Pack rat middens reveal south-central Idaho plant history

Associated Press

IDAHO FALLS – Scientists studying pack rat middens at the City of Rocks National Reserve in south-central Idaho have determined that the area’s iconic pinyon pine trees first arrived about 2,800 years ago and became prevalent about 700 years ago.

That’s just one example of what scientists have learned by studying the middens in the area that date back 45,000 years.

Julio Betancourt of the U.S. Geological Survey has studied pack rat middens in the U.S. West and South America.

“They’re hard as a rock, kind of black, big chunks, sometimes huge chunks of pack rat deposits that have been consolidated, cemented by the crystalized urine of pack rats,” Betancourt told the Post Register. “The pack rats are just collecting tons of stuff and don’t go very far to collect it – mostly within 50 (yards). The spatial resolution is really high. We’ve studied all kinds of things in these deposits. The principal thing is plant remains.”

The middens can be dated, giving a snapshot of the local ecology over time.

The U.S. Geological Survey keeps a database of pack rat middens for western North America.

The middens are typically found in small caves or under overhangs where they’re protected from rain and snow. Rock climbers help scientists recover some middens.

One midden from the area dated to about 10,000 years ago is on display at the City of Rocks visitor center in Almo.

To study what middens contain, the middens are placed in water to dissolve for a week or two. The plant remains are separated and put on a paper plate to dry.

“Many of the plant fragments are in pretty good shape,” Betancourt said. “We can identify oftentimes the species.”

He said the middens have allowed scientists to track climate and vegetation changes at the City of Rocks.

“At fairly high resolutions we can tell the timing of plant migrations,” Betancourt said. “We’ve done that for Utah juniper, and we’ve done that for the two main species of pinyon pine and for ponderosa pine in the central Rockies.

“One of the things that’s always interested me are these isolated outposts of particular species.”

The middens also give clues about how wildfires have played a role over time.

“What comes first, the change in vegetation or the shift in fire regime?” Betancourt said. “And also, the erosion in sedimentation regime?”

More recently, scientists have been using advances in science to study genetic material from middens. Betancourt said that can identity pathogens, and even other types of animals that came in contact with the middens.

“You can start getting a little bit better view of the whole biotic community contributing,” Betancourt said.