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UBC Theses and Dissertations

Complexity, cost and quality of ecosystem models and their impact on resilience : a comparative analysis, with emphasis on marine mammals and the Gulf of St. Lawrence Morissette, Lyne


The development of an ecosystem approach is essential to improve understanding of the nature and dynamics of exploited marine ecosystems and the complexity of species interactions. A database of 393 Ecopath models was compiled to investigate questions pertaining to (1) the uncertainty in model parameters, (2) the relationship between ecosystem complexity and stability, (3) the effect of marine mammals on ecosystem productivity, and (4) the lack of recovery of commercially caught predatory fish species. Reviewing the strengths and weaknesses of Ecopath methods showed that few published models have addressed uncertainty even when the tools were provided with the software. Estimating uncertainty (with confidence intervals for parameters) was improved by combining the Ecopath approach with other modelling techniques. The second analyses, using 50 ecosystem models, showed that quality of input data affects conclusions concerning the relationship between complexity and stability in ecosystems. Findings further support the notion that stability also increases as community complexity (expressed as ascendancy and omnivory) increases. The third analyses used seven ecosystem models to explore whether the presence of marine mammals and their overlap with fisheries for food resources affects overall ecosystem productivity, and whether their extirpation could decrease productivity. Simulation results were equivocal about whether extirpation of marine mammals from ecosystems would benefit fisheries, while mixed trophic impact analyses suggested that beneficial predation does occur in these ecosystems. Finally, a case-study of four Ecopath models from the Gulf of St. Lawrence addressed the collapse in biomass of predatory fish and the potential causes of the lack of recovery of these species. Analyses revealed that the ecosystem structure shifted from one previously dominated by piscivorous groundfish during the mid-1980s to one dominated by small-bodied pelagic species during the mid-1990s. Part of the total mortality of groundfish was accounted for by seal predation when fishing mortality was reduced. These analyses collectively demonstrate the value of investigating ecological questions applicable to marine ecosystems using a multi-model comparative approach.

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