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Which additional watersheds will become vulnerable to irreversible change?
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During the long period of natural selection, prior to human disturbance, populations in the linked terrestrial/aquatic system evolved complex feedbacks that permitted a relatively stable system. The further the systems are forced away from this natural state, the greater the probability that the systems will be unable to respond stably to natural disturbances and normal variations in environmental conditions.
It is not even theoretically possible to predict the critical threshold beyond which the ecological system will move to a new and undesired state. However, one can estimate how far the watersheds have already been moved from their
natural state. As human activities add stressors (e.g., chemical pollutants), extract resources (e.g., lumber), and change landcover (e.g., fragmentation) the natural feedbacks are disrupted and the system becomes more vulnerable to radical and potentially irreversible change.
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The watersheds that are furthest from the natural state in 2020 remain concentrated in the eastern urban complex. The watersheds adjacent to Pittsburgh and Raleigh are no longer among the top 20. Five watersheds, indicated in dark blue, have become more vulnerable to irreversible change. These watersheds are located in suburban areas around the eastern urban complex. However, three of these watersheds were already among the top 20 at the present time. The two new watersheds that appear to enter the top vulnerability class are Lower Susquehanna-Swatara and Shenandoah.
No single factor causes the degradation in the two watersheds and both show a similar pattern. Total forest cover only changes by 5-9% of current values. Riparian forest shows a 4-15% change and Wetlands decrease by 13-14% of current values. The most distinct change is in interior forest habitat. More than 50% of the current interior forest is lost on both watersheds.
The stressors that result in the degradation are varied. There is a 3-8% increase in human land cover and a 20-31% increase in population in these suburban areas. There is a small increase in nutrient inputs to the aquatic systems. There is also a notable increased in terrestrial exotic species (~50% over current values) and in aquatic exotics (~20% over current values).
In the case of the urban areas shown in red in both maps, the natural ecological systems have become dominated by societal controls and are unlikely to change spontaneously. However, it should also be recognized that these watershed have already been so altered that they are unlikely to return to their pre-human natural state if human controls were removed. They may already have been subjected to irreversible change.
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