62 Marine Ecosystems, Human Impacts on greenhouse gases, is part of a long-term climatic trend In any case, marine species with different geographical origins would have different responses to water temperature alterations (Castilla and Camus, 1992) Moreover, in the case of the oceans, water temperature modifications would be just one of the potential factors affecting the distribution of species For instance, temperature effects on the turbulence of the ocean waters, and their association with wind stress, may have major implications for plankton dispersal Also, the predicted north–south interhemispheric asymmetry, due to the thermal inertia in the south, must be considered before drawing firm conclusions on marine species latitudinal shifts (Bernal, 1994) Furthermore, since the ocean is affected simultaneously by several climate forces (including anthropogenic greenhouse effects), it is difficult to determine the real cause of any observed change, such as that in surface seawater temperature Shifts in marine populations, community structure, and their functioning represent the integrated response of species assemblages to nonanthropogenic long-term climate changes superimposed on the effects of numerous short-term factors, including anthropogenic forcing Conclusions This article discussed several marine examples in which direct anthropogenic and nonanthropogenic impacts (or combinations), such as species extraction and oceanic water temperature modifications, caused drastic ecological shifts on marine benthic intertidal, subtidal, and coastal–oceanic communities, and thereby modified species diversity and the functioning of associated communities Interestingly, extreme conservation measures (e.g., the establishment of no-take areas) to protect species, habitat, community, or ecosystem may also cause drastic modifications in the functioning of marine communities and drive communities into alternative ecological states (Castilla et al., 1994; Estes et al., 1998; Castilla, 1999) This article highlighted that anthropogenic activities (e.g., mariculture) and impacts (e.g., overfishing) on different ecological categories of species (predator, keystone, engineer, invasive, and competitive dominant) translate into differential responses and functioning at the species diversity and community level The unique ecological role played by humans and their apex keystone position in trophic webs were discussed Acknowledgments The author acknowledges financial support received through the Ca´tedra Presidencial en Ciencias, Chile (awarded in 1997) and the Pew Charitable Fund as a Pew Fellow in Marine Conservation (1997) Minera Escondida Ltd supported fieldwork in Antofagasta, Chile Comments and suggestions on a preliminary manuscript made by a referee are acknowledged See also: Aquaculture Intertidal Ecosystems Marine and Aquatic Communities, Stress from Eutrophication Marine Ecosystems Resource Exploitation, Fisheries References Alverson DL, Freeberg MH, Murawski SA, and Pope JG (1994) A Global Assessment of Fisheries Bycatch and Discards FAO Fisheries Technical Paper No 339 Rome: Food and Agriculture Organization Anderson JL (1997) The growth of salmon aquaculture and the emerging new world order of the salmon industry In: Pikitch E, Huppert DD, and Sissenwine M (eds.) 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