Trophic cascades promote threshold-like shifts in pelagic marine ecosystems

TLDR

Fisheries that remove large predatory fish can trigger cascading effects through the food web, yet the consequences for ecosystem functioning and resilience remain debated. The study argues that management decisions should incorporate a food‑web perspective. Field data from the Baltic Sea show that the collapse of cod triggered a threshold shift in ecosystem functioning, with sprat abundance above ~17 × 10¹⁰ individuals decoupling zooplankton dynamics from hydrology and potentially preventing cod recovery.

Abstract

Fisheries can have a large impact on marine ecosystems, because the effects of removing large predatory fish may cascade down the food web. The implications of these cascading processes on system functioning and resilience remain a source of intense scientific debate. By using field data covering a 30-year period, we show for the Baltic Sea that the underlying mechanisms of trophic cascades produced a shift in ecosystem functioning after the collapse of the top predator cod. We identified an ecological threshold, corresponding to a planktivore abundance of approximately 17 x 10(10) individuals, that separates 2 ecosystem configurations in which zooplankton dynamics are driven by either hydroclimatic forces or predation pressure. Abundances of the planktivore sprat above the threshold decouple zooplankton dynamics from hydrological circumstances. The current strong regulation by sprat of the feeding resources for larval cod may hinder cod recovery and the return of the ecosystem to a prior state. This calls for the inclusion of a food web perspective in management decisions.