1997-06-13_REPORT - M1981302 (11)

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ungulates from excessive congregation in riparian habitats throughout the year. As the stream channel continues to change over time, becoming deeper relative to an orchid site due to downcutting or sediment deposition or moving laterally farther away from an orchid site, seasonal hydrology also changes so that an orchid site experiences drier conditions. This causes the vegetation community to become dominated by upland grass and forb species. With time, riparian trees may establish and shade orchid sites as well . Under these conditions, the orchid is less competitive and begins to die out. Ute ladies'-tresses are expected to be scattered along stream systems and associated floodplain areas with appropriate hydrology. A particular orchid occurrence will persist as long as hydrologic and vegetation community conditions remain favorable. The longevity of an orchid occurrence at any particular location likely ranges from a few years to more than 100 years. Thus, over decades, it may not be possible to determine exactly where an orchid population will be encountered along a stream because the stream channel and associated riparian area will always be changing. However, as long as these dynamic conditions continue, the orchid will dependably occur along the stream system where favorable habitat is found. As mentioned, the model described above is consistent with what is now known about the habitat preferences of the Ute ladies'-tresses. However, many populations of the orchid, particularly those along the Front Range of Colorado, exist under habitat conditions that are maintained by management activities such as irrigation and grazing rather than by natural stream processes. Other populations occur in association with isolated seeps and springs. Many aspects of this model have yet to be verified. It is expected that this model will be refined as new information becomes available through ongoing and proposed research. Based on this model , the continued existence of the orchid along a stream system requires either (1) direct manipulation of habitat to maintain necessary hydrologic and vegetation community conditions (e.g. , by irrigation or stream channel manipulation, and mowing, grazing, or other vegetation management methods) or (2) assurance of the continual creation and evolution of favorable habitat conditions resulting from natural stream dynamics. Of these options, the latter, ensuring the conditions that allow natural stream dynamics to create and maintain preferred orchid habitat, is in the long run the most dependable and ecologically desirable way to guarantee the viability of the orchid in perpetuity. F. Reasons for Listing Orchid species are never common. The Ute ladies'-tresses historically occurred over a wide range but was distributed as scatterings of small populations in suitable habitat within this range. It never dominated local vegetation communities. As previously described, the orchid depends upon natural stream processes, and likely also natural ungulate population levels and behavior, to create and maintain habitat. Both of these environmental 10