The Functioning of Marine Ecosystems

There is considerable evidence that environmental variability plays a major role in controlling abundance and distribution of marine populations and that fisheries alter ecosystem functioning and state. This overview documents emergent ecosystem-level ecological patterns and answers meaningful questions regarding the exploitation of marine resources. Do marine ecosystems function differently from terrestrial systems? Are there multiple stable marine ecosystem states? In marine ecosystems, does removal of top predators result in fundamental changes in the plankton (top-down “trophic cascades”), as observed in lakes? Alternatively, are marine ecosystems characterized by bottom-up control such that fishing predatory fish does not disturb community structure and function? Does heavy exploitation of forage species, such as anchovies and sardines, cause changes in the functioning of upwelling ecosystems? The key to answering these questions and exploring whether general principles apply lies in understanding the mechanisms responsible for these observed emergent patterns. Different types of energy flow in marine ecosystems are reviewed: bottom-up control (control by primary producers), top-down control (control by predators) and wasp-waist control (control by dominant species). Answers to the questions depend on the different energy flow mechanisms assumed to operate. No general theory can be ascribed to the functioning of marine ecosystems. However, tentative and partial generalisations are proposed, for example that bottom-up control predominates, top-down control plays a role in dampening ecosystem-level fluctuations, trophic cascades seldom occur, and waspwaist control is most probable in upwelling systems. Regime shifts, alternation and large-scale synchronised fluctuations in fish stocks, stability of fish communities and emergent features such as size spectra are potentially important patterns when assessing states and changes in marine ecosystems. New and meaningful indicators, derived from our current understanding of marine ecosystem functioning, can be used to assess the impact of fisheries and to promote responsible fisheries in marine ecosystems.