Exploring macroecology of cephalopods in a changing climate through trait-based approaches

Cephalopods are important predators and prey in the global oceans and have a key role in the transfer of energy through marine ecosystems. As a result, marine ecosystem structure and function is affected by the trophic role of cephalopods. Individual body size strongly influences the trophic role of marine organisms and quantifying trophic position-individual body size relationships (trophic allometries) is a powerful approach to facilitate size-structured ecosystem modelling. Despite the importance of cephalopods in marine ecosystems and the documentation of their abundance increasing globally, they are poorly represented in marine ecosystem and food webs models. Trophic allometries are well studied for fishes but remain relatively unexplored for cephalopods, while the functional role of cephalopods in these systems is relatively poorly understood. This is a major gap that may hinder the management of marine ecosystems moving forward in a changing climate. My thesis presents a trait-based approach to first empirically quantify trophic ecology of oceanic cephalopods and to then implement a trait-based framework for cephalopods in size-structured ecosystem models. This allowed me to assess the implications for ecosystem structure and function when cephalopod realism is increased in this powerful modelling framework, while also highlighting the difference in ecosystem response to fishing and warming when resolved cephalopod functional groups are used. In Chapter 2 I quantified the trophic allometries for oceanic cephalopods using stable isotope analyses and found that inferred activity-level and individual body size were the best explanatory factors. Contrary to established theory, not all cephalopods are voracious predators, as I found low activity-level cephalopods have a distinct trophic role, with lower trophic positions and little-to-no ontogenetic increases. These findings facilitated the more realistic representation of cephalopod trait-based groups in size-structured ecosystem models in Chapters 3 and 4. I tested a variety of feeding and growth assumptions using an established size-structured model in Chapter 3, using trait-based functional groups for cephalopods based on activity level. Ecosystems modelled with more realistic cephalopod groups had different size spectra, lower overall biomass, increased stability, and higher turnover when compared to control models. The emergent trophic allometries also matched our Chapter 2 empirical findings well. Importantly, the differences between the control and the cephalopod resolved models are mainly driven by the distinct feeding traits. The effects of different levels of fishing effort and different magnitudes of warming were tested in Chapter 4, where I found ecosystems with realistic cephalopod groups had higher biomass stability overall when compared to a control model, as well as being more stable in response to warming and displaying different responses fishing levels. This thesis outlines how distinct cephalopod functional groups, largely driven by feeding trait differences, have alternate responses to fishing and warming when compared to models using commonly used assumptions. Understanding cephalopods' 'life in the fast lane' and how it is changing due to fishing and climate will be an important part of understanding marine ecosystem response to change.