Testing ecological models: the meaning of validation

The ecological literature reveals considerable confusion about the meaning of validation in the context of simulation models. The confusion arises as much from semantic and philosophical considerations as from the selection of validation procedures. Validation is not a procedure for testing scientific theory or for certifying the ‘truth’ of current scientific understanding, nor is it a required activity of every modelling project. Validation means that a model is acceptable for its intended use because it meets specified performance requirements.Before validation is undertaken, (1) the purpose of the model, (2) the performance criteria, and (3) the model context must be specified. The validation process can be decomposed into several components: (1) operation, (2) theory, and (3) data. Important concepts needed to understand the model evaluation process are verification, calibration, validation, credibility, and qualification. These terms are defined in a limited technical sense applicable to the evaluation of simulation models, and not as general philosophical concepts. Different tests and standards are applied to the operational, theoretical, and data components. The operational and data components can be validated; the theoretical component cannot.The most common problem with ecological and environmental models is failure to state what the validation criteria are. Criteria must be explicitly stated because there are no universal standards for selecting what test procedures or criteria to use for validation. A test based on comparison of simulated versus observed data is generally included whenever possible. Because the objective and subjective components of validation are not mutually exclusive, disagreements over the meaning of validation can only be resolved by establishing a convention.     

Aerosol composition as a function of haze and humidity levels in the Southwestern U.S.

The observed composition of visibility-reducing aerosols in the Grand Canyon region is summarized in climatological terms. Observations are from SCENES, a measurement program extending from 1984 to 1989. Results are presented as average mass balances stratified by various factors.Aerosols were found to exhibit substantial seasonal variation, but little systematic diurnal variation. Crustal material was a dynamic component, and peaked during springtime. Aerosol composition, but not total concentration, depended strongly on ambient relative humidity, with crustal material augmented at low humidities and sulfates augmented at high humidities. Total fine-particle concentrations correlated strongly with light scattering, as expected; however, little association between chemical composition and light scattering was observed.     

Role stress,physical symptomatology,and turnover intentions: A causal analysis of three alternative specifications

Causal relationships involving role stressors, physical symptomatology, and turnover intentions were examined within three alternative specifications. These specifications stemmed from Beehr and Newman's (1978) and Schuler's (1982) models of role stress and Locke's (1976) theory of job satisfaction. The results, derived from LISREL VI analyses, indicate that a revised Schuler model is the most plausible specification. Findings are discussed in terms of implications for understanding role stress and employee withdrawal.     

Use of Ordination Techniques to Follow Community Succession from Oil Impact to Recovery in the Field

Much effort has been directed toward elucidating pollution effects on marine benthic communities (Pearson and Rosenberg, 1978; Sanders et al., 1980; NAS 1985). Less effort has been directed at methods of data analysis which will identify distribution patterns and relationships between communities. Traditional community structure summary parameters such as species richness and various indices of diversity utilize only part of the information contained in a data set and are not very useful in elucidating relationships between communities of animals.     

Songbird diversity along an urban powerline right-of-way in Rhode Island

The potential of powerline rights-of-way to provide songbird habitat in an urbanizing environment was assessed over a 7-month.period. Comparisons of species richness and total number of birds observed were made with a nearby residential development. The right-of-way, for 7 years free from herbicide manipulation, exhibited a greater number of species (but fewer individuals) and higher bird species diversity (BSD) than the residential area. The importance of mixed-shrub communities for the provision of niches for songbirds was demonstrated by increased BSD in residential vacant lots and sections of the right-of-way containing a variety of vegetative profiles. Recommendations are presented on right-of-way vegetation management to enhance songbird utilization.     

Relating environmental modeling to policy issues and decisions

This paper describes a linear program-based computer model which has been developed to identify the most effective spending patterns for environmental cleanup based on public preferences and spending limitations. The inputs to the model include the costs of removing each residual studied from its various sources. The objective function is the total “value of removal”. The factor added for each residual consists of a value for the residual multiplied by the quantity or fraction removed.Results of the analysis relate to and support current and projected policy decisions by the U.S. Environmental Protection Agency (EPA) for air and water programs. In addition, the results of the analysis suggest that EPA undertake a redirection of the Nation's resources among residual control programs.     

Neue Reaktor-Technologien

It is doubtful that large-scale industrial utilization of the fast breeding reactor will ever be possible. It is, however, probable that the thorium reactor, for which three plans already exist, will be realized. There is enough thorium to supply thorium reactors up to the next glacial period.