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  • EwEugAnOverviewOfEcosim

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     1== 3.1 An overview of Ecosim == 
     3Ecosim provides a dynamic simulation capability at the ecosystem level, with key initial parameters inherited from the base Ecopath model. See [[Using Ecosim. Part 1 Ecosim inputs.htm|Ecosim inputs]] and [[Using Ecosim. Part 2 Ecosim outputs.htm|Ecosim outputs]] for links to instructions for using Ecosim. 
     5The key computational aspects are in summary form: 
     7  * Use of mass-balance results (from Ecopath) for parameter estimation; 
     9  * Variable speed splitting enables efficient modelling of the dynamics of both ‘fast’ (phytoplankton) and ‘slow’ groups (whales); 
     11  * Effects of micro-scale behaviours on macro-scale rates: top-down vs. bottom-up control incorporated explicitly. 
     13  * Includes biomass and size structure dynamics for key ecosystem groups, using a mix of differential and difference equations. As part of this EwE incorporates: 
     15  * Multi-stanza life stage structure by monthly cohorts, density- and risk-dependent growth; 
     17  * Stock-recruitment relationship as ‘emergent’ property of competition/predation interactions of juveniles. 
     19Ecosim uses a system of differential equations that express biomass flux rates among pools as a function of time varying biomass and harvest rates, (for equations see Walters et al., 1997; Walters et al., 2000; Christensen and Walters, 2004). Predator prey interactions are moderated by prey behaviour to limit exposure to predation, such that biomass flux patterns can show either bottom-up or top down (trophic cascade) control (Walters et al., 2000. By doing repeated simulations Ecosim allows for the fitting of predicted biomasses to time series data.