Protection promotes energetically efficient structures in marine communities

The sustainability of marine communities is critical for supporting many biophysical processes that provide ecosystem services that promote human well-being. It is expected that anthropogenic disturbances such as climate change and human activities will tend to create less energetically-efficient ecosystems that support less biomass per unit energy flow. It is debated, however, whether this expected development should translate into bottom-heavy (with small basal species being the most abundant) or top-heavy communities (where more biomass is supported at higher trophic levels with species having larger body sizes). Here, we combine ecological theory and empirical data to demonstrate that full marine protection promotes shifts towards top-heavy energetically-efficient structures in marine communities. First, we use metabolic scaling theory to show that protected communities are expected to display stronger top-heavy structures than disturbed communities. Similarly, we show theoretically that communities with high energy transfer efficiency display stronger top-heavy structures than communities with low transfer efficiency. Next, we use empirical structures observed within fully protected marine areas compared to disturbed areas that vary in stress from thermal events and adjacent human activity. Using a nonparametric causal-inference analysis, we find a strong, positive, causal effect between full marine protection and stronger top-heavy structures. Our work corroborates ecological theory on community development and provides a quantitative framework to study the potential restorative effects of different candidate strategies on protected areas. The sustainability of marine communities is critical for supporting many bio-physical processes, including human well-being. Human impacts, including land-based and ocean-based activities that modify, exploit, and pollute marine habitats, have been reported to alter species composition and biomass. Marine protected areas (MPAs) have been established for the conservation of target species, including the protection of economic resources and biodiversity. However, we know little about the cause-effect relationship between MPAs and community-wide properties of marine communities under climate change and anthropogenic disturbances. Yet, this knowledge is central to increase our understanding of the development and potential restoration of marine communities. Here, we combine big data, ecological theory, and nonparametric causal-inference analysis to provide key evidence of the cause-effect relationship between MPAs and the structure of fish communities. Specifically, we find that fully protected areas directly increase by the probability that fish communities display a stronger top-heavy structure relative to limits imposed by their environmental contexts, supporting more biomass per unit of energy flow. Because restoration and conservation efforts require interventions, it then becomes necessary to increase our understanding of cause-effect relationships between disturbance and community composition.