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

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.Black Sea ͉ disturbance ͉ foodweb control ͉ invasive species ͉ top predators

The importance of within-system spatial variation in drivers of marine ecosystem regime shifts

“…In the Black Sea, a pronounced regime switch from a predator-dominated to a planktivore-dominated system has been documented [52,53]. As described by Aristotle in his Historia Animalium [54], the main pelagic predators (bonito, bluefish and mackerel, but also bluefin tuna in the past) migrate in the spring from the southern Black Sea and Mediterranean to feed during the warm season along the most productive area of the basin-the large and shallow northwest shelf.…”

Effects of Altered Offshore Food Webs on Coastal Ecosystems Emphasize the Need for Cross-Ecosystem Management

“…In particular, decreased stocks of predatory fish have generated strong increases in their prey, medium-sized or ''meso-'' predators (i.e., ''mesopredator release''), changing the interactions between higher trophic levels considerably (e.g., Myers et al 2007;Baum and Worm 2009). In some instances, there are documented cascading effects from such mesopredator increases on lower levels in the pelagic food web, including communitywide decreases of zooplankton, and increases in jellyfish and phytoplankton (e.g., Frank et al 2005;Daskalov et al 2007;Casini et al 2008).…”

Past and future challenges in managing European seas

“…Interannual variation in climate and climate change affect marine ecosystems by, e.g., the poleward species range expansions, changes in local species compositions due to physiological intolerance to new conditions (e.g., a shift from marine to brackish or freshwater species with decreasing salinities) and arrival of nonindigenous species, observed across a large number of marine ecosystems (Beaugrand et al 2002, Drinkwater 2002, Daskalov et al 2007, Drinkwater et al 2010. However, more specific changes in climate conditions and, consequently, in the marine environment are often largely determined by the location and general characteristics of the sea (Philippart et al 2011).…”

“…The pelagic ecosystem of the Black Sea underwent major regime shifts, occurring first because of overfishing of top predators and middle trophic levels, causing system-wide trophic cascades (Daskalov et al 2007). 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.…”