Interactions between Predators and Mt. Graham Red Squirrels (Tamiasciurus hudsonicus grahamensis)

Abstract

Predation in populations of small mammals is a rarely observed event (Pearson, 1985); however, the role of predators in population regulation often is considered to be important. Documentation and assessment of mortality factors are especially critical when considering the population dynamics of threatened or endangered species. Herein, we document interactions of the endangered Mt. Graham red squirrel with potential predators. The Mt. Graham red squirrel (Tamiasciurus hudsonicus grahamensis) is an isolated subspecies found only on the highest peaks of the Pinalefio Mountains, Graham Co., Arizona (Hoffmeister, 1986). The squirrel is slightly smaller in body size and has a slightly narrower skull (United States Fish and Wildlife Service, 1993) than other red squirrels found in North America. Genetic evidence also supports the distinctiveness of this geographically isolated subspecies (Sullivan and Yates, 1994). The squirrel was reported to be extinct in the 1960s (Minckley, 1968) but was rediscovered in 1972 (Spicer et al., 1985) and listed as endangered in 1987. In fall 2000, the total population of the Mount Graham red squirrel was estimated at approximately 470 individuals (T. Snow, Arizona Department of Game and Fish, pers. comm.). Mount Graham red squirrels inhabit sprucefir and mixed conifer forests (United States Fish and Wildlife Service, 1993). Red squirrels larderhoard seed cones into piles known as middens, typically near the center of their territory (Steele, 1998). Preferred midden locations include areas of thick foliage, dense canopies, and high stem densities (Smith and Mannan, 1994). Depending on season and availability, the squirrels feed mainly on conifer seeds, fungi, bone, and conifer twigs (Coronado National Forest, 1988). Nest sites typically are located in large decaying snags (Froehlich and Smith, 1990). Our monitored area, approximately 250 ha, incorporates both mixed conifer forest dominated by Douglas-fir (Pseudotsuga menziesii), ponderosa pine (Pinus ponderosa), corkbark fir (Abies lasiocarpa var. arizonica) and southwestern white pine (Pinus strobiformis) and sprucefir forest composed principally of Englemann spruce (Picea engelmannii) and corkbark fir. During the course of other investigations (Young, 1994), observations of potential predators were recorded nearly monthly from 1989 until 1996 and quarterly thereafter. A minimum of 41,000 h of field observation, including 3,269 h of focal animal observations, was expended. Eighty-three observations of predators were made during monitoring periods (Table 1). Eight species of potential predators (6 avian, 2 mammalian) were observed. Five predation attempts, 1 successful and 4 unsuccessful, were recorded. Evidence of 6 possible predation attempts was documented and 5 potential predator species were observed in or near middens but did not attempt predation. On 19 October 1989, 2 juvenile red squirrels (presumed siblings) chased each other as a Mexican spotted owl (Strix occidentalis lucida) unsuccessfully attacked at 1354 h MST. After 1 min the squirrel chase resumed and the owl looked on from a perch 15 m away. Eight min later the owl made a second, and this time successful, attempt. Carrying the squirrel by the neck, the owl flew 30 m east, perched in a tree for a few minutes and then left the area. Im-