For certain types of anthropogenic effects, ecological consequences of changes in marine biodiversity are at least partially known. These effects include fisheries activities, physical habitat destruction, eutrophication, releases of toxic chemicals, and transport of exotic species [ Norse, 1993]. There are also more subtle human causes of change in biodiversity whose potential ecological consequences are largely unknown. Some examples are global climate change and associated effects such as sea-level rise and changes in global ocean circulation, alteration of terrestrial vegetation input to the ocean due to the removal of coastal forests, and alteration of deep-sea biodiversity due to a changes in whale carcass supply associated with whaling.
Discussed in this section are examples of a relatively well-understood and a poorly understood effect of human activities on biodiversity. The first example---the recent introduction of the Chinese clam to San Francisco Bay, California [ Carlton et al., 1990]---indicates how a striking contemporary change in biodiversity can have important effects at the ecosystem level. The second example---the potential effect of whaling on deep-sea biodiversity [ Butman et al., 1995]---suggests that a human cause of biodiversity change may be spatially and temporally very remote from the marine habitat affected, and yet the biological consequences are potentially substantial indeed.