Paul S Giller Alan P Covich Katherine C Ewel Robert O Hall Jr and David M Merritt

Society obtains great benefits from properly functioning ecosystems in the form of provisioning (e.g., food), supporting (e.g., waste processing, sustained supplies of clean water), and enriching (e.g., recreation) services, all of which are provided at multiple scales and at no charge to society. Freshwater benthic ecosystems often play important and unique roles in providing many of these services (see Chapter 3, Table 3.1), but the number and magnitude of anthropogenic stressors that threaten these services is growing rapidly. Sustainable development, which "meets the needs of the present generation without compromising the ability of future generations to meet their own needs" (World Commission on Environment and Development 1987), depends on our ability to manage and maintain these ecosystems and the services they provide. In order to achieve this end, we need a better understanding of how benthic ecosystems function and are structured, as well as stronger integration of management with ecological studies. Having insulated ourselves from many natural ecosystems through technology, we often fail to appreciate the beneficial "workers" that sustain "nature's economy" upon which ecosystem services depend.

We now appreciate that degradation of freshwater sediments, which harbor the biota essential to benthic ecosystem processes, will in turn degrade water quality and a range of other services. We also understand that because of the strong linkage between freshwater ecosystems and the landscapes they drain (Giller & Malmqvist 1998), changes in land use and other activities in the catchment can contribute to such degradation. Many

We are very grateful to three anonymous referees for their constructive insights and comments. We are also grateful to Willem Goedkoop for his contributions to the discussions and ideas that initiated this chapter.

current water management practices, such as flood control, water diversion and detention, channelization, and irrigation, affect the hydrological cycle at local to catchment scales. Over the past several hundred years, humans have built thousands of kilometers of diversion canals, channels, and levees to divert water for society's use. Humans have drained wetlands for urban development and agriculture, and have dammed rivers for water abstraction and the generation of hydroelectric power. Although these activities are intended to provide certain important services to the human population, they also significantly degrade many other services, the values of which become evident only when they are lost or destroyed. The examples of the demise of the Aral Sea in Central Asia (Micklin 1992) due to the diversion of inflowing freshwater streams, and the ongoing threats from paper mill effluent to the species-rich and globally unique freshwater biota of Lake Baikal in southeast Siberia, are clear cases in point. On a larger scale, climate change, an unintended consequence of human activities, also alters the hydrological cycle, threatening freshwater habitats and organisms (and hence a range of services) throughout the world (Palmer et. al. 1997; Lake et al. 2000; Wall et al. 2001). The threats of such activities on sustainable development are clear.

The various types and importance of ecosystem services in fresh waters, and the role of benthic biodiversity in the delivery of these services, are presented in Chapter 3, along with a discussion of the balance between ecological and economic values. In this chapter, we will briefly review the various threats to freshwater benthic ecosystems and the important benthic species that help sustain ecosystem services. We also consider the vulnerability of these services, using a number of case studies to illustrate the cascading effects of overexploitation and the subsequent loss or degradation of other services. These case studies also illustrate how benthic organisms and the ecosystem services they perform can be used to enhance management and maintain the overall health and sus-tainability of freshwater systems.

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