pogenic stresses in the oceans (e.g., Kinne, 1984; Sherman et al., 1990; Beatley, 1991; GESAMP, 1991; Clark, 1992; Thayer, 1992; Norse, 1993; C.H. Peterson, 1993; Suchanek, 1994). Significant criteria in recognizing those stresses most important to changes in biodiversity include (1) their ubiquity across many different marine habitats, (2) their duration and magnitude, and (3) their degree of reversibility. Moreover, the strength and impact of many human perturbations will vary because of interactions with natural environmental conditions and changes. In turn, alterations to biodiversity may result from (1) the cumulative effects of one or more stresses, (2) the synergistic effects of two or more stresses, or (3) the cumulative or synergistic interactions between natural and human stresses.

Given the above criteria for identifying critical environmental issues, the committee believes the most important agents of present and potential change to marine biodiversity at the genetic, species, and ecosystem levels are the following five activities:

• fisheries operations,
• chemical pollution and eutrophication,
• alteration of physical habitat,
• invasions of exotic species, and
• global climate change.

Singly or in combination, these human perturbations can lead to transformation of energy flow patterns and many other fundamental alterations in system structure and function (e.g., Boxes 5 and 6). Human activities have further led to the global extinction of marine mammals, birds (Vermeij, 1993), and invertebrates (Carlton, 1993), although little is known about the number of threatened, endangered, or extinct marine invertebrates or fish (e.g., Lovejoy, 1980). Many species of marine animals have been hunted to commercial and ecological extinction (Norse, 1993), with potential genetic consequences and concomitant implications for management and conservation programs (Ehrlich and Ehrlich, 1981). Particularly difficult management issues and questions emerge for species that are sliding along the continuum from threatened to endangered.

There is no one operational definition of "serious change" in the oceans—rather, the seriousness of a change is a function of the balance between the magnitude and persistence (endurance) of a perturbation on the one hand, and the ability of a given system to recover from that disturbance when its effects are reduced or stopped on the other. It is evident that a thorough understanding of the composition and functioning of an ecosystem is fundamental to recognizing changes in that system. However, it will not always be possible to separate long-term natural variation or cyclic changes from human impacts and the potentially large synergistic interactions between them. The ubiquity and magnitude of human perturbations have already reduced—and in some areas eliminated—opportunities to study pristine habitats or communities within habitats.