Since early in this century when rainbow trout were introduced, and possibly even before, brook trout have been retreating upstream in these mountains. In the late 19th century, brook trout occurred as low as 500 meters (1,600 feet); now they are found mostly above 915 meters (3,000 feet). The effects of logging and competition from rainbows are the most frequently suggested reasons for this retreat. Logging, which began on a large scale in the Smokies about 1900, brought with it many fires. The resulting exposure to full sunlight caused the warming of low-elevation sections of streams. Erosion of the denuded land added heavy loads of sediment to the streams. These changed conditions, and possibly heavy fishing pressure, apparently speeded the disappearance of brook trout from the lower elevations. Rainbows were introduced and proved able to survive. In the ’20s and ’30s it was noted that rainbows occurred in streams up to about the upper limit of logging, and that brook trout occupied streams above that point. Now, however, streams are once again shaded by forests their full length; but brook trout, instead of moving back down, seem to have retreated higher upstream. It appears that the larger, more aggressive rainbows somehow prevent brook trout from reoccupying their lost waters. The National Park Service is concerned for the future of the Smokies’ one species of native trout, and especially for the few isolated populations of brookies that may still remain unmixed with populations of brook trout introduced from other parts of the country. On some streams, waterfalls provide effective barriers to the advance of rainbow trout, and use of artificial barriers for this purpose has been considered. Stringent fishing regulations may help the easily caught brook trout—and the gluttonous poaching that sometimes eliminates large numbers of brook trout from long stretches of a stream must be stopped. This type of management problem, how to preserve native species and reduce the impact of exotic ones, is common in national parks. It is only one aspect of a larger problem: How do we maintain natural ecosystems in parks? This basic aspect of the national park idea is difficult to implement in a country where human influence is so ubiquitous.