These case studies provide a spectrum of examples that demonstrate not only the ways in which ecosystem services are provided by freshwater benthic species, but also how they are vulnerable to human activities. The studies also provide some lessons that can be carried over into creating improved management of complex, interconnected ecosystems. A central feature of vulnerability of these benthic species is that, although freshwater is widely available, it is often extremely and unevenly distributed. Consequently, there are significant geographical disparities in the frequency and intensity of threats to the benthic biota and their associated ecosystem services. The trade-offs between ecological and economic values that are facing managers will be drastically different in the arid zones of Africa or India than in the arid western United States.
Trade-offs can be complex in wet or dry regions when exploitation of an ecosystem for one service eventually becomes a disservice relative to other needs. When managing for optimizing one service entails obstructing or even destroying the capacity to enjoy another service, either from the same ecosystem or from another ecosystem, planners must rationalize benefits from each service as well as the possibility of mitigation of effects in advance.
Managers often focus on a single problem and then seek to enhance a single ecosystem service to resolve the problem. For instance, designating the Catskill Mountains as a protected watershed for supplying New York City with fresh water provides a complex case study for other cities to consider. Will this approach establish a sustainable system for obtaining potable water without other unintended consequences? It is not clear what the effects of deflecting inflows for New York City's use will have on the Hudson River. Will the complexity of habitats that would have supported a greater number and diversity of fish and benthic infauna be affected by this alteration of flow? Will saline waters move farther upstream on the tidally altered portions of the Hudson River during droughts and thus affect other water supplies or certain benthic species' roles in providing other needed ecosystem services? Another poignant example of our inability to manage "single-service contracts" with freshwater aquatic ecosystems is the increased mercury contamination in the Everglades, now the scene of dramatic and expensive efforts to restore the suite of ecosystem services that it once provided. Restoring and preserving watersheds, redirecting wastewater to specially constructed wetland ecosystems, and guarding against the introduction of alien species are important goals—but complete analysis also requires comprehensive studies of inputs from the airshed. Mercury contamination from rainfall containing metals derived from burning fossil fuels persists as a major issue even if water pollution and hydrology can be managed to sustain the benthic biota. The Pantanal provides a positive example of a vast and complex landscape that continues to sustain high productivity in a mixture of wetland ecosystems that change shape and chemistry as wet and dry seasons alternate. Although the Pantanal is probably a fragile collection of interdependent ecosystems, and important parts may yet be lost to the threats that impinge on it, its example impresses on us the reality that assaults to a benthic community may ultimately be repairable. This is the hope for many of the severely stressed freshwater systems in Asia and Africa that have lost most of their most important natural provisioning and support ecosystem services (especially provision of potable clean water) through excessive inputs of pollutants. Generally, freshwater ecosystems are resilient to many kinds of short-term threats, once the perturbation stops and recovery becomes possible (Resh et al. 1988; Jansson et al. 1999). Much of this resilience and resistance can be attributable to the benthic community, which seems to provide a stabilizing interface between the physical environment and the nonbenthic community, and hence many of the services the freshwater ecosystems provide.
Risk analyses, to help balance our demands on valuable ecosystems more effectively, depend on the knowledge of what human activities are damaging, how such damage can be avoided, and the extent to which ecosystem services that are currently impaired can be restored. In order to offer such advice, we also need information on what governs the production of ecosystem services, the role of biodiversity in the sustainability and level of the services, and how production of services changes under altered conditions. Can we rank the threats in order to provide some guidance as to what actions are the most important to avoid and what are the most useful for beneficial restoration? We might speculate that geomorphic alteration is the most serious, as freshwater systems are not resilient to this sort of change. Chemical pollution and local extinctions can be more easily mitigated against and recovery is usually rapid. Large-scale watershed/catchment perturbations (such as changing land use) and resultant hydrochemical changes are far more significant again than local point source pollution. Invasive species may or may not be significant, depending on how they interact with the native communities and the scale of activity and population growth. Pulse disturbances (which occur over a rea-
sonably limited spatial and temporal scale), be they hydrological (such as natural drought or flood) or chemical (such as pollution events), have limited long-term impact due to the high resilience of most freshwater systems. On the other hand, longer-term directed press disturbances (such as acidification, eutrophication, human-induced climate change, and hydrologic regulation) will have a greater impact on the ecological communities and hence on the provision of ecosystem services. The extent to which biodiversity provides some "insurance" against such changes is not clear at present, but the evidence from evolution suggests that some species may adapt to change while others may become extinct, thus, the provision of some ecosystem services may remain. For freshwater systems, however, this insurance is also at risk, as available information suggests that freshwater biodiversity has declined much faster over the past 30 years than either terrestrial or marine biodiversity. The greatest effects appear to be in the densely populated regions of the tropics (particularly South and Southeast Asia) and in dryland areas (Jenkins 2003). This complex linkage within and among ecosystems, like the example of the River Rhine, whose water quality has been improved enough to see the reappearance of many species of aquatic insects and fishes, will benefit from continued long-term monitoring and analysis of complex trade-offs inherent in management decisions.
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