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1.
Y Adini  D Sagi 《Spatial Vision》1992,6(1):61-77
Human ability to identify simultaneously two targets in the visual field is severely limited. Previous studies have shown that orientation identification of two targets takes twice the time needed for one target. Here we asked whether this seriality is imposed by the decision requirement of the task or by such stimulus properties as target spatial separation and similarity. Observers had to identify the orientations (vertical vs horizontal) of two Gabor patches presented at random positions. Performance on this double-task experiment was compared with performance on each of the tasks when carried out alone. We varied the spatial separation between the two targets for targets having identical or different spatial-frequencies and found that the orientation of two targets having different frequencies could be identified in parallel when occupying the same spatial position but not when separated in space by 4 deg of visual angle or more. Targets having the same frequency could be identified in parallel even when separated by 8 deg, demonstrating that decision factors do not impose seriality. This result can be taken as evidence for the existence of a grouping process operating prior to orientation identification. This grouping process operates according to classical Gestalt rules (proximity, similarity) and enables parallel attentive processing of large input chunks.  相似文献   

2.
由于土壤碳通量在空间分布上具有很强的异质性,传统的采样方法难以对区域土壤碳通量进行精确估算,因此确定适当的采样策略对区域土壤碳通量的估算具有重要意义.本文提出一种逐点递增式采样的区域剖分部署策略(RDPG):设定初始采样点,使用改进的凸包插值算法构造Delaunay三角网,根据邻近已知采样点插值计算三角形各边垂直平分线的交点的离散度,选择离散度最大的点作为新增采样点.采用该方法对变异系数为0.42~0.59的仿真试验区域进行多次试验,结果表明:在相同试验条件下,RDPG布局策略能够获得比随机采样和均匀采样策略更高的区域土壤碳通量估算准确度.RDPG方法考虑了区域土壤碳通量的空间异质性,提高了区域土壤碳通量拟合精度.  相似文献   

3.
Investigating the drivers of diet quality is a key issue in wildlife ecology and conservation. Fecal near infrared reflectance spectroscopy (f‐NIRS) is widely used to assess dietary quality since it allows for noninvasive, rapid, and low‐cost analysis of nutrients. Samples for f‐NIRS can be collected and analyzed with or without knowledge of animal identities. While anonymous sampling allows to reduce the costs of individual identification, as it neither requires physical captures nor DNA genotyping, it neglects the potential effects of individual variation. As a consequence, regression models fitted to investigate the drivers of dietary quality may suffer severe issues of pseudoreplication. I investigated the relationship between crude protein and ecological predictors at different time periods to assess the level of individual heterogeneity in diet quality of 22 marked chamois Rupicapra rupicapra monitored over 2 years. Models with and without individual grouping effect were fitted to simulate identifiable and anonymous fecal sampling, and model estimates were compared to evaluate the consequences of anonymizing data collection and analysis. The variance explained by the individual random effect and the value of diet repeatability varied with seasons and peaked in winter. Despite the occurrence of individual variation in dietary quality, ecological parameter estimates under identifiable or anonymous sampling were consistently similar. This study suggests that anonymous fecal sampling may provide robust estimates of the relationship between dietary quality and ecological correlates. However, since the level of individual heterogeneity in dietary quality may vary with species‐ or study‐specific features, inconsequential pseudoreplication should not be assumed in other taxa. When individual differences are known to be inconsequential, anonymous sampling allows to optimize the trade‐off between sampling intensity and representativeness. When pseudoreplication is consequential, however, no conclusive remedy exists to effectively resolve nonindependence.  相似文献   

4.
We develop a general theory of organism movement in heterogeneous populations that can explain the leptokurtic movement distributions commonly measured in nature. We describe population heterogeneity in a state-structured framework, employing advection-diffusion as the fundamental movement process of individuals occupying different movement states. Our general analysis shows that population heterogeneity in movement behavior can be defined as the existence of different movement states and among-individual variability in the time individuals spend in these states. A presentation of moment-based metrics of movement illustrates the role of these attributes in general dispersal processes. We also present a special case of the general theory: a model population composed of individuals occupying one of two movement states with linear transitions, or exchange, between the two states. This two-state "exchange model" can be viewed as a correlated random walk and provides a generalization of the telegraph equation. By exploiting the main result of our general analysis, we characterize the exchange model by deriving moment-based metrics of its movement process and identifying an analytical representation of the model's time-dependent solution. Our results provide general and specific theoretical explanations for empirical patterns in organism movement; the results also provide conceptual and analytical bases for extending diffusion-based dispersal theory in several directions, thereby facilitating mechanistic links between individual behavior and spatial population dynamics.  相似文献   

5.
A model is presented to explore how the form of selection arising from competition for resources is affected by spatial resource heterogeneity. The model consists of a single species occupying two patches connected by migration, where the two patches can differ in the type of resources that they contain. The main goal is to determine the conditions under which competition for resources results in disruptive selection (i.e., selection favoring a polymorphism) since it is this form of selection that will give rise to the evolutionary diversification of resource exploitation strategies. In particular, comparing the conditions giving rise to disruptive selection when the two patches are identical to the conditions when they contain different resources reveals the effect of spatial resource heterogeneity. Results show that when the patches are identical, the conditions giving rise to disruptive selection are identical to those that give rise to character displacement in previous models. When the patches are different, the conditions giving rise to disruptive selection can be either more or less stringent depending upon demographic parameters such as the intrinsic rate of increase and the migration rate. Surprisingly, spatial resource heterogeneity can actually make forms of evolutionary diversification such as character displacement less likely. It is also found that results are dependent on how the resource exploitation strategies and the spatial resource heterogeneity affect the population dynamics. One robust conclusion however, is that spatial resource heterogeneity always has a disruptive effect when the migration rate between patches is low.  相似文献   

6.

Background

Neonatal mortality contributes a large proportion towards early childhood mortality in developing countries, with considerable geographical variation at small areas within countries.

Methods

A geo-additive logistic regression model is proposed for quantifying small-scale geographical variation in neonatal mortality, and to estimate risk factors of neonatal mortality. Random effects are introduced to capture spatial correlation and heterogeneity. The spatial correlation can be modelled using the Markov random fields (MRF) when data is aggregated, while the two dimensional P-splines apply when exact locations are available, whereas the unstructured spatial effects are assigned an independent Gaussian prior. Socio-economic and bio-demographic factors which may affect the risk of neonatal mortality are simultaneously estimated as fixed effects and as nonlinear effects for continuous covariates. The smooth effects of continuous covariates are modelled by second-order random walk priors. Modelling and inference use the empirical Bayesian approach via penalized likelihood technique. The methodology is applied to analyse the likelihood of neonatal deaths, using data from the 2000 Malawi demographic and health survey. The spatial effects are quantified through MRF and two dimensional P-splines priors.

Results

Findings indicate that both fixed and spatial effects are associated with neonatal mortality.

Conclusions

Our study, therefore, suggests that the challenge to reduce neonatal mortality goes beyond addressing individual factors, but also require to understanding unmeasured covariates for potential effective interventions.  相似文献   

7.
长三角典型区占补耕地土壤肥力的时段特征   总被引:3,自引:0,他引:3  
基于1984、2000和2016年卫星遥感影像,分析了江苏省常熟市耕地占补时空变化特征;基于ArcGIS和土壤采样数据,利用隶属度函数综合法评价了1984和2000年耕地土壤肥力;采用叠加分析,揭示了1984-2000、2000-2016年间该地区占补耕地土壤肥力特征.结果表明: 1984-2016年间,常熟市耕地面积减少了1/3,1984-2000年被水域和居民点工矿用地分别占用了51.7%和41.4%,2000-2016年只居民点工矿用地就占用了80.8%.1984-2000年间,常熟市耕地土壤肥力上升的面积占65.5%,仅3.9%下降;南部地区土壤肥力均高于其他地区.研究期间耕地数量和质量占补不平衡,前时段“占多补少、占优补劣”形势严峻,后时段“占多补少、占劣补劣”特征明显.耕地减少的趋势并未扭转,异地补充明显,耕地占补平衡应当关注异地补充耕地质量和土壤肥力时段性特点.  相似文献   

8.
To understand empirical patterns of phenotypic plasticity, we need to explore the complexities of environmental heterogeneity and how it interacts with cue reliability. I consider both temporal and spatial variation separately and in combination, the timing of temporal variation relative to development, the timing of movement relative to selection, and two different patterns of movement: stepping‐stone and island. Among‐generation temporal heterogeneity favors plasticity, while within‐generation heterogeneity can result in cue unreliability. In general, spatial variation more strongly favors plasticity than temporal variation, and island migration more strongly favors plasticity than stepping‐stone migration. Negative correlations among environments between the time of development and selection can result in seemingly maladaptive reaction norms. The effects of higher dispersal rates depend on the life history stage when dispersal occurs and the pattern of environmental heterogeneity. Thus, patterns of environmental heterogeneity can be complex and can interact in unforeseen ways to affect cue reliability. Proper interpretation of patterns of trait plasticity requires consideration of the ecology and biology of the organism. More information on actual cue reliability and the ecological and developmental context of trait plasticity is needed.  相似文献   

9.
Fungi impact humans and the environment in many ways, for good and ill. Some fungi support the growth of terrestrial plants or are used in biotechnology, and yet others are established or emerging pathogens. In some cases, the same organism may play different roles depending on the context or the circumstance. A better understanding of the relationship between fungal biochemical composition as related to the fungal growth environment is essential if we are to support or control their activities. Synchrotron FTIR (sFTIR) spectromicroscopy of fungal hyphae is a major new tool for exploring cell composition at a high spatial resolution. Brilliant synchrotron light is essential for this analysis due to the small size of fungal hyphae. sFTIR biochemical characterization of subcellular variation in hyphal composition will allow detailed exploration of fungal responses to experimental treatments and to environmental factors.  相似文献   

10.
Habitat variation and population regulation in Sparrowhawks   总被引:2,自引:0,他引:2  
I. NEWTON 《Ibis》1991,133(S1):76-88
In a study area in south Scotland, Sparrowhawks did not occupy the available nesting places at random, but more often used those places where breeding success was highest (here called high-grade places). Most birds stayed on particular nesting places for only one year, but others stayed up to 8 years. Some birds moved from low- to high-grade places as they aged. Continued occupancy of certain places was thus produced by many different individuals occupying such places in rapid succession, but most staying for only one breeding season.
On the most used (high-grade) nesting places pairs produced more than enough young per breeding attempt to offset the average annual mortality, but on the less used (low-grade) places they produced too few. Low-grade nesting places therefore acted as a sink, whose occupancy could be maintained only by continual immigration. Over the study area as a whole, the population was in balance, with reproduction matching mortality.
Habitat changed over periods of 15–30 years, as woodland matured. Nesting places in young woods, with small densely-growing trees, showed the highest occupancy and nest success. Both aspects of performance declined as the woodland aged, and trees became larger and more widely spaced. Long-term stability in nest numbers and success in the study area as a whole was associated with a system of rotational forest management, which ensured a continuing availability of young woods.
It is proposed that spatial variation in habitat quality is involved in the regulation of breeding numbers. Removal experiments confirmed the presence of non-breeders, which could attempt to breed when high-grade nesting habitat was made available to them, but otherwise remained as non-breeders despite the presence of vacant low-grade habitat. This situation, involving an interaction between habitat quality and bird quality, probably occurs in some other raptors too.  相似文献   

11.
香菇自然群体中个体间的空间分布及其遗传联系*   总被引:1,自引:0,他引:1  
代江红  林芳灿 《菌物学报》2001,20(1):100-106
应用体细胞不亲和性反应、交配型因子分析和基因组DNA的RAPD分析,研究了一个分布于方圆约1km的6根倒木上的18个香菇野生菌株间的遗传差异。结果表明,该群体大多数菌株间配对(80.4%)体细胞不亲和,而同一倒木菌株间配对的体细胞亲和率达 62.5%。不同倒木菌株间未发现体细胞亲和的配对。该群体存在 11个特异的A因子和7个特异的B因子。同一倒木的菌株有的交配型因子相同,有的则不同。不同倒木的菌株大多数交配型因子不同,未发现交配型因子完全相同的菌株。RAPD分析显示,体细胞亲和的菌株,交配型因子完全相同的菌株,在基于DNA相似系数的遗传相关聚类中,首先聚为小类。总起来看,在自然群体中,香菇个体间的遗传差异与其空间分布之间存在一定的联系,随着空间距离的增大,菌株间的异质性相应增高。  相似文献   

12.
R K Chesser 《Genetics》1998,150(3):1309-1327
This study assesses factors that influence the rates of change of organelle gene diversity and the maintenance of heteroplasmy. Losses of organelle gene diversity within individuals via vegetative segregation during ontogeny are paramount to resultant spatial and temporal patterns. Steady-state losses of organelle variation from the zygote to the gametes are determined by the effective number of organelles, which will be approximately equal to the number of intracellular organelles if random segregation prevails. Both rapid increases in organelle number after zygote formation and reductions at germ lines will reduce variation within individuals. Terminal reductions in organelles must be to very low copy numbers (<5) for substantial losses in variation to occur rapidly. Nonrandom clonal expansion and vegetative segregation during gametogenesis may be effective in reducing genetic variation in gametes. If organelles are uniparentally inherited, the asymptotic expectations for effective numbers of gametes and spatial differentiation will be identical for homoplasmic and heteroplasmic conditions. The rate of attainment of asymptote for heteroplasmic organelles, however, is governed by the rate of loss of variation during ontogeny. With sex-biased dispersal, the effective number of gametes is maximized when the proportional contributions of the sex having the higher dispersal rate are low.  相似文献   

13.
喀斯特木论自然保护区旱季土壤水分的空间异质性   总被引:19,自引:0,他引:19  
采用经典统计学和地统计学方法,分段研究了喀斯特木论自然保护区典型峰丛坡地和洼地旱季表层土壤水分(0~5 cm和5~10 cm)的空间异质性.结果表明:研究区旱季表层土壤水分仍然较高,总体上具有良好的半方差结构;坡地和洼地0~5 cm和5~10 cm土层土壤水分的空间分布均符合指数模型,同一立地条件下两土层土壤含水量具有相似的空间结构和分布格局,相同土层不同立地条件下的差异明显;坡地土壤水分含量的空间相关性中等且连续性好,具有明显的斑块分布格局,其Moran I值的变化相对缓慢;洼地土壤水分含量具有强烈的空间自相关,但变程很短,其Moran I值的波动较大,斑块比较破碎.地形、微地貌、降雨、人为干扰特别是植被是保护区土壤含水量空间变异的重要影响因素,保存完好的原始森林对土壤水分空间异质性具有良好的调控作用.  相似文献   

14.
We analyze how spatial heterogeneity in host density affects the advance of vector-borne disease. Infection requires vector infestation. The vector spreads only between hosts occupying the same neighborhood, and the number of hosts varies randomly among neighborhoods. Simulation of a spatially detailed model shows that increasing heterogeneity in host abundance reduces pathogen prevalence. Clumping of hosts can limit the advance of the vector, which inhibits the spread of infection indirectly. Clumping can also increase the chance that the pathogen and vector become physically separated during the initial phase of the epidemic process. The latter limitation on the pathogen's spread, in our simulations, is restricted to small interaction neighborhoods. A mean-field model, which does not maintain spatial correlations between sites, approximates simulation results when hosts are arrayed uniformly, but overestimates infection prevalence when hosts are aggregated. A pair approximation, which includes some of the simulation model's spatial correlations, better describes the vector infestation frequencies across host spatial dispersions.  相似文献   

15.
An experimental approach was used to examine the effects of spatial nutrient heterogeneity and planting density on the sizes of plants within populations of Abutilon theophrasti. Planting locations were generated using random numbers and replicated among populations growing on two different scales of heterogeneity and homogeneous soils. The same quantity of nutrients (dehydrated cow manure) was added to each population, regardless of the spatial nutrient distribution. The higher density was achieved by adding additional planting locations to those present at the lower density. Plant biomass was compared among ten planting locations present in all populations. Plants in seven locations were smaller at the higher density, but the spatial distribution of nutrients affected plant size in only two locations. At the population level, the higher density reduced mean plant biomass and increased both total biomass and the coefficient of variation in biomass, a measure of size inequality. Only when populations on both scales of heterogeneity were together compared with those on homogeneous soils were population-level measurements found to be significantly affected by soil treatment; heterogeneity resulted in decreased total biomass and an increase in the coefficient of variation, apparently due to an increase in the number of small plants in the population. These results, together with the finding that fine root biomass increased in nutrient-enriched patches, suggest that on heterogeneous soils most plants were able to access nutrient patches.  相似文献   

16.
Evidence of a latitudinal gradient in spider diversity in Australian cotton   总被引:1,自引:0,他引:1  
The most common explanation for species diversity increasing towards the tropics is the corresponding increase in habitats (spatial heterogeneity). Consequently, a monoculture (like cotton in Australia) which is grown along a latitudinal gradient, should have the same degree of species diversity throughout its range. We tested to see if diversity in a dominant cotton community (spiders) changed with latitude, and if the community was structurally identical in different parts of Australia. We sampled seven sites extending over 20° of latitude. At each site we sampled 1–3 fields 3–5 times during the cotton growing season using pitfall traps and beatsheets, recording all the spiders collected to family. We found that spider communities in cotton are diverse, including a large range of foraging guilds, making them suitable for a conservation biological control programme. We also found that spider diversity increased from high to low latitudes, and the communities were different, even though the spiders were in the same monocultural habitat. Spider beatsheet communities around Australia were dominated by different families, and responded differently to seasonal changes, indicating that different pest groups would be targeted at different locations. These results show that diversity can increase from high to low latitudes, even if spatial heterogeneity is held constant, and that other factors external to the cotton crop are influencing spider species composition. Other models which may account for the latitudinal gradient, such as non‐equilibrium regional processes, are discussed.  相似文献   

17.
桂西北喀斯特地区石灰土养分空间变异特征   总被引:7,自引:2,他引:5  
从区域尺度出发,将整个桂西北喀斯特地区作为研究区域,以大面积野外调查采样和室内分析数据为基础,采用经典统计学和地统计学方法,探索喀斯特区域内石灰土养分的空间异质性和分布格局。结果表明:桂西北喀斯特地区石灰土pH为弱变异,其他养分变异系数为30%—75%,变异程度中等。石灰土各养分的空间变异特征不同,土壤有机质(SOM)、速效磷(AP)和速效钾(AK)的最佳拟合模型依次为球状模型、高斯模型和线性模型,pH、碱解氮(AN)、全氮(TN)、全磷(TP)和全钾(TK)的最佳拟合模型均为指数模型。SOM、AP、TN和TP表现为中等空间相关性,空间差异是由随机因素和结构性因素共同作用引起的;pH、AN、AK和TK呈弱空间相关性,空间变异主要受随机因素的控制;SOM和TN的空间变异尺度较大,pH、AN和TK的空间变异尺度最小。SOM、TN、TP和AP养分含量呈自西向东逐渐减少的趋势,pH、AN、AK和TK在区域尺度上的分布没有呈现一定的规律性。相关分析结果表明,在区域大尺度下,地形因子(如海拔、坡度和岩石裸露率等)和植被类型是影响桂西北喀斯特石灰土空间变异特征的重要因素。  相似文献   

18.
Starting from the experimental situation that in each of a environments all mp combinations between m maternal and p paternal genotypes are raised in b replications (blocks), a linear model is formulated for a quantitative character. On the basis of this model formulae for the analysis of variance are given. The expectations of mean squares as well as the estimations of effects (general and specific combining ability and their interaction terms with environments) and their variances are given for different combinations of fixed and random factors.  相似文献   

19.
The human body is formed by trillions of individual cells. These cells work together with remarkable precision, first forming an adult organism out of a single fertilized egg, and then keeping the organism alive and functional for decades. To achieve this precision, one would assume that each individual cell reacts in a reliable, reproducible way to a given input, faithfully executing the required task. However, a growing number of studies investigating cellular processes on the level of single cells revealed large heterogeneity even among genetically identical cells of the same cell type. Here we discuss the sources of heterogeneity in mammalian systems; how cells ensure reliable processing of information despite fluctuations in their molecular components; and what could be the benefit of cell-to-cell variability for mammalian cells.  相似文献   

20.

Paddy fields are essential habitats for frogs. We evaluated the impacts of both farmland consolidation including agricultural road improvement and farmland abandonment on the two Rana species using a model incorporating spatial autocorrelation. A sampling unit consists of several paddy fields that share a ditch and are isolated from other blocks by roads or other land covers. We surveyed 619 blocks in an area of about 1000 km2 from the plain to the mountains of Toyota City in central Japan. Among them, 124 blocks included at least a flooded paddy field where frogs could lay eggs. R. ornativentris and R. japonica bred in 50 and 25 blocks, respectively. We constructed models to explain the presence/absence of two species by GLM (non-spatial model) and hierarchical Bayesian model with INLA (spatial model) that includes spatial autocorrelation as a random effect. Explanatory variables of the local scale were the altitude, location of the paddy field (yatsuda (valley bottom paddy fields) or non-yatsuda), farmland consolidated or not consolidated, and under cultivation or abandoned. Those of the landscape scale were areas of forest and paddy fields, and road density in 14 circles with different radius from 50 to 2000 m. Both species’ distribution had significant spatial autocorrelation. The spatial model had a higher discriminative ability than the non-spatial model. Farmland consolidation and the forest area in the 400 m radius had a positive effect on R. ornativentris. Altitude and road density in the 50 m radius had negative effects, cultivation had a positive effect, and farmland consolidation and yatsuda had no or negative effects on R. japonica. R. ornativentris was threatened by farmland abandonment, but the urbanization and/or farmland consolidation threatened R. japonica.

  相似文献   

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