Body size is a fundamental axis of marine ecosystem structure and function, linking individual metabolism and trophic interactions to ecosystem biomass and production. Fishing and climate change can both alter the distribution of abundance across the body size spectrum, but potentially in different ways. Size-structured marine ecosystem models therefore provide an opportunity to examine how responses to different pressures are distributed across the body-size spectrum, and whether these patterns can help detect and attribute ecosystem change.
Here, we use the Dynamic Benthic Pelagic Model (DBPM), a global size-structured marine ecosystem model representing coupled pelagic and benthic food webs, to examine historical ecosystem change around Australia. Building on the FishMIP 2.0 detection and attribution framework, we compare simulations incorporating historical fishing and transient climate forcing with counterfactual simulations to characterise background variability in the absence of these drivers. We examine how fishing and climate change alter biomass across the body-size spectrum, when these changes become detectable above background variability, and whether particular size classes provide earlier or clearer signals of ecosystem change than total biomass. We also examine how these responses vary spatially around Australia. By reconstructing how fishing and climate have shaped the size structure of Australian marine ecosystems, this analysis provides a historically informed picture of their contemporary state and a baseline for comparing future projections under alternative climate and sustainable development pathways.