Overfishing of top predators eroded the resilience of the Black Sea system regardless of the climate and anthropogenic conditions

Abstract

It is well known that human activities, such as harvesting, have had major direct effects on marine ecosystems. However, it is far less acknowledged that human activities in the surroundings might have important effects on marine systems. There is growing evidence suggesting that major reorganization (i.e., a regime shift) is a common feature in the temporal evolution of a marine system. Here we show, and quantify, the interaction of human activities (nutrient upload) with a favourable climate (run-off) and its contribution to the eutrophication of the Black Sea in the 1980s. Based on virtual analysis of the bottom-up (eutrophication) vs. top-down (trophic cascades) effects, we found that an earlier onset of eutrophication could have counteracted the restructuring of the trophic regulation at the base of the food web that resulted from the depletion of top predators in the 1970s. These enhanced bottom-up effects would, however, not propagate upwards in the food web beyond the zooplankton level. Our simulations identified the removal of apex predators as a key element in terms of loss of resilience that inevitably leads to a reorganization. Once the food web has been truncated, the type and magnitude of interventions on the group replacing the apex predator as the new upper trophic level have no effect in preventing the trophic cascade. By characterizing the tipping point at which increased bottom-up forcing exactly counteracts the top-down cascading effects, our results emphasize the importance of a comprehensive analysis that take into account all structuring forces at play (including those beyond the marine system) at a given time.

Interaction between top-down and bottom-up control in marine food webs

“…The dominant signals were modeled using Generalized Additive Models in fully additive (GAM) and also a threshold (tGAM) formulation (18,19) that allows for changes in the relationship between a response term and an explanatory variable as a function of another variable. The models were used to hindcast the data and to conduct simulations under scenarios of external forcing based only on the initial conditions of each food web component.…”

“…To address this question, an advanced statistical modeling approach (18,19) was developed incorporating the interactions between three phytoplankton measures (abundance of diatoms and dinoflagellates and a greenness index), three zooplankton groups (the large copepods Calanus finmarchicus and Calanus helgolandicus as well as an assemblage of small copepod species), four forage fish species (herring, sprat, sandeel, and Norway pout), four piscivorous fish species (cod, haddock, whiting, and saithe), and one seabird group. The model incorporates direct and indirect responses of these groups to…”

Past and future challenges in managing European seas

“…The overfishing of the pelagic top predators in the 1970s and of planktivorous fish in the 1990s resulted in regime shifts and caused changes in the abundance of the zooplankton, jellyfish, and phytoplankton, as well as in surface oxygen and phosphate concentrations. Llope et al (2011) identified the removal of top predators as a key element in terms of loss of resilience that inevitably leads to reorganization of the food web. Historically, the fishery-driven trophic cascade first disturbed the structure of the system from higher trophic levels and then the already disturbed food web was further degraded by eutrophication.…”

“…Historically, the fishery-driven trophic cascade first disturbed the structure of the system from higher trophic levels and then the already disturbed food web was further degraded by eutrophication. Simulation results demonstrated that increased productivity could have been more efficiently handled by a more complex ecosystem including viable top predators (Llope et al 2011). For that reason, a regeneration of the food web structure by rebuilding the top predator stocks could improve the system's ability to counterbalance fluctuations driven by climate or eutrophication.…”

Models with a porpoise

“…Fisheries scientists suggest that perhaps 70% of the harbor porpoise population in the Black Sea has already been lost through direct and indirect effects (8). With the concomitant loss of top predators in the Black Sea ecosystem, the short-beaked common dolphin (Delphinus delphis) and the common bottlenose dolphin (Tursiops truncatus), reversing a regime shift now may not even be possible (9,10).…”