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Results for “Carrying Capacity”

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At least 73 records · Page 4Linked to original sources
PubMed2006

Population growth rate and carrying capacity for springtails Folsomia candida exposed to ivermectin.

Forecasting the effects of stressors on the dynamics of natural populations requires assessment of the joint effects of a stressor and population density on the population response. The effects can be depicted as a contour map in which the population response, here assessed by population growth rate, varies with stress and density in the same way that the height of land above sea level varies with latitude and longitude. We present the first complete map of this type using as our model Folsomia candida exposed to five different concentrations of the widespread anthelmintic veterinary medicine ivermectin in replicated microcosm experiments lasting 49 days. The concentrations of ivermectin in yeast were 0.0, 6.8, 28.8, 66.4, and 210.0 mg/L wet weight. Increasing density and chemical concentration both significantly reduced the population growth rate of Folsomia candida, in part through effects on food consumption and fecundity. The interaction between density and ivermectin concentration was "less-than-additive," implying that at high density populations were able to compensate for the effects of the chemical. This result demonstrates that regulatory protocols carried out at low density (as in most past experiments) may seriously overestimate effects in the field, where densities are locally high and populations are resource limited (e.g., in feces of livestock treated with ivermectin).

Animals↗
PubMed2000

Carrying capacity and demographic stochasticity: scaling behavior of the stochastic logistic model.

The stochastic logistic model is the simplest model that combines individual-level demography with density dependence. It explicitly or implicitly underlies many models of biodiversity of competing species, as well as non-spatial or metapopulation models of persistence of individual species. The model has also been used to study persistence in simple disease models. The stochastic logistic model has direct relevance for questions of limiting similarity in ecological systems. This paper uses a biased random walk heuristic to derive a scaling relationship for the persistence of a population under this model, and discusses its implications for models of biodiversity and persistence. Time to extinction of a species under the stochastic logistic model is approximated by the exponential of the scaling quantity U=(R-1)(2) N/R(R+1), where N is the habitat size and R is the basic reproductive number.

Animals↗
PubMed1977

[The lateral tension band--a method of osteosynthesis of patella fractures for immediate carrying capacity (author's transl)].

The tensions caused by means of flexors and tensors on the intact and by various procedures operated fractured patella have been determined. The disadvantages of the cerclage and the eccentric ventral tension band, both of them mainly usual, are exposed. Interfragmentary measurements of pression power in a corpse knee as well as breaking tests with a model patella and first experiences by operation with a new method confirm the reflexions. Principles for a modus of osteosynthesis for fractures of the patella are realized by the lateral tension-band. All loading inclined fragment distractive powers thereby are taken up by production a high-level interfragmentary compression, so that immediate postoperative loading is possible.

Biomechanical Phenomena↗
PubMed1992

Effect of animal husbandry on herbivore-carrying capacity at a regional scale.

All significant properties of the herbivore trophic level, including biomass, consumption and productivity, are significantly correlated with primary productivity across a broad range of terrestrial ecosystems. Here we show that livestock biomass in South American agricultural ecosystems across a 25-fold gradient of primary productivity exhibited a relationship with a slope essentially identical to unmanaged ecosystems, but with a substantially greater y-intercept. Therefore the biomass of herbivores supported per unit of primary productivity is about an order of magnitude greater in agricultural than in natural ecosystems, for a given level of primary production. We also present evidence of an increase in livestock body size with primary productivity, a pattern previously characterized in natural ecosystems. To our knowledge this is the first quantitative documentation at a regional scale of the impact of animal husbandry practices, such as herding, stock selection and veterinary care, on the biomass and size-structure of livestock herds compared with native herbivores.

Agriculture↗
PubMed1986

The physiologic reserve in oxygen carrying capacity: studies in experimental hemodilution.

The mechanisms by which the body attempts to avoid tissue hypoxia when total body oxygen delivery is compromised during acute anemia are reviewed. When the hematocrit is reduced by isovolemic hemodilution the compensatory adjustments include an increase in cardiac output, redistribution of blood flow to some tissues, and an increase in the whole body oxygen extraction ratio. These responses permit whole body oxygen uptake to be maintained until the hematocrit has been lowered to about 10%. Several factors are discussed which contribute to the increase in cardiac output during acute anemia including the reduction in blood viscosity, sympathetic innervation of the heart, and increased venomotor tone. The latter has been shown to be dependent on intact aortic chemoreceptors. With respect to peripheral vascular responses, the rise in coronary and cerebral blood flows which occur following hemodilution is proportionally greater than the increase in cardiac output while the opposite is true for kidney, liver, spleen, and intestine. Skeletal muscle does not contribute to a redistribution of blood flow to more vital areas during acute anemia despite its relatively large anaerobic capacity. Overall, peripheral compensatory adjustments result in an increased oxygen extraction ratio during acute anemia which reflects a better matching of the limited oxygen supply to tissue oxygen demands. However, some areas such as muscle are relatively overperfused which limits an even more efficient utilization of the reduced oxygen supply. Studies of the response of the microcirculation and the extent to which sympathetic vascular controls are involved in peripheral blood flow regulation are necessary to further appreciate the complex pattern of physiological responses which help ensure survival of the organism during acute anemia.

Anemia↗
PubMed2006

Blood viscosity maintains microvascular conditions during normovolemic anemia independent of blood oxygen-carrying capacity.

Responses to exchange transfusion with red blood cells (RBCs) containing methemoglobin (MetRBC) were studied in an acute isovolemic hemodiluted hamster window chamber model to determine whether oxygen content participates in the regulation of systemic and microvascular conditions during extreme hemodilution. Two isovolemic hemodilution steps were performed with 6% dextran 70 kDa (Dex70) until systemic hematocrit (Hct) was reduced to 18% (Level 2). A third-step hemodilution reduced the functional Hct to 75% of baseline by using either a plasma expander (Dex70) or blood adjusted to 18% Hct with all MetRBCs. In vivo functional capillary density (FCD), microvascular perfusion, and oxygen distribution in microvascular networks were measured by noninvasive methods. Methylene blue was administered intravenously to reduce methemoglobin (rRBC), which increased oxygen content with no change in Hct or viscosity from MetRBC. Final blood viscosities after the entire protocol were 2.1 cP for Dex70 and 2.8 cP for MetRBC (baseline, 4.2 cP). MetRBC had a greater mean arterial pressure (MAP) than did Dex70. FCD was substantially higher for MetRBC [82 (SD 6) of baseline] versus Dex70 [38 (SD 10) of baseline], and reduction of methemoglobin to oxyhemoglobin did not change FCD [84% (SD 5) of baseline]. P(O2) levels measured with palladium-meso-tetra(4-carboxyphenyl)porphyrin phosphorescence were significantly changed for Dex70 and MetRBC compared with Level 2 (Hct 18%). Reduction of methemoglobin to oxyhemoglobin partially restored P(O2) to Level 2. Wall shear rate and wall shear stress decreased in arterioles and venules for Dex70 and did not change for MetRBC or rRBC. Increased MAP and shear stress-mediated factors could be the possible mechanisms that improved perfusion flow and FCD after exchange for MetRBC. Thus the fall in systemic and microvascular conditions during extreme hemodilution with low-viscosity plasma expanders seems to be, in part, from the decrease in blood viscosity independent of the reduction in oxygen content.

Anemia↗