The role of species diversity in regulating ecosystem processes and services in sedimentary systems has received considerably less attention than its role in terrestrial systems (Estes & Peterson 2000). Although there are many examples of living organisms that play critical roles in providing services and functions, there is little evidence that biodiversity per se is critical for the delivery of services and functions. In many instances, it is likely that the availability of specific functional groups is most important in providing a given service or function (Tables 4.2a—4.2c). The benthic biota of estuaries are the least diverse of the marine sediment realms, but specific groups perform valuable functions: they create habitat, trap and retain sediments (e.g., rooted vegetation), maintain water quality (e.g., filter-feeding bivalves), contribute to aeration of subsurface sediments (e.g., bioturbators/burrowing crabs), and shunt production from the microbial decomposers to higher trophic levels (e.g., grazing snails and amphipods). There is some evidence that diversity decreases variability in rates of nutrient recycling and there are complementary effects of diversity on function, but there is no consistent relationship between species richness and function (Emmerson et al. 2001; see also Biles et al. 2003). However, few experiments to test these questions have been conducted in marine systems. Experiments with hard substrate communities have suggested that species diversity enhances resistance to invasive species (Stachowicz et al. 1999), but similar experiments are lacking for estuarine sediments. Nonetheless, estuaries have a public visibility that seems to confer a high value to the limited species diversity for aesthetic, recreational, and scientific reasons.
The role of species diversity on the continental slope is not well documented in provision of trophic support, nutrient cycling, and waste disposal/detoxification, but it is clear that multiple species are involved. In cases where multiple species are eliminated by hypoxic events, for example, the loss of key sedimentary functions has resulted (Rabalais et al. 1996), but it could be argued that loss of functional groups, rather than species, is more important (Elmgren & Hill 1997). In estuarine and shelf ecosystems, the diversity of structure-forming species often contributes to habitat diversity, which subsequently increases the diversity of species that utilize that habitat and therefore may enhance key services such as food production (Auster et al. 1996).
The high diversity of infaunal species in the deep sea raises many questions about rates and redundancy that are largely unanswered (Snelgrove & Smith 2002). The relative importance of species diversity for the efficiency of the deep-sea functions discussed above has not been tested experimentally. In general, measures of macrofaunal density, biomass, or diversity have been poor predictors of functions such as bioturbation, whereas particulate organic carbon (POC) flux and densities of selected megafauna can be good predictors (Smith 1992; Smith & Rabouille 2002).
One formidable challenge is to determine whether diversity at the level of habitats, functional groups, species, genes, or gene expression (functional genomics) is most critical for sustaining ecological processes and services. Recent research has considered the role of landscape configuration (Archambault & Bourget 1999) and the effect of anthropogenic modifications and structures on estuarine biodiversity (Chapman & Bulleri 2003). These foci have potential applications for restoring and conserving biodiversity in the face of growing pressures for increased coastal development; they also have potential consequences for processes and services.
Theory based on the terrestrial literature suggests that if each species performs a function slightly differently, then sediments with high diversity are likely to achieve the most effective function (i.e., sampling effect) (Loreau et al. 2001; Zedler et al. 2001). Interspecific facilitative interactions are particularly likely to enhance functions in areas with low oxygen, high sulfides, food scarcity, physical disturbance, or other stressors (Levin et al. 2001b). Structures on the sea floor such as polychaete feeding mounds, tracks in sediments from surface burrowers, and discarded shell material provide heterogeneity, which facilitates adults and juveniles of many deep-sea species, providing food, substrate, and refugia (Levin et al. 1997; Snelgrove & Smith 2002).
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