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Empirical Modelling (empirical + modelling)
Selected AbstractsGastropods on Submersed Macrophytes in Yangtze Lakes: Community Characteristics and Empirical ModellingINTERNATIONAL REVIEW OF HYDROBIOLOGY, Issue 6 2006Hai-Jun Wang Abstract Epiphytic gastropods in Yangtze lakes have suffered from large-scale declines of submersed macrophytes during past decades. To better understand what controls gastropod community, monthly investigations were carried out in four Yangtze lakes during December, 2001,March, 2003. Composed of 23 species belonging to Pulmonata and Prosobranchia, the community is characterized by the constitution of small individuals. The average density and biomass were 417 ± 160 ind/m2 and 18.05 ± 7.43 g/m2, with maxima around August. Submersed macrophyte biomass is shown to be the key factor affecting species number, density, and biomass of gastropods. Accordingly, a series of annual and seasonal models yielding high predictive powers were generated. Preference analyses demonstrated that pulmonates and prosobranchs with different respiratory organs prefer different macrophyte functional groups. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Predicting the impacts of an introduced species from its invasion history: an empirical approach applied to zebra mussel invasionsFRESHWATER BIOLOGY, Issue 6 2003Anthony Ricciardi SUMMARY 1.,Quantitative models of impact are lacking for the vast majority of known invasive species, particularly in aquatic ecosystems. Consequently, managers lack predictive tools to help them prioritise invasion threats and decide where they can most effectively allocate limited resources. Predictive tools would also enhance the accuracy of water quality assessments, so that impacts caused by an invader are not erroneously attributed to other anthropogenic stressors. 2.,The invasion history of a species is a valuable guide for predicting the consequences of its introduction into a new environment. Regression analysis of data from multiple invaded sites can generate empirical models of impact, as is shown here for the zebra mussel Dreissena polymorpha. Dreissena's impacts on benthic invertebrate abundance and diversity follow predictable patterns that are robust across a range of habitat types and geographic regions. Similar empirical models could be developed for other invaders with a documented invasion history. 3.,Because an invader's impact is correlated with its abundance, a surrogate model may be generated (when impact data are unavailable) by relating the invader's abundance to environmental variables. Such a model could help anticipate which habitats will be most affected by invasion. Lack of precision should not be a deterrent to developing predictive models where none exist. Crude predictions can be refined as additional data become available. Empirical modelling is a highly informative and inexpensive, but underused, approach in the management of aquatic invasive species. [source] Empirical modelling of the DEM/USD and DEM/JPY foreign exchange rate: Structural shifts in GARCH-models and their implicationsAPPLIED STOCHASTIC MODELS IN BUSINESS AND INDUSTRY, Issue 1 2002Helmut Herwartz Abstract We analyse daily changes of two log foreign exchange (FX) rates involving the Deutsche Mark (DEM) for the period 1975,1998, namely FX-rates measured against the US dollar (USD) and the Japanese yen (JPY). To account for volatility clustering we fit a GARCH(1,1)-model with leptokurtic innovations. Its parameters are not stable over the sample period and two separate variance regimes are selected for both exchange rate series. The identified points of structural change are close to a change of the monetary policies in the US and Japan, the latter of which is followed by a long period of decreasing asset prices. Having identified subperiods of homogeneous volatility dynamics we concentrate on stylized facts to distinguish these volatility regimes. The bottom level of estimated volatility turns out be considerably higher during the second part of the sample period for both exchange rates. A similar result holds for the average level of volatility and for implied volatility of heavily traded at the money options. Copyright © 2002 John Wiley & Sons, Ltd. [source] Testing for Neglected Nonlinearity in Cointegrating Relationships,JOURNAL OF TIME SERIES ANALYSIS, Issue 6 2007Andrew P. Blake C32; C45 Abstract., This article proposes pure significance tests for the absence of nonlinearity in cointegrating relationships. No assumption of the functional form of the nonlinearity is made. It is envisaged that the application of such tests could form the first step towards specifying a nonlinear cointegrating relationship for empirical modelling. The asymptotic and small sample properties of our tests are investigated, where special attention is paid to the role of nuisance parameters and a potential resolution using the bootstrap. [source] Simulation of the mean urban heat island using 2D surface parameters: empirical modelling, verification and extensionMETEOROLOGICAL APPLICATIONS, Issue 3 2009Bernadett Balázs Abstract The spatial distribution of the annual mean urban heat island (UHI) intensity was simulated applying empirical models based on datasets from urban areas of Szeged and Debrecen, using simple and easily determinable urban surface cover variables. These two cities are situated on the Alföld (Great Hungarian Plain) and have similar topographic and climatic conditions. Temperature field measurements were carried out, Landsat satellite images were evaluated, and then one- and multiple variable models were constructed using linear regression techniques. The selected multiple-parameter models were verified using independent datasets from three urban settlements. In order to obtain some impression of the mean UHI patterns in other cities with no temperature measurements available, the better model was extended to urban areas of four other cities situated in geographical environments similar to Szeged and Debrecen. The main shortcoming of typical empirical models, namely that they are often restricted to a specific location, is overcome by the obtained model since it is not entirely site but more region specific, and valid in a large and densely populated area with several settlements. Copyright © 2009 Royal Meteorological Society [source] The Delphi process , an expert-based approach to ecological modelling in data-poor environmentsANIMAL CONSERVATION, Issue 1 2006D. C. MacMillan Abstract Resource managers are involved in difficult decisions that affect rare species and habitats but often lack relevant ecological knowledge and experience. Ecological models are increasingly being looked to as a means of assisting the decision-making process, but very often the data are missing or are unsuited to empirical modelling. This paper describes the development and application of the Delphi approach to develop a decision support tool for wildlife conservation and management. The Delphi process is an expert-based approach to decision support that can be used as a means for predicting outcomes in situations where ,absolute' or ,objective' models are unavailable or compromised by lack of appropriate data. The method aims to develop consensus between experts over several rounds of deliberation on the assumption that combining the expertise of several individuals will provide more reliable results than consulting one or two individuals. In this paper the approach is used to engineer soft knowledge on the conservation requirements of capercaillie Tetrao urogallus, an endangered woodland grouse, into a model that can be used by forests managers to improve the quality of forest habitat for capercaillie over extensive commercial forest areas. This paper concludes with a discussion of the potential advantages and disadvantages of Delphi and other soft knowledge approaches to ecological modelling and conservation management. [source] Ammonia reduction in seawater by Yucca schidigera extract: efficacy analysis and empirical modellingAQUACULTURE RESEARCH, Issue 8 2010Roberto A Santacruz-Reyes Abstract Yucca schidigera is a plant native to southwestern USA and Mexico. Its extract has been used in the livestock industry to control ammonia accumulation in animal holding facilities, and to reduce ammonia concentration in animal excreta. This study investigated the potential and effectiveness of Y. schidigera extract (YUPE) for ammonia reduction in seawater. A dose,response experiment was conducted to determine the effect of different concentrations of YUPE at 0, 18, 36, 72 and 108 mg L,1 on total ammonia nitrogen (TAN) at 1, 3 and 9 mg L,1. At a higher YUPE dosage rate, higher TAN reduction was observed, and TAN reduction was highest during the first 12 h, and decreased thereafter. A stepwise multiple linear regression that included the initial TAN, YUPE concentration and time was developed, which accurately predicted empirical TAN concentrations. Applications of this model for ammonia management strategies were formulated for hypothetical tiger prawn (Penaeus monodon) rearing conditions. YUPE's efficacy for ammonia reduction, natural origin and safety make YUPE a potentially suitable compound for water quality management in mariculture. [source] Effective global regularity and empirical modelling of direct, inverse, and mixed demand systemsCANADIAN JOURNAL OF ECONOMICS, Issue 2 2009Keith R. McLaren Abstract This paper utilizes the notion of ,effective global regularity' and the intuition stemming from Cooper and McLaren (1996)'s General Exponential Form to develop a family of ,composite' (product and ratio) direct, inverse and mixed demand systems. Apart from having larger regularity regions, the resulting specifications are also of potentially arbitrary rank, which can better approximate non-linear Engel curves. We also make extensive use of duality theory and a numerical inversion estimation method to rectify the endogeneity problem encountered in the estimation of the mixed demand systems. We illustrate the techniques by estimating different types of demand systems for Japanese quarterly meat and fish consumption. Ce mémoire utilise la notion de régularité effective globale et l'intuition émergeant de la Forme Exponentielle Générale de Cooper et McLaren (1996) pour développer une famille de systèmes de demandes (directes, inverses et mixtes) composites (produit et ratio). En plus d'avoir de plus vastes régions de régularité, les spécifications qui en résultent sont aussi potentiellement de rang arbitraire, ce qui permet une meilleure approximation des courbes d'Engel non linéaires. On fait aussi un usage extensif de la théorie de la dualité, et d'une méthode numérique d'estimation de l'inversion pour corriger le problème d'endogénéité qu'on rencontre dans l'estimation des systèmes de demandes mixtes. On illustre les techniques en estimant différents types de systèmes de demandes trimestrielles de viande et de poisson par les consommateurs au Japon. [source] | ||||||||||||||||