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. Maximum Allowable Disturbance Areas for Preble's Meadow Jumping Mouse in Ditch-side Habitat A White Paper for the Collaborative Planning Process March 8, 1999 Written by Chris Pague for the PMJM Science Team I. BACKGROUND The U. S. Fish and Wildlife Service (USFWS) is attempting to determIne if some ditch maintenance activities do not reach the threshold of "take" (Endangered Species Act as amended in 1988) regarding the Preble's meadow jumping mouse (PMJM) or its habitat. Discussions between the USFWS, ditch users, and the Science Team (represented by the author) occurred on two occasions. As a result, the USFWS is drafting a letter that will provide guidance for ditch maintenance activities relative to implementation of the Endangered Species Act and PMJM. The conversation precipitated specific questions about the significance of small habitat patches and the maximum area that could be impacted without having deleterious effects on an individual PMJM. . Specifically, the USFWS wants to know the minimum habitat patch size that would be potentially occupied by a PMJM and how much alteration of habitat could be sustained in occupied habitat. These questions were forwarded to the PMJM Science Team. The team discussed and debated this issue on three separate occasions. The issue of management of ditches and potential restrictions is significant for its potential social and economic repercussions. PMJM does inhabit at least some suitable habitat along ditch corridors (e.g., Hygiene, North Fork Cache la Poudre, and South Boulder Creek). . However, our understanding of the distribution along ditches is meager, since sampling of ditches has been relatively rare. Only a small percentage of total occupied habitat (in Colorado) occurs along ditches. But in a few populations, as noted above, ditch-side vegetation is occupied. It is the general opinion of the Science Team that historical/ongoing patterns of ditch maintenance, at least in these populations, are generally supportive of PMJM populations. This was reflected in an exercise to determine the most significant threats to PMJM. Ditch maintenance ranked as a low priority in all 5 "subareas" in Colorado (pMJM 1998, unpub. Data). This was based on the existing threat to the populations, not to individual mice. In fact, in one case (in the vicinity of Hygiene) ditch locations and resultant vegetation may have prevented a local extirpation (albeit incidentally). The presence of mice in ditch-maintained habitats presents a conflict since PMJM persists because of the activity while individual mice may be threatened by the same activities. .