Timothy H. Heaton
Department of Earth Sciences and Physics, University of South Dakota, Vermillion, SD 57069
Department of Paleobiology, Smithsonian Institution, Washington, D.C. 20560
The Alexander Archipelago poses a complex island biogeographic puzzle, with each island being home to a unique subset of mainland species. Prince of Wales Island, for example, contains just over half the species of terrestrial mammals that inhabit southeast Alaska. Since glaciers once filled the straits that now separate the islands, colonization may have been easier in the past for some species, and therefore the current distribution may be relectual rather than an equilibrium between dispersal and extinction. Islands of the Alexander Archipelago are closely spaced, however, usually about 5 km, and large mammals such as deer and bears are known to swim such distances. Why some large-bodied species are absent from some islands is therefore a mystery.
Grizzly bears (Ursus arctos) and black bears (Ursus americanus) both inhabit southeast Alaska, but the two species rarely coexist on offshore islands. Prince of Wales Island, in spite of its large size and proximity to the mainland, has only black bears, and these bears parallel the habits of grizzlies by including fish in their diets. Since grizzlies tend to dominate black bears wherever the two coexist, it was once thought that grizzlies must never have colonized the island.
Last year we reported on a fossil bear den being excavated from El Capitan Cave on the northern part of Prince of Wales Island (Heaton and Grady 1992a, 1992b). During July, 1992, we opened the sealed den entrance and conducted a full excavation of the site, recovering parts of at least four black bears and three grizzly bears. This site is located in a glacial valley near the bay below El Capitan Peak. Skulls of two additional grizzlies (both juveniles) were recovered by cavers from deep shafts in Blowing in the Wind Cave in the alpine karst on El Capitan Peak. These fossils demonstrate that grizzlies did in fact colonize Prince of Wales Island and occupied diverse habitats there before disappearing.
The large bear previously reported from El Capitan Cave (Heaton and Grady 1992a, 1992b) has now been positively identified as an enormous, aged grizzly. It was radiocarbon dated at 9,760+/-75 yr B.P. (AA-7794). Its remains, including cranium and upper dentition, were scattered in the rubble near the entrance of the bear den. A second individual, somewhat smaller, was later identified from the same site, but too much glue was used during preparation of the single femur shaft to allow for dating. A third grizzly, smaller still, is represented by many heavily chewed but beautifully preserved elements found farther inside the den passage. This animal was radiocarbon dated at 12,295+/-120 yr B.P. (AA-10445), the oldest from the site. Of the two juvenile grizzlies from Blowing in the Wind Cave, the smaller one contains almost no collagen and is therefore undatable. The larger one has excellent preservation and was radiocarbon dated at 9,995+/-95 yr B.P. (AA-10451), an age bracketed by the El Capitan Cave grizzlies. The oldest black bear from El Capitan Cave was radiocarbon dated at 11,565+/-115 yr B.P. (AA-10448), so black and grizzly bears coexisted on the island for at least 1,800 years.
These data raise as many questions as they answer about bears and island populations. First, it is unusual to find both black and grizzly bear remains spanning a long time interval from the same den. Remains of both species have been found together in caves but are not likely synchronous (Emslie and Czaplewski 1985, Grady 1988, Heaton 1988). Grizzlies rarely den in caves, and that helps explain their generally poor fossil record compared with black bears (Kurtén and Anderson 1980). The second enigma is the extinction of grizzlies on Prince of Wales Island. The fact that grizzlies outnumber black bears from the two caves and that their record is older offers the hint that grizzlies reached the island first and outnumbered black bears for a time. Given that black bears, a native American species, have generally not fared well in areas invaded by grizzlies, especially in coastal Alaska, their lone survival on Prince of Wales Island is strange indeed.
Grizzly remains in upper Alaska date to the early Wisconsinan, but the earliest dates south of the ice are around 13,000 yr B.P. (Kurtén and Anderson 1980). It is highly unlikely that grizzlies could have reached the Alexander Archipelago prior to--or survived there during--the last glacial. Local ice-free conditions during the height of glaciation have been documented on the Queen Charlotte Islands farther south (Warner et al. 1982), but the Alexander Archipelago is thought to have been fully covered by the Cordilleran Ice Sheet until at least 14,000 yr B.P. (Clague 1991). The Quaternary history of southeast Alaska has not received the attention that coastal British Columbia has, but this is changing. Since El Capitan Cave is located in a glacial valley, our date of 12,295+/-120 yr B.P. (AA-10445) on a grizzly bear provides a new minimum age for substantial deglaciation on northern Prince of Wales Island, which is near the center of the Alexander Archipelago.
We thank Kevin Allred, Steve Lewis, Paul Matheus, Dan Monteith, and other members of the Tongass Caves Project for help in finding and collecting the fossils. Jim Baichtal has been instrumental in attracting scientists to the karst of southeast Alaska, and we appreciate his support. Funding for radiocarbon dates, travel, and supplies was provided by Tongass National Forest and the National Geographic Society.
Clague, J. J. 1991 Quaternary Glaciation and Sedimentation. In Geology of the Cordilleran Orogen in Canada, edited by H. Gabrielse and C. J. Yorath, pp. 419-434. Geological Survey of Canada, Geology of Canada 4.
Emslie, S. D., and N. J. Czaplewski 1985 A New Record of Giant Short-faced Bear, Arctodus simus, from Western North America with a Re-evaluation of its Paleobiology. Natural History Museum of Los Angeles County, Contributions in Science 371:1-12.
Grady, F. 1988 Vertebrate Fossils. In Caves of the Organ Cave Plateau, Greenbrier County, West Virginia, edited by P. J. Stevens, pp. 51-55. West Virginia Speleological Survey Bulletin 9.
Heaton, T. H. 1988 Bears and Man at Porcupine Cave, Western Uinta Mountains, Utah. Current Research in the Pleistocene 5:71-73.
Heaton, T. H., and F. Grady 1992a Preliminary Report on the Fossil Bears of El Capitan Cave, Prince of Wales Island, Alaska. Current Research in the Pleistocene 9:97-99.
Heaton, T. H., and F. Grady 1992b Two Species of Bear Found in Late Pleistocene/Early Holocene Den in El Capitan Cave, Prince of Wales Island, Southern Alaska Coast (Abstract). Journal of Vertebrate Paleontology 12:32A.
Kurtén, B., and E. Anderson 1980 Pleistocene Mammals of North America. Columbia University Press, New York.
Warner, B. G., R. W. Mathewes, and J. J. Clague 1982 Ice-free Conditions on the Queen Charlotte Islands, British Columbia, at the Height of Late Wisconsin Glaciation. Science 218:675-677.