Timothy H. Heaton
Professor of Earth Sciences
University of South Dakota
The vertebrate fossils recovered as part of the Tongass Caves Project have enormous significance and have filled a crucial geographical gap in the study of the North American fauna. Their excavation has been an exciting project featuring one breakthrough after another. Southeast Alaska, once thought to be an insignificant place that only recently became colonized by mammals, now appears to have been an Ice Age refugium and possibly a corridor for coastal colonization.
Island biogeography is a most interesting topic. The study of variation among island populations was one of the key factors that led Charles Darwin to his theory of evolution. Since Darwin's time, mathematical models have been developed to analyze island diversity in order to determine modes of colonization and rates of extinction. Island systems which have a predictable diversity based on island size and distance from the mainland are said to be in equilibrium, suggesting a constant colonization rate. Systems lacking such equilibrium are thought to represent relict distributions from some former period when colonization was easier. I first became familiar with island biogeography when studying the Great Basin with its mountain "islands" separated by broad desert valleys. The Alexander Archipelago poses an even more complex puzzle that is only beginning to be elucidated.
Southeast Alaska has special significance for two reasons. First, it lies half way between two large Ice Age refugia (Asia/Beringia and southern North America) that were biogeographically separated by continental glaciers. Second, it lies on a coastline that may have housed minor refugia with access to marine food sources. The recent glacial cover combined with rain forest conditions made any fossil record seem hopelessly unlikely, so the region remained largely unstudied while upper Alaska and the lower 48 states became famous for their bizarre Ice Age fossils (including extinct ground sloths, mammoths, mastodons, horses, camels, saber-tooth cats, etc.). But the situation changed when the Tongass caves were found to be sites of exceptional fossil preservation.
The irony of working the Alaskan islands is that their fossil diversity is unusually low, and yet a large share of the species recovered are exotic and therefore significant. Of the five large mammal species found at El Capitan Cave, two (brown bear and red fox) no longer inhabit the island. The only two large mammal species recovered from Bumper Cave (brown bear and caribou) fit in this category. How did fox and caribou manage to colonize offshore islands when today they aren't even found on the nearby mainland? The conspicuous absence of brown bears in the southern Alexander Archipelago used to be attributed their recent invasion from the north and the possibility that they had never managed to reach those islands. But our cave excavations have turned up more remains of brown bears than all other large mammals combined! Obviously some important pieces of the puzzle have been missing.
While there are still more questions than answers, all the data seem to be pointing in a certain direction: the Alexander Archipelago was not completely stripped of its mammals during the Ice Age, but contained habitable land that acted as a coastal refugium. This situation is nearly certain with respect to brown bears. Postglacial remains have been found in the two caves mentioned above on Prince of Wales Island and in Enigma Cave on Dall Island. Even more significant was the recovery of a 35,000 year old femur from On Your Knees Cave, together with a 42,000 year old tibia of a black bear, because these remains predate the last peak of glaciation. Genetic work by Gerald Shields and Sandra Talbot in Fairbanks suggests that the living brown bears of the northern Alexander Archipelago represent a distinct clade with a long history of reproductive isolation. The simplest explanation for all these data is an Ice Age refugium in southeast Alaska.
We do not yet have an adequate fossil record to determine whether animals like foxes and caribou (or even current island inhabitants like black bear and deer) survived the last glaciation on Prince of Wales Island. But even if they did not, the coastal refugium theory helps explain their postglacial extinction in the area. As the ice melted and long-term biogeographic barriers disappeared, species expanded their ranges and came into competition from three directions (from the north, from the south, and from the coastal refugium). Retreating glaciers may have made access to some islands rather easy. Such a situation would have led to high species diversity, but only temporarily until ecological stability was reached through competition and extinction.
The goal of our future paleontological work is two-fold. First we need to establish a much more thorough mammalian record across the peak of the last Ice Age. This will help to distinguish the glacial survivors from the postglacial invaders. Second, we want to determine the antiquity of Man in the islands. The most enthusiastic proponents of the coastal refugium theory have not been biologists but archaeologists, for the coastal Pacific is one of the most likely routes for the entry of humans into North America. The presence or absence of Ice Age coastal refugia is critical to this theory, and our fossil cave mammals have provided the first long-awaited evidence that such refugia existed. I will be working closely with archaeologist Jim Dixon as the cave excavations continue.
My brief 1995 field season with Dave Love, Kevin Allred, and Jim Baichtal produced a key find toward each of these goals. The jaw and ulna of a small seal (probably ringed seal, which now only inhabits northern Alaska) were found in On Your Knees Cave and dated at 17,500 years old, right at the last glacial peak. This suggests that the cave was open then and not covered by ice. In Kushtaka Cave we found the tip of a bone spear point together with bones of two black bears that have been dated at 8,700 years old. This appears to be the oldest known archaeological record from the island, and it helps us know what to look for as we carefully search older deposits.
I feel that we've only seen the tip of the iceberg so far. What has been found to date tells us what questions to ask as well as what to look for and where to look for it. From this point on the research will become much more interdisciplinary. Glacial and sea level geologists will locate the likely shorelines and potential glacial refugia. Cavers will search for caves that might contain fossil remains. Paleontologists and archaeologists will meticulously excavate the deposits and identify the remains. And physicists, chemists, and geneticists will determine the ages, diets, and relationships using state-of-the-art laboratory techniques. These combined methods will piece together the complex story of the Alexander Archipelago and tell us many new things about the animal and human history of our continent. I look forward to working with many of you as this important work proceeds.