Environmental Engineering Reference
In-Depth Information
Chapter 5
Steady State Models of Ecological Systems:
EcoPath Approach to Mass-Balanced System
Descriptions
Matthias Wolff and Marc Taylor
Abstract We describe the fundamentals and applications of trophic models of
ecological systems and show how a simple mass balance approach of the early
1980s was further developed into a very advanced complex software package,
freely available on the internet (Ecopath with Ecosim, EwE,
http://www.ecopath.
org
). Through its three decades of evolution, the approach became increasingly
popular, with over three hundred Ecopath models being published to date. During
its first 10-15 years, the approach was mainly used as a tool to integrate ecological
and fisheries data to understand and visualize the trophic flow structure of ecosys-
tems, thereby allowing for the meaningful comparisons between systems. Later
(since the mid-1990s) it was increasingly used to explore ecosystem changes under
the impact of management or climate impact scenarios. This evolution from a more
descriptive mass-balance to a simulation modelling tool was enabled through
fundamental changes in the mathematical architecture: the original version of
Ecopath was based on linear algebra for input-output analysis to investigate the
properties of steady state networks, while in the recent version, coupled differential
equations are used for each of the defined system compartments. The new model
architecture thus allows the magnitude of flows in and out of compartments to
change over time, which makes simulations of changes possible. Foraging arena
theory was also taken into account and prey biomass was allowed to vary in its
availability (vulnerability) to predators. If the predators consumption is mainly
determined by prey availability (bottom-up control, low vulnerability), predator
biomass would greatly respond to changes in prey biomass, while under a situation
of strong predator (top-down) control, changes in predator biomass would greatly
impact its prey (high vulnerability). Since EwE also allows for specifying different
resource use types (e.g. types of fisheries or other resource uses) and economic
variables associated to them (operational costs, number of people employed etc.),
different management regimes can also be explored in terms of their socio-economic
