day patterns of biodiversity therefore must be embedded firmly in an historical perspective, underscoring the importance, for example, of knowing if a given species was introduced or is native to the environment in which it now occurs (Carlton, 1989; Valentine and Jablonski, 1993) and of retrospective analyses in general (discussed in Chapter 6).

Understanding and predicting changes in biodiversity due to effects of human activities requires consideration of the time scales of variation of physical processes relative to biological processes. This consideration defines the relevant regional-scale of study for a given community or habitat. This marine biodiversity initiative is envisioned as a decadal-scale program. Given this time frame and physical considerations, important life-history features of the organisms include, for example, generation time, larval type (direct development or planktonic), dispersal period in the plankton (for pelagic larvae or spores of benthic organisms), and resting stage duration (for dormant benthic stages of planktonic organisms). Generation time, for example, is one criterion dictating the ultimate duration of a study whose goal is to understand changes in biodiversity due to effects of human activities.

Moreover, spatial scales tend to increase as time scales increase. Thus, for example, time scales associated with various physical oceanographic processes define advection length scales for planktonic larval transport, and the frequency and duration of resuspension events for benthic resting stages. Based on larval transport considerations alone, then, regional spatial scales would tend to be smaller for polar compared with temperate benthic invertebrate communities because the proportion of direct developers tends to increase with latitude (Thorson, 1950; Mileikovsky, 1971; Strathmann, 1985).

Defining the Most Meaningful Scales of Study

Ultimately, a variety of criteria, based on all life-history stages of the organisms, the oceanographic processes relevant to these stages, and the critical environmental issues in the region, must be used in defining the maximum spatial and temporal scales of study (e.g., Box 5). Once the maxima have been defined, shrewd insight will be needed to select the most meaningful smaller-scales of study. A compelling terrestrial example is the 12-year study of Tilman and Downing (1994; but see also Givnish, 1994 and Tilman et al., 1994) on the relationship between the biodiversity of vascular plants (manipulated by nitrogen fertilization) and the stability of the grassland ecosystem (in response to drought). This marine biodiversity program would emphasize inclusion of biological and oceanographic processes at the largest relevant scales—recognizing the connectivity among sites within a geographic region—while acknowledging the critical importance of ecological processes and physical-biological coupling at smaller, site-specific scales. This initiative does not recommend studying all processes at all scales within a selected geographic region. Rather, this initiative



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