Human pressures and the emergence of novel marine ecosystems

The progressive expansion of human activities is causing unprecedented changes to marine ecosystems. In some cases, the extent of these changes may be so large as to generate ecosystems in which structure and functioning have no historical analogs (i.e. novel ecosystems). Here, we review the main abiotic and biotic drivers of change in the marine realm and provide a critical assessment of ecosystems for which there is empirical evidence of human-induced shifts into novel states. Our review indicates that there is evidence for human agency of changes, no-analog species composition and threshold crossing for a variety of marine ecosystems, including intertidal rocky shores, temperate and tropical reefs, estuaries and pelagic systems, while there is no definitive proof of irreversibility of changes. We discuss how a more thorough recognition of the novel ecosystem concept may help initiate conservation and restoration efforts in each of these systems.