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Because of GLOBEC's focus on population dynamics, species‐level research is central to the programme, and most field, retrospective and modeling studies were directed at target species defined on the basis of their suitability for use in the comparative approach or their trophic role in the ecosystem. Target species may be economically significant due to their contribution to local, regional, and national economies through subsistence, commercial enterprise, and use by indigenous peoples. Target species of conservation significance may be the subjects of regional, national, or international conservation agreements. Target species of social or cultural significance have value to human communities because of their historical, aesthetic, educational, or recreational value. GLOBEC target species are heavily weighted towards marine pelagic organisms, particularly zooplankton. However, vertebrates with largely (seabirds and seals) or wholly (whales) pelagic life histories have been studied in some ecosystems, as have anadromous fish whose life history is not entirely marine. Here, this chapter reviews major groups of GLOBEC target species: Calanus and other large copepods, salmonids, cod, small pelagic fish, and large apex predators.

The biodiversity of the oceans, including the striking variation in life forms from microbes to whales and ranging from surface waters to hadal trenches, forms a dynamic biological framework enabling the flow of energy that shapes and sustains marine ecosystems. Society relies upon the biodiversity and function of marine systems for a wide range of services as basic as producing the seafood we consume or as essential as generating much of the oxygen we breathe. Perhaps most obvious is the global seafood harvest totalling over 100 Mt yr–1 (82 and 20 Mt in 2008 for capture and aquaculture, respectively; FAO 2009) from fishing effort that expands more broadly and deeper each year as fishery stocks are depleted (Pauly et al. 2003). Less apparent ecosystem services linked closely to biodiversity and ecosystem function are waste processing and improved water quality, elemental cycling, shoreline protection, recreational opportunities, and aesthetic or educational experiences (Cooley et al. 2009). There is growing concern that ocean acidification caused by fossil fuel emissions, in concert with the effects of other human activities, will cause significant changes in the biodiversity and function of marine ecosystems, with important consequences for resources and services that are important to society. Will the effects of ocean acidification on ecosystems be similar to those arising from other environmental perturbations observed during human or earth history? Although changes in biodiversity and ecosystem function due to ocean acidification have not yet been widely observed, their onset may be difficult to detect amidst the variability associated with other human and non-human factors, and the greatest impacts are expected to occur as acidification intensifies through this century. In theory, large and rapid environmental changes are expected to decrease the stability and productivity of ecosystems due to a reduction in biodiversity caused by the loss of sensitive species that play important roles in energy flow (i.e. food web function) or other processes (e.g. ecosystem engineers; Cardinale et al. 2006). In practice, however, most research concerning the biological effects of ocean acidification has focused on aspects of the performance and survival of individual species during short-term studies, assuming that a change in individual performance will influence ecosystem function.