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Show for huge populations and many species of birds have developed migratory behavior that capitalizes on these resources. Variability within natural limits is good for ecosystem productivity. ( SRC, 1999c) Sustainability and Ecosystem Health Sustainability is " a system's ability to maintain its structure ( organization) and function ( vigor) over time in face of external stress ( resilience) "( Costanza, 1992). A system responds to outside stresses and nature of the stress may even be more significant ( Smol, 1995). ( SRC, 1999c) A well buffered system is able to maintain its structure and function ( SRC, 1999c). Biodiversity is another important consideration for planners and managers. A " healthy" ecosystem is one that existed before significant anthropogenic impact ( Smol, 1992), but ecosystem health is difficult to define. As environmental conditions change, the system adjusts to these changes. Therefore an " unhealthy" condition is beyond the natural range of fluctuation due to conditions resulting from some human- induced modification of the system. ( SRC, 1999c) Ecosystem Impacts There are several types of environmental impacts that managers consider in planning and managing for important natural resources. Managers consider short- and long- term, immediate and site- specific impacts. There are also adverse, unavoidable, irreversible and irretrievable impacts. Direct impacts are the result of circumstances or activities that occur at the same time and place and hence alter a system. Indirect impacts are further removed but are still reasonably foreseeable and influence a system. Cumulative impacts occur when there are multiple effects on the same values. Gradual impacts occur on resources when combined with past, present and future actions ( BLM, 1998). There are many direct, indirect and cumulative impacts to GSL and its environs. The following list cites a few examples of human- related direct and indirect GSL impacts: Dikes and causeways Brine shrimp harvesting Exotic species introduction Mineral extraction Oil and gas production Lake level modification Recreational activities Grazing Discharges/ Accidental spills Upstream water allocation Water and air quality Population growth Wetland- nutrient loading Loss of GSL wetlands Agriculture activities Road salts Mosquito abatement Trash and pollution 105 |