Packrat Paleo in Bears Ears National Monument

Editor's note: This guest post comes to us from Dr. Allison Stegner, a paleontologist who has conducted fieldwork within what is now Bears Ears National Monument.After you have read Dr. Stegner's post, learn more about SVP's involvement with BENM here.

“Here it comes!” I call down to my field assistant, Molly, as I begin to lower a 5-gallon bucket full of sediment on a rope pulley system we have cobbled together. Small rocks cascade off the cliff surface below me, zinging into the brush, and a moment later I feel the weight of the bucket suddenly relieved as Molly grabs it and frees it from the rope. Molly and I have been excavating a small packrat midden at the back of a 20-foot-deep cave in what is now known as Bears Ears National Monument (BENM; at the time of excavation it was the Butler Wash Wilderness Study Area, just a mile or two south of the Canyonlands National Park Needles District). This cave is home to an industrious—living—packrat, who, within the last day, has clipped squawbush leaves and set them out to dry on a shelf of rock beside a grand pile of juniper sticks. I have mapped and begun excavating a 1x1 meter square to the northwest of this stick pile: these sediments don’t represent the packrat’s residence, but debris falling off the stick pile has accumulated here and been buried over time. Some of this debris includes small vertebrate bones, likely brought to the mouth of the cave by owls and then dragged to this dark recess by packrats. It is a hot day, the dust is fine and smells like packrat urine, and my hands and knees are pincushions of transparent cactus spines from a heap of cactus paddles that the resident packrat has collected and fastidiously maintained.

Above: Looking northeast to the Butler Wash Wilderness Study Area, now northwest Bears Ears National Monument

Above: Allison lowering buckets of excavated sediments from Butler Wash Cave.

When belly-crawling over rodent scats, loose cactus spines, and miscellaneous sticks and berries lovingly accumulated by packrats, I sometimes reflect on how unexpected my career as a Quaternary paleoecologist has been. I never anticipated how fundamental rodents, coyote scats, and sand would be to my work. I have been excavating and studying packrat middens in BENM and the surrounding areas of southeastern Utah for half a decade now, using the fossilized bone and plant material to answer questions about biodiversity conservation today. And while the medium is unusual, middens provide a window into the past that allows me to quantify and qualify the ways in which the modern fauna of BENM is similar, and different, from the fauna that persisted here for the last 10,000 years.

When most informed people think of paleontology in Utah, they think of impressive trackways, crocydiliforms, AllosaurusUtahraptor—and rightly so: BENM and the surrounding areas in southeastern Utah, western Colorado, and northern Arizona, are remarkable for their richly fossiliferous Jurassic and Triassic deposits (see Rob Gay’s excellent guest blog for PLOS at http://blogs.plos.org/paleocomm/2017/02/14/paleontology-of-bears-ears-national-monument-utah/ to learn more). Outcrop from these time periods abounds in southeastern Utah, and vegetation is sparse in this desert environment, so fossils are relatively easy to find. Quaternary paleontology has a very different, but equally important relationship to hard-rock geology: certain Paleozoic and Mesozoic outcrops are prime real estate for packrats and for the preservation of their middens. In BENM, the Slickrock member of the Entrada (Jurassic), Navajo Sandstone (Jurassic), and the White Rim and Cedar Mesa members of the Cutler formation (i.e., the white Cutler sandstones that form curvaceous, sometimes, dare I say, blobby outcrops; Permian) weather in a way that often creates large alcoves, caves, or overhangs where packrats like to live, where carnivores and avian predators like to roost, and where eolian deposition is rapid. These are the kinds of sites where middens accumulate lots of bone and where they are protected for thousands of years. So, while BENM represents unparalleled territory for the study of Mesozoic fossils, it is also ideal for large packrat middens.

Beyond the geology, BENM is a fascinating place for studying Quaternary biogeography. Three major laccolithic mountains ranges (the Abajos—now included within BENM—as well as the La Sals and the Henrys) penetrate the sandstone layer cake of southeastern Utah. These ranges have never been connected to one another or to other high elevation zones, and each is home to multiple endemic mammalian subspecies. Add an elevation and vegetation gradient from desert through alpine at the very highest elevations in the Abajos, and the Colorado River which divides the La Sals and Abajos from the Henrys, and you have an incredible natural laboratory for biogeography and macroecology. In addition to collecting fossil middens, I have also surveyed small mammals in BENM and surrounding areas since 2014, and have trapped the Great Basin pocket mouse (Perognathus mollipilosus), a species never previously reported east of the Colorado River. Is the range of this species expanding, and if so, was it able to readily cross the river via human infrastructure, or does this migration predate our bridges and highways? This is a question than could be answered using ancient DNA analysis of pocket mouse fossils, which are common in the deposits I have excavated. Packrat middens, in turn, raise biogeographic questions that can be answered by detailed trapping surveys: in a site near the northwestern corner of BENM, I have collected jaw elements from the desert shrew (Notiosorex crawfordii), a cryptic species not presently considered resident in this part of Utah—is this species truly extralimital or have we simply never surveyed it? If it does still persist in the area, what does that tell us about its ecology?

Above: Pinon mouse captured (and released) in BENM.

But the primaryreason I study packrat middens in BENM is because they can reveal how species respond to environmental change. The species preserved in middens are, by and large, still extant today, so an understanding of how they responded to past periods of rapid climate change can tell us a great deal about how they will respond during rapid climate change today. But understanding the impacts of recent climate change is complicated by the fact that human population, resource use, and land conversion have dramatically increased simultaneous with climate change. BENM represents an extraordinary opportunity to disentangle the relative impacts on communities and species of pre-industrial human populations, large spatial scale land use, like grazing, and climate change. Parts of BENM were far more densely populated 1000 years ago than they are today. We have considerable knowledge of the human history of BENM, and there are even indications of overlap between humans and mammalian megafauna: for example, petroglyphs that depict soft-tissue details of proboscideans, detail that could only be known by people who saw the living animals. Quaternary vegetation and tree ring history in BENM has been studied in detail, so we know also how vegetation and precipitation has changed in parts of BENM. Early American settlers often kept detailed records of livestock numbers and land initiatives, like rodent eradication. It should go without saying that there is vastly more to learn about this region, but wherever these lines of evidence do come together, we can now identify the relative impacts of humans and climate change.

Above: Weasel (Mustela) jaw found at Butler Wash Cave.

Once we have accumulated 3 or 4 four buckets of sediment, I clamber down from the cave mouth to help screen. Molly hoists a bucket, poised to dump half the sediment onto our stacked screens. But before she pours, she shrieks and points: a 2-inch-long, nearly translucent scorpion is sitting in the middle of the screen, pinchers raised and ready for combat. I take the handle of the screen and clumsily whip it off the stack—the scorpion flies into a rounded clump of blackbrush, a clump we eye suspiciously for the rest of the afternoon. Then Molly pours, and we rake our hands over the sediments, pulling out bone fragments: jackrabbit, jackrabbit, kangaroo rat, jackrabbit, snake vertebra—the usual suspects. Every once in a while, something exciting, like a bat tooth or a weasel jaw—things to take to the comparative collection back on campus.

Above: After a spring rainstorm, looking toward the Butler Wash Study Area from Middle Park, northwestern BENM.

Posted: 7/6/2017 10:53:17 PM by andyfarkeadmin | with 0 comments
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