Regime shifts in exploited marine food webs: detecting mechanisms underlying alternative stable states using size-structured community dynamics theory

Abstract

Many marine ecosystems have undergone ‘regime shifts’, i.e. abrupt reorganizations across trophic levels. Establishing whether these constitute shifts between alternative stable states is of key importance for the prospects of ecosystem recovery and for management. We show how mechanisms underlying alternative stable states caused by predator–prey interactions can be revealed in field data, using analyses guided by theory on size-structured community dynamics. This is done by combining data on individual performance (such as growth and fecundity) with information on population size and prey availability. We use Atlantic cod ( Gadus morhua ) and their prey in the Baltic Sea as an example to discuss and distinguish two types of mechanisms, ‘cultivation-depensation’ and ‘overcompensation’, that can cause alternative stable states preventing the recovery of overexploited piscivorous fish populations. Importantly, the type of mechanism can be inferred already from changes in the predators' body growth in different life stages. Our approach can thus be readily applied to monitored stocks of piscivorous fish species, for which this information often can be assembled. Using this tool can help resolve the causes of catastrophic collapses in marine predatory–prey systems and guide fisheries managers on how to successfully restore collapsed piscivorous fish stocks.

Win‐wins for biodiversity and ecosystem service conservation depend on the trophic levels of the species providing services

“…Leopold et al., ). As an example, the effect of overfishing on predators can cascade down to impact lower trophic levels, decreasing food web stability or driving regime shifts (Gårdmark et al., ). Under this scenario, including top‐down effects could alter our results depending on the number of trophic levels in the ecosystem.…”

Hypoxic areas, density-dependence and food limitation drive the body condition of a heavily exploited marine fish predator

“…We, therefore, also estimated the biomass of the prey sizes most suitable for each length-class of cod by using the size-dependent attack rate (i.e. the rate at which cod of a certain size feed on a certain size of their prey; [40]) as done in [41]. …”

A holistic view of marine regime shifts

“…They can be related to direct anthropogenic impact on a system, e.g. overfishing and associated top predator species removal/reduction, introduction of alien, invasive species, species responses to nutrient enrichment and related eutrophication, or acidification and associated reduced biological calcification [11,43,51,[71][72][73]. While the dominance of physical or trophic control is still hotly debated in the pelagic domain [29], this discussion may be antiquated, as a multitude of studies from a diversity of habitats now show that both controls usually exist in parallel, and their dominance is strongly context dependent [66,[74][75][76][77][78].…”