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

Russell Davis University of Arizona, Tucson

David E. Brown Arizona Game and Fish Department, Phoenix, Arizona

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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 , 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 () (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 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 Plateau in the at least portions of its present northern range early or middle Holocene (Betancourt and is certainly as well supported by available evi- Van Devender 1981), when the squirrel may dence as any to the contrary. or may not have been present. The chronol- ogy of certain records over the past 100 years, Dispersal Across however, suggests that Aberts squirrel may non-ponderosa plne habitat not have reached parts of its present range north of the 36th parallel (Fig. 1) until quite Weighing against any hypothesis involving recently. a role of post-Pleistocene dispersal is the For example, even though first recorded in present disjunct distribution of isolated mon- southern Colorado in 1875 (Armstrong 1972), tane patches of ponderosa pine forest in which 430 Great Basin Naturalist Vol. 49, No. 3

this squirrel now occurs (Fig. 1; Lomolino by many hikers and hunters. Those who knew et al. 1989). Northward post-Pleistocene dis- this mountain and its game had persal by this squirrel beyond the 36th paral- been confident of the absence of these distinc- lel could have taken place through continuous tive and obvious squirrels prior to the first ponderosa pine habitat only if ponderosa pine record of their occurrence (in 1969 and 1970) arrived in the north prior to the disjunction in three of the forest patches (Brown 1984). of the massive late-Pleistocene forest. If dis- About 10 years later their presence was re- persal occurred when vegetational conditions ported about 32 km further south in this same were similar to those existing today, then range in another patch of ponderosa pine much of this must have taken place across where there had been no previous sign of apparent barriers of "unsuitable" (non-pon- them. The ponderosa pine forest nearest to derosa pine) habitat. The presence of such the Mazatzal Mountains is at least 50 km to apparent barriers is especially critical to our the north. suggestion of the possibility of recent range Other evidence is provided from a recon- expansion into Utah and into the southwest- struction of the distribution of vegetation ern portions of Colorado. The Abajo Moun- occurring in late Pleistocene (Lomolino et al. tains in southeastern Utah, the only Utah lo- 1989). If this reconstruction is valid, then cality in which these squirrels occur, are even in late Pleistocene the distribution of separated by a gap of about 30 km of non- Abert's squirrel could not have been continu- ponderosa-pine habitat from the nearest ous and unfragmented. There apparently was forests of southwestern Colorado (Lomolino never a continuous, late-Pleistocene connec- et al. 1989). Likewise, the ponderosa pine tion between the extensive boreal conifer forests in southwestern Colorado, north of the forest of central and northern Arizona-New San Juan Mountains, are small and disjunct Mexico and the austral conifer forest of north- (Armstrong 1972) and comparably isolated. ern Mexico. The closest approximation of

It is logical, then, to postulate that at least such a connection occurred in southeastern some gaps of non-ponderosa-pine habitat Arizona where forests were present on mon- were likely to have been present whenever tane islands surrounded by woodlands. These the northward, post-Pleistocene dispersal of forests were only slightly less isolated then Abert's squirrel occurred. To be plausible, a than they are today (Lomolino et al. 1989). dispersal hypothesis such as this requires ver- Consequently, if dispersal through this region ification that these squirrels are capable of occurred at any time during the past 30,000 crossing such apparent barriers. Evidence for years, it could only have taken place because this comes from several sources. the squirrel was able to cross non-pon- First, even though reproducing popula- derosa-pine habitat barriers and island-hop tions of Abert's squirrel are restricted to from one montane island of ponderosa pine ponderosa pine forests, Hoffmeister (1986) to another. It is interesting to note that indicates that 20% of the locality records in while populations of this squirrel must have Arizona are within pinyon-juniper wood- occurred on the mountains in southeastern

lands. Consequently, there is no question that Arizona at some time or another, these even- individuals can exist at least temporarily in tually became extinct. It occurs now only on a pinyon-juniper habitat (Pederson et al. 1976) few of these mountains as the result of trans- and should be capable of dispersal through planting (Davis and Brown 1989). Such local this habitat at least for short distances from extinction is not unexpected; these habitat one ponderosa pine forest to another. islands are small. In addition, in central Arizona there is an Even stronger evidence for the ability of example of a natural range expansion (not this squirrel to disperse through non-pon- assisted by transplanting; Davis and Brown derosa-pine habitat was provided by biogeo- 1989) that must have occurred through inter- graphical experiments resulting from a trans- vening habitats of chaparral and woodland. In planting program conducted by the Arizona the Mazatzal Mountains there are a few small Game and Fish Department, beginning in patches of ponderosa pine. These had been 1940-41. Abert's squirrel was known to be examined previously by personnel of the Ari- absent from several isolated mountain ranges zona Game and Fish Department and visited containing ponderosa pine, and it was thought July 1989 Davis, Brown: Aberts Squirrel 431

HABITAT Conifer Forest 1/tM Conifer Woodland Evergreen I I Woodland lm^m\ Chaparral IMS Grassland ESSS1 Desertscrub

Fig. 5. The distribution of Abert's squirrel in Arizona (modified from Brown 1984), with examples of dispersal and colonization. Circles indicate sites of transplants (Davis and Brown 1989). A-D are transplants that were followed by colonization of nearby, isolated forests; and the straight lines with arrows for A-C are the shortest possible routes of dispersal; intervening habitats are indicated (from Brown and Lowe 1983). At D, the site of transplant was Mt. Logan, with subsequent dispersal through pinyon-juniper woodland to three nearby, isolated forests 8-12 km distant.

appropriate to transplant this squirrel into the (C). The intervening habitats include oak and larger of these. In several instances following pinyon-juniper woodlands, interior chapar- grassland (Fig. 5). In release, the squirrel then dispersed through ral, and even semidesert each case, the sources of dispersing squirrels intervening nonforest habitats to colonize could have been from elsewhere than the sites other smaller, adjacent forests from which it of transplanting as indicated in Figure 5; if this was also known to have been absent previ- were the case, however, the distances in- ously (Fig. 5). In these experiments, the time volved would then be even greater and the at which colonization was detected following intervening habitats no less harsh (Fig. 5, and transplanting increased with the distances be- Brown and Lowe 1983). These are examples of tween the isolated forests: 8 to 12 km in about natural dispersal and subsequent coloniza- 10 years (D in Fig. 5), 23 km in about 20 years tion; additional transplanting was not in- (A), 29 km in 30 years (B), 57 km in 40 years volved (Davis and Brown 1989). 432 Great Basin Naturalist Vol. 49, No. 3

Observations of individuals of any spe- Pleistocene dispersal unimpeded by habitat cies dispersing through unsuitable habitat are barriers. rare. However, Cooper (1987) observed an Abert's squirrel is absent from most of the Abert's squirrel crossing alpine tundra, 350 m ponderosa pine forests of North America elevation above treeline and at least 1,250 m (compare Figs. 1 and 3). In some cases this above the ponderosa pine forest. Alpine tun- absence can be explained as the result of local dra on mountain tops would seem to be even extinction in small forests (as in the mountains less suitable habitat for a tree squirrel than the of southeastern Arizona) or as a result of exces- woodlands at lower elevations in which indi- sively low dispersal rates to the more isolated viduals occasionally occur and through which forests (Fig. 2). In addition, there is always the much of the dispersal shown in Figure 5 must typical enigma of dispersal; some habitat and have taken place. distance barriers are crossed while other bar- riers, apparently with higher probabilities of Summary, Conclusion, and Discussion dispersal, are not (Fig. 2). In any case, the lack of complete overlap of

Because all reproducing populations of the distributions of these two species should Sciurus aberti are restricted to ponderosa come as no surprise. Abert's squirrel is depen- pine forests, and since ponderosa pine is not dent upon ponderosa pine, but the reverse known north of the 36th parallel in the late relationship does not exist. These two species Pleistocene, two conclusions emerge: (1) this have very different characteristics. Abert's squirrel could not have been north of the 36th squirrels have poorer dispersal ability and a parallel prior to the arrival of ponderosa pine, lower probability of successful colonization and (2) post-Pleistocene dispersal must have following dispersal. Also, reproducing popu- played a major role in determining the mod- lations of these mammals have much greater ern distribution of the squirrel north of this resource-area requirements. latitude. While much of such dispersal may In addition, their absence north of southern have occurred prior to the fragmentation of Wyoming suggests the possibility that these the vast northern forest of the late Pleistocene squirrels may be unable to exist in more (but after the arrival of ponderosa pine), there northern climates and thus may have a nar- remains the possibility that some dispersal of rower range of ecological tolerance. On the Abert's squirrel may have taken place consid- other hand, if the distribution of Abert's squir- erably later when modern fragmented con- rel is as dynamic as we have proposed, and if ditions existed. In fact, although unproven they have reached both southern Wyoming (and though alternate possibilities exist), and southeastern Utah only quite recently (as there is evidence suggesting that this squir- the chronology of records suggests), then we rel may have arrived rather recently in cer- have no way yet of knowing what their poten- tain isolated portions of the northern part of its tial may be for future expansion to either the range. north or west (compare Figs. 1 and 3). In any case, once habitat conditions This study involves only a single species of reached today's norms, no matter how much mammal, but the phenomenon demonstrated later dispersal may have occurred, at least may have broad implications. If post-Pleis- some must have taken place across gaps in the tocene dispersal can so significantly influence distribution of ponderosa pine. Evidence the current distribution of Sciurus aberti, a from several sources clearly demonstrates species with an obligatory relationship with that such gaps would not have prevented dis- montane forests dominated by ponderosa persal and that these squirrels are presently pine, it could have a comparable influence on capable of colonizing isolated forests. the distribution of other small, mountain- Historical legacy may have been involved dwelling, nonflying mammals—especially in the distribution of this species south of the those less restricted to montane habitats. 36th parallel, and local extinction has appar- The yellow-nosed cotton rat (Sigmodon ently occurred at least on the montane islands ochrognathus) is such a species, and it has in southeastern Arizona. All of the current been shown to have recently expanded its range of this squirrel north of the 36th paral- range (Davis and Dunford 1987, Davis and

lel, however, must have resulted from post- Ward 1988). If this phenomenon should occur July 1989 Davis, Brown: Abert's Squirrel 433

Davis, , S Bissell 1989. distribution widely, then post-Pleistocene dispersal has R and J. The of Abert's squirrels in Colorado. Southwestern Nat. played a major role in determining the mod- 34(2): in press. ern patterns of distribution of montane forest Davis. R , and D E Brown 1989. Documentation of the mammals of the American Southwest. Analy- transplanting of Abert's squirrels. Southwestern ses of both community- and species-level pat- Nat. 33: 490-492. terns and evidence that many forest-dwelling Davis, R, and C Dunford. 1987. An example of contem- porary colonization of montane islands by small, species occasionally occur in woodlands and nonflving mammals in the American Southwest. habitats have provided other low-elevation Amer. Nat. 129: 398-406. strong support for this hypothesis (Davis et al. Davis, R., C Dunford, and M. V Lomolino 1988. Mon- 1988, Lomolino et al. 1989). tane mammals of the American Southwest: the possible influence of post-Pleistocene coloniza-

tion. J. Biogeogr. 15: 841-848.

Acknowledgments Davis, R , and O. G Ward 1988. A vacant "Microtus niche" now occupied by the yellow-nosed cotton Several people reviewed preliminary rat {Sigmodon ochrognathus) on an isolated moun-

versions of this paper and provided helpful tain in southeastern Arizona. J. Mamm. 69: 362-365. suggestions: D. Armstrong, J. Betancourt, Durrant, S D 1952. Mammals of Utah, and E. L. Cockrum, S. Cross, M. Lomolino, distribution. Univ. Kansas Publ., Mus. Nat. Hist. J. Mead, B. Patterson, B. Sheehy, B. Sidner, 6: 1-549. and T. Van Devender. C. Dunford and P. S. Goldman, E A 1928. The Kaibab or white-tailed squir-

Martin helped with a major revision that rel. J. Mammal. 9: 127-129. Hall, 1981. A field study of the Kaibab squirrel in eventually resulted in the present version. J G National Park. Wildl. Monogr. 75. Discussions with Betancourt, Mead, and J. J. Harris, A H 1985. Late Pleistocene vertebrate paleoe- T. Van Devender, regarding ponderosa pine cology of the West. University of Texas Press, distribution during the late Pleistocene, were Austin. 293 pp. invaluable. Figures 1-3 were prepared by Hoffmeister, D. F 1986. Mammals of Arizona. Univer- sity of Arizona Press and the Arizona Game and J. Abbott; Figures 4 and 5 were prepared by Fish Dept, Tucson. 602 C. Ernstein. pp.

Hoffmeister, D Y . and V E Diersinc 1978. Review of the tassel-eared squirrels of the subgenus Otosciu- rus. Mammal. 59: 402-413. Literature Cited J. Keith. J D 1965. The Abert squirrel and its dependence on ponderosa pine. Ecology 46: 150-163. Armstrong. D M 1972. Distribution of mammals in

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Patton, R , and Green 1970. Abert's squirrels Brown, D E , and C H Lowe 1983. Map: biotic com- D W munities of the Southwest. General Tech. Rept. prefer mature ponderosa pine. USDA For. Serv. RM-78. Rocky Mt. Forest and Range Expt. Sta., Res. Note RM-169, Rocky Mt. For. and Range USDA Forest Serv., Fort Collins, Colorado. Expt. Sta., Fort Collins, Colorado. A. Brown, L N 1965. Abert's squirrels in southern Pederson, J. C, R. N. Hasenyager, and W Heggen. 1976. Habitat requirements of the Abert squirrel Wyoming. J. Mamm. 46: 516. Cole, K. L. 1982. Late Quaternary zonation of vegetation (S. aberti navajo) on the Monticello District, in the eastern Grand Canyon. Science 217: Manti-LaSal National Forest of Utah. Publ. No. 1142-1145. 76-9, Utah State Div. of Wildl. Resources.

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RaSMUSSEN, D. I 1941. Biotic communities of the Kaibab Turbak, G 1987a. Prisoner of geography. Natl. Wildl. Plateau, Arizona. Ecol. Monogr. 11: 229-275. 24: 14-16. 1987b. The squirrel that time forgot. Arizona Rasmussen. D I , D E Brown, and D Jones 1975. Use Highways 63: 12-13. of ponderosa pine by tassel-eared squirrels and a Van Devender, T R, L Betancourt, and key to determining evidence of their use from that J M. Wimberlv 1984. Biogeographic implications of red squirrels and porcupines. Arizona Game of a packrat midden sequence from the Sacra- and Fish Dept. Wildl. Digest 10: 1-12. mento Mountains, south-central New Mexico. Spaulding, W. G. 1984. The last glacial-interglacial cycle: Quaternary Res. 22: 344-360. its effects on woodlands and forests in the Ameri- Van Devender. T R , R S Thompson, and J L Betan- can West. Pages 42-69 in R. N. Lanner, ed., Proc. court 1987. Vegetation history of the deserts of Eighth North Amer. Forest Biol. Workshop. southwestern North America; the nature and tim- Dept. Forest Research, Utah State University, ing of the late Wisconsin-Holocene transition. Logan. 169 pp. Pages 323-352 in W. F. Ruddiman and H. E. Stephenson, R. L. 1975. Reproductive biology and food Wright, Jr., eds.. North America and adjacent of Abert's squirrels in central Arizona. Un- habits oceans during the last deglaciation. The geology of published master's thesis, Arizona State Univer- North America, Vol. K—3. Geol. Soc. Amer., sity, Tempe. 66 pp. Boulder, Colorado. 501 pp.

Stephenson, R. L . and D. E. Brown 1980. Snow cover Wells, P V 1983. Paleobiogeography of montane islands as a factor influencing mortality of Abert's squir- in the Great Basin since the last glaciopluvial.

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Thompson, R. S., and T. R. Van Devender 1982. Late Wettstein. P J., AND J S States. 1986. The major histo- Pleistocene vegetational records from desert compatibility complex of tassel-eared squirrels. II. grassland in the Santa Catalina Mountains, Ari- Genetic diversity associated with Abert squirrels. zona. Amer. Quaternary Assoc. Abstr. 7: 167. Immunogenetics 24: 242-250.