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Know the West

Scientists dig up the past in packrat middens

The animals’ sturdy nests can preserve clues about the climate for 50,000 years or more.


On a bright, late-summer day in southwest Montana, Julio Betancourt gazes through binoculars across a dry slope rimmed by limestone cliffs. A cave-like divot in the rock catches his attention. 

“I see middens,” he says, meaning packrat middens — the nests that the long-tailed nocturnal rodents construct with material from trees and other vegetation. Excited, he uses a technical term: “I can actually see the amberat.”

For Betancourt, a U.S. Geological Survey senior scientist who studies climate variability and ecological change, middens provide a window into the past. Packrats drink no water, but produce a viscous plant-derived urine, which they excrete on their nests. When this dries, it forms amberat — an asphalt-like crust that can preserve plant material for tens of thousands of years. A well-preserved midden is a snapshot of plant communities from as far back as the last ice age.

Over the past decades, Betancourt has chiseled samples from hundreds of middens, some more than 50,000 years old. Back then, glaciers were scouring Western mountain ranges, global temperatures were several degrees cooler, and familiar trees like ponderosa pine hadn’t yet spread across the West. By noting middens’ age and location, scientists can map how plant species migrated as the ice age transitioned, starting around 12,000 years ago, to the warmer, arid climate of the modern West.

Now, middens may help us understand how plant communities will respond to human-caused climate change. “Most places in the world lack the detailed historical knowledge that middens provide here,” says Betancourt. “We’re damn lucky” to have them.


In 1979, Julio Betancourt holds a midden he collected from a cliff in New Mexico’s Chaco Canyon.
Courtesy Julio Betancourt

In 1961, two scientists studying the biological effects of nuclear detonations at the Nevada Test Site climbed a nearby mountain. They were surprised to find no juniper trees on top, but spotted what turned out to be a midden — full of juniper twigs. Radiocarbon dating pegged the twigs at about 10,000 years old.

The duo later found other middens as old as 40,000 years in the area. When they published their conclusion that juniper had climbed the area’s mountains and then vanished as the climate became warmer and drier, they mentioned the middens’ “peculiar varnish-like coating” and noted that they may hold “unique value” in the study of ancient climate.

Scientists began poking around rock crannies in the Sonoran and Mohave deserts, the Great Basin and the Grand Canyon, using the middens they found to plot the spread of plant species across the landscape in the wake of the most recent ice age. Betancourt and others charted the northward march of ponderosa and the spread of piñon-juniper forests to the Colorado Plateau.

Although a modern-day forest may look ancient, “there’s a lot of instability,” says Betancourt. Generally, the midden record shows some plant species invading while others retreat, not only in rhythm with broad climatic trends but also with acute droughts and wet periods.

For example, Utah juniper jumped suddenly from southern Wyoming into Montana during a two-millennia dry period beginning around 7,500 years ago, stalled during a wet spell, then backfilled as the climate again began drying 2,800 years ago. Similarly, piñon pine leaped 25 miles to Dutch John Mountain on the Utah-Wyoming border around the year 1200, hung on during a decades-long drought that killed most of the long-dominant juniper, and finally took over during the wetter 1300s.

Such ecological elasticity is reason for “cautious optimism” as climate change likely brings a new scale of drought and other disturbance, says Stephen Jackson, who heads the U.S. Geological Survey’s Southwest Climate Science Center. “We know that with enough time, species can migrate” to areas with suitable climate, rather than perish, and that even small populations like the Dutch John Mountain piñons can weather severe drought and spring back.

But, Jackson adds, the accelerating pace of climate change may mean that plant species “aren’t necessarily going to have those decades or centuries to make the adjustment.” He and Betancourt, teaming up on a study published last November, estimate that a widespread Western subspecies of ponderosa could lose half its current habitat by 2060 due to warming. They suggest one possible solution: deliberately translocating it to places — such as farther north in Canada — where it is more likely to endure future climate, with past climate-driven jumps helping to identify those areas.

Back in Montana, Betancourt continues his midden hunt. By midday, we’ve found a few gamey middens, none with the promising crust of old age. As if for a consolation prize, Betancourt pulls out several baggies of duff-like plant material from his luggage — ice-age midden matter, washed of the amberat. As a passerby eyes us suspiciously, Betancourt opens one and assures me that ice-age samples such as this one smell of Pinesol. I inhale deeply, imagine future juniper flowing across these foothills. Packrats will surely glean those trees’ twigs, caching a story yet to be told.