Small Fossil Mammals Reveal Wounds Left by the Last Great Extinction


A golden-mantled ground squirrel (Spermophilus lateralis), photographed in Bryce Canyon National Park, Utah. Though abundant at the Samwell Cave Popcorn Dome, California site during the Late Pleistocene, its numbers in the area decline at the beginning of the present Holocene epoch.

"One of the penalties of an ecological education", the naturalist Aldo Leopold once wrote, "is that one lives alone in a world of wounds." Few knew this better than he did. Despite becoming a celebrated advocate of wilderness for its own sake during the early twentieth century, Leopold began his career by hunting down large "pests" which bothered farmers - namely wolves, bears, and mountain lions. He had helped to create the very wounds he would later work to mend, but even the landscape Leopold knew had already been battered by events which took place thousands of years before.

The wolves, moose, bears, elk, big cats, and bison of the American west are impressive beasts, but they are only the remnants of a richer assemblage of large mammals which existed in North America until around 12,000 years ago. It was at that time that Saber-toothed cats, "short-faced" bears, giant ground sloths, and mammoths (among other large mammals) disappeared, part of the most recent mass extinction in earth's history. Just why these animals died out, however, is hotly debated. Rapid climate shifts, hunting by humans, hyperdisease, asteroid impact, and other hypotheses have all been forwarded in recent years, with hungry, hungry humans and the development of a warmer, wetter global climate being the prime suspects. Yet there is more to the story of the Pleistocene extinction that what became of the mammoth and mastodon. As a recent Nature paper by paleontologists Jessica Blois, Jenny McGuire, and Elizabeth Hadly points out, what happened to small mammals which scurried around the Pleistocene world can help us understand what caused the deep ecological wounds still visible today.

The trouble with solving the mystery of a mass extinction is that the fossil record only provides snapshots of what transpired. It is rare that deposits which span the duration of an extinction - from pre-extinction communities to tattered remnants - are found, but paleontologists have discovered one such place in northern California called Samwell Cave Popcorn Dome. This location not only bridges the divide between the end of the Pleistocene and the beginning of our present epoch, the Holocene, but it also contains the remains of many small mammal species which have allowed paleontologists to track the abundance of species over time.

At first glance, the species of small mammals from Samwell Cave Popcorn Dome appear to have done better than the largest of the megafauna - all of the species represented in the cave are still alive today. This does not mean that they were immune from the catastrophic events of the time. During the Pleistocene the small mammal community was relatively even, with each species represented by numerous individuals (there were few "rare" species), but at the beginning of the Holocene wider disparities opened. Suddenly some small mammal species were dominant while others became scarce. In fact, some species disappeared from the site entirely, causing a drop in diversity from 12 to 8 species (signaling local absence but not total extinction). Whereas they had been present at the site during the Pleistocene, the Mazama pocket gopher and mountain beaver were gone by the Holocene, with species like ground squirrels, chipmunks, and voles making up a smaller proportion of the community than they had previously.

The shifting abundance of small mammals occurred in two pulses, the first of which came between about 15,000 and 14,000 years ago. Some species, such as deer mice and Botta's pocket gopher, became more abundant during this time, while others, such as the Mazama pocket gopher, declined, but no species disappeared entirely. Then came another extinction pulse between 11,000 and 7,500 years ago. During this event some species were extirpated from the area while others suffered population reductions, bringing the community close to its modern-day makeup. The prolific nature of many small mammal species made them resistant to total extinction, but they were not invulnerable to drastic ecological change.

But what caused these shifts in the first place? There is no reason to suppose that these small mammals suffered from "overkill" as has been proposed for large mammals, and instead it appears that climate change was the primary culprit. As the cold, arid climate of the Pleistocene rapidly shifted to a warmer and wetter one, some species were locally wiped out, and it is not surprising that the species of pocket gophers and mountain beavers which once inhabited Samwell Cave Popcorn Dome now live in cooler areas. Furthermore, during the ecological pulses deer mice populations surged. As these mice are prolific generalists which often invaded disturbed habitats, their burgeoning numbers during the transition hints that major climate-driven ecological shifts were underway during the close of the Pleistocene. Perhaps the changes were even reinforced through the extinction of the megafauna. Apex predators and megaherbivores literally change the landscape through controlling populations of other animal species, dispersing seeds, and other activities, so perhaps the small mammal community was close to the bottom of an ecological cascade triggered by the loss of larger animals.

The modern distribution of the small mammals found around Samwell Cave Popcorn Dome is a signal of the ecological damage left by the end-Pleistocene extinction, and our species continues to pick at the wounds. Between habitat destruction and anthropogenic climate change, our species is again putting substantial pressure on these mammals and other species around the world - it is little wonder that "crisis" species which thrive is disturbed habitats, such as deer mice, are continuing to do so well. Things have not settled or slowed down. Instead it would appear that our species has picked up where the causes of the Late Pleistocene mass extinction left off, and in the fullness of geologic time the miniscule gap between the end of the Pleistocene and today may seemingly be obliterated, leaving the fossil record to tell an even more catastrophic tale of extinction.

Blois, J., McGuire, J., & Hadly, E. (2010). Small mammal diversity loss in response to late-Pleistocene climatic change Nature, 465 (7299), 771-774 DOI: 10.1038/nature09077

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