Ecology of Introduced Abert's Squirrels - Final Report

Koprowski, John

A major threat to biological diversity is the intentional or accidental introduction of nonnative species to ecosystems (Primack 1993). Such introductions can disrupt the function of their new ecosystem through a number of mechanisms to include the spread of disease, competition, predation, reproductive success, and induced habitat change. Because exotic species did not evolve in their new ecosystem, the results of translocations are unpredictable. In some cases, the species is so poorly adapted to the new environs that the introduced population becomes extinct, and the impact on the ecosystem in negligible. However, some exotics are quite successful and have a profound and negative impact on the ecosystem (Primack 1993). For example, the prognosis for the European red squirrel (Sciurus vulgaris) in the British Isles is poor, and the likely outcome extinction due to the introduction of the eastern gray squirrel (Sciurus carolinensis) from the USA over 120 yr ago (Rushton et al. 1997). Exotic species are a problem in many management settings; however, in fragmented environments introduced species can be especially problematic. National parks often serve as such relicts of habitat. Abert’s squirrels (Sciurus aberti), a large diurnal tree squirrel of 600- 800g, were introduced to several montane forests by the Arizona Game & Fish Department in 1940-41 to increase hunting opportunities. The introductions were remarkably successful in establishing this squirrel; one site was the Santa Catalina-Rincon Mtn complex (Brown 1984). Introductions were stopped when concerns were expressed over the impact of the exotic tree squirrels on the uncommon native Catalina gray squirrel (Sciurus arizonensis catalinae: Cockrum 1960). This unique subspecies is smaller and morphologically distinct from the nearest population to the south (Hoffmeister 1986) and has likely been isolated on these mountains without congeneric competitors since the last ice age some 7,000 to 10,000 years ago (VanDevender & Spaulding 1979). The introduced Abert’s squirrel has been implicated in the decline of the uncommon native in the last half of the 20th century (Lange 1960, Brown 1984); however, the evidence is only anecdotal. Both species rely on the reproductive tissues of trees for survival, and the potential for ecological overlap is immense (Best & Riedel 1995, Nash & Seaman 1977). Moreover, Abert’s squirrels are known to reduce cone crops of ponderosa pine by as much as 55% (Allred et al. 1994), a known food source of Catalina gray squirrels (Best & Riedel 1995). As a result, the potential for ecologically significant interactions between the species is high. In 1991, Catalina gray squirrels were afforded protection as a Category 2 species (Drewry 1991) in recognition of potentially precarious status and the dearth of life history information. This formal status was retained until the abolition of the category by the USFWS after which the Coronado National Forest considered the species to be sensitive. Both species occur in the higher elevations of the East Unit of Saguaro National Park, where their distribution and populations are poorly known. The impact of a potentially invasive species must be documented to assess the consequences of its presence in the forest ecosystems of Saguaro NP.