1. Ecological evaluation of ecosystem service. Society has alternative uses for fresh water, which are associated with competing demands for particular quantities and qualities of water. For example, in Central Asia, increased diversions of water from the Amu and Syr Darya Rivers expanded production of irrigated agriculture and other upstream uses beginning in the early 1960s, but resulted in major declines in fish production and water-based transportation after the Aral Sea partially dried and became more saline. The results included loss of aquatic species and endangerment of human health in and around what had been the fourth largest lake in the world (Williams 2002). Another example of these trade-offs for competing demands for fresh waters led to ecosystem degradation resulting from lower lake levels in Mono
Lake, east of the Sierra Nevada Mountains of California. Demands for fresh water increased rapidly in the Los Angeles Basin and water was diverted from the Owens River in the Mono Lake Basin. These diversions to Los Angeles resulted in fewer breeding sites for migratory waterfowl and changes in lake food webs as salinity increased (Hart 1996). Local community action eventually restored the integrity of the lake ecosystem (Loomis 1987, 1995). These sorts of trade-offs require careful ecological evaluation of the full range of ways in which water allocations and species loss may alter ecosystem services. Ecologists and economists are beginning to quantify trade-offs among different uses of ecosystems influencing water quality and yield (Whigham 1997; Loomis et al. 2000). As is discussed below, more research is needed to evaluate how loss of species diminishes or eliminates critical ecosystem services.
2. Economic evaluation of ecosystem services. Many natural freshwater ecosystem processes have definable economic values (Abramovitz 1998; Pearce 1998) as well as the non-use, existence, and aesthetic values that must also be evaluated in ways that reflect the importance of protecting benthic species and their habitats. Methods to determine economic values include market pricing, contingent evaluation, cost-benefit analysis, consideration of replacement costs, and prices of substitute goods and services, if any (National Research Council in press). Values of ecosystem services such as the production of high-quality drinking water or storm mitigation (protection of river banks and lake shores by riparian vegetation) can be estimated by determining how much people are willing to pay for, or, if possible, to replace these services (Cleveland et al. 2001; Daily & Ellison 2002). The costs of comparable substitutes (e.g., in the form of engineered replacements for natural benthic ecosystems) provide one means to evaluate the economic values of some natural services provided by sediment-dwelling organisms. For example, building filtration plants to provide clean drinking water for New York City could cost from US$2 to 8 billion (Foran et al. 2000; O'Melia et al. 2000; Gandy 2002), while protection of the watershed's natural communities of benthic invertebrates and improved riparian management is likely to save many or all of the costs of building filtration plants (Daily & Ellison 2002). On-going studies of water quality and stream invertebrates by ecologists at the Stroud Water Research Center in Pennsylvania, USA, are documenting these ecosystem services (Bern Sweeney, personal communication 2003). Replacement costs of wetlands that provide natural filtration are also generally large (Mitsch & Gosselink 1993; Bedford 1996; Williams 1999) because of the complex nature of these ecosystem processes performed by numerous sediment-dwelling species of plants and animals. For example, more than US$500 million is being spent to restore 11,500 hectares along 90 km of the old river channel of the Kissimmee River Basin in Florida (Dahm et al. 1995; Toth et al. 1998). Services from the natural meandering river and its floodplain were lost in the 1960s when 167 km of the river was channelized and 21,000 hectares of wet lands were drained (Whalen et al. 2002). Other attempts to replace lost services with artificially constructed wetlands have had limited success, especially if native species and habitat structures are not included in the design (Zedler 2000; Bonilla-Warford & Zedler 2002; Stevenson & Hauer 2002). Similarly, as discussed in Chapter 6, attempts to restore ecosystem services provided by benthic organisms in European rivers (following industrial pollution, channelization, and dam building) continue to face serious constraints (Cioc 2002). In many cases there are no satisfactory and sustainable substitutes for natural ecosystem services.
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