Great Basin Naturalist Volume 49 Number 3 Article 13 7-31-1989 Role of post-Pleistocene dispersal in determining the modern distribution of Abert's squirrel Russell Davis University of Arizona, Tucson David E. Brown Arizona Game and Fish Department, Phoenix, Arizona Follow this and additional works at: https://scholarsarchive.byu.edu/gbn Recommended Citation Davis, Russell and Brown, David E. (1989) "Role of post-Pleistocene dispersal in determining the modern distribution of Abert's squirrel," Great Basin Naturalist: Vol. 49 : No. 3 , Article 13. Available at: https://scholarsarchive.byu.edu/gbn/vol49/iss3/13 This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. ROLE OF POST-PLEISTOCENE DISPERSAL IN DETERMINING THE MODERN DISTRIBUTION OF ABERT'S SQUIRREL Russell Davis 1 and David E. Brown 2 Abstract. —Abert's squirrel is a forest-dwelling mammal, dependent upon ponderosa pine, that now ranges irom southern Wyoming to northern Mexico. During the late Pleistocene, ponderosa pine and this squirrel occurred no further north than central Arizona and New Mexico. In consequence, the present range of the squirrel north of the 36th parallel must have been the result of post-Pleistocene (Holocene) dispersal. If such dispersal took place after the fragmentation of the northern montane conifer forest, at least some leakage across barriers of unsuitable (non-pon- derosa pine) habitat must have occurred. Dispersal following transplanting "experiments" has shown that such barriers can be crossed; other evidence is provided that suggests this may occur sufficiently often to produce significant changes in distribution within short periods of time. Thus, explanations for the distribution of Abert's squirrel, based only on historical legacy and local extinctions, are found to be insufficient. An alternative explanation is proposed in which post-Pleistocene dispersal also plays an important role. The present distribution of the Aberts edulis) woodland, Douglas-fir (Pseudotsuga squirrel, Sciurus aberti, extends discontinu- menziesii) and spruce-fir (Picea-Abies) habi- ously from northern Mexico to southern tats (Nash and Seaman 1977, Brown 1984, Wyoming (Fig. 1). These disjunct populations Hoffmeister 1986), numerous studies have have been thought to be Pleistocene relicts shown that reproducing populations are re- (McKee 1941). Presumably, during the late stricted to forests dominated by ponderosa Pleistocene Abert's squirrel was widely dis- pine (Pinus ponderosa) (Rasmussen 1941, tributed throughout a vast coniferous forest. Keith 1965, Patton and Green 1970, Patton With Holocene aridity, this forest, with its 1975, Rasmussen et al. 1975, Stephenson Aberts squirrel component, fragmented and 1975, Pederson et al. 1976, Stephenson and retreated to higher elevations. Supposedly, Brown 1980, Hall 1981, Brown 1984, Peder- since this arboreal squirrel is restricted to son and Welch 1985). Dependence on a base coniferous forests, dispersal across the result- of ponderosa pine is a characteristic trait of the ing woodland gaps was impossible. In conse- species and occurs in each of the now geo- quence, the squirrel was thought to be a help- graphically isolated populations (compare less prisoner of geography (Goldman 1928, Figs. 1 and 3). Turbak 1987a) and of a fragmented post-Pleis- With the reasonable assumption (Wettstein tocene forest (McKee 1941); the distribution and States 1986) that there have never been since the Pleistocene could only be influenced any unique populations of Abert's squirrel ca- by local extinction. pable of reproduction and continued survival However, a study of presence and absence in the absence of ponderosa pine (and which showed a distributional pattern for Abert's later independently developed the ponderosa squirrel that was consistent with one influ- pine dependency now characteristic of the enced strongly by dispersal (Fig. 2). In conse- entire species), we can be confident that this quence, the traditional historical legacy expla- squirrel has never ranged beyond the geo- nation for the present distribution of Abert's graphical limits of the distribution of pon- squirrel now requires reexamination. derosa pine. Squirrel Dependence on History of the Distribution of Ponderosa Pine Ponderosa Pine and Aberts Squirrel in the Southwestern United States While individual Aberts squirrels are occasionally observed in pinyon pine (Pinus Ponderosa pine has had a long and complex 'Department of Ecology and Evolutionary Biology, University ol Arizona, Tucson, Arizona 85721. 2Arizona Game and Fish Department, Box 9099, Phoenix, Arizona 85068 Present address: 3118 W. McLellen Blvd., Phoenix, Arizona 85017. 425 426 Great Basin Naturalist Vol. 49, No. 3 Fig. 1. The modern distribution of Abert's squirrel (modified from Hoffmeister and Diersing 1978, Brown 1984, and Davis and Bissell 1989). The dashed line for 36° North latitude designates the northern limit of ponderosa pine in late Pleistocene. Introductions are not included. July 1989 Davis, Brown: Aberts Squirrel 427 105 1^=0.66 Pa <0.001 P =0.01 d o o 10- o o 50 100 150 200 250 Isolation (km) Fig. 2. Effects of area and isolation on the presence and absence of Aberts squirrel on 27 forest islands in the American Southwest (Lomolino et al. 1989; see also Lomolino 1986). Darkened circles indicate presence; open circles indicate absence. Probabilities that insular occurrence is not influenced by area (extinction) P.,, and not influenced by distance (dispersal) = P^. history in western North America. Fossil there into Utah, Colorado, and Wyoming. plant parts similar to those of this species Eventually, the vast northern conifer forest of have been recorded from middle- and late- the late Pleistocene (Lomolino et al. 1989) Miocene deposits in Nevada (Axelrod 1986). contracted into the isolated montane patches In the Pleistocene, these trees apparently that exist today, now dominated at middle occurred as far north as southern Wyoming elevations by ponderosa pine. The rapid ex- during the Sangamonian interglacial (Baker pansion of ponderosa pine into the huge, 1986), but during the late Wisconsin glacial modern range it now occupies (Fig. 3) repre- they were restricted in the United States to sents one of the most remarkable disper- refugia in southern Arizona and New Mexico. sal events of the Holocene (Van Devender During late Pleistocene, ponderosa pine is etal. 1984). known no further north than 36 degrees North The northward movement of ponderosa latitude (Betancourt and Van Devender 1981, pine allowed a corresponding movement of Cole 1982, Thompson and Van Devender Aberts squirrel. Populations of these squir- 1982, Wells 1983, Betancourt 1984, Spaul- rels now present in northern Arizona, New ding 1984, Van Devender et al. 1987). In Mexico, Colorado, Utah, and Wyoming must consequence, Aberts squirrel would not have had their origin from populations that have been further north in late Pleistocene had previously been present in ponderosa than north central Arizona and New Mexico pine forests south of the 36th parallel. (Fig. 1). In northwestern Arizona, the squirrel some- Beginning in early Holocene, climatic how reached the north Kaibab (Fig. 1). Con- changes again favored dispersal of ponder- trary to both traditional and popular opinion osa pine northward beyond the 36th parallel (Goldman 1928, Turbak 1987a, 1987b), the pop- into northern Arizona-New Mexico, and from ulation now in the north Kaibab apparently 428 Great Basin Naturalist Vol. 49, No. 3 Fig. 3. Distribution of ponderosa pine in Mexico and a selected portion of the United States (Little 1971). While individual Abert's squirrels occasionally occur in habitats other than those dominated by ponderosa pine, at this scale any suggestion of the occurrence of populations of these squirrels (Fig. 1) outside the range of ponderosa pine is an artifact resulting from errors in the estimations of the distributions of one or the other of these two species. July 1989 Davis. Brown: Aberts Squirrel 429 Fig. 4. Distribution of Aberts squirrel in Colorado (Davis and Bissell 1989). Cross-hatched area is Armstrong's estimate (1972), and black dots represent records he compiled. Localities indicated by open circles are the sites of post-1972 records; several sites close together are shown by a single open circle. could not be a Pleistocene relict; there is no it was not reported in southeastern Utah until evidence of the existence of ponderosa pine 1947 (Durrant 1952) nor in Wyoming until forests north of the Colorado River until after 1963 (Brown 1965). Based on observational the end of the Pleistocene. records, a recent expansion of range has been In the absence of fossil evidence (Harris reported in the southern and southwestern portions of Colorado (Fig. 4). Given its high 1985), there is no way of knowing when visibility, it is this squirrel could the northward post-Pleistocene dispersal of unlikely that have escaped detection in all of these localities Aberts squirrel began nor when the distribu- throughout historic time until its recent, for- tion of this species reached its modern extent. tuitous discovery. A hypothesis suggesting Forests dominated by ponderosa pine proba- the relatively recent arrival of the squirrel in bly developed on the Colorado