Environmental Engineering Reference
In-Depth Information
outcome. If spatially explicit data on biomass distribution of model groups and
dispersal rate values are available, the Ecospace module of the EwE package may
be used to derive at a spatially explicit, dynamic trophic model able to simulate how
the management of one part of the system area may affect other parts. The
incorporation of these spatial dynamics is of particular interest in the exploration
of management questions, such as in the size and placement of Marine Protected
Areas (MPAs) (this application is not further described in this chapter).
5.1
Introduction
The steady state trophic modelling approach came into focus in the late 1970s,
when the evidence was increasing that any strong single species fishery (like the
Cod fishery in the North Atlantic) substantially affects other parts of the ecosystem
that are trophically linked with the target resource. People sought to quantify
trophic interactions within aquatic ecosystems and different multispecies
approaches and models evolved. Among the pioneering work was the North Sea
Model of Anderson and Ursin (AU model, 1977), which includes the dynamics of
nutrients, phytoplankton, zooplankton and age-structured Beverton-Holt yield
models for the main fish groups of the system. The full model was based on 308
differential equations and required an enormous amount of biological information
for model formulation. The Multispecies Virtual Population Analysis (MSVPA)
followed shortly in the early 1980s and the AU model also represents a multispecies
extension of traditional single species fisheries stock assessment models. It is based
on values of predation mortality based on catch-at-age data; predator ration and
predator diet information and allows estimating the predation mortalities produced
by predators on prey species and the annual consumption of prey by predators.
While the approaches mentioned, and other related ones, were developed from age
structured population models, the ECOPATH trophic modelling concept, developed
during the early 1980s by Polovina (
1984
), followed a different, more holistic
approach: it uses biomass pools of functional groups, which are connected through a
predator-prey diet matrix. Energy flow through the food web is balanced by equating
the biomass production with internal consumption and exports to the fishery of the
different compartments. This approach allows integrating a large number of species
into ecological functional groups, requires much less biological information and is
thus applicable for data - sparse situations that are often found in tropical waters.
During the first decade of its genesis, ECOPATH was greatly enriched through the
input of Ulanowicz's (
1986
, 1997) ideas of network growth and development of
natural systems and through substantial contributions by Christensen and Pauly
(1992), which were all integrated into the ECOPATH II software. In the mid-1990s
Carl Walters joined the team and developed a simulation software, then called
ECOSIM (Walters et al. 1997). It was based on ECOPATH II, but replaced the simple
linear algebraic equations by coupled differential equations, thus allowing compart-
ment biomasses to change over time as a response to changes in mortality or
