Modelling eco-evolutionary dynamics of species' traits in size-structured ecosystems

Human impacts on natural systems are driving rapid ecological and evolutionary changes. In the ocean, we know that fishing and climate change are affecting the functional traits of species and restructuring ecosystems. However, the extent to which evolutionary adaptations can buffer ecosystems against these impacts is not well understood. This information is needed to improve knowledge of how ecosystem function and biodiversity are being affected by a rapidly changing ocean. This thesis is centred around the introduction of evolutionary dynamics into trait-based models. Trait-based models capture functional biodiversity of ecosystems by resolving traits within and across multiple species. However, current trait-based models do not allow for traits to change through adaptation and natural selection on the same time scales as ecological processes. Yet there is growing recognition that evolution can act on the same time scales as ecology. The key aim of the thesis is to extend trait-based size spectrum models, widely used to assess impacts of fishing and climate change on fish communities, by exploring how species adaptation can alter population and ecosystem responses to environmental and human-induced change. I develop an eco-evolutionary simulation model that introduces new phenotypes into a dynamic trait- and size-based community model. I then apply the model to three theoretical case-studies, each focusing on the adaptation of a key trait thought to be under strong selection from external drivers. Firstly, I explore the case of fisheries induced evolution or how fishing drives the adaptation of maturation size in fish. Here, the study confirms previous empirical findings and single-species based predictions that size selective fishing will generally drive a reduction in maturation size for large fish species. However, due to interacting forces of predation and competition, this effect was not found for small species and was sometimes reversed for medium-sized species. Secondly, I examine species' thermal performance as an adaptative trait under a warming climate. This study shows that in the short-term (~50 years), under a projected 'high emissions' climate scenario, marine species' phenotypic diversity helps to buffer against warming temperature by species slowly adapting towards more generalist thermal strategies. However, at the projected rate of warming of 3.5° per 100 years over 200 years of simulation, phenotypic diversity eventually collapsed as as adaptation could not keep up with rapid change and specialised phenotypes disappeared. Thirdly, I investigate the driver of food limitation to explore how feedbacks between predation and competition might lead to species-specific differences in the relative prey size preferences of fish. This chapter asks why some large marine fish (e.g. large planktivores) feed on much smaller prey relative to their own body size compared to others (e.g. predatory sharks). The model predicts that low resource availability drives an adaptation towards smaller relative prey size and that apex carnivores, who feed on relatively larger prey, only emerges as an alternative strategy if enough energy is available to transfer up the food webs. In summary, the model developed in this thesis has been used to show that there is significant scope for adaptations in size-structured multispecies systems. Some of the general patterns predicted by the model were consistent with macroecological patterns for fish. Future work would be needed to develop and test the model to support biodiversity policy and ecosystem-based management.