From population trends to biodiversity changes: synthesizing multi-species signals of collapse

Katherine Hébert , Alexandre Fuster-Calvo, Camille Levesque, Dominique Gravel

Taking the pulse of biodiversity is no small feat, and abundance time series are some of the most valuable data we have to measure change. Biodiversity changes are often complex, with some species declining while others grow, leading to a reorganisation of the network of interactions between species. The infamous decline of Atlantic cod in the Newfoundland shelf groundfish community between 1981 and 2013 is an example of such complexity. Here, we develop a new approach to track the multispecies signals before and after the cod decline, offering a community-oriented alternative to species abundance indicators like the Living Planet Index. We derive theory-based indicators and build a model to quantify the short-term stability and long-term reorganisation of communities with an explicit estimation of uncertainty. These indicators together reveal how severe declines in dominant species like Atlantic cod, combined with community-wide biomass changes, generated periods of instability and recovery that reshuffled the community more profoundly than previously thought. We find that the community was already facing reorganisation before the collapse and only recently recovered towards a desired state. By grounding indicators in ecological theory, we are better equipped to understand the process, and not just the pattern, of change.