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Results for “Spatial heterogeneity”

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PubMed2004

Processes leading to a spatial aggregation of Echinococcus multilocularis in its natural intermediate host Microtus arvalis.

The small fox tapeworm (Echinococcus multilocularis) shows a heterogeneous spatial distribution in the intermediate host (Microtus arvalis). To identify the ecological processes responsible for this heterogeneity, we developed a spatially explicit simulation model. The model combines individual-based (foxes, Vulpes vulpes) and grid-based (voles) techniques to simulate the infections in both intermediate and definite host. If host populations are homogeneously mixed, the model reproduces field data for parasite prevalence only for a limited number of parameter combinations. As ecological parameters inevitably vary to a certain degree, we discarded the homogeneous mixing model as insufficient to gain insight into the ecology of the fox tapeworm cycle. We analysed five different model scenarios, each focussing on an ecological process that might be responsible for the heterogeneous spatial distribution of E. mulitlocularis in the intermediate host. Field studies revealed that the prevalence ratio between intermediate and definite host remains stable over a wide range of ecological conditions. Thus, by varying the parameters in simulation experiments, we used the robustness of the agreement between field data and model output as quality criterion for the five scenarios. Only one of the five scenarios was found to reproduce the prevalence ratio over a sufficient range of parameter combinations. In the accentuated scenario most tapeworm eggs die due to bad environmental conditions before they cause infections in the intermediate host. This scenario is supported by the known sensitivity of tapeworm eggs to high temperatures and dry conditions. The identified process is likely to lead to a heterogeneous availability of infective eggs and thus to a clumped distribution of infected intermediate hosts. In conclusion, areas with humid conditions and low temperatures must be pointed out as high risk areas for human exposure to E. multilocularis eggs as well.

Animals↗

PubMedDate not supplied

[On methodological aspects of ecological experiments (comments on M. V. Kozlov publication)].

The meaning of conception of pseudoreplication in ecological experiments is discussed. The replications are found to be pseudo if factors causing random variation act on such replications as on one unit. For example, preudoreplications could appear if a single flipped coin would be split up. The absence of random spatial distribution of experimental and control fields could be considered as a mistake because the experimental results could interfere with spatial heterogeneity. However, there are no pseudoreplications in this case, because the variations of ecological systems are produced by factors that are different in different places and each organism responds to these factors individually. The pseudoreplication should be created by multiple determination of a response of an individual organism. Also recommendations for applications of randomized block design and analysis of covariance for field trials in plant communities are proposed. Randomized blocks unite the experimental and control fields occupying the identical place in the series of spatial heterogeneity. The influence of such heterogeneity on random variation of determined parameters could be excluded by this approach. The heterogeneity considered only at the scale of used blocks. As far as control and experimental fields belongs to a single set, the series of heterogeneities, that comprises the range of their parameters, is identical. The random sorting of the large amount of experimental and control samples consisting of the same number of replications will produces replications in the same place of the series of general heterogeneity of investigated set. The replications occupying the same places of the sorted series form "virtual block". The population of such blocks includes the majority of random variation and allows one to reveal even very weak experimental effects by variance analysis.

Analysis of Variance↗

PubMed2003

Development, survival and fitness performance of Helicoverpa zea (Lepidoptera: Noctuidae) in MON810 Bt field corn.

Helicoverpa zea (Boddie) development, survival, and feeding injury in MON810 transgenic ears of field corn (Zea mays L.) expressing Bacillus thuringiensis variety kurstaki (Bt) Cry1Ab endotoxins were compared with non-Bt ears at four geographic locations over two growing seasons. Expression of Cry1Ab endotoxin resulted in overall reductions in the percentage of damaged ears by 33% and in the amount of kernels consumed by 60%. Bt-induced effects varied significantly among locations, partly because of the overall level and timing of H. zea infestations, condition of silk tissue at the time of egg hatch, and the possible effects of plant stress. Larvae feeding on Bt ears produced scattered, discontinuous patches of partially consumed kernels, which were arranged more linearly than the compact feeding patterns in non-Bt ears. The feeding patterns suggest that larvae in Bt ears are moving about sampling kernels more frequently than larvae in non-Bt ears. Because not all kernels express the same level of endotoxin, the spatial heterogeneity of toxin distribution within Bt ears may provide an opportunity for development of behavioral responses in H. zea to avoid toxin. MON810 corn suppressed the establishment and development of H. zea to late instars by at least 75%. This level of control is considered a moderate dose, which may increase the risk of resistance development in areas where MON810 corn is widely adopted and H. zea overwinters successfully. Sublethal effects of MON810 corn resulted in prolonged larval and prepupal development, smaller pupae, and reduced fecundity of H. zea. The moderate dose effects and the spatial heterogeneity of toxin distribution among kernels could increase the additive genetic variance for both physiological and behavioral resistance in H. zea populations. Implications of localized population suppression are discussed.

Animals↗

PubMed2002

Adaptive diversification of germination strategies.

Evolution of the germination rate (the proportion of newly produced and dormant seeds that germinates every year) of annual plants is investigated, when the environment is temporally stochastic and spatially heterogeneous. The environment consists of two habitats with synchronous stochastic variation in the annual yield and permanent difference in constant seed survival rates. Density dependence operates within the habitats, which are connected via restricted seed dispersal. We find that instead of a single common evolutionarily stable strategy the coexistence of several germination strategies is possible and that in an initially monomorphic population evolutionary branching may occur. During evolutionary branching the population undergoes disruptive selection and splits into two branches of different lineages that converge to the evolutionarily stable coalition of different germination strategies. It is shown that spatial heterogeneity and restricted dispersal are essential for evolutionary branching. Disruptive selection on the germination rate presents yet another possibility for parapatric speciation.

Adaptation, Physiological↗

PubMed1999

Combretastatin A-4 phosphate as a tumor vascular-targeting agent: early effects in tumors and normal tissues.

The potential for tumor vascular-targeting by using the tubulin destabilizing agent disodium combretastatin A-4 3-0-phosphate (CA-4-P) was assessed in a rat system. This approach aims to shut down the established tumor vasculature, leading to the development of extensive tumor cell necrosis. The early vascular effects of CA-4-P were assessed in the s.c. implanted P22 carcinosarcoma and in a range of normal tissues. Blood flow was measured by the uptake of radiolabeled iodoantipyrine, and quantitative autoradiography was used to measure spatial heterogeneity of blood flow in tumor sections. CA-4-P (100 mg/kg i.p.) caused a significant increase in mean arterial blood pressure at 1 and 6 h after treatment and a very large decrease in tumor blood flow, which-by 6 h-was reduced approximately 100-fold. The spleen was the most affected normal tissue with a 7-fold reduction in blood flow at 6 h. Calculations of vascular resistance revealed some vascular changes in the heart and kidney for which there were no significant changes in blood flow. Quantitative autoradiography showed that CA-4-P increased the spatial heterogeneity in tumor blood flow. The drug affected peripheral tumor regions less than central regions. Administration of CA-4-P (30 mg/kg) in the presence of the nitric oxide synthase inhibitor, N(omega)-nitro-L-arginine methyl ester, potentiated the effect of CA-4-P in tumor tissue. The combination increased tumor vascular resistance 300-fold compared with less than 7-fold for any of the normal tissues. This shows that tissue production of nitric oxide protects against the damaging vascular effects of CA-4-P. Significant changes in tumor vascular resistance could also be obtained in isolated tumor perfusions using a cell-free perfusate, although the changes were much less than those observed in vivo. This shows that the action of CA-4-P includes mechanisms other than those involving red cell viscosity, intravascular coagulation, and neutrophil adhesion. The uptake of CA-4-P and combretastatin A-4 (CA-4) was more efficient in tumor than in skeletal muscle tissue and dephosphorylation of CA-4-P to CA-4 was faster in the former. These results are promising for the use of CA-4-P as a tumor vascular-targeting agent.

Animals↗

PubMed1988

Spatial map of salts and saccharides on dog tongue.

Front and rear regions of dog tongue were tested for spatial heterogeneity with respect to monovalent chloride salts (NaCl, KCl, and NH4Cl) and sweet tastants (sucrose, D-glucose, and L-glucose) by placing them in Ussing chambers where the open-circuit potential, Voc, and short-circuit current, Isc, were measured. The responses of Voc and Isc to 0.5 M NaCl were greater in the front of the tongue than in the rear, whereas the responses of Voc and Isc to 0.5 M sucrose were greater in the rear of the tongue than in the front. These results are similar to those of published neurophysiological measurements from the chorda tympani nerve and thalamus. These data suggest that the spatial heterogeneities seen in higher taste centers are present in the isolated lingual epithelium where primary taste transduction events occur.

Ammonium Chloride↗

PubMed1999

Spatial distribution and habitat characterization of anopheline mosquito larvae in Western Kenya.

Studies were conducted to characterize larval habitats of anopheline mosquitoes and to analyze spatial heterogeneity of mosquito species in the Suba District of western Kenya. A total of 128 aquatic habitats containing mosquito larvae were sampled, and 2,209 anopheline and 10,538 culicine larvae were collected. The habitats were characterized based on size, pH, distance to the nearest house and to the shore of Lake Victoria, coverage of canopy, surface debris, algae and emergent plants, turbidity, substrate, and habitat types. Microscopic identification of third- and fourth-instar anopheline larvae did not yield any Anopheles funestus or other anophelines. A total of 829 An. gambiae s.l. larvae from all habitats were analyzed further by rDNA-polymerase chain reaction to identify individual species within the An. gambiae species complex. Overall, An. arabiensis was the predominant species (63.4%), and An. gambiae was less common (31.4%). The species composition of An. gambiae s.l. varied significantly among the sampling sites throughout Suba District. The larval habitats in the southern area of the district had a higher proportion of An. gambiae than in the northern area. Multiple logistic analysis did not detect any significant association between the occurrence of anopheline larvae and habitat variables, and principal component analysis did not identify key environmental factors associated with the abundance of An. gambiae. However, significant spatial heterogeneity in the relative abundance of An. gambiae within the Suba district was detected. When the effect of larval habitat locality was considered in the analysis, we found that the distance to the nearest house and substrate type were significantly associated with the relative abundance of An. gambiae. Future studies integrating detailed water chemistry analysis, remote sensing technology, and the ecology of predators may be required to further elucidate the mechanisms underlying the observed spatial variation of anopheline larval distribution.

Animals↗

PubMed1999

[How much blood flow is required by the myocardium?].

OBJECTIVES: The myocardium of the left ventricle exhibits spatial heterogeneity of blood flow under physiological conditions. This study was designed to investigate, whether oxygen supply is jeopardized in low flow areas (blood flow < 50% of mean) under physiological conditions and whether areas of high flow (> 150% of mean) exhibit perfusion in excess of demand ("luxury perfusion"). METHODS: The study was performed in anesthetized and ventilated beagle dogs. Local blood flow was reduced by mechanically narrowing of the r. circumflexus of the left coronary artery; myocardial blood flow was measured by the tracer-microsphere technique, free concentrations cellular adenosine by the SAH-technique, regional metabolism of substrates by the desoxyglucose-technique. RESULTS: Low flow areas exhibited normal oxygenation of the myocardium, while in high flow areas no luxury perfusion could be demonstrated. CONCLUSION: Myocardial blood flow and metabolism demonstrate significant spatial heterogeneity. There appears to be no absolute threshold of blood flow, where regional myocardial ischemia develops. Probably biochemical evidence of myocardial ischemia is determined by a local ratio of oxygen supply and demand.

Adenosine↗

PubMed2005

Computer simulation of the reentrant cardiac arrhythmias in ischemic myocardium.

Computer simulation was performed to determine how reentrant activity could occur due to the spatial heterogeneity in refractoriness induced by the regional ischemia. Two regional ischemic models were developed by decreasing the intracellular ATP concentration, reducing conductance of the inward Na+ current and increasing the extracellular K+ concentration on the two-dimensional sheet. Operator splitting method was used to integrate the models. The vulnerability to reentry was estimated from the timings of premature stimuli on the constructed models, which could result in unidirectionally propagating action potentials. Two kinds of sustained spiral waves and their Pseudo-Electroscardiograms were observed in numerical simulation. The results showed that the dispersion of refractory period increased with ischemic aggravation, and led to augment of the vulnerable window. A permature stimulation within the vulnerable window could easily induce spiral reentry. The Pseudo-Electrocardiograms of the spiral waves exhibited monomorphic tachycardiac waveforms. Thus, the spatial heterogeneity in refractoriness could be a substrate for reentrant ventricular tachyarrhythmias on the regional ischemic tissue.

Computer Simulation↗

PubMed2000

Low-pass filtering, a new method of fractal analysis: application to PET images of pulmonary blood flow.

The pattern of a spatial structure that repeats itself independently of the scale of magnification or resolution is often characterized by a fractal dimension (D). Two-dimensional low-pass filtering, which may serve as a method to assess D, was applied to functional images of pulmonary perfusion measured by positron emission tomography. The corner frequency of a low-pass filter is inversely proportional to the resolution scale. The method was applied to three types of images: random noise images, synthetic fractal images, and positron emission tomographic images of pulmonary perfusion. Images were processed with two-dimensional low-pass filters of decreasing corner frequencies, and a spatial heterogeneity index, the coefficient of variation, was calculated for each low-pass-filtered image. The natural logarithm of the coefficient of variation scaled linearly with the natural logarithm of the resolution scale for the PET images studied (average R(2) = 0.99). D ranged from 1.25 to 1.36 for the residual distribution of pulmonary perfusion after vertical gradients were removed by linear regression. D of the same data without removal of vertical gradients ranged from 1.11 to 1.14, but the fractal plots had systematic deviations from linearity and a lower linear correlation coefficient (R(2) = 0. 96). The method includes all data in the lung field and is insensitive to the effects of misregistration. We conclude that low-pass filtering offers new insights into the interpretation of D of two-dimensional functional images as a measure of the frequency content of spatial heterogeneity.

Fractals↗

PubMed2002

Polymorphic reentrant ventricular tachycardia in the isolated rabbit heart studied by high-density mapping.

BACKGROUND: The role of dispersion of refractoriness and reentry for the genesis of polymorphic ventricular tachycardia (VT) has recently become emphasized. We investigated the mechanisms of polymorphic arrhythmias in a 2D preparation confining an area of prolonged refractoriness. METHODS AND RESULTS: In 16 Langendorff-perfused rabbit hearts, a sheet of left ventricular epicardium was obtained by a cryoprocedure. Enhanced spatial heterogeneity in a refractory period was created by cooling a central region (diameter=12 mm). This markedly prolonged the refractory period (by 36+/-14 ms) inside but only slightly prolonged it (by 5+/-11 ms) outside the cooled area (n=6). During a control procedure, programmed stimulation with up to 3 premature stimuli induced an episode of monomorphic VT in only 1 of 10 hearts. During regional cooling, episodes of polymorphic VT with a maximum duration of 35 seconds could be induced in all hearts. High-resolution mapping (229 electrodes) of epicardial activation revealed that polymorphic VT was caused by a functional reentrant circuit located partially within the region of prolonged refractoriness. The reentrant wavefront was continuously shifting along the border of the cooled region, resulting in beat-to-beat changes in the excitation pattern. Spontaneous termination of polymorphic VT occurred either by a shift of the reentrant circuit outside the cooled region or by a block in the central common pathway during figure-of-8 reentry in the region of prolonged refractoriness. CONCLUSIONS: A shifting functional reentrant circuit was the underlying mechanism of polymorphic VT in a substrate of enhanced spatial heterogeneity of refractoriness.

Animals↗

PubMed2004

In situ measurement of water absorption by fine roots of three temperate trees: species differences and differential activity of superficial and deep roots.

The spatial heterogeneity of water uptake by fine roots under field conditions was analyzed in situ with miniature sap flow gauges in a mature beech-oak-spruce mixed stand. Sap flow rate (J), sap flow density (Jd), and root surface-area-specific flow rate (uptake rate, Js) were measured for eight to 10 small-diameter roots (3-4 mm) per species in the organic layer (superficial roots) and in the mineral soil (30-80 cm, deep roots) during four months in summer 1999. We calculated Js by relating J to the surface area of the section of the fine root system distal to the position of the gauge on the root. When measured synchronously, roots of the three species did not differ significantly in mean Js, although oak roots tended to have lower rates. However, Jd decreased in the sequence spruce > beech > oak in most measurement periods. Microscopic investigation revealed differences in fine root anatomy that may partly explain the species differences in Jd and Js. Oak fine roots had a thicker periderm than beech and spruce roots of similar diameter and spruce roots had fewer fine branch rootlets than the other species. Synchronously recorded Jd and Js of nearby roots of the same tree species showed large differences in flow with coefficients of variation from 25 to 150% that could not be explained by patchy distribution of soil water. We hypothesize that the main cause of the large spatial heterogeneity in root water uptake is associated with differences between individual roots in morphology and ultrastructure of the root cortex that affect root radial and root-soil interface conductivities. The high intraspecific variation in Js may mask species differences in root water uptake. Superficial roots of all species typically had about five times higher Jd than deep roots of the same species. However, Js values were similar for superficial and deep roots in beech and spruce because small diameter roots of both species were more branched in the organic layer than in mineral soil. In oak, deep roots had lower Js (maximum of 100 g m(-2) day(-1)) than superficial roots (about 1000 g m(-2) day(-1)). We conclude that temperate tree species in mixed stands have different water uptake capacities. Water flow in the rhizosphere of forests appears to be a highly heterogeneous process that is influenced by both tree species and differences in uptake rates of individual roots within a species.

Fagus↗

PubMed2001

Spatially correlated dynamics in a simulated glass-forming polymer melt: analysis of clustering phenomena.

In recent years, experimental and computational studies have demonstrated that the dynamics of glass-forming liquids are spatially heterogeneous, exhibiting regions of temporarily enhanced or diminished mobility. Here we present a detailed analysis of dynamical heterogeneity in a simulated "bead-spring" model of a low-molecular-weight polymer melt. We investigate the transient nature and size distribution of clusters of "mobile" chain segments (monomers) as the polymer melt is cooled toward its glass transition. We also explore the dependence of this clustering on the way in which the mobile subset is defined. We show that the mean cluster size is time dependent with a peak at intermediate time, and that the mean cluster size at the peak time grows with decreasing temperature T. We show that for each T a particular fraction of particles maximizes the mean cluster size at some characteristic time, and this fraction depends on T. The growing size of the clusters demonstrates the growing range of correlated motion, previously reported for this same system [C. Beneman et al. Nature (London) 399, 246 (1999)]. The distribution of cluster sizes approaches a power law near the mode-coupling temperature, similar to behavior reported for a simulated binary mixture and a dense colloidal suspension, but with a different exponent. We calculate the correlation length of the clusters, and show that it exhibits similar temperature- and time-dependent behavior as the mean cluster size, with a maximum at intermediate time. We show that the characteristic time of the maximum cluster size follows the scaling predicted by mode-coupling theory (MCT) for the beta time scale, revealing a possible connection between spatially heterogeneous dynamics and MCT.

Journal Article↗

PubMed2003

Pair-edge approximation for heterogeneous lattice population models.

To increase the analytical tractability of lattice stochastic spatial population models, several approximations have been developed. The pair-edge approximation is a moment-closure method that is effective in predicting persistence criteria and invasion speeds on a homogeneous lattice. Here we evaluate the effectiveness of the pair-edge approximation on a spatially heterogeneous lattice in which some sites are unoccupiable, or "dead". This model has several possible interpretations, including a spatial SIS epidemic model, in which some sites are occupied by immobile host-species individuals while others are empty. We find that, as in the homogeneous model, the pair-edge approximation is significantly more accurate than the ordinary pair approximation in determining conditions for persistence. However, habitat heterogeneity decreases invasion speed more than is predicted by the pair-edge approximation, and the discrepancy increases with greater clustering of "dead" sites. The accuracy of the approximation validates the underlying heuristic picture of population spread and therefore provides qualitative insight into the dynamics of lattice models. Conversely, the situations where the approximation is less accurate reveals limitations of pair approximation in the presence of spatial heterogeneity.

Algorithms↗

PubMed1992

Spatial d/h heterogeneity of leaf water.

The mean deltaD value of petiole water of Pterocarpus indicus Willd (deltaD = -9.0 +/- 2.5 per thousand, n = 3) was not significantly different from the mean value of stem water (-8.3 +/- 2.8 per thousand, n = 3). deltaD values of main vein water ranged from -11.1 to + 12.0 per thousand (n = 14) and increased along the main vein from petiole to the tip of leaves. Mesophyll water was highly enriched in deuterium (mean deltaD = +32.0 +/- 2.0 per thousand, n = 19) when compared with stem, petiole, and vein water. deltaD values of mesophyll water for different areas of the lamina, however, were not homogenous and could differ by as much as 20 per thousand.

Journal Article↗

PubMed2004

Landscape eco-environmental research on littoral zone in China.

Littoral zone is a special land/landscape type. As an important kind of land resource in support, the use of littoral zone is vital to eastern coastal areas in China. And the research on littoral zone relates to the key theory of landscape ecology. Based on the theory of landscape ecology, the littoral zone was divided into four types: mud flat, sand beach, bench, and biological flat. The distribution of each type in China is pointed out. As a typical open system, littoral zone has six landscape ecological characteristics: (1) high sensitivity to disturbance; (2) distinct edge effect; (3) spatial aggregation of natural resources; (4) frequently spatial oscillation; (5) obviously spatial heterogeneity; and (6) noticeably spatial differentiation. Some proposals are also put forward on the land use and development of littoral zone for environmental protection and environmental management.

China↗

PubMed2003

Transmural heterogeneity of calcium handling in canine.

Spatial heterogeneity of the action potential and its influence on arrhythmia vulnerability is known. However, heterogeneity of intracellular calcium handling and, in particular, its effect on the electrophysiological substrate is less clear. Using optical mapping techniques, calcium transients and action potentials were recorded simultaneously from ventricular sites across the transmural wall of the arterially perfused canine left ventricular wedge preparation during steady-state baseline pacing and rapid pacing. During baseline pacing, the decay of intracellular calcium to diastolic levels and calcium transient duration were slower (70%, P<0.005) and longer (20%, P<0.005), respectively, closer to the endocardial surface compared with the epicardial surface. Tissue samples isolated from the left ventricular wall demonstrate that sarcoplasmic reticulum Ca2+ ATPase (SERCA2a) expression was significantly less in the subendocardial and midmyocardial layers compared with the subepicardial layer. In contrast, no significant difference in the transmural expression of Na+-Ca2+ exchanger was observed. During rapid pacing, calcium transient alternans and increased levels of diastolic intracellular calcium were significantly greater (P<0.01) closer to the endocardium (101%+/-62% and 41%+/-15%, respectively) compared with the epicardium (12%+/-7% and 12%+/-14%, respectively). In conclusion, cells closer to the endocardium exhibit a slower decay of intracellular calcium compared with cells near the epicardium, which may be due in part to reduced expression of SERCA2a. As a possible consequence, calcium transient alternans and increased diastolic levels of intracellular calcium may occur preferentially closer to the endocardial surface.

Action Potentials↗

PubMed2004

Translocation of carbon by Rhizoctonia solani in nutritionally-heterogeneous microcosms.

Responses of Rhizoctonia solani to spatial heterogeneity in sources of carbon, and associated translocation of carbon (C), were studied in a simple microcosm system comprising two discrete domains of agar gels separated on a glass slide and overlain with a porous membrane. Two arrangements of the gel pairs were used, one containing two equally large resources (representing 'homogeneous' conditions) and one containing a large and a negligible resource (representing 'heterogeneous' conditions). The nutrient sources were a standard mineral salt medium with or without glucose as sole C source. The fungus was inoculated onto one domain and growth responses determined by direct measurement of biomass. Translocation of C was quantified by use of 13C-enriched glucose. This substrate was either added to the agar at the outset, when studying newly developing colonies, or as a pulse into already established colonies. When growing in heterogeneous conditions, the fungus actively translocated C from a glucose-containing domain to sustain growth in the adjacent region lacking such a resource. In homogeneous conditions there was evidence of passive translocation (diffusion), but the fungus preferentially used local resource to maintain growth. Active translocation was only observed in newly growing colonies, whereas passive translocation occurred in both growing and established colonies. When the fungus was pulsed with a 13C-enriched glucose solution after 10 d growth, 2.5 times more 13C was taken up by the fungus grown in heterogeneous than homogeneous conditions, suggesting uptake exceeded local demands. In heterogeneous conditions, the total amount of 13C enriched glucose taken up by the fungus was independent of the location of the enriched glucose in the underlying medium. When the nylon membrane was replaced by Cellophane (an additional C source), degradation of the membrane and an increase in biomass occurred only in the heterogeneous system. The possible implications for these results in soil systems are discussed.

Biomass↗