Maintaining Biodiversity Of Freshwaters

From an ecosystem perspective, the reason to preserve biodiversity is to preserve options for the future. The environment is certain to change because of natural fluctuations in the physical forcing of the biosphere and large-scale anthropogenic effects. We cannot predict which building blocks will be essentia! for maintaining ecosystem or landscape processes in future environments. It is prudent to preserve taxa that may be crucial for ecosystem processes under new conditions. At a large scale, a diversity of ecosystems representing a range of biogeochemical processing characteristics will be essential for assembling reasonably self-sustaining landscape in an altered world.

A related reason to conserve biodiversity is to preserve the resistance, resilience and compensatory capacity of indigenous assemblages. Exotic species in freshwaters create management problems that cost large amounts of money {Drake et al. 1989; National Research Council 1992). Intact ecosystems may resist invasion or recover from invasion more rapidly than depauperate ones. For example, all of the Laurentian Great Lakes were invaded by alewife. which became the primary forage base for salmonids (Christie 1974). Alewife populations cycle explosively, but may be undergoing a long-term decline due to overstocking of salmonids, a series of cold winters and other factors. In Lake Michigan, effects of the alewife decline are compensated by expansion of the native club populations. Lake Ontario, in contrast, has lost its native club populations and a sustained alewife decline could lead to collapse of the fishery.

In freshwaters, managing at the scale of whole ecosystems (freshwaters and their catchments) is inevitable. Freshwaters tend to be insular ecosystems with strong internal interactions, so it is difficult to manage isolated components without considering feedbacks to other parts of the ecosystems. The worldwide homogenization of freshwaters has a few common drivers, all of which act at the landscape or ecosystem scales. Experience in aquatic ecosystem restoration shows that these large-scale drivers must be addressed, or restorations will be unstable and require continual maintenance (National Research Council 1992).

If societies want to sustain a diversity of freshwater ecosystem types, it is necessary to constrain inputs of silt and nutrients to levels commensurate with regional geochemistry, to allow a diversity of hydrologic regimes, to manage for diverse fish communities (which may entail high variance among different freshwater ecosystems on the landscape), to limit the spread of exotic species, and to decrease the release of pollutants. From a policy standpoint, these steps may be most feasible in smaller lakes and streams. In landscapes where such systems are abundant, it may be possible to sustain a diversity of ecosystem types. In large lakes and rivers, where stakeholders are numerous and a single ecosystem is in question, policy issues are much more difficult (Lee 1993).

Reliable scientific guidelines for restoring and sustaining freshwater biodiversity are within our grasp. Worldwide patterns in the major stressors, ecosystem responses and certain strong community interactions suggest that a set of general solutions to freshwater problems can be found. Research programs to develop these solutions will be large in scale (whole watersheds and freshwater ecosystems for enough time to judge baseline variability and responses to manipulations or restorations), recognize humans as dynamic, interactive components, and include explicit mechanisms for assessment of learning and predictive capability.

The rate of change in ecosystems may now exceed the rate at which scientists can fully understand them. The limited scientific resources available for work on biodiversity should be channeled toward the scale of the problem. If wc are going to conserve freshwater biodiversity, we need to get started now. We propose the following hypothesis to guide freshwater conservation and research.

Maintaining or restoring biodiversity of freshwater ecosystem types will require the maintenance or restoration of biodiversity in terrestrial landscapes of biodiversity in terrestrial landscapes of the catchments. By restoring or maintaining ecosystem biodiversity, the biodiversity of the constituents (communities, populations, species and genotypes) of ecosystems is restored or maintained.

This hypothesis requires us to "learn by doing" (Walters and Holling 1990). The watershed is the minimal spatial unit. Management actions at the ecosystem scale (watershed land use, hydrologic interventions, pollution control and fisheries practices) pose the greatest policy challenges. Once these are underway, they establish a matrix for maintaining or restoring other components of freshwater systems. In the process of testing the hypothesis, we will initiate conservation of freshwater biodiversity at large scales. These projects will become management experiments that allow us to learn which methods are most successful at landscape, ecosystem, community, population and genetic levels.

Some objections to this approach are easily foreseen and can be addressed now. Some critics will caution that a program of rigorous mechanistic experimentation would eventually allow us to proceed in large-scale conservation programs with more confidence. This argument will be echoed by factions interested in delaying the sacrifices required by conservation. However, learning by doing and rigorous experimentation are not mutually exclusive. We see many opportunities for rigor in the context of large-scale conservation programs. More importantly, if action is delayed until the science has advanced sufficiently, many conservation opportunities will be lost, or systems will have degraded to the point where far more expensive and difficult restorations are necessary.

Institutions, including large research programs, can develop habits that impede learning from experience (Hiiborn 1992; Levin 5992; Schindler 1992). On the other hand, many positive examples exist of rapid progress in large-scale management experiments (Holling 1978; Kitchell 1992; National Research Council 1992; Lee 1993). The success of biodiversity conservation programs depends on far more than scientific information. We have the opportunity to reduce key uncertainties, with important implications in the context of large-scale conservation programs, if societies have the will to sustain freshwater biodiversity, the general direction of the first steps is clear. We can take those steps and learn in the process.

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