, and critically examined observations attempting to link organochlorines to reproductive and population effects in marine mammals. There is no firm basis to conclude that the contaminants reviewed herein have affected baleen whale populations. Although more information on contaminants in baleen whales would be of value, as a matter of priority, research and management actions should focus on reducing human-caused mortality and increasing habitat carrying capacity for these species.

Animals↗
PubMed1990

Mechanical function of the human lumbar interspinous and supraspinous ligaments.

The mechanical function of the interspinous and supraspinous ligaments has been examined by simulating, on excised specimens, the deformation caused during forward flexion of the spine in real life. The load extension curves showed that during the first half of flexion the ligaments carried very little load but towards the end of the range of flexion they resisted up to 134N. When the supraspinous ligament was sectioned the interspinous ligament alone resisted 75% of this load. Testing at high strain rates showed an increase in load-carrying capacity of up to 30%. The maximum extension moment that can be produced by these ligaments was calculated to be approximately 7 Nm, or 5% of the moment the back muscles can produce across any intervertebral joint. Hence, during active lifting, these ligaments in isolation provide little mechanical assistance.

Aged↗
PubMed1993

Intramedullary nailing of the cortical bone osteotomies in rabbits with self-reinforced poly-L-lactide rods manufactured by the fibrillation method.

Right femoral, cortical bone osteotomies of 42 rabbits were fixed with fibrillated, self-reinforced (SR) poly-L-lactide (PLLA) rods, 4.5 mm in diameter. Follow-up times were from 3 to 48 wk. None of the rods broke during this period. There was one non-union at three weeks and one non-union at 12 weeks. Radiographic, histologic, microradiographic and oxytetracyclic fluorescence studies showed normal healing. Shear load carrying capacity of the osteotomy plane was measured from 30 bone samples; the shear force at breaking increased during the follow-up time to 450 N, while the mean shear force of intact, left-side control femurs was 440 N. Before shear tests of the bones, the SR-PLLA rods were taken away from the medullary cavities of the femurs. The rods were also tested mechanically. About 25% of the initial 136 MPa shear-strength of the rods was left after 24 wk. The results show that fibrillated SR-PLLA rods are strong enough to be used in intramedullary nailing of femoral cortical bone osteotomies in rabbits.

Animals↗
PubMed1983

Population ecology of rotifers as a bioassay tool for ecotoxicological tests in aquatic environments.

The population dynamics of the monogonont rotifer Brachionus rubens were used under controlled experimental conditions as a sensitive bioassay for toxic substances in sublethal doses. Even slight reductions in organism fertility and life expectancy were reflected at the integrated level of population dynamics. Serving as bioindicators for the standard test are the population parameters "intrinsic rate of natural increase" (r), "carrying capacity" (k), "frequency" (f), and "pregnancy" of the density oscillations (p). The optimal spreading power (steepest slope of the response curves) lies at different concentration areas for each parameter, so that they can be used specifically in standard tests.

Aniline Compounds↗
PubMed1985

The use of cohorts and populations in chronic toxicity studies with Daphnia magna: a cadmium example.

Two semistatic life table experiments with Daphnia magna were carried out on reconstituted and Lake IJssel water. The "nontoxic concentrations" for cadmium with respect to the intrinsic rate of natural increase, derived from age-specific survival and fecundity were 1 and 3.2 micrograms/liter, respectively. Body length appeared to be a sensitive parameter. A third intermittent-flow experiment was started with small, exponentially growing populations. These populations had a stable age distribution, were composed of cohorts of different ages and showed an almost perfect logistic growth. Cadmium was shown to reduce the upper numerical limit (carrying capacity) for D. magna and was inversely related to this parameter: log Y = 2.85 -0.20 log [Cd]; r = -0.99. A "nontoxic concentration" could not be established. Based on the "background" concentration of cadmium, a freshwater quality criterion of 0.1 microgram/liter is proposed. The results are used to discuss several shortcomings of the current methods. Finally it is stated that the introduction of the concepts of population dynamics in reproduction tests with D. magna is a realistic step towards ecotoxicology.

Actuarial Analysis↗
PubMed1982

Ecology of dogs in the Central Philippines in relation to rabies control efforts.

Rabies is endemic in dogs but has no other host as reservoir in the Philippines. Essential to programmes of rabies control is an understanding of the ecology of dogs and their relationships to human society. In the central Philippines, where this study was conducted before and during rabies control efforts, most households in rural 'barangays' or communities owned one or more dogs which served as guards, as scavengers and, in some, as a source of meat--but not as pets to be handled and played with. A second important category of dogs comprised those which foraged semi-self-sufficiently but were recognized by the barangay residents as belonging to the community. The number of these dogs was determined by the carrying capacity of the community, but their population was dependent upon cast-off owned dogs as their fecundity was below maintenance levels. Killing of any dogs, except for meat or in the event of suspicion of rabies, was of low social acceptance. In urban centres, upper class residents owned dogs as guards or pets but their employees, who often lived within their compounds, related to their dogs as did rural residents. Middle class families maintained greater control over their dogs. In lower class urban districts, a continual influx of dogs which on occasion were incubating rabies, plus the large size of many such districts, enabled them to serve as continual foci of infection. In this setting, rabies control was most effectively achieved by teams of vaccinators going house to house, capturing and injecting all dogs whether owned or belonging to the community. The immunization of 80% or more of the dogs was readily achieved and brought rabies under complete control.

Animals↗
PubMed1991

Drug induced perturbations in tumor blood flow: therapeutic potential and possible limitations.

Chemical modulation of tumor blood flow has until recently received relatively little attention as a therapeutic tool. Developments in the last few years, both in technology and in drug development, have changed this perspective. Fluorescence activated cell sorting techniques have provided evidence for the existence of acutely hypoxic cells resulting from transient fluctuations in microregional tumor blood flow in experimental tumor systems. We have used such techniques to assess the effects of three systemically administered agents, nicotinamide, flunarazine and Flusol-DA, on the amount of acute hypoxia in the SCCVII tumor. The most effective agent identified in this study is the benzamide analog nicotinamide. We suggest that compounds which modulate such hypoxia could well have a role in radiation therapy, particularly when combined with techniques which increase the oxygen carrying capacity of the blood. The potential of tumor blood flow reduction to improve the effectiveness of bioreductive agents administered alone or in combination with radiation and/or hyperthermia, is well established in experimental systems. Further data are presented, which show that combining hydralazine and the beta-blocker propranolol can provide greater reduction in tumor blood flow than observed with hydralazine alone. Potential limitations of drug induced reduction in tumor blood flow are discussed including the possibility of inducing hypoxia in normal tissues.

Animals↗