Houitte_de_la_Chesnais_whole_thesis.pdf (2.59 MB)
Role of cephalopods in the structure and functioning of marine ecosystems
thesisposted on 2023-05-28, 09:39 authored by Houitte de La Chesnais, T
An increase in cephalopod abundance worldwide has been observed over recent decades, possibly a function of cephalopods' response to the effects of climate change and the reduction in predation by human exploitation on predators of marine systems. Additionally, there has been a renewal of fisheries interest due to the development of markets for cephalopod products and the reduction of finfish stocks. Assessing their role in food webs is thus essential for understanding and anticipating upcoming changes in ecosystem functioning and services. From this perspective, ecosystem models can be used as powerful tools to understand the impact of a group of species on the system, and to anticipate future developments and foresee the future dynamics. However, these models generally fail to represent cephalopods ecology adequately. This thesis thus aimed to tackle the issue of representing cephalopods ecology in ecosystem models and to use these new methods to assess their role in ecosystems and the potential changing of this role under climate change: Identifying the limits of ecosystem models when it comes to representing cephalopods ecology; Tackling the issue by improving the existing features of the Atlantis modelling framework and developing additional tools to represent the particularity of cephalopods ecology; Testing the sensitivity of the resulting model to various ecological processes and cephalopod life-history; Subjecting the model to an environmental change scenario and analysing the implications for cephalopods and other groups. A preliminary investigation into the representation of cephalopod ecology in ecosystem models around the world asserts the limitations of our understanding of their impact on the system. I find that despite the frequent lack of data on cephalopod ecology, structural improvements to models could already bring valuable additional insights on their role in the ecosystem. I then focused on an ecosystem model of the South East of Australia region to investigate the role of cephalopods in the food web. New features were added to the Atlantis modelling framework and many mechanisms disregarded until now were used to adequately represent the life-history traits of cephalopods; their exponential growth, their opportunistic and voracious feeding, their predation on relatively large prey; their ontogenic ecological shifts. The model was then subjected to a series of sensitivity tests to cephalopod parameters to identify the key ecological processes of this group and their impact on the structure and functioning of marine food web. This exercise revealed the potential of ecosystem models to adequately simulate cephalopod life-history traits, and pointed to the key importance of the growth capacity of squid species to drive their role in the ecosystem. The ecosystem model is also used to assess the impact of climate change on cephalopod ecology and on ecosystem functioning. An increase in temperature over 50 years, following one of the IPCC scenarios, is forced on the model. Results indicate that upcoming changes will benefit squid species as they take advantage of food-web changes better than many forage fishes that are both their competitors and their prey. The abundance of arrow squid, an oceanic squid, increases by about 15%, and the abundance of calamari, a neritic squid, increases by about 9% by the end of the climate change scenario. This leads to a substantial ecosystem change that could impact many species of interest for fisheries, and should therefore be taken in to consideration for the future of marine exploitation.
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