Our effort was to explore the system-level consequences of human-caused changes in open-ocean biodiversity. Of the 15 biomes examined in the course of the SCOPE project, the open ocean is the most underdescribed. It is in many ways premature to describe ccosystem function when we have not become familiar with all of the components. Due to the taxonomic structure of marine assemblages, when an open-ocean creature is newly discovered there is a reasonable likelihood that an entire family, class or even phylum has been overlooked: the recent description of a new phylum, the Loricifera, is but one example. Few of the new groups turn out to be especially rare, and some are astonishingly abundant, albeit unfamiliar. The novelty of new taxa from the open ocean is much higher than that for new rain forest species, heightening an already extreme imbalance of attention and resources between rain forest and pelagic systematics. A true understanding of the significance of diversity to open-ocean processes will only come after careful experimental manipulations. Unfortunately, we are simultaneously altering so many of the ocean's species, primarily through resource extraction, that we may never have a baseline against which to understand how humans are actually impacting this biome.
The picture of marine functional diversity that emerges is one of a rich mélange not only of species, but also of basic body plans and metabolic capabilities. The biota may be highly adaptable, but functional equivalency is not necessarily very high, with the possible exception of the species-rich deep-sea benthos. In general, the dynamics of marine systems, and in particular pelagic systems, are likely to change considerably if species are eliminated, even a relatively few species. Extinction probabilities may be much lower for pelagic than for terrestrial organisms, but functional similarity is probably also lower. Any one marine extinction, once it occurs, is expected to have a more profound impact on the system than the loss of a terrestrial species. Buddemeir (cited in Culotta 1994) warned that "You don't want to get trapped into a linear comparison of terrestrial and marine ecosystems. . . The marine system is less extinction-prone, but if you do start getting extinctions, it means you've got a problem on a much larger scale. The rules are different in the sea". Even at the subspecific level, functional diversity exists among marine organisms, even among cetaceans, where we would least expect it because of their enhanced buffering capacity (large size, endothermy, high vagility) (Perrin 1991). This suggests that different functional roles are strongly selected for in the open ocean, and that the loss of even one species of cetacean could result in a significant change to that ecosystcm. A similar argument could be constructed for sharks. Populations of cetaceans and sharks are depressed around the world today, and the current pelagic assemblage may already reflect this loss.
Recent consensus holds that marine systems are organized, and function, in fundamentally different ways than terrestrial systems (Steele 1985, 1991; Ray and Grassle 1991; Holling 1992; Holling et ai 1994). Direct services to humans include the production of food and oxygen, and a dumping ground for toxic and non-toxic wastes, although indirect services, such as climate regulation, may be more important over the long term.
Certain organisms play key roles in the open ocean, and their disappearance would have particularly strong ramifications. The likely key organisms include those that transport nutrients from epipelagic systems to deeper ecosystems (e.g. salps, large vertebrates, rhizsolenid diatoms), and organisms that modulate extreme environments (chiefly microorganisms) such as sulfate reducers, methanogens and fermenters, and organisms that modify the physical structure of the deep-sea benthos. The impact of global reduction in apex consumers will be possibly the single greatest impact that humans will have over the short term on the productivity of pelagic ecosystems. Over the long term, changes in ocean-atmosphere coupling through the selective elimination of specific phytoplankton (for example through UV-B
radiation, climate change) may be of even greater importance. As for impacts arising from the loss of deep-ocean, soft-sediment species, nothing can be surmised at this time.
In many parts of the world, particularly around the Pacific Rim, the continental shelf is reduced to a few kilometers. Under these circumstances, the open ocean virtually abuts the land. This is the oceanic biomc in which the lag between coastal signal and oceanic impact is the shortest, and functional links the strongest (e.g. the guano-anchovy link in South America). It is misleading to treat these systems as strictly coastal. Indeed, these situations may offer the ideal monitoring outposts for both signs of change in pelagic biodiversity, and the impacts of such change on ecological processes and human affairs.
The productivity of pelagic fisheries is one key ecosystem process that oceans provide to humans. Changes in the species composition of landings have been much greater than fluctuations in the overall catch rate of all fishes. Whether one regards these species shifts as changes in ecosystem process depends on the scale examined. In several regions managed as one system, such as the Grand Banks or the Yellow Sea, there have been significant changes in species composition over a short period of time. However, there probably has not been much change in total fish productivity . With respect to processes provided to humans, this is not a trivial matter. There is strong cultural and economic value associated with each specific fishery. In the North Atlantic, the cod fishery cannot be replaced easily with a dogfish or skate fishery. A tourist industry based on whale watching is another example of a human service that depends on a stable pelagic community of specific species. The benefits derived from ecosystems, especially those linked to human culture, usually depend on processes that operate at much smaller scales than those typically associated with ecosystems.
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