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Ecosystem models describe the structure and function of an ecosystem. Ecosystem structure is the set of components, functional groups in this case, that exist and interact in the ecosystem and the web of links between them. The ecosystem currency used in Ecopath models is biomass, the weight of living matter, and ecosystem function describes the movement of biomass from one component to another.

Ecosystem models for fisheries management purposes have ranged in complexity from enhanced single-species models to fully articulated food web models including physiological, spatial and environmental processes. At the simple end of the spectrum are several models which include marine mammal predation in fish stock assessments. The other extreme, a fully realized ecosystem simulation, has rarely been satisfactorily completed. The ecopath modelling approach lies in the middle.

The amount and variety of data required to construct a fully detailed ecosystem model are virtually never available. One of the first steps is to simplify the ecosystem model by either excluding certain aspects or ‘pooling’ together ecologically related groups. The ecopath approach does both of these. First, it only considers the trophic, i.e. feeding, interactions between groups within the model. Secondly, it defines ‘functional’ groups in the model. This may be a single species, a group of species or in some cases it may only be a certain life-history stage of a species or group of species. The key aspect of a functional group is that all the members can be treated as being trophically similar; they are at the same trophic level; they all eat the same prey groups and are eaten by the same predators.

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