Trophic cascades triggered by overfishing reveal possible mechanisms of ecosystem regime shifts

TLDR

Regime shifts, such as those observed in the Black Sea where heavy fishing, eutrophication, and alien species invasions disrupted a once predator‑dominated ecosystem, highlight the complex responses of marine systems to human pressures and the need for more sophisticated management approaches. The study aims to use natural experiments in the Black Sea to uncover the mechanisms driving regime shifts and to challenge current practices by developing explanatory models that better integrate conservation and ecosystem management. The authors examined changes in the Black Sea ecosystem—treated as natural experiments—to identify the mechanisms underlying regime shifts. The study detected two major shifts: one from predator depletion and another from an outbreak of the alien comb jelly *Mnemiopsis leidyi*, both triggered by intense fishing and producing system‑wide trophic cascades.

Abstract

Large-scale transitions between alternative states in ecosystems are known as regime shifts. Once described as healthy and dominated by various marine predators, the Black Sea ecosystem by the late 20th century had experienced anthropogenic impacts such as heavy fishing, cultural eutrophication, and invasions by alien species. We studied changes related to these "natural experiments" to reveal the mechanisms of regime shifts. Two major shifts were detected, the first related to a depletion of marine predators and the second to an outburst of the alien comb jelly Mnemiopsis leidyi; both shifts were triggered by intense fishing resulting in system-wide trophic cascades. The complex nature of ecosystem responses to human activities calls for more elaborate approaches than currently provided by traditional environmental and fisheries management. This implies challenging existing practices and implementing explanatory models of ecosystem interactions that can better reconcile conservation and ecosystem management ideals.