Thursday, July 25, 2013

Two western garter snakes proposed as threatened

Thamnophis rufipunctatus
The U.S. Fish and Wildlife Service propose to  list the northern Mexican gartersnake (Thamnophis eques megalops) and narrow-headed gartersnake (Thamnophis rufipunctatus) as  threatened taxa on July 10, 2013. Comments are accepted on this proposal until September 9, 2013.

The northern Mexican gartersnake had a limited historical distribution in New Mexico that consisted of scattered locations throughout the Upper Gila River watershed in Grant and western Hidalgo Counties, including the Upper Gila River, Mule Creek in the San Francisco River sub-basin, and the Mimbres River. The species was detected at only 2 of 11 historical localities along the northern-most part of its range from which the subspecies was previously known. The only viable northern Mexican gartersnake populations in the United States where the subspecies remains reliably detected are all located in Arizona.  In New Mexico, the northern Mexican gartersnake may occur in extremely low population densities within its historical distribution; limited survey effort is inconclusive to determine extirpation. The status of the northern Mexican gartersnake on tribal lands, such as those owned by the White Mountain or San Carlos Apache Tribes, is poorly known due to historically limited survey access and less is known  about the current distribution of the northern Mexican gartersnake in Mexico due to limited access to information on survey efforts and field data from Mexico.

After no confirmed sightings in nearly twenty years, scientists discovered three northern Mexican gartersnakes along a stretch of the Gila River in southwestern New Mexico in June. After nearly three years of intensive searching, scientists with the BioPark Zoo in Albuquerque discovered three young males along the Gila, suggesting  at least one viable reproducing population of the northern Mexican gartersnake exists in the region. Northern Mexican gartersnakes inhabit riparian wetland and prey on tadpoles and small fishes. More than 90% of their habitat has disappeared due to overgrazing, water diversions, wildfires and drought.
A proposed diversion of the Gila would alter the river’s modest flood pattern and transport  water to an off-channel reservoir via a pipeline. This would  damage some of the last  remaining habitat in the Gila drainage.

The historical distribution of the narrow-headed gartersnake ranged across the Mogollon Rim and along its associated perennial drainages from central and eastern Arizona, southeast to southwestern New Mexico at elevations ranging from 2,300 to 8,000 ft (700 to 2,430 m). The species was historically distributed in headwater streams of the Gila River sub-basin that drain the Mogollon Rim and White Mountains in Arizona, and the Gila Wilderness in New Mexico; major sub-basins in its historical distribution included the Salt and Verde River sub-basins in Arizona, and the San Francisco and Gila River sub-basins in New Mexico Researchers suspect the species was likely not historically present in the lowest reaches of the Salt, Verde, and Gila rivers, even where perennial flow persists. Numerous records for the narrow-headed gartersnake (through 1996) in Arizona are maintained in the GFD’s Heritage Database. The narrow-headed gartersnake as currently recognized does not occur in Mexico. Recently narrow-headed gartersnakes were detected in only 5 of 16 historical localities in Arizona and New Mexico surveyed in 2004 and 2005. Population densities have noticeably declined in many populations, as compared to previous survey efforts.

The narrow-headed gartersnake  populations have likely deteriorated as a result of declines in resident fish communities due to heavy ash from wild fires and sediment flows, but subsequent survey data have not been collected. If the Whitewater Creek and Middle Fork Gila River populations did decline as a result of these factors, only three remaining populations of this species remain viable today across their entire distribution. Unnaturally large wildfires have become increasingly common across the Mogollon Rim of Arizona and New Mexico where the narrow-headed gartersnake historically occurred.

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