Metabolism, pace of life, and the dynamics of size-structured populations and communities: The case of fast-living squid

Global warming and fisheries exploitation are impacting marine ecosystems. Predicting future changes in marine ecosystems is one of the most urgent challenges in marine science. However, marine ecosystems are complex, and the classic species-based approach has difficulties predicting these global changes because species and species interactions change dramatically among ecosystems. The trait-based approach provides an efficient tool for understanding ecosystems. This theory ignores the membership of individuals to a species but rather assumes that traits, features of individuals, interact together. There are probably as many traits as there are species, and there is no consensus in marine ecology about which individual traits explain most of the variability across ecosystems. To make global predictions of marine ecosystem processes and services such as commercial landings, we must understand what traits drive marine populations and ecosystems and how they affect global ecosystem dynamics. The most abundant populations at high trophic levels (teleost fish, sharks and mammals) are size-structured, with individuals growing from offspring size to adult size. The dynamic of size-structured populations depends on the size range between offspring and adults and how fast individuals grow through this size range. The size strategies, i.e., variation in adult:offspring size, or in growth strategies, are known and well described; However, little is known about their joint effects on population and ecosystem. Yet, global models based on size and traits often do not consider variation in growth rate. In my Ph.D. I investigate how life history traits of growth and size – the adult:offspring size ratio and the somatic growth rate – drive the dynamic and the structure of high-trophic level marine populations and communities.
This PhD thesis is composed of three manuscripts that answer different aspects of the structuring role of size and growth traits. I developed in my first paper a general theory deriving population scaling rules using individual-level metabolism and size traits. I further reviewed how these traits vary among marine groups and showed that population-level predictions of maximum population growth rate change depending on growth and size strategies employed within taxa. Fastgrowing species always have fast population growth, however, small species do not always grow faster than large ones. In my second manuscript, I investigated how fast-living strategies structure ecosystems taking the example of the fast-living squids. I developed an extension of the FEISTY framework (Fisheries Size and Functional Type model) that includes squids. I show that squid biomass strongly depends on pelagic secondary production and that ecosystem structures (trophic interactions and standing stock biomass) change in the presence of squid. In my third manuscript, I used the FEISTY framework to understand the recent global increase in squid biomass. I showed that squid biomass increase is not attributed to a rise in temperature (as previously proposed) but rather due to the loss of top predators. The results also suggested that the recent increase of squids in ecosystems likely caused a decline in upper trophic level biomass and increased metabolic losses in systems.
This thesis addresses several aspects of the impact of size growth strategies in structuring populations and ecosystems using squid as a main result of an individual representation of physiological processes. I first show that strategies regarding growth exist in nature and are an important driver of potential population growth. I further demonstrate that the composition in growth strategies drives a large part of the high-trophic level biomass. This shows that better global prediction could be reached by incorporating several growth and size strategies. Additionally, this thesis gives the first proof-of-concept of the ecological properties of squid based on their life history strategies. Further, I developed a new version of the food-web framework FEISTY, including squid. This new framework could later be used to predict the global biogeography of squid and fish, carbon fluxes, and future changes in squid global landings with global warming.