Protection from harvesting promotes energetically efficient structures in marine communities

AbstractThe 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 communities (with small basal species being the most abundant and large apex predators the least 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 protection from harvesting promotes 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 harvested 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, as a natural experiment, the structures observed within fully protected marine areas compared to harvested areas across 299 geographical sites worldwide 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 protection from harvesting and top-heavy structures. Our work corroborates ecological theory on community development and provides a framework for additional research into the restorative effects of protected areas.