Towards end-to-end models for investigating the effects of climate and fishing in marine ecosystems
Introduction
Various models have been used to investigate the effects of climate and fishing on marine ecosystems. However, effective management of living marine resources still requires more reliable predictions of the combined effects of climate and fishing, especially in the context of the adoption of an ecosystem approach to fisheries and the ongoing concerns about the long-term effects of climate change. Climate and fishing affect different components and attributes of the food web in different ways, for example, light on phytoplankton growth, temperature on zooplankton spatial distribution (Beaugrand et al., 2002), currents on recruitment success of small pelagic fish (Mullon et al., 2003), fishing on the size distribution of fish (Shin et al., 2005) and oxygen concentration on tuna distribution (Cayré and Marsac, 1993). Fishing and the environment also have indirect effects on organisms when direct effects propagate through the food web. Therefore, to assess the direct and indirect effects of climate and fishing on ecosystem dynamics, an adequate model should represent the key linkages among ecosystem components from the bottom to the top of the food web.
The recent emphasis on the potential value and development of end-to-end models (IMBER, 2005) has reflected a change in the perspective of fisheries and marine ecologists who are working together more closely than at any time in the last 2–3 decades. End-to-end refers to a specific approach to ecosystem modelling which (1) aims to represent the entire food web and the associated abiotic environment, (2) requires the integration of physical and biological processes at different scales, (3) implements two-way interaction between ecosystem components and (4) accounts for the dynamic forcing effect of climate and human impacts at multiple trophic levels. Developing end-to-end models is proving to be a challenging task. First, the complexity of an end-to-end model can be unmanageable given the number of species or functional groups that need to be represented, their biological variability, and the number and nature of interactions linking these components. Second, the spatio-temporal scales at which processes occur differ by many orders of magnitude, since they are associated with the organisms represented, possibly from viruses to whales. Third, fishing and the environment can affect most organisms in the ecosystem either directly and/or indirectly, increasing the number and complexity of processes to be represented.
The development of end-to-end models would improve our ability to quantify the interplay between climate and fishing effects. As such, the objectives of this paper are to review the main process-based approaches of marine ecosystem modelling, to identify their strengths and weaknesses in the context of end-to-end modelling and to propose further steps to move towards the development of end-to-end models. We focus on existing models with features that might contribute to end-to-end modelling, taking account of the parts of the food web that are represented, how fishing and environment effects are treated, and the effects of model structures on the ways in which these effects might propagate through the modelled food web.
Historically, components of marine ecosystems have been modelled independently and in different ways (Franks, 2002, Latour et al., 2003), reflecting research issues which focused on selected components. For example, biogeochemical models were developed for assessing carbon fluxes in the ocean and for understanding plankton dynamics in variable environments (Franks, 2002, Arhonditsis and Brett, 2004). In contrast, the larger organisms in marine ecosystems were originally studied because of their economic and social importance. As a result, fisheries models usually focused on single species or small groups of species and, with few notable exceptions (e.g., Andersen and Ursin, 1977), other components of the ecosystem and the effects of abiotic factors were not considered. There is a long history of using different approaches to model the dynamics of ecosystem components motivated by scientific understanding, institutional and agency objectives, etc. As a result, the development of end-to-end models needed to explore the interplay between climate and fishing at the ecosystem scale has progressed more slowly (e.g., deYoung et al., 2004). Here, we review the properties of models that focus on different components of the food web and consider the extent to which they can be extended or coupled to support an end-to-end approach, taking account of differences in the discretization methods (level of aggregation of organisms, e.g., population, functional group, trophic level), the representation of forcing factors and the spatial dimension used. Based on this analysis we suggest approaches for developing end-to-end models, based on structures that could easily be adapted for application in a range of ecosystems and that would help to meet the objectives of understanding the effects of climatic variation and change and developing the science needed to support the implementation of an ecosystem approach to fisheries. This review is intended to complement many of the detailed reviews of existing models (e.g., Hollowed et al., 2000, Whipple et al., 2000, Latour et al., 2003) by placing them in an end-to-end context.
Section snippets
Biogeochemical models
Initially, biogeochemical models represented the marine ecosystem in a simple way, by modelling fluxes of nitrogen, considered to be the main limiting nutrient of primary production, between three compartments: nutrients (N), phytoplankton (P) and zooplankton (Z) (Steele, 1974, Evans and Parslow, 1985, Wroblewski et al., 1988, Franks, 2002). Adding a detritus compartment (D) which indirectly represents the bacterial activities of decomposition and mineralization, we obtain the classical
Extent of the represented food web
One of the main differences between the models reviewed in this paper is the extent of the representation of the food web (Fig. 1). Some models focus only on a subset of the food web, either the plankton components (NPZD-type models), part of the fish community (MSVPA, Osmose) or a selection of trophic levels (Hermann et al., 2001, Sourisseau, 2002). Conversely, other models aim to represent the whole food web, or at least a large part, considering large functional groups from the plankton
Conclusion
Existing models have been developed from diverse perspectives (e.g., plankton-oriented or fish-oriented) with different objectives, forcing factors and formalism. None of the existing models in their current state provide the framework necessary to develop an end-to-end approach for assessing the combined impacts of fishing and climate change on the ecosystem. Models of low trophic levels, which represent mainly the plankton community through functional groups are well suited for exploring
Acknowledgements
This is a contribution to EUR-OCEANS Network of Excellence funded by the European Commission (contract FP6-511106). Morgane Travers was supported by a EUR-OCEANS scholarship. We thank John Field, Christian Mullon, Manuel Barange, Xabier Irigoien, Patrizio Mariani and three anonymous reviewers for their helpful comments and discussions on earlier version of this manuscript.
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