is all but certain that they do, but we have far too little information about such matters, which may affect the overall functioning of these aquatic ecosystems and also be important to the success or failure of threatened and endangered species.

Example 3. In a broader way, we may ask how riparian wetlands, which are extensions of the littoral zones of streams and rivers, influence the functioning of the actual littoral zones and their adjacent water bodies. Wetzel (1990, 1992) has provided a general account of effects on carbon cycling, but much remains to be done in terms of the cycles of nutrients and toxins and their effects upon species composition, productivity, food chains, etc. The invasion of North American wetlands by the Eurasian exotic purple loosestrife (Thompson et al., 1987), as it affects the functioning of adjacent streams and lakes, is a specific case worthy of further investigation in this context. Also relevant are the movements of organisms such as insects, amphibians, and furbearing mammals to and from uplands, wetlands, and adjacent littoral areas of streams and lakes.

Example 4. In a related question, what is the influence of streams and lakes on the plant and animal communities of adjacent wetland and upland ecosystems? This is an important matter that concerns the design of buffer zones around lakes and the management of riparian zones.

Example 5. How will effects of global warming on wetlands influence ecosystem structure and function in the streams and lakes that receive inputs from them (for instance, of nutrients such as nitrogen and phosphorus, acidifying agents such as organic (humic) acids and sulfuric acid, or stored metal and organic pollutants)? Meyer and Pulliam (1992) provide some interesting insights that suggest future research directions.

Example 6. An extremely important problem in landscape ecology (Risser, 1987), as well as in regional and global studies, is that of scaling up from purely local studies of individual ecosystems to develop understanding at the landscape or higher level. It is here that interactions among aquatic (and upland) ecosystems are of major importance, because they must be taken into account at each successive step in the scaling process if we are to understand adequately, for instance, nutrient cycling and the processing of toxic contaminants, as well as their influence on species composition, productivity, carbon storage, etc.—all vital components in the functioning and management of the biosphere.

In many cases, processes of ecosystem interaction will be nonlinear and may involve threshold and stochastic phenomena, making it necessary to study them over long periods for adequate understanding (Likens, 1989). For instance, consider the influence of peatlands on small lakes in the same catchment and receiving drainage from them. The studies of Gorham et al. (1986) indicate that the influence of such peatlands on the concentrations of colored organic acids in the lakes increases in nonlinear fashion with the ratio of peatland area to lake area. Even a very small proportion

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