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101.
Spinosad is a reduced-risk insecticide with a novel mode of action that provides an alternative to older classes of insecticides such as organophosphates, carbamates and pyrethroids. A comprehensive ecological risk assessment for spinosad use in US cotton crops is presented within a framework of tiered levels of refinement following the guidelines of the US EPA for ecological risk assessments. Toxicity information for a variety of species is documented and utilized, environmental concentrations estimated, and risk characterizations in the form of risk quotients are quantified. Results indicate that spinosad use in cotton does not exceed the most conservative Tier I levels of concern (LOC) values for groundwater, mammals and birds or acute risk to aquatic organisms. Use of very conservative Tier I screening methods resulted in exceeding LOC values for chronic exposure for some aquatic organisms, thus prompting further refinement. When the exposure prediction was refined using less conservative, Tier II mechanistic environmental fate transport models to predict offsite transport and environmental concentrations, chronic risk was not predicted for these species. Spinosad is acutely toxic to bees under laboratory conditions, but toxicity of residue studies and field studies indicate that under actual use conditions the impact on bees is minimal. 相似文献
102.
对黄瓜组培根分泌物的化感潜势进行可拓评价, 得出黄瓜组培根分泌物对菜豆幼苗生长有显著促进作用, 而对其它3种作物有明显的毒害作用, 毒害程度为黄瓜>甜瓜>番茄。同时, 建立了黄瓜组培根分泌液的化感效应的数学模型RE = [ k1Rp0 / ( k2 - k1 ) ] ( e- k1t - e- k2t ) ( e- k1a - e- k2a ) , 说明了黄瓜组培根分泌液的化感效应与组培根培养时间( t) 以及根分泌液浓度( a) 显著相关。黄瓜根分泌物对4种受体作物显示出显著的化学干扰差异, 论证了植物种内(间) 发生的干扰作用与植物的品种(系) 有关, 从理论上得出菜豆可以作为黄瓜的间套作物或下茬作物。 相似文献
103.
Dividing regions into manageable landscape units presents special problems in landscape ecology and land management. Ideally,
a landscape should be large enough to capture a broad range of vegetation, environmental and disturbance dynamics, but small
enough to be useful for focused management objectives. The purpose of this study was to determine the optimal landscape size
to summarize ecological processes for two large land areas in the southwestern United States. We used a vegetation and disturbance
dynamics model, LANDSUMv4, to simulate a set of nine scenarios involving systematically varied topography, map resolution,
and model parameterizations of fire size and fire frequency. Spatial input data were supplied by the LANDscape FIRE Management
Planning System (LANDFIRE) prototype project, an effort that will provide comprehensive and scientifically credible mid-scale
data to support the National Fire Plan. We analyzed output from 2,000 year simulations to determine the thresholds of landscape
condition based on the variability of burned area and dominant vegetation coverage. Results show that optimal landscape extent
using burned area variability is approximately 100 km2 depending on topography, map resolution, and model parameterization. Variability of dominant vegetation area is generally
higher and the optimal landscape sizes are larger in comparison to those features determined from burned area. Using the LANDFIRE
project as a case study, we determined landscape size and map resolution for a large mapping project, and showed that optimal
landscape size depends upon geographical, ecological, and management context.
This paper was written and prepared by U.S. Government employees on official time, and therefore is in the public domain and
not subject to copyright. The use of trade or firm names in this paper is for reader information and does not imply endorsement
by the U.S. Department of Agriculture of any product or service. 相似文献
104.
Stochastic modeling provides a useful tool in managing biological variation in the postharvest chain. In the current study, the fruit-to-fruit variability in the postharvest firmness of apples was modeled. Apples from three cultivars (‘Jonagold’, ‘Braeburn’, and ‘Kanzi’) were harvested at different levels of maturity, and stored at different temperatures and controlled atmosphere (CA) conditions. By using a kinetic model describing firmness breakdown as a function of time, temperature, controlled atmosphere conditions and endogenous ethylene concentration, the main stochastic variables were identified as the initial firmness and the rate constants for firmness breakdown and ethylene production. Treating these variables as random model parameters, the Monte Carlo method was used to simulate the propagation of the fruit-to-fruit variability in flesh firmness within a batch of apples during storage under different CA conditions and subsequent shelf-life exposure. The model was validated using independent data sets from apples picked in a different season. The model developed in this study can be used to predict the probability of having apples of certain firmness after long term storage for different scenarios of temperatures and CA conditions. 相似文献
105.
106.
介绍了在三维机械CAD系统Solid Edge中,建立常用零件参数化特征库的技术以及参数化特征的使用方法,着重探讨了将驱动尺寸与变量表结合起来提高常用零件建模效率的技术。 相似文献
107.
108.
张锡滨 《林业机械与木工设备》2004,32(11):36-37
提出了一种基于成组技术思想,针对不同类型双颚板散货抓斗工作过程计算机仿真的综合建模方法。用同一模型同时描述三种不同类型的抓斗,通过改变初始输入条件,来反映不同抓斗的工作规律。 相似文献
109.
A mathematical model for generating land-use allocation alternatives for agroforestry systems 总被引:2,自引:0,他引:2
Guillermo A. Mendoza 《Agroforestry Systems》1987,5(4):443-453
Agroforestry has been considered a viable land-use system particularly in the tropical regions. In developing or designing agroforestry systems, various concerns have to be addressed namely: (1) economic; (2) biological and physical; and (3) silvicultural and ecological considerations. This paper addresses these concerns from an interdisciplinary perspective. A land-use allocation model which ensures the simultaneous consideration of these concerns is proposed. 相似文献
110.
Mathieu Fortin Gaétan Daigle Chhun-Huor Ung Jean Bégin Louis Archambault 《European Journal of Forest Research》2007,126(4):573-585
This study proposes a within-subject variance-covariance (VC) structure to take into account repeated measurements and heteroscedasticity
in a context of growth modeling. The VC structure integrates a variance function and a continuous autoregressive covariance
structure. It was tested on a nonlinear growth model parameterized with data from permanent sample plots. Using a stand-level
approach, basal area growth was independently modeled for red spruce (Picea rubens Sarg.) and balsam fir [Abies balsamea (L.) Mill.] in mixed stands. For both species, the implementation of the VC structure significantly improved the maximum
likelihood of the model. In both cases, it efficiently accounted for heteroscedasticity and autocorrelation, since the normalized
residuals no longer exhibited departures from the assumptions of independent error terms with homogeneous variances. Moreover,
compared with traditional nonlinear least squares (NLS) models, models parameterized with this VC structure may generate more
accurate predictions when prior information is available. This case study demonstrates that the implementation of a VC structure
may provide parameter estimates that are consistent with asymptotically unbiased variances in a context of nonlinear growth
modeling using a stand-level approach. Since the variances are no longer biased, the hypothesis tests performed on the estimates
are valid when the number of observations is large. 相似文献