Packrats and Paleobotany

By Iris Maplewood (she/they)

Packrats may be a nuisance to homeowners, but they are a paleobotanist’s best friend.  Paleobotanists study plants that lived thousands or even millions of years ago.  Some are still alive today, like the piñon pine, which has been in the Great Basin for well over 10,000 years.  Others are long extinct.  Plants can form fossils, but more often than not, they decay.  That’s why we can throw our leaves in compost bins and make new soil so quickly.  To make a fossil that survives, plant matter has to be physically or chemically preserved, protected from the elements, and remain undisturbed until we find it.  

Enter the packrat.  More specifically, their bathrooms. 

The genus Neotoma includes 23 species of pack rats across the world.  Eight are native to North America.  Neotoma lepida, the desert woodrat, is native to Nevada.  Desert woodrats have a wide range of habitats, from southeast Oregon in the U.S. to Baja California in Mexico.  They build cone-shaped dens against the base of shrubs or in rock crevices.  The nickname “packrat” comes from their propensity to hoard any debris they can find close to their nests.  This includes animal droppings, bones, pine cones, bits of twine, bottle caps, baby pacifiers, and more.  If they can carry it, they’ll hoard it.  Perhaps they find their own abode rather crowded after a while, so it’s no surprise that each rat creates multiple dwellings.  

Inside each dwelling is, of course, a bathroom, also called a midden.  Packrats, including the desert woodrat, set aside part of their homes to defecate and urinate…right on top of their own woodland treasures.  Over time, the assorted cones, twigs, and other debris get packed down and sealed up in layers of dried urine.   This mass of plant matter and dried urine is called an amberat.  The urine seals and protects the plant matter from decay.  A den built under a cactus may not last very long, but a den in a narrow rock crevice or dry cave may be protected enough to last for tens of thousands of years.  

Desert woodrats tend to forage no more than 100 yards from their dens.  If their botanical trinkets make it into the midden, they create a snapshot of what plants lived around them at the time of collection.  Even better, while only one woodrat occupies a den at a time, the dens may be used by successive generations.  Instead of just one snapshot, the accumulated amberat becomes a slideshow of local wildlife as long as the midden is inhabited.  Carbon dating pinpoints the age of each plant sample.  Paleobotanists can then learn which plants grew where, for how long, whether their range fluctuated with climatic conditions, and when they may have gone extinct. 

The Great Basin has undergone a long series of climatic changes.  The Pleistocene Epoch lasted from 2.6 million years ago to 11,700 years ago.  It is known as the “Great Ice Age,” for good reason.  Chilly temperatures and massive glaciers swept across North America multiple times.  Retreating glaciers left massive lakes in their place.  This was followed by the Holocene Epoch, a time of much warmer temperatures which continues to the present day.  The lakes of the Great Basin began to dry up.  There were still stretches of cooler temperatures, but nowhere near that of the Ice Age.  

Desert woodrat middens show how various plants responded to the changing climate.  About 26,000 years ago, the Great Basin was still a much colder climate filled with massive lakes.  The range of subalpine conifers, which thrive just below the treeline, were as much as 1,000 meters (or 3,281 feet) downslope of their range today.  Bristlecone pine, limber pine, and others grew in more limited stands.  In the western portion of the Great Basin, juniper grew in lower elevations close to lakes formed by rainfall.  These juniper sites were likely dominated by Utah juniper, but some middens show potential hybrids with other junipers like the California or Rocky Mountain juniper.  The Douglas fir and Rocky Mountain juniper were entirely absent from the Basin.  Their range was farther south, in southern Nevada and southeast California.  

As the Pleistocene drew to a close and the climate began to warm, plant ranges began to shift.  Steppe plants like sagebrush expanded their range into the Mojave and Sonoran deserts, as well as creeping into higher elevations.  Shadscale dominated northern areas of the now-drier Great Basin.  Coniferous forests were driven further and further into higher elevations.  Cold-loving limber pine, along with Engelmann spruce and certain junipers, began a slow decline.  They were replaced by snakeweed.  Snakeweed can still tolerate cold, but not as much.  Engelmann spruce, for example, can handle temperature ranges of -45.6-32.2°C, or -50-90°F; snakeweed prefers 5-35°C, or 41-95°F.  In southeast Nevada, limber pine and Douglas fir were replaced by the Gambel oak and Utah juniper.  In the southern Great Basin, plants adapted to moist environments were increasingly replaced with desert vegetation.  By mid-Holocine, about 7,800 years ago, Utah juniper and single-leaf piñon grew in robust stands throughout Nevada’s mountains.  

These shifts occurred at different rates depending on the location, but the fluctuating tree ranges were fairly predictable.  For example, when the Great Basin entered a cool, moist period 4,400-2,700 years ago, the same types of shifts occurred.  Cold-loving trees shifted further downslope, but when the warm period ended, it drew the treeline back up to modern levels.  

Currently, sagebrush and shadscale dominate low elevation vegetation.  The lower mountain slopes of the northern Great Basin are covered in juniper forests, while the same elevations in the southern Great Basin are dominated by piñon-juniper forests.  Stands of white fir, Douglas fir, and ponderosa pine are scattered across the Great Basin in the south and east.  Bristlecone pine, Engelmann spruce, and limber pine still grow at higher elevations.  As our global climate continues to change, however, the range of these plants may fluctuate in line with past trends. 

Paleobotanists aren’t the only ones studying middens.  Woodrats dragged animal remains to their middens, as well.  Paleontologists use middens to study ancient animals and overall ecology.  Learning how ecosystems respond to climate change may give us insight into how to protect our modern wildlife.  Packrats may not be welcome houseguests, but they are an invaluable resource in studying the past and predicting the future of the Great Basin.  

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About the Author

Iris Maplewood (she/they)

Iris graduated from the University of California, Irvine with a B.A. in Anthropology and a minor in Biology.  She worked in education as a teacher and tutor before switching careers to conservation.  She volunteered and worked for Conserve Southwest Utah in St. George, and is excited to serve her first AmeriCorps term with us as the Rosewood Nature Center Host.  She is going back to school to earn a degree in either forestry or environmental science.  In her free time, she can be found hiking or playing with her two cats.