Currently being revised for Quaternary Research
An Ice Age Refugium for Large Mammals in the Alexander Archipelago, Southeast Alaska

Timothy H. Heaton
Department of Earth Sciences
University of South Dakota
Vermillion, South Dakota 57069

Sandra L. Talbot and Gerald F. Shields
Institute of Arctic Biology
University of Alaska,
Fairbanks, Alaska 99775, USA

ABSTRACT

Genetic and paleontological evidence are combining to provide a new and surprising picture of mammalian biogeography in southeast Alaska. Prior to our study, the brown and black bears of the Alexander Archipelago were considered postglacial immigrants that never had overlapping ranges. Vertebrate fossils from caves on Prince of Wales Island now demonstrate that brown and black bears coexisted there (and even inhabited the same caves) both before and after the last glaciation. Differences in mtDNA sequences suggest that living brown bears of the Alexander Archipelago comprise a distinct clade and are more closely related to polar bears than to their mainland conspecifics. We conclude that brown bears, and perhaps other large mammals, have continuously inhabited the archipelago for at least 40,000 years and that habitable refugia were therefore available throughout the last glaciation.

Until recently the prehistory of the Alexander Archipelago has been the subject of much speculation but little study. Archaeologists have considered it a possible route for human travel to the New World--a route that would have circumvented the great ice sheets of Canada (Heusser, 1960; Fladmark, 1979, 1983; Dixon, 1993). But geologists and biologists have traditionally discounted the possibility of habitable areas in the archipelago during glacial maxima (Prest, 1969; Nasmith, 1970; Klein, 1965). The controversy has centered around the question of whether non-glaciated refugia existed, with access to marine food sources, that were large enough to support populations of large mammals, including humans. Studies on modern and fossil bears from the Alexander Archipelago are providing the first conclusive evidence that such refugia existed (Fig. 1).

David R. Klein (1965:7) began his historic paper on the mammal distributions of the Alexander Archipelago with the following declaration:

A study of Kodiak Island mammals also discounted Ice Age refugia in favor of postglacial colonization (Rausch, 1969), and a study of Queen Charlotte Island mammals left the issue unresolved (Banfield, 1961; Cowan and Guiguet, 1965; Cowan, 1969). Klein (1965:15) went on to explain the distribution of brown bears, Ursus arctos, and black bears, U. americanus, of the Alexander Archipelago in terms of postglacial invasions from the north and south, respectively, and competition between the two species:

During the Wisconsin glaciation the present land areas of the coastal regions of Alaska bordering the Gulf of Alaska were virtually completely overridden by ice. The now existing flora and fauna of the region have presumably become established in the 10,000 years since the recession of the ice. . . . The present distribution of mammals in this region, although complicated by the phenomenon of insularity, reflects the sequence of their arrival and their relationship to specific refugia.

Klein's conclusions, which represented the prevailing consensus prior to our work, entail two postulates that have now been proven incorrect, as discussed below: 1) that brown bears never inhabited Prince of Wales Island and other islands of the southern archipelago, and 2) that the brown bears of the northern archipelago are closely related to their mainland counterparts.

Postglacial fossil deposits

The extensive karstlands of Prince of Wales Island were largely ignored as a scientific resource until cavers Kevin and Carlene Allred began exploring and mapping the caves of Prince of Wales Island in 1987. A joint project between the National Speleological Society and Tongass National Forest emerged from their efforts, and in 1990 the first vertebrate fossil remains were discovered in a remote upper passage of El Capitan Cave, the largest known cave in Alaska. In 1992 a sealed entrance to the cave was reopened and a full excavation conducted (Heaton and Grady, 1992, 1993). Along with four black bears, the remains of at least three brown bears were recovered, one of which was gigantic. The cave appears to have been a den site for both species of bears, showing that they once coexisted on the island.

A similar den site in Bumper Cave was excavated in 1994 and found to contain remains of at least nine brown bears, mostly females and juveniles (Heaton and Love, 1995). Brown bear remains have also been found in two natural trap caves: Blowing in the Wind Cave on El Capitan Peak and Enigma Cave on Dall Island. Table 1 provides pertinent data about the cave sites, and Table 2 is a list of radiocarbon dates obtained thus far. The total minimum number of individuals (MNI) for brown bears from postglacial deposits of the southern archipelago is now 15.

The caves of Prince of Wales Island have preserved remains of two other extirpated mammalian species in addition to Ursus arctos. The jaw of a red fox (Vulpes vulpes) was found in El Capitan Cave, and the partial metacarpal of a caribou (Rangifer tarandus) was recovered from Bumper Cave. Neither species currently inhabits the Alexander Archipelago or the nearby Alaskan mainland (Klein, 1965; Hall, 1981). A dwarf form of caribou inhabited the Queen Charlotte Islands into historic times before going extinct (Banfield, 1961; Cowan and Guiguet, 1965; Cowan, 1969).

Postglacial extinctions suggest that island diversity was once much greater than at present. There are two possible explanations for this higher diversity:

The failure of brown bears to occupy the islands south of Frederick Sound when access became available may be a result of prior occupancy by the black bear. A previously established species obviously has an advantage over a similar form attempting to occupy the same ecological niche.
1) Island colonization was easier in early postglacial times due to ice bridges and/or lower sea level.
2) Some species survived in the archipelago during glaciation and thereafter had to endure changing climate and/or competition with invading species from the mainland.

Klein (1965) rejected the second explanation for large mammals but specifically invoked the first, especially for bears. New data now suggest that the second explanation may be more accurate.

Mid-glacial fossil deposits

Postglacial fossils are useful for establishing colonization history and dating local glacial retreat, but they alone cannot resolve the question of Ice Age refugia in the Alexander Archipelago. Fortunately, older fossil deposits have now been discovered.

On Your Knees Cave is located in the extreme northwest corner of Prince of Wales Island adjacent to Sumner Strait (Heaton, 1995a; Fig. 1). Seal remains from the cave date near the peak of Fraser (late Wisconsin) Glaciation, suggesting that the area was ice free (Table 2). Remains of brown and black bear from On Your Knees Cave date to the period preceding Fraser Glaciation, called the Olympia Nonglacial Interval in British Columbia (Fig. 2). Since brown bears apparently did not reach the lower 48 states until about 12,000 years ago (Kurten and Anderson, 1980; Mustoe and Carlstad, 1995), their early presence in the Alexander Archipelago is surprising. This suggests that, at least for bears, the glaciated coastal islands were less inhibitive to travel than was mainland North America.

Stable carbon isotope values on fossil bones give an indication of an animal's diet. The brown bear from On Your Knees Cave has the highest value of any bear analyzed from the islands thus far (Table 2), and this indicates an unusually large marine component to the animal's diet (Fry and Sherr, 1984; Heaton, 1995b). However, the black bear from that cave has a low isotopic value typical of black bears, and this indicates a primarily terrestrial diet (Bocherens et al., 1994). Marine feeders in the archipelago, such as brown bears and otters, would likely have a great advantage in surviving periods of glaciation, and this must be kept in mind when considering theories of continuous habitation.

Devil's Canopy Cave on Prince of Wales Island has also produced a mammalian fauna predating Fraser Glaciation, though only rodents have been recovered thus far (Heaton, 1995a). The most significant find is marmot, Marmota sp., an animal not currently found on the island or in postglacial deposits. The only sample analyzed from the cave is beyond the range of radiocarbon dating (Table 2), so unfortunately the glacial context is unknown. This cave suggests the possibility that fossil deposits in the archipelago could have survived multiple glaciation events. Field work over the next several years will focus on these older deposits in hopes of obtaining a complete faunal record for the last 50,000 years.

Genetic Studies

The fossil record of brown and black bears in the Alexander Archipelago obtained thus far still leaves ambiguous the question of whether these bears inhabited the islands throughout the period of Fraser Glaciation or merely colonized the archipelago more than once. We have compared the DNA sequences of three mitochondrial genes and associated non-coding segments of nearly 200 extant brown bears from all geographic regions of Alaska and some regions of northeastern Siberia and Turkey, and these data, combined with the fossil evidence herein, suggest continuous habitation of the archipelago.

The DNA sequences suggest two very distinct clades for Ursus arctos, one composed exclusively of brown bears from Admiralty, Baranof, and Chichagof Islands (ABC Islands) of the northern Alexander Archipelago (Fig. 1) and another comprising all other populations (Talbot and Shields, 1996; Fig. 3). The brown bears of the ABC Islands are fixed for a minimum of 27 point mutations of which two are transversions in the cytochrome b gene alone. This suggests long-term isolation (550,000-700,000 years) from other populations of brown bears (Talbot and Shields, 1996; Shields and Kocher, 1991; Cronin et al., 1991)). This long-term isolation is also apparent when the DNAs of ABC bears are compared with those of more than 300 brown bears from throughout their world-wide range (Waits et al., 1996). The most logical explanation for these results is that the bears of the ABC Islands are the relic of an invasion of Ursus arctos from Asia into Alaska prior to the Fraser Glaciation. This invasion may never have extended beyond Alaska, and apparently it died out (except in the Alexander Archipelago) prior to the more extensive postglacial invasion (Figs. 2 and 3).

The mitochondrial data also suggest that the polar bear is the closest extant relative of the ABC Island bears (Talbot and Shields, 1996; Taberlet and Bouvet, 1992). Kurten suggested from fossil evidence that polar bears arose from a coastal form of brown bear possibly in northeastern Siberia (Kurten, 1964, 1968), and our data are consistent with this. However, our novel observation of paraphyly of brown bears with respect to polar bears will have to be tested using other genetic markers, including those of nuclear genes, before a full analysis can be made.

Conclusions

Since brown bears of the Alexander Archipelago were previously thought to be postglacial immigrants from the Alaskan mainland (Klein, 1965), the distinct genetic makeup of the ABC Island bears and the recovery of pre-Fraser age Ursus arctos remains from On Your Knees Cave both came as complete surprises. The antiquity of the On Your Knees Cave specimen suggests that it is part of the modern ABC Island lineage and thereby documents that this lineage inhabited the Alexander Archipelago at an early date. By far the simplest explanation for all these data is continuous habitation of brown bears in the archipelago throughout Fraser Glaciation, as the Cordilleran Ice Sheet surrounding this area would have been a major barrier to travel. The ABC Island bears are a living relic of that habitation, and postglacial fossil brown bears from Prince of Wales and Dall Islands probably also belong to this clade. Future genetic studies on fossils may confirm this.

This combination of paleontological and genetic evidence offers strong documentation for habitable coastal refugia in the Alexander Archipelago during the latest glaciation event. The fossil record even demonstrates that brown bears had a more extensive range in the islands than at present--both before and after the last glacial maximum. This suggests the possibility that glaciation may have actually promoted rather than inhibited coastal range extension by brown bears. The combination of marine foods and ice bridges may have provided a coastal corridor for this species not available in interglacial or postglacial times. This suggests that at least some unglaciated land was always available for denning and food supplement, and other species of mammals may have survived glaciation in such refugia as well.

Figures 4 and 5 contrast two models of mammal invasion of new island habitat following glaciation. The theory of postglacial colonization of the Alexander Archipelago from the Alaska mainland, advocated by Klein (1965), can be termed the Inland Colonization Model (Fig. 4). It may still be the best explanation for many island mammals, but it can no longer be applied exclusively. The theory outlined in this paper for brown bears can be termed the Coastal Colonization Model (Fig. 5; Heaton, 1995c). It now provides the best explanation for brown bears, and it may also apply to black bears and other mammalian species.

As Calvin J. Heusser (1960) stated at the end of his book on Late Pleistocene North America, no discussion of this topic is complete without considering humans. A recent linguistic study reached conclusions strikingly similar to ours, namely that coastal southern Alaska housed an isolated refugium during the last glaciation (Rogers et al., 1991). While our study provides no direct evidence of human antiquity in the Alexander Archipelago, the evidence from brown bears furnishes a close analog. Humans and brown bears are both omnivores and have similar habitat requirements. If marine foods and coastal den sites were available for Ursus arctos, then the minimum requirements for seafaring Homo sapiens were almost certainly available.

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ACKNOWLEDGEMENTS

We thank Frederick Grady, David Love, Kevin Allred, James Baichtal, Terry Fifield, and Judy Gust for their generous help. Funding for the paleontological excavations and radiocarbon dates was provided by the National Geographic Society, Tongass National Forest, National Speleological Society, and University of South Dakota. The genetic work was supported by a University of Alaska at Fairbanks Chancellor's Graduate Fellowship, a University of Alaska Resource Fellowship, and a Boone and Crockett Grant-in-Aid for Wildlife Research. We thank Joseph A. Cook, E. James Dixon, R. Dale Guthrie, and David R. Klein for providing helpful criticisms on the manuscript.
Fig. 1. Index map of the Alexander Archipelago, southeast Alaska. Currently brown bears inhabit the northern (ABC) islands while black bears inhabit most islands south of Frederick Sound. Both species inhabit the mainland. The star shows the karst area of Prince of Wales Island where fossils of brown and black bears have been found in mid-glacial and postglacial cave deposits.

Fig. 2. Time line of interpreted events in southeast Alaska from 50,000 years B.P. to the present. The early invasion of bears is represented by the fossils of On Your Knees Cave and the living brown bears of the ABC Islands. Mainland bears represent the later invasion. The affinity of the postglacial bears of the southern archipelago has not yet been determined, but their early fossil record at El Capitan Cave and other sites suggests that they may also represent the early invasion.

Fig. 3. Neighbor-joining tree for the major lineages of brown bears and polar bears based on complete sequences of the mitochondrial cytochrome b gene and analyzed by the Kimura (1980) two parameter distance model. DNA sequence from the cytochrome b gene of a sun bear (Helarctos {= Ursus} malayanus) was used as an out-group reference.

Fig. 4. Idealized diagrams of southeast Alaska during and following Fraser Glaciation illustrating the Inland Colonization Model advocated by Klein (1965). Under this model there were no glacial survivors in the Alexander Archipelago, and the mainland and island populations have a postglacial common ancestry.

Fig. 5. Idealized diagrams of southeast Alaska during and following Fraser Glaciation illustrating the Coastal Colonization Model presented for Ursus arctos in this paper. Under this model the mainland and island populations have a separate glacial and postglacial history and are therefore more distinct genetically.

Table 1. List of limestone caves from which vertebrate remains have been recovered in the southern Alexander Archipelago indicating the minimum number of individuals (MNI) recovered of brown bears (Ursus arctos) and black bears (U. americanus). Only El Capitan Cave and Bumper Cave have been thoroughly excavated thus far. The first six caves listed contain only postglacial remains. On Your Knees Cave contains bears that predate Fraser Glaciation as well as other remains of glacial and postglacial age. The list of other vertebrate remains recovered is not exhaustive and excludes micromammals. Note that black bears, river otters (Lutra canadensis), and fish (mostly tide pool species eaten by otters) are commonly associated in coastal caves (den sites) but are not found at higher elevations. Brown bear remains are found in caves at all elevations.

Table 2. List of radiocarbon dates and associated stable carbon isotope values obtained thus far from vertebrate fossils in the southern Alexander Archipelago. The caves are located on northern Prince of Wales Island unless otherwise indicated (Fig. 1)

Timothy H. Heaton: E-mail, Home page, Phone (605) 677-6122, FAX (605) 677-6121