Field Notes
U.S. Fish & Wildlife Service
Field Notes Entry   
Service and Rogers State University Begin USACE-Funded Study to Monitor the Effect of Cave Passage Modification on the Microclimate of a Cave Used by a Gray Bat Maternity Colony
Southwest Region, February 4, 2008
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Richard Stark of the Oklahoma ES Field Office; Steve Hensley, manager of the Ozark Plateau National Wildlife Refuge; and Dr. Keith Martin, Rogers State University, deployed relative humidity and temperature loggers and a light intensity logger in Beaver Dam Cave located in Delaware County, Oklahoma, to gather baseline microclimate data of the cave prior to implementing a conservation measure.  The water level in Beaver Dam Cave, which is used by a maternity colony of gray bats, is affected by water levels in Grand Lake.  The Federal Energy Regulatory Commission (and Grand River Dam Authority) and the U. S. Army Corps of Engineers control the power and flood pools of the reservoir, respectively.  The Service anticipates take of gray bats within Beaver Dam Cave near Grand Lake due to flooding associated with the lake operation that can result in inundation of the main flyway.  During an exit count effort conducted on July 19, 2007, two smaller (<1-foot wide and tall), higher elevation passages were found that potentially would allow bats to exit and re-enter the cave and roost room while the main flyway is inundated.  However, it is highly likely that the small size of the passages currently limits their use as a flyway, especially when considering the size of the bat colony (about 12,000 bats).  A conservation measure currently being discussed would consist of increasing the size of these passages.  Higher elevation passages would greatly enhance the ability of bats to enter/exit the cave and would reduce the chance of take by providing an exit for the bats when the main flyway is inundated.  The possibility of take of non-volant young also would be reduced because the enlarged high passages would allow easier movement of adult females in and out of the cave.  The adult females, therefore, could continue to take care of their non-volant young should the main flyway become inundated during the early summer.  Although enlarging the cave passage would be implemented as a conservation measure for the gray bat and we believe this action would be beneficial, it is important to monitor the effect of the proposed cave passage modification on the microclimate of the cave.  Gray bats have very specific cave requirements, and only those caves with the appropriate microclimates are used as maternity roosts and hibernacula.  As a result of their very specific requirements, it is estimated that less than 5% of available caves provide suitable roosting habitat.  Anthropomorphic modification of cave passages possibly could affect the internal microclimate by altering air flow.  The Service anticipates working with the U. S. Army Corps of Engineers and the Grand River Dam Authority to 1) continue to obtain baseline temperature, humidity, and light intensity data of the cave during the winter and summer seasons prior to enlarging the passages, 2) enlarge the higher elevation passages during the fall/winter when bats are not present, 3) subsequently monitor the effect of the passage modification on the temperature, humidity, and light intensity of the cave, and 4) conduct exit counts during the maternity season following passage modification to determine if the gray bat colony accepts the modification. 

Contact Info: Martin Valdez, 505-248-6599, martin_valdez@fws.gov

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