Niche breadth

The epipelagic world is more variable over small spatial and temporal scales, containing generalists with broad diets which are widely distributed in their biogeographic region (low among-region diversity; Angel 1993; McGowan and Walker). Broad niche structure among pelagic species does not imply that the niches are overlapping, i.e. that there is high species redundancy. For some marine groups, e.g. cetaceans, the lack of functional redundancy is fairly obvious: each species is highly distinct, and there is even significant (and perhaps reiterative) differentiation within species, in the bottlenose dolphin. Tursiops, for example. !t is no accident that we have no trouble in recognizing functional differences among whales, but tend, erroneously, to lump the microbes, together. McGowan and Walker (1992} presented evidence that species of zoopiankton do not to shift in rank abundance over time. Some shifts do occur, but rare forms tend not to displace abundant ones, and abundant species do not become rare. If the ranking of species dominance in pelagic systems is indeed robust, functional redundancy would appear to be low among abundance classes. In contrast, the bcnthic communities may be more specialized, with species having restricted dispersal, and communities with high among-region diversity (Grassle 1989; Grassle and Maciolek 1992). Pelagic ecosystems may be more fine-grained than bcnthic ecosystems.

Each group of organisms tends to be adapted to its particular environment and the scaling of natural disturbances that occur in that ecosystem. Novel perturbations caused by humans, such as exploitation, mining or pollution, will affect the system in ways that it is not used to adjusting to. In the advent of such novel disturbances, generalists with high dispersal will tend to increase the stability of the system by being able to respond more quickly to the changes in the environment (resilience in the sense of Pimm 1991). Moreover, the mixing of the epipelagic biome by oceanic currents will "quickly" restore perturbed systems to all but very large-scale disturbances (McGowan and Walker 1993). The fate of benthic taxa may depend more on their own dispersal capabilities. Buzas and Culver (1995) recently compared the distribution of present-day benthic foraminiferans that exist in the fossil record with the distribution of those that do not occur in the fossil record. Taxa that are ubiquitously distributed throughout the biogeographic regions are more likely to be found in the fossil record, suggesting that well-dispersed taxa are more likely to persist through time.

56.5 BIODIVERSITY AND ECOLOGICAL PROCESSES: EMPIRICAL EVIDENCE

It cannot be emphasized enough that knowledge about the relationship between biodiversity and ecosystem processes is very primitive. Despite the lack of concrete evidence, some people feel very strongly that changes in the structure of food webs, through changes in the abundance of biodiversity, will have significant impacts on both the state and the rates of ecosystem processes in open oceans. It is tempting to make such generalizations because of the ocean's unique characteristics, and because any change in the configuration of the open ocean could have a very large impact on any or all of the ecosystem processes and, through climate change, on terrestrial ecosystems as well. Because of the lack of knowledge about specific biodiversity impacts on many of the ocean's processes, discussion about certainties is limited. We have attempted to expose the most likely impacts of changes in biodiversity on ecosystem processes. The following list of open-ocean processes is not exhaustive, but should provide a basic list of the important processes operating in this biome (Figure 16.6).

16.5.1 Nutrient regeneration in the pelagic zone (microbial loop)

The pelagic microbial loop allows organic detritus to enter a detritivore food chain while still suspended in the water column, leading to local regeneration

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