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1.
Density-dependent population regulation is important in many natural populations, and in those populations, high population density is a likely stressor. In amphibians, previous laboratory studies with tadpoles suggest that corticosterone, the main glucocorticoid stress hormone, is one of the key regulators of density-dependent growth. To test this relationship in natural settings, we manipulated wood frog (Rana sylvatica) tadpole density at three levels in outdoor mesocosms and used a capture stress protocol to examine the hormonal stress response. In addition, we used the same capture protocol in six natural ponds (three high density and three low density). In the mesocosms, there was an increase in corticosterone levels in tadpoles following 1 h of confinement at weeks 1, 2, and 5. However, while tadpoles maintained at higher densities were smaller after metamorphosis, density did not alter mean levels of corticosterone obtained during confinement, and baseline levels of corticosterone did not differ between the densities. Similarly, in natural ponds, density did not correlate with baseline corticosterone or mean corticosterone levels obtained during confinement. We suggest that the physiological response to density may vary across the range of natural densities and that the role of corticosterone may be limited to periods of extreme high density, such as during pond-drying events.  相似文献   

2.
A new model for the catabolism of very low density lipoprotein will be proposed following a brief discussion of the relevant information about its metabolism. This model is based on the assumption that each very low density lipoprotein derivative has a distinct structural feature which determines its biological fate. Since this unique determinant may reside in only a small portion of the particles, such as apoprotein, the different very low density lipoprotein derivatives may possess similar or even identical electrophoretic mobility or hydrated density. For this reason, we propose that very low density lipoprotein derivatives be defined according to their putative position in the metabolic pathways rather than according to their hydrated density or their electrophoretic behavior. Thus it is recommended that biochemical separation methods based on principles other than electrophoretic mobility or hydrated density be used to isolate, purify and define very low density lipoprotein derivatives. The position that a lipoprotein particle occupies in the catabolic pathways should be determined by its ability to interact with liver cells or its ability to become converted to low density lipoprotein. This new nomenclature would eliminate unnecessary confusion and stimulate more research toward elucidating the unique structural feature of each very low density lipoprotein derivative.  相似文献   

3.
Migration is thought to maximize growth by enabling individuals to escape from density dependence, but this has rarely been tested at the individual level in natural populations. We employed linear mixed modeling of the spacing between consecutive scale growth rings to reconstruct individual growth profiles of a paradigmatic fish migrant, the sea trout (Salmo trutta) and related these to estimates of year class strength over a 13‐year period. Variation in scale growth was 1.3 times greater among individuals than within individuals in freshwater and 10 times greater at sea. Scale growth was inversely related to year class strength, both in freshwater (before migration) and at sea (after migration). Competition for patchily distributed resources is the most plausible explanation of the negative density‐dependent growth observed in freshwater and, to a lesser extent, in the marine environment. Our study provides some of the strongest evidence for a role of density dependence in determining partial migrations because although migrants can maximize growth by moving into the sea, they do not appear to become free from density dependence constraints completely. This has implications for conservation and suggests that sea trout and other anadromous fish displaying partial migrations may not be best managed on a river by river basis, but rather from a broader, coastal perspective.  相似文献   

4.
Cortical bone tissue is an anisotropic material characterized by typically five independent elastic coefficients (for transverse isotropy) governing shear and longitudinal deformations in the different anatomical directions. It is well established that the Young’s modulus in the direction of the bone axis of long bones has a strong relationship with mass density. It is not clear, however, whether relationships of similar strength exist for the other elastic coefficients, for they have seldom been investigated, and the results available in the literature are contradictory. The objectives of the present work were to document the anisotropic elastic properties of cortical bone at the tibia mid-diaphysis and to elucidate their relationships with mass density. Resonant ultrasound spectroscopy (RUS) was used to measure the transverse isotropic stiffness tensor of 55 specimens from 19 donors. Except for Poisson’s ratios and the non-diagonal stiffness coefficient, strong linear correlations between the different elastic coefficients \((0.7 < {r^{2}} < 0.99)\) and between these coefficients and density \((0.79 < {r^{2}} < 0.89)\) were found. Comparison with previously published data from femur specimens suggested that the strong correlations evidenced in this study may not only be valid for the mid-tibia. RUS also measures the viscous part of the stiffness tensor. An anisotropy ratio close to two was found for damping coefficients. Damping increased as the mass density decreased. The data suggest that a relatively accurate estimation of all the mid-tibia elastic coefficients can be derived from mass density. This is of particular interest (1) to design organ-scale bone models in which elastic coefficients are mapped according to Hounsfield values from computed tomography scans as a surrogate for mass density and (2) to model ultrasound propagation at the mid-tibia, which is an important site for the in vivo assessment of bone status with axial transmission techniques.  相似文献   

5.
Debate continues in theoretical ecology over whether and why the scaling exponent of biomass–density (MN) relationship varies along environmental gradients. By developing a novel geometric model with assumptions of allometric growth at the individual level and open canopy at the stand level, we propose that plant height–crown radius and canopy coverage–density relationships determine the above-ground MN relationship in stressful environments. Results from field investigation along an aridity gradient (from eastern to western China) confirmed our model prediction and showed that the above-ground MN scaling exponent increased with drought stress. Therefore, the ‘universal’ scaling exponents (−3/2 or −4/3) of the MN relationship predicted by previous models may not hold for above-ground parts in stressful environments.  相似文献   

6.
Density-dependent constraints on fecundity or survival are critical for the regulation and stability o f all populations. Helminth parasites are no exception to this rule. In medical helminthology, it has been widely assumed that the most effective density-dependent constraints act upon parasite fecundity and are the result of intra-specific parasite competition or acquired immunity to infection. In this article, Anne Keymer and Andrew Slater advocate a more detailed examination of the evidence on which this assumption rests.  相似文献   

7.
Through comparison with ab initio reference data, we have evaluated the performance of various density functionals for describing pi-pi interactions as a function of the geometry between two stacked benzenes or benzene analogs, between two stacked DNA bases, and between two stacked Watson-Crick pairs. Our main purpose is to find a robust and computationally efficient density functional to be used specifically and only for describing pi-pi stacking interactions in DNA and other biological molecules in the framework of our recently developed QM/QM approach "QUILD". In line with previous studies, most standard density functionals recover, at best, only part of the favorable stacking interactions. An exception is the new KT1 functional, which correctly yields bound pi-stacked structures. Surprisingly, a similarly good performance is achieved with the computationally very robust and efficient local density approximation (LDA). Furthermore, we show that classical electrostatic interactions determine the shape and depth of the pi-pi stacking potential energy surface.  相似文献   

8.
  • 1.1. Significant changes in morphology and distribution of red blood cells were observed in blood from adult goldfish held at two loading densities (0.80 and 4.19 fish cm/1 water).
  • 2.2. Changes associated with fish held at high density included decreased counts of red blood cells and thrombocytes and an increase in lymphocytes and total leucocytes.
  • 3.3. Smaller red blood cells were also found in fish held at high density.
  相似文献   

9.
1. Body condition (defined as the relative amount of energy reserves in the body) is an animal trait with strong ecological implications. In some animal taxa (e.g. arthropods), the external volume of the body part in which most nutrients are stored (e.g. abdomen) is used interchangeably with body mass to estimate body condition, making the implicit assumption that abdomen residual volume is a good surrogate of residual mass. However, the degree of correlation between these two measures should largely depend on the density of the nutrients stored. 2. We simulated two food-supplemented experimental groups of animals, each storing a slightly different amount of lipids either in their abdomens or in their entire bodies, and explored (i) how different estimates of condition were able to detect fixed differences between the groups; and (ii) how the amount of lipids stored could affect the outcome of non-intrusive measures of condition on a dichotomous variable (e.g. survival, mating success). We found that density body condition (body mass statistically controlled for structural body size and body volume) has much greater power to detect differences between experimental groups or effects on binary response variables than do classic mass/size or volume/size condition indices. 3. Using data on Lycosa tarantula (L.), a burrowing wolf spider, we report dramatic differences among these three indices in their ability to detect sex differences in the effect of feeding treatment on body condition at maturity. In particular, a plot of residual mass against residual volume reflecting nutrient density suggests that poorly fed spiders are nutritionally unbalanced, since well-fed spiders invest in nutrients of very different density. 4. Furthermore, using data on Scathophaga stercoraria (L.), the yellow dung fly, we found that an index of density condition was better at distinguishing condition differences among three populations than were mass or volume condition estimates alone. 5. We propose that including these three surrogates of condition (mass, volume and density) will substantially improve the accuracy of non-intrusive estimates of body condition, thus providing more powerful tools with direct application in a wide range of disciplines.  相似文献   

10.
Boosting kernel density estimates: A bias reduction technique?   总被引:1,自引:0,他引:1  
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11.
Electrical taste stimulus: current intensity or current density?   总被引:1,自引:0,他引:1  
Ajdukovie  D. 《Chemical senses》1990,15(3):341-347
In electrical taste stimulation, current intensity is usuallyconsidered to be a simple equivalent of the taste stimulus intensity,this being analogous to the concentration of a chemical solution.However, there are indications that current density might bea better correlate of the true electrical taste stimulus. Inthe present study the relationship between the size of the stimulatedhuman tongue area, the electrical current intensity, and themagnitude of elicited taste sensations was examined. Tests insix subjects showed that in order to achieve the standard sourtaste magnitude when using electrode sizes that increased insize, the current intensity also had to be increased, but currentdensity decreased. This might be attributed to spatial summationwhich is plausible only if current density was taken as a realelectrical taste stimulus. Summation degree was less than complete,indicating that the density might be a predominant, but notthe only factor in electrical stimulus efficacy.  相似文献   

12.
Salmonid population dynamics: stability under weak density dependence?   总被引:1,自引:0,他引:1  
S Einum 《Oikos》2005,110(3):630-633
Lobón-Cerviá and Rincón analysed the dynamics of a stream-resident brown trout ( Salmo trutta ) population using stock-recruitment relationships, and reported no indication of density dependence but strong effects of water discharge on juvenile production. The authors concluded to have demonstrated an example of a population where "… endogenous density-dependent mechanisms played, at most, a limited role" for population dynamics (p. 643), and that the population persists with "… little or no operation of endogenous regulatory mechanisms" (p. 641). Further, they "… question the extent to which generalizations claiming that most natural animal populations should be subject to the operation of regulatory feedback mechanisms hold for fish populations" (p. 644). If these conclusions were accurate, it should radically alter our view of fish population dynamics and how such populations should be managed. However, here I argue that these conclusions should be treated cautiously, and suggest alternative interpretations which are more consistent with theory and previous empirical research. Furthermore, using the study of Lobón-Cerviá and Rincón as an illustrative example, I suggest that additional insight into population dynamics may be attained by shifting the focus away from the conventional absolute distinction between density-dependent and -independent factors towards studies that address the potential for interactions between external forces and density.  相似文献   

13.
Animal space use patterns can be affected by the intra- and interspecific density of individuals competing for resources, with home ranges generally decreasing with increasing population density. By applying spatially explicit capture–recapture models implemented in the R package secr, we study whether home ranges of co-occurring yellow-necked mice, Apodemus flavicollis, and bank voles, Myodes glareolus, are related to population density of (a) conspecifics (intraspecific density), (b) the other sympatric species, A. flavicollis or M. glareolus (interspecific density), or (c) total rodent density (A. flavicollis plus M. glareolus). Home ranges of both species were negatively related to intraspecific population density, and were not related to interspecific density or total rodent density. Given that rodents tend to reduce home ranges if resources are abundant, this pattern may merely result from the higher abundance of resources generally associated with high density populations, if the two species were responding to different subsets of resources. However, intraspecific density could directly reduce home ranges, because conspecifics are more likely to interfere with each other due to the overlapping of space use patterns. Therefore, results suggest complementary space or resource use patterns between species, with consequent weak competition and niche differentiation. Across several years and population densities, home ranges of the two co-occurring rodents thus appear to be affected by conspecifics only, suggesting that the two species may coexist in the study area owing to limited space or resource use overlap.  相似文献   

14.
Mechanisms underlying density–area relationships (correlations between population density and patch size) have rarely been tested experimentally. It is often assumed that higher density on large patches is evidence that large patches are high quality (i.e. have greater survival and reproduction), but the same pattern could result from disproportionate movement from small to large patches. Movement-based and within-patch processes must be experimentally separated to show that large patches are indeed of higher quality, but few studies have done so. We experimentally tested movement-based and within-patch hypotheses to explain the positive density–area relationship observed for a saproxylic (decayed wood-dependent) beetle, Odontotaenius disjunctus Illiger (Coleoptera: Passalidae). In separate experiments we quantified (1) immigration into and (2) finite growth rate within logs (=patches) that varied in size and density of conspecific beetles. A log was 11.7-fold [95 % confidence interval (CI) 3.4–40.8) and 10.5-fold (95 % CI 2.7–40.9) more likely to contain a new immigrant if it was large or contained a conspecific pair of beetles, respectively. Neither log size nor conspecific density was associated with changes in finite growth rate that would lead to higher density: decreased log size and increased conspecific density reduced finite growth rate in direct proportion to the loss in available resources per mating pair. We conclude that movement behavior rather than habitat quality is responsible for the positive density–area relationship for O. disjunctus. An important implication of our results is that population density is an imperfect indicator of habitat quality.  相似文献   

15.
Intraspecific density regulation influences the synchronization of local population dynamics through dispersal. Spatial synchrony in turn may jeopardize metapopulation persistence. Joining results from previous studies suggests that spatial synchrony is highest at moderate over-compensation and is low at compensating and at very strong over-compensating density regulation. We scrutinize this supposition of a unimodal relationship using a process-based metapopulation model with explicit local population dynamics. We extend the usually studied range of density regulation to under-compensation and analyse resulting metapopulation persistence. We find peaks of spatial synchrony not only at over-compensatory but also under-compensatory density regulation and show that effects of local density compensation on synchrony follow a bimodal rather than unimodal relationship. Persistence of metapopulations however, shows a unimodal relationship with a broad plateau of high persistence from compensatory to over-compensatory density regulation. This range of high persistence comprises both levels of low and high spatial synchrony. Thus, not synchrony alone jeopardizes metapopulation persistence, but only in interplay with high local extinction risk. The functional forms of the relations of density compensation with spatial synchrony and persistence are robust to increases in dispersal mortality, landscape dynamics, or density dependence of dispersal. However, with each of these increases the maxima of spatial synchrony and persistence shift to higher over-compensation and levels of synchrony are reduced. Overall, for over-compensation high landscape connectivity has negative effects while for under-compensation connectivity affects persistence positively. This emphasizes the importance of species life-history traits for management decisions with regard to landscape connectivity: while dispersal corridors are essential for species with under-compensatory density regulation, they may have detrimental effects for endangered species with over-compensation.  相似文献   

16.
AIM: To evaluate the bone mineral density at lumbar spine and at femoral neck in a group of young adults in whom Graves' disease developed during childhood and adolescence. PATIENTS AND METHODS: We examined 28 patients (5 male, 23 female, age 20.9 +/- 3.3 years) who were 11.8 +/- 2.9 years old at the onset of Graves' disease. They were treated either with methimazole (14 patients) or with methimazole plus l-thyroxine (14 patients). At the time of the investigation, 13 patients were considered cured following antithyroid treatment, 2 were still on antithyroid drugs, 3 were on replacement therapy with l-thyroxine because of hypothyroidism, and 10, treated either surgically or with (131)I, were on replacement therapy. The bone mineral density was measured at the lumbar spine (L2-L4) and at the femoral neck, using dual-energy X-ray absorptiometry. RESULTS: The spinal bone mineral density SD score was -0.28 +/- 1.02, the femoral neck bone mineral density SD score was 0.36 +/- 1.02, and both were not different from zero (NS). We did not find any correlation between the bone mineral density of the femoral neck and that of the lumbar spine and the clinical parameters. CONCLUSION: Graves' disease, beginning in childhood and adolescence, when appropriately treated, does not affect attainment of peak bone mass.  相似文献   

17.
In this paper, we investigate the abnormalities of electroencephalograph (EEG) signals in the Alzheimer’s disease (AD) by analyzing 16-scalp electrodes EEG signals and make a comparison with the normal controls. The power spectral density (PSD) which represents the power distribution of EEG series in the frequency domain is used to evaluate the abnormalities of AD brain. Spectrum analysis based on autoregressive Burg method shows that the relative PSD of AD group is increased in the theta frequency band while significantly reduced in the alpha2 frequency bands, particularly in parietal, temporal, and occipital areas. Furthermore, the coherence of two EEG series among different electrodes is analyzed in the alpha2 frequency band. It is demonstrated that the pair-wise coherence between different brain areas in AD group are remarkably decreased. Interestingly, this decrease of pair-wise electrodes is much more significant in inter-hemispheric areas than that in intra-hemispheric areas. Moreover, the linear cortico-cortical functional connectivity can be extracted based on coherence matrix, from which it is shown that the functional connections are obviously decreased, the same variation trend as relative PSD. In addition, we combine both features of the relative PSD and the normalized degree of functional network to discriminate AD patients from the normal controls by applying a support vector machine model in the alpha2 frequency band. It is indicated that the two groups can be clearly classified by the combined feature. Importantly, the accuracy of the classification is higher than that of any one feature. The obtained results show that analysis of PSD and coherence-based functional network can be taken as a potential comprehensive measure to distinguish AD patients from the normal, which may benefit our understanding of the disease.  相似文献   

18.
Cationic liposomes (CLs) are one of the most employed nonviral nanovector systems in gene therapy. However, their transfection efficiency is strongly affected by interactions with plasma components, that lead to the formation of a “protein corona” onto CL surface. The interactions between nanoparticles entering the body and biomolecules have an essential role for their biodistribution. Because the knowledge of proteins adsorbed onto vector surface could be useful in the screening of new, more efficient and more biocompatible liposomal formulations, the behavior of three CLs with different membrane charge densities was investigated. The proteins of the three coronas were identified by nano-liquid chromatography-tandem mass spectrometry, and quantified with label-free spectral counting strategy. Fibrinogen displayed higher association with CLs with high membrane charge density, while apolipoproteins and C4b-binding protein with CLs with low membrane charge density. These results are discussed in terms of the different lipid compositions of CLs and may have a deep biological impact for in vivo applications. Surface charge of nanoparticles is emerging as a relevant factor determining the corona composition after interaction with plasma proteins. Remarkably, it is also shown that the charge of the protein corona formed around CLs is strongly related to their membrane charge density.  相似文献   

19.
Negative density dependent selection on individuals in prey aggregations (negative DDS, the preferential selection by predators of spatially isolated prey) is assumed to contribute in many cases to the evolution and maintenance of aggregation. Both positive and negative DDS on prey groups have been documented in nature but there is no existing framework to predict when each of these forms of natural selection is most likely. By exploiting the tendency of artificial neural networks to exhibit consumer-like emergent behaviours, I isolate at least two environmental factors impinging on the consumer organism that may determine which form of density dependent natural selection is shown: the distribution of prey group size attacked by the predator and the spatial conformation (dispersed or compacted) of the prey group. Numerous forms of DDS on artificial prey (positive, negative, and non-DDS) are displayed through different combinations of these factors. I discuss in detail how the predictions of the model may be tested by empiricists in order to assess the usefulness of the framework presented. I stress the importance of understanding DDS on prey groups given the recent emergence of these systems as test beds for ideas on biological self-organisation.  相似文献   

20.
An azo bridge (–N?=?N–) can not only desensitize explosives but also dramatically increase their heats of formation and explosive properties. Amino and nitro are two important high energy density functional groups. Here, we present calculations on 1-nitro-1-triazene (NH2–N?=?N–NO2). Thermal stability and detonation parameters were predicted theoretically at CCSD(T)/6-311G* level, based on the geometries optimized at MP2/6-311G* level. It was found that the p?→?π conjugation interaction and the intramolecular hydrogen bonding that exist in the system together increase the thermal stability of the molecule. Moreover, the detonation parameters were evaluated to be better than those of the famous HMX and RDX. Finally, the compound was demonstrated to be a high energy density material.  相似文献   

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