Mount Graham Red Squirrel


Mount Graham Red Squirrel

Mount Graham red squirrel

Photo by Paul Young 



The Mount Graham red squirrel (Tamiasciurus hudsonicus grahamensis) is one of 25 subspecies of red squirrels found throughout North America. The Mount Graham subspecies, which is found only on the Pinaleño (Graham) Mountains of southeastern Arizona, was thought to have been extinct in the 1950's, but small numbers of squirrels were "rediscovered" in the 1970's. The squirrel was added to the federal endangered species list in 1987 by the U.S. Fish and Wildlife Service, when the estimated population in 1986 was fewer than 400.

The Mount Graham subspecies has been isolated from other subspecies of red squirrels since the end of the Pleistocene glacial periods. It is still rather unclear if the Mt. Graham red squirrel is distinct or not from red squirrels elsewhere. Studies on genetic data are in progress.

 Initially, some biologists believed that the Mount Graham red squirrel could only survive in spruce-fir habitat on the Pinaleño range, resulting in the designation of 1700 acres of the highest elevations (10,000 feet) as critical habitat. This area was given refugium status with access restricted by a permit system administered by the U.S. Forest Service. Since 1989, significant numbers of red squirrels have been found at lower elevations on the mountains, calling into question the validity of designating only spruce-fir as critical habitat and the effectiveness of the refugium restrictions. Currently red squirrels are found throughout the mixed-conifer and spruce-fir habitat zones, from elevations of about 7800 feet on the north and east slopes to 10,720 feet on High Peak. The greatest number of squirrel middens and squirrels are found within the transition zone between mixed-conifer and spruce-fir habitat. The autumn 1993 census found about 55% of the total population in this zone, with about 25% and 20% in the spruce-fir and mixed-conifer zones respectively.

Hypothesis assumed for the biological assessment:

  • Squirrels main habitat is in the spruce-fir zone
  • When squirrel is rare (pop. numbers down) the bulk of population is in the spruce-fir zone
  • Some squirrels are in the mixed conifer but this zone is not as important as the spruce-fir zone



Main squirrel habitat


What the current data actually shows:

  • More squirrels are in the mixed conifer than in the spruce-fir zone
  • When squirrel is rare (pop. numbers down) bulk of population is in the mixed conifer zone
  • Spruce is important, but the key habitat is mixed conifer
Squirrel habitat



The U.S. Forest Service, in conjunction with the U.S. Fish and Wildlife Service, Arizona Game and Fish Department, and The University of Arizona biologists, has conducted a biennial census of the red squirrel population since 1987. From 1989 to the present, the Mount Graham population has increased and decreased in numbers that correspond to changes in the conifer seed crop, the squirrels' primary food resource. This result is not surprising since studies of red squirrels in other geographic regions have clearly demonstrated that red squirrel populations are controlled by their food supply. Since 1986, the U.S. Forest Service census has produced estimates of the total Mount Graham red squirrel population: mgrscensus2010.pdf

As part of its permit for the Mount Graham International Observatory, the University is required to fund a monitoring program that is charged with determining whether construction of the observatory is negatively impacting the squirrel population. The monitoring program, a separate entity from the observatory, currently employs five full-time biologists to monitor the red squirrel population around the observatory site. The studies have found no indication of differences between the observatory and its road area and the control area. There are no signs of any effect of the observatory construction on the red squirrel.