共查询到20条相似文献,搜索用时 31 毫秒
1.
Identifying and counting the total number of biological species observed to date, and plotting versusa measure of the effort used to record them, gives rise to a species accumulation curve. Interest typically is concerned with estimating the total number of species in the area of study, having observed only the accumulation curve, having no information on species frequencies. This article considers the problem of optimally stopping the sampling process. We use a sequential procedure with a fixed maximum horizon for accumulation. A utility function based on the number of new species to be observed and the effort saved from the maximum horizon is adopted, and a workable algorithm based on backward induction is obtained. An example in accumulation of bat species is also presented. 相似文献
2.
Chris Gennings Walter H. Carter Julie A. Campain Dong-soon Bae Raymond S. H. Yang 《Journal of Agricultural, Biological & Environmental Statistics》2002,7(1):58-73
Exposure to mixtures of chemicals is an important and relevant environmental issue. Of particular interest is the detection and characterization of departure of biological effects from additivity. Methodology based on the assumption of additivity is used in fitting single-chemicaldata. Interactionsare determined and characterized by making comparisons between the observed and predicted responses at mixtures along a fixed ratio ray of the component substances. Two simultaneous tests are developed for testing for any departure from additivity. Multiple comparisons procedures are used to compare observed responses to that predicted under additivity. A simultaneous confidence band on the predicted responses along the mixture ray is also developed. The methods are illustrated with cytotoxicity data that arise when human epidermal keratinocytes are exposed to a mixture of arsenic, chromium, cadmium, and lead. Synergistic, antagonistic, and additive cytotoxicities were observed at different dose levels of the four-metal mixture. 相似文献
3.
Elosa Diaz-Frances Luis G. Gorostiza 《Journal of Agricultural, Biological & Environmental Statistics》2002,7(3):335-349
Soberon and Llorente proposed using the mean S(t; ϕ) of a pure birth process as a model for species accumulation functions and obtained ordinary least squares point estimates for the vector of parameters ϕ. They provided useful biological interpretations for S(t; ϕ) as well as for the birth rates of the process; they also expressed the strong need for an objective tool to compare different models for a given species accumulation data set. We show that the functions S(t; ϕ) proposed by Soberon and Llorente are generally not means of pure birth processes but that they can be well approximated by the mean of a suitable nonhomogeneous pure birth process B(t). We suggest using S(t; ϕ) and an upper bound for the (unknown) variance of the approximating pure birth process to make statistical inferences about ϕ. A nonlinear regression model with mean S(t; ϕ) and normal errors, possibly correlated, is proposed using a likelihood approach. This model can be compared with other relevant models under consideration by means of their corresponding likelihood ratio, providing a useful, objective, and quantitative tool for comparing different models in the light of the observed data. The suggested approach is exemplified with two species accumulation data sets. 相似文献
4.
For species that are still widespread, obtaining accurate and precise measures of population change inevitably means gathering representative sample data rather than undertaking a complete census. In the UK, a system of raising ‘alerts’ utilises stochastic models for such data to identify species in rapid (>50%) or moderate (25-50%) decline across various temporal and spatial scales. Considerable improvements in interpretation can be made by explaining annual fluctuations in terms of explicit population models (rather than trends of an arbitrary mathematical form); through the simultaneous modelling of data from a complete or partial census with those providing information on the demographic rates employed in these models; and through adopting a Bayesian rather than a frequentist statistical approach. A Bayesian approach is natural for quantifying, in the form of a probability, the support provided by the data for assigning a species to each of the categories. Based on territory mapping and ringing data for the lapwing Vanellus vanellus, we describe such an approach. Trends are estimated more precisely than those under models previously employed in the alerts context. Some smoothing is induced, but realistic responses to years of severe weather are retained, and these are expressed also via model-averaged trends in key demographic parameters. We discuss the conservation implications for this declining species, and the wider potential arising from the ability to quantify confidence that population change has exceeded a threshold either generating conservation concern or justifying a subsequent programme of action for recovery. 相似文献
5.
在我国的北方地区植被与水资源之间的关系是森林植被构建的关键性问题.如何依据水分条件的承载力,构建水分稳定的森林植被是一项重要研究.以北京妙峰山侧柏、油松为研究对象,应用大型可称量式蒸渗仪测定林木个体耗水规律,并在此基础上进行尺度扩展,分析2种林分群体耗水特征.研究发现:在晴天条件下,侧柏和油松蒸腾强度的日变化趋势均为单峰曲线;在典型阴天和雨天条件下,2树种间蒸腾速率差异明显,受云量动态变化影响,蒸腾强度的日变化趋势为多峰曲线.通过相关分析发现,在选定的环境因子中太阳辐射通量、大气水汽压饱和差、空气温度、土壤含水率和土壤水势均与林木蒸腾速率呈正相关,而空气相对湿度与林木蒸腾速率呈负相关.另外,利用边材面积作为尺度转换因子实现由单木到林分的尺度扩展,根据测定的样木蒸腾量,推算得出侧柏林6-9月总蒸腾量为321.43 mm,油松林的总蒸腾量为192.83 mm. 相似文献
6.
7.
东南烟稻轮作区烤烟临界氮浓度稀释曲线的建立与验证 总被引:1,自引:0,他引:1
8.
We evaluated the usefulness of a genetic linkage between the U1 and U2 small nuclear RNAs for species identification. Six soles belonging to the genera Solea, Dicologlossa, and Microchirus were studied. A simple methodology based on two single PCRs is described. Reproducible band profiles were generated for all samples. This rapid and discriminatory molecular method is highly promising for determining the authenticity of sole fillets in the food industry. 相似文献
9.
黄土旱塬不同氮肥用量下冬小麦干物质累积和氮素吸收利用过程研究 总被引:8,自引:0,他引:8
利用旱作长期定位施肥试验研究了不同氮肥用量对冬小麦干物质累积和氮素吸收利用的影响,结果显示,不同处理下干物质累积变化趋势都呈"S"型曲线,且冬小麦各生育期干物质量,随氮肥用量的增加而增大,说明氮肥对促进冬小麦干物质累积作用显著。干物质累积速率均呈现明显的单峰曲线,拔节-灌浆阶段累积速率最大,是干物质累积的重要时期。小麦植株含氮量和氮素累积量都随氮肥用量增加而升高,在冬前-拔节期和开花-灌浆期两个阶段,冬小麦植株氮素累积量较大,累积速率快,是氮素吸收利用的两个关键阶段。 相似文献
10.
Chris Gennings W. Hans Carter Richard A. Carchman Michael J. DeVito Jane Ellen Simmons Kevin M. Crofton 《Journal of Agricultural, Biological & Environmental Statistics》2007,12(1):96-111
When an interaction has been detected among the chemicals in a mixture, it may be of interest to predict the interaction threshold.
A method is presented for estimation of an interaction threshold along a mixture ray which allows differences in the shapes
of the dose-response curves of the individual components (e.g., mixtures of full and partial agonists with differing response
maxima). A point estimate and confidence interval for the interaction threshold may be estimated. The methods are illustrated
with data from a study of a mixture of 18 polyhalogenated aromatic hydrocarbons (PHAHs) in rats exposed by oral gavage for
four consecutive days. Serum total thyroxine (T4) was the response variable. Previous analysis of these data demonstrated
a dose-dependent interaction among the 18 chemicals in the mixture, with additivity suggested in the lower portion of the
dose-response curve and synergy (greater than additive response) in the higher portion of the dose-response curve. The present
work builds on this analysis by construction of an interaction threshold model along the mixture ray. This interaction threshold
model has two components: an implicit additivity region and an explicit region that describes the departure from additivity;
the interaction threshold is the boundary between the two regions. Estimation of the interaction threshold within the observed
experimental region suggested evidence of additivity in the low dose region. Total doses of the mixture that exceed the upper
limit of the confidence interval on the interaction threshold were associated with a greater-than-additive interaction. 相似文献
11.
Mario Hasler Kathrin Böhlendorf 《Journal of Agricultural, Biological & Environmental Statistics》2013,18(4):578-593
Agricultural experiments often have a completely randomized design, and multiple, correlated variables are measured. This paper addresses an appropriate statistical evaluation. A multivariate t-distribution is used for the calculation of multiplicity-adjusted p-values and simultaneous confidence intervals. The number of the multiple variables as well as their correlations are taken into account this way. We consider ratios of means instead of differences, and comparisons versus the overall mean instead of all-pair comparisons. A data set from a greenhouse experiment with glucosinolates of several cultivars of Chinese cabbage (Brassica rapa subsp. pekinensis) is used as an example. Related code based on the R-package SimComp is presented. This package allows a wide application in many agricultural experiments with a similar design. 相似文献
12.
关中平原夏玉米临界磷浓度稀释曲线构建与磷营养诊断 总被引:1,自引:1,他引:0
13.
Gerrit Gort Wim J. M. Koopman Alfred Stein Fred A. van Eeuwijk 《Journal of Agricultural, Biological & Environmental Statistics》2008,13(2):177-198
AFLP is a frequently used DNA fingerprinting technique that is popular in the plant sciences. A problem encountered in the
interpretation and comparison of individual plant profiles, consisting of band presence-absence patterns, is that multiple
DNA fragments of the same length can be generated that eventually show up as single bands on a gel. The phenomenon of two
or more fragments coinciding in a band within an individual profile is a type of homoplasy, that we call collision. Homoplasy
biases estimates of genetic similarity. In this study, we show how to calculate collision probabilities for bands as a function
of band length, given the fragment count, the band count, or band lengths. We also determine probabilities of higher order
collisions, and estimate the total number of collisions for a profile. Since short fragments occur more often, short bands
are more likely to contain collisions. For a typical plant genome and AFLP procedure, the collision probability for the shortest
band is 25 times larger than for the longest. In a profile with 100 bands a quarter of the bands may contain collisions, concentrated
at the shorter band lengths. All calculations require a careful estimate of the monotonically decreasing fragment length distribution.
Modifications of Dice and Jaccard coefficients are proposed. The principles are illustrated on data from a phylogenetic study
in lettuce. 相似文献
14.
D. A. Elston M. J. Brewer B. Martay A. Johnston P. A. Henrys J. R. Bell R. Harrington D. Monteith T. M. Brereton K. L. Boughey J. W. Pearce-Higgins 《Journal of Agricultural, Biological & Environmental Statistics》2017,22(4):427-445
Weather has often been associated with fluctuations in population sizes of species; however, it can be difficult to estimate the effects satisfactorily because population size is naturally measured by annual abundance indices whilst weather varies on much shorter timescales. We describe a novel method for estimating the effects of a temporal sequence of a weather variable (such as mean temperatures from successive months) on annual species abundance indices. The model we use has a separate regression coefficient for each covariate in the temporal sequence, and over-fitting is avoided by constraining the regression coefficients to lie on a curve defined by a small number of parameters. The constrained curve is the product of a periodic function, reflecting assumptions that associations with weather will vary smoothly throughout the year and tend to be repetitive across years, and an exponentially decaying term, reflecting an assumption that the weather from the most recent year will tend to have the greatest effect on the current population and that the effect of weather in previous years tends to diminish as the time lag increases. We have used this approach to model 501 species abundance indices from Great Britain and present detailed results for two contrasting species alongside an overall impression of the results across all species. We believe this approach provides an important advance to the challenge of robustly modelling relationships between weather and species population size.Supplementary materials accompanying this paper appear online. 相似文献
15.
Heritability quantifies the extent to which a physical characteristic is passed from one generation to the next. From a statistical perspective, heritability is the proportion of the phenotypic variance attributable to (additive) genetic effects and is equal to a function of variance components in linear mixed models. Relying on normal distribution assumptions, one can compute exact confidence intervals for heritability using a pivotal quantity procedure. Alternatively, large-sample properties of the restricted maximum likelihood (REML) estimator can be used to construct asymptotic confidence intervals for heritability. Exact and asymptotic intervals are compared loineye muscle area measurements and balanced one-way random effects models having groups of correlated responses. In some cases the two interval methods yield vastly different results and the REML-based confidence interval does not maintain the nomiral coverate value even for seemingly large sample sizes. For finite sample size applications, the validity of the REML-based procedure depends on the correlation structure of the data. 相似文献
16.
Aude Migeon Pierre Richaud Frédéric Guinet Michel Chalot Damien Blaudez 《Water, air, and soil pollution》2009,204(1-4):89-101
Metal accumulation was investigated in a range of woody species that were planted on Cd-, Zn- and Pb- polluted sites in North of France. The study is unique in that we directly compare a large number of woody species (25). The highest accumulation of Zn and Cd was found in the Salicaceae family members with up to 950 mg Zn kg?1 dry weight (DW) and 44 mg Cd kg?1 DW in leaves of Populus tremula × Populus tremuloides. Zn content was positively correlated with Cd content, both in leaves and stems. Pb concentration was generally low and was species-independent. Oak and birch species accumulated more Mn as compared to other woody species. A seasonal variation in metal accumulation could be found. Although soil compositions and metal bioavailabilities differed amongst the experimental sites chosen in this study, variation of metal concentrations within a given species was small. High bioconcentration factors for poplar and willow suggested the high potential of these species over other woody species for metal accumulation. Taken together, these data suggest that poplar and willow species are good candidates for phytoremediation programmes. 相似文献
17.
We explore two methods that correct for differential sampling effort when estimating the true number of species in a region based on samples such as those typically recorded in museum or conservation databases. The two methods are: (1) a phenomenological model that relies on a saturating sampling curve; and (2) a model based on a lognormal distribution of species abundances. We test these methods using a database for the butterflies of Oregon and find that the distribution of high-diversity areas, using the estimated, or “asymptotic”, diversities, is strikingly different from the geographic pattern one would deduce if the raw data were used, without correcting for differential sampling effort. Further, we show that differences in accuracy exist between the two estimation procedures, and that these differences are aggravated at small sample sizes; we argue that estimates based on the lognormal distribution should be preferred because they can offer substantial improvement over analyses based solely on the raw data, generally without risking overestimation. Lastly, using both the database and estimated values of butterfly diversity, we show that the distribution of endangered and numerically rare butterflies rarely coincides with “hotspots” or centers of biodiversity. Thus, protecting regions of Oregon rich in overall butterfly diversity will not normally protect the bulk of rare or endangered butterfly species. 相似文献
18.
Walter W. Piegorsch Kelly A. Richwine 《Journal of Agricultural, Biological & Environmental Statistics》2001,6(3):305-325
In studies examining patterns of mutational damage, the primary variables of interest are expressed typically as discrete counts within defined categories of damage. The multinomial distribution is a common model for such data settings. Of interest is statistical comparison of the pattern, or spectrum, of mutation among the various mutant categories. A specific question in such instances is the identification of differences among the spectral categories, identified, e.g., via simultaneous confidence intervals for pairwise differences in or ratios of the multinomial response probabilities. Herein, a selection of possible methods for constructing such intervals is described and compared via computer simulations to determine which are most appropriate for practical use in the evaluation of mutant spectra. 相似文献
19.
M. A. A. COX 《European Journal of Soil Science》1983,34(4):649-658
The conventional method used to investigate the pore structure of a core of wet sand is to construct a moisture characteristic curve. The plot summarizes the dimensionless volume obtained from the sample at a given suction pressure. We assume here that pore diameters are selected from an arbitrary normal distribution. The problem of estimating the parameters of this distribution reduces to the solution of a quadratic equation and substitution of the resulting value into a polynomial. There is no necessity to plot the characteristic curve, the only values being needed are the first two moments of the raw distribution. 相似文献
20.
The quantification of abundance and species richness of soil microarthropods is most often severely hindered by extraordinary data variability, highly skewed frequency distributions, many extreme and zero counts, and small sample sizes. We developed a composite sampling technique to enhance the confidence of abundance and species richness estimates. Many soil cores (n ≥ 100) are sampled, animals extracted, the extracts pooled, mixed, and subsamples (aliquots) taken. Compared to the standard (separate sampling units), no microarthropods were lost or mechanically damaged during the compositing procedure. The confidence of abundance estimates was substantially greater in the composite than in the standard, although not for taxa of low abundance (< ≈ 103 ind.m−2). Moreover, compositing was the superior technique in estimating species richness. The number of sampling units needed to recover a certain number of species with the composite was 70% of the standard method. We conclude that composite sampling is a promising alternative to the standard technique and may help to increase the generally low confidence of microarthropod field data. Finally, potential limitations of composite plans are discussed: a great number of field cores from an unbiased sampling plan have to be composited; comparisons between composites of unequal size should be avoided; all information on the variation among field cores is lost by compositing; parallel measurements of fauna and other variables in the same cores are not possible. 相似文献