160-7 Mass extinction but not rapid climate change drives permanent ecological state changes in marine fish communities

Abstract:

Human impacts on marine systems – from over-fishing to rapid climate change – have the potential to have profound and perhaps irreversible impacts on marine ecosystems. Modern ecosystems can exhibit ecological hysteresis, switching between states along an environmental change that are not reversible along the same environmental axis, hinting that there may be processes driving long-term ecological stability. Dynamic systems theory suggests that ecosystems should exhibit periods of stability marked by rapid transitions between stable states, but to date, there are very few empirical datasets that have sufficient temporal scope to capture these predicted ecological dynamics. Here, we demonstrate that despite only approximately 5% of fish tooth morphotypes going extinct at the end of the Cretaceous, the extinction marks the transition between two distinct stable states: the Late Cretaceous and Eocene, with a directed transitional ecological phase in the Paleocene. This dataset, which spans 75-42 Ma in the South Pacific Gyre, fulfills the five quantitative criteria required in dynamic systems theory to be considered “true” stable states: compositional distinctness, statistical separation, differential attractor geometry, directed transitional dynamics, and irreversibility over the entire record. These patterns are robust when accounting for evolutionary drift by considering the community composition based on large-scale morphological groups, which can be interpreted as ecological guilds, rather than individual morphotypes. The long-term stable state observed in the Eocene spans several major climatic events of the Paleogene, including short term warming events such as the Paleocene-Eocene Thermal Maximum and other Early Paleogene hyperthermals, as well as long-term warming and cooling of the Early Eocene Climate Optimum, suggesting that at this scale, ecological stability is robust to both rapid and long-term climate perturbations, but can be disrupted by major extinction events, even when the total extinction is quite small. These results suggest that selective removal of specific parts of the pelagic ecosystem - including overfishing of large predators from whales to sharks to tunas and billfishes, may have a far more destabilizing impact on pelagic fish community stability than the rapid warming driven by anthropogenic activities. 

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