Scaling fish energetics from individuals to communities

Two distinct biological currencies - energy and materials - are essential to life because both are required for the maintenance, growth and reproduction of organisms. Modelling ecological phenomena on the basis of these currencies therefore holds potential for developing a deeper understanding of how the availability of energy and materials in the environment constrains life, in all its diversity, across space and time. In this dissertation, I use the Metabolic Theory of Ecology (MTE) as a framework to explore how individual energetics influences biological processes at distinct levels of organization, from individuals to communities. Particularly, I explore how body mass, environmental temperature, and other variables constrain (i) metabolic rates and growth rates of individuals, and thereby influence (ii) densities of populations at different trophic levels and (iii) the standing biomass and energy fluxes of communities that differ substantially in species diversity. I use fishes to address these questions because they encompass the highest species richness among vertebrates, they encompass more than seven orders of magnitude in body mass, and they occupy diverse habitats that vary substantially in thermal regime across the globe. At the individual level, MTE predictions are generally well supported, although deviations attributable to differences among taxa are clearly noted. By contrast, at the population and community levels, while I do find evidence of energetic constraints, deviations from MTE-derived predictions are frequently observed, highlighting the importance of factors other than individual energetics. I conclude by discussing the implications of these findings to climate change biology and ecosystem dynamics, and highlight avenues for future research.