Which came first, non-equilibrium dynamics or human exploitation?

Background/Question/Methods

Human exploitation may push system dynamics from equilibrium to disequilibrium. For example, simple ecological models show populations can go from steady-state to chaotic as a result of increasing intrinsic growth rates. Such a change may result from harvesting, and studies have shown fish populations demonstrate such responses. On the other hand, non-equilibrium dynamics may be part and parcel of natural biological cycles and ecological interactions. Studies of human heart rhythms demonstrate healthy hearts produce signals that are chaotic; such dynamics may help healthy hearts cope with perturbations. Here, we use nonlinear time series forecasting models to classify dynamics as equilibrium or non-equilibrium. Drawing on datasets from a variety of systems, we investigate whether those associated with human exploitation are more likely to exhibit non-equilibrium, nonlinear behavior.

Results/Conclusions

One example from fisheries, analyzing 145 species from the Pacific and Atlantic oceans, demonstrates that exploited species are far more likely to have non-equilibrium dynamics than unexploited species. A second example from an oak forest in Arkansas demonstrates trees with non-equilibrium dynamics were significantly more likely to survive a wood-boring beetle outbreak, a phenomenon expected to increase under anthropogenic climate change. A third example from mast seeding in plants shows non-equilibrium dynamics may be a natural phenomenon. Finally, a fourth example shows fish populations changed from non-equilibrium dynamics to equilibrium dynamics as fishing pressure increased.  These contrasting examples illustrate that patterns are not clearly aligned with human exploitation, and additional biological considerations are important. While non-equilibrium dynamics significantly decrease our ability to make predictions about future behavior, they also may be a coping mechanism for responding to unpredictability of natural systems. Ultimately, non-equilibrium dynamics are not normatively "bad", but it is challenging to determine whether their existence is inherent or a response to human intervention.