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The effects of model structure and complexity on the behaviour and performance of marine ecosystem models

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Fulton, Elizabeth Ann (2001) The effects of model structure and complexity on the behaviour and performance of marine ecosystem models. PhD thesis, University of Tasmania.

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Abstract

Despite increasing use of ecosystem models, the effects of model structure and
formulation detail on the performance of these models is largely unknown. Two
biogeochemical marine ecosystem models were constructed as the foundation of a study
considering many aspects of model simplification. The models use a trophic web that is
resolved to the level of functional groups (feeding guilds), and includes the main pelagic
and benthic guilds from primary producers to high-level predators. Both models are
process based, but the Integrated Generic Bay Ecosystem Model (IGBEM) is highly
physiologically detailed, while Bay Model 2 (BM2) uses simpler general assimilation
equations. Both models compare well with real systems under a wide range of
eutrophication and fishing schemes. They also conform to general ecological
checkpoints and produce spatial zonation and temporal cycles characteristic of natural
systems. The performance of IGBEM is not consistently better than that of BM2,
indicating that high levels of physiological detail are not always required when
modelling system dynamics. This was reinforced by a section of the study that fitted
BM2, IGBEM and an existing ecosystem model (ECOSIM) to Port Phillip Bay. The
predictions of all three models lead to the same general conclusions across a range of
fishing management strategies and scenarios for environmental change. Models that are less resolved or use simpler formulations have lower
computational demands and can be easier to parameterise and interpret. However,
simplification may produce models incapable of reproducing important system
dynamics. To address these issues simplified versions of BM2 and IGBEM were
compared to the full models to consider the effects of trophic complexity, spatial
resolution, sampling frequency and the form of the grazing and mortality terms used in
the models on the performance of BM2 and IGBEM. It was clear in each case that some
degree of simplification is acceptable, but that using models with very little resolution or very simplistic linear grazing and mortality terms is misleading, especially when
ecosystem conditions change substantially. The research indicates that for many facets
of model structure there is a non-linear (humped) relationship between model detail and
performance, and that there are some guiding principles to consider during model
development. Developmental recommendations include using a sampling frequency of 2
—4 weeks; including enough spatial resolution to capture the major physical
characteristics of the ecosystem being modelled; using quadratic mortality terms to
close the top trophic levels explicitly represented in the modelled web; aggregating
species to the level of functional groups when constructing the model's trophic web, but
if further simplification of the web is necessary then omission of the least important
groups is preferable to further aggregation of groups; giving careful consideration to the
grazing terms used, as the more complex lolling type responses may be sufficient; and
if an important process or linkage is not explicitly represented in the model, or is poorly
known, then a robust empirical representation of it should be included.
The work presented here also has implications for wider ecological topics (e.g.
the stability-diversity debate) and management issues. Consideration of the effects of
trophic complexity on model performance under a range of environmental conditions
supports the ecological "insurance hypothesis", but not the existence of a simple
relationship between diversity and stability. The biological interactions captured in the
web are a crucial determinant of ecosystem and model behaviour, but simple aggregate
measures such as diversity, interaction strength and connectance are not. Similarly, the
work on the effects of spatial resolution on model performance indicates that spatial
heterogeneity is a crucial system characteristic that contributes to many of the emergent
properties of the system.
The comparison of the full models with each other, and with ECOSIM, leads to
five general conclusions. First, shallow enclosed marine ecosystems react more strongly to eutrophication than to fishing. Second, a selected set of indicator groups can signal
and characterise the major ecosystem impacts of alternative management scenarios and
large-scale changes in environmental conditions. Third, policies focusing on the
protection of a small sub-set of groups (especially if they are concentrated at the higher
trophic levels) can fail to achieve sensible ecosystem objectives and may push systems
into states that are far from pristine. Fourth, multispecies and ecosystem models can
identify potential impacts of anthropogenic activities and environmental change that a
series of single species models cannot. Finally, and most importantly, the use of a single
"ultimate" ecosystem model is ill advised, but the comparative and confirmatory use of
multiple "minimum-realistic" models is very beneficial.

Item Type: Thesis (PhD)
Keywords: Marine ecology
Copyright Holders: The Author
Copyright Information:

Copyright 2001 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2002. Includes bibliographical references

Date Deposited: 09 Dec 2014 00:07
Last Modified: 11 Mar 2016 05:54
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