The software described here is designed to help you construct a (simple or complex) model of the trophic flows in an ecosystem. Aquatic ecosystems will be emphasized because the approach presented here was initially applied to marine and freshwater ecosystems, but it can also be applied to terrestrial ecosystems, such as e.g. farming systems (Dalsgaard et al 1995).
The word "model" has several meanings; for scientists, and more specifically for biologists working at the ecosystem level, "models" may be defined as consistent descriptions, emphasizing certain aspects of the system investigated, as required to understand their function.
Thus, models may consist of a text ("word models") or a graph showing the interrelationships of various components of a system. Models may also consist of equations, whose parameters describe "states" (the elements included in the models) and "rates" (of growth, mortality, food consumption, etc.), of the elements of the model. The behavior of mathematical models is difficult (often impossible) to explore without computers. This is especially the case for "simulation models", i.e., those representations of ecosystems which follow, through time, the interactive behavior of the (major) components of an ecosystem.
Traditional simulation models are difficult to build, and even more difficult to get to realistically simulate, without "crashing", the behavior of a system over a long period of time. This is one reason why many biologists shy away from constructing such models, or even interacting with "modelers" (who, often being non-biologists, may have scant knowledge of the intricate interactions between living organisms). However, "modeling" does not necessarily imply "simulation modeling". There are various ways of constructing quantitative models of ecosystems which avoid the intricacies of traditional simulation modeling, yet still give most of the benefits of fully-fledged modeling, viz.:
To avail of these and other related advantages without having to get involved in traditional simulation modeling, one's models can be limited to describing the situation prevailing during a certain “average” period, during which mass balance is assumed.
This limitation is not as constraining as it may appear at first sight. It is consistent with the work of most aquatic biologists, whose state and rate estimates represent “averages”, applying to a certain period (although this generally is not stated). The approach proposed here is thus to use state and rate estimates for single species in a multispecies context, to describe aquatic ecosystems in rigorous, quantitative terms, during the (arbitrary) period to which their state and rate estimates apply.
In many cases, the period considered will be a typical season, or a typical year, but the state and rate estimates used for model construction may pertain to different years. Models may represent a decade or more, during which little changes have occurred.
When ecosystems have undergone massive changes, two or more models may be needed, representing the ecosystem before, (during), and after the changes. This can be illustrated by an array of models of the Peruvian upwelling ecosystem representing periods before and after the collapse of the anchoveta fishing there (Jarre et al. 1991a). Several other examples for this may be found in Christensen and Pauly (1993).
Where there are seasonal changes to be emphasized, different models may be constructed for each season, or for extreme situations (“summer” vs. “winter”). As an example Baird and Ulanowicz (1989) constructed four trophic models describing the seasons in Chesapeake Bay, and an “average” model to represent the whole year. The same idea can be applied to aquaculture situations, where a pond and its producers and consumers can be described for instance at the beginning, midpoint, and end of a growing season. Examples of this can be found in Christensen and Pauly (1993).
Judicious identification of periods long enough for sufficient data to be available, but short enough for massive changes of biomass not to have occurred, will thus solve most problems associated with the lack of an explicit time dimension. Moreover, when a build-up of biomass is known to have occurred, this can be considered explicitly as “accumulated biomass”, a component of biological production.
The Ecopath system is built on an approach presented by Polovina (1984a) for the estimation of the biomass and food consumption of the various elements (species or groups of species) of an aquatic ecosystem, and subsequently combined with various approaches from theoretical ecology, notably those proposed by R.E. Ulanowicz (1986), for the analysis of flows between the elements of ecosystems.
Once a model of the type discussed here has been built it can be used directly for simulation modeling thanks to a model, EcoSIM, developed by Carl Walters, Fisheries Centre, University of British Columbia. EcoSIM is presently being integrated with Ecopath, and is a simple simulation model intended for studies of the impact of different fishing regimes on the biological components of ecosystems. See EcoSIM.
Descriptions of the Ecopath software has been published (Christensen and Pauly 1992, 1995) - reprints are available from the authors. In addition a description in French of the Ecopath model can be requested from ICLARM. English, French and Spanish versions of the manual to Version 2.+ has been published, while a draft Portuguese version of this manual is available but not published.
At present a printed manual for the Windows version of Ecopath is under publication. Until appearing we hope that the present help system can alleviate the lack of a printed manual. Please contact us for further information (v.christensen@cgnet.com).
Send mail to v.christensen@cgnet.com with questions or comments