Beaver Dams Can Last Centuries, 1868 Map Shows

Beavers aren't just busy — they're swamped. But while building and maintaining a marsh can take time, it's apparently worth the investment. The rodents' ecosystem-shaping homes have long been known for their durability, and a recent study offers unique evidence that individual beaver dams can persist for centuries.

That evidence comes via an 1868 map (see below) commissioned by Lewis H. Morgan, a prominent American anthropologist who also worked as a railroad director. While overseeing a rail project through Michigan's Upper Peninsula in the 1860s, Morgan came across something that amazed him: "a beaver district, more remarkable, perhaps, than any other of equal extent to be found in any part of North America."

Morgan went on to study these beavers for years, resulting in his 396-page tome "The American Beaver and His Works." Published in 1868, it included a map of 64 beaver dams and ponds spread across roughly 125 square kilometers (48 square miles) near the city of Ishpeming, Michigan. And now, a fresh look at Morgan's map has revealed that most of the beaver dams are still there.

Checking In, 150 Years Later

This map shows beaver dams near Ishpeming, Michigan, in 1868. (Image: Lewis Henry Morgan/Internet Archive)

"We haven't known much about the long-term resilience of beaver populations, but this map allowed us to look back in time in a pretty unique way," study author and South Dakota State ecologist Carol Johnston tells Science Magazine's David Malakoff.

When Johnston first learned of Morgan's map during her postdoctoral work, she noticed its age and detail stood out from most beaver-dam data. Curious how the dams fared over the past century and a half, she decided to see for herself.

Using aerial imagery, Johnston pieced together a modern update of Morgan's map. She realized 46 of the 64 dams and ponds were still there, or about 72 percent. Some dams seemed abandoned, and while every one may not have housed beavers continuously since 1868, Johnston is nonetheless impressed.

"This remarkable consistency in beaver pond placement over the last 150 years is evidence of the beaver's resilience," she writes in the journal Wetlands.

Other research has hinted at even longer resilience. A 2012 study, for example, found that some beaver dams in California date back more than 1,000 years. One of those dams was first built around 580 AD, making it older than China's Tang Dynasty or the earliest-known English poetry. Later evidence shows the same dam was in use around 1730, when beavers apparently made repairs to it. It was finally abandoned after suffering a breach in 1850 — some 1,200 years after its initial construction.

Beavers' Turbulent History

Despite all their resilience, however, both of Earth's beaver species — the North American (Castor canadensis) and Eurasian (Castor fiber) — were obliterated by human trappers from the 1600s to 1800s. Beavers have been building ecosystems in North America for the past 7 million years or so, and even longer in Eurasia, but demand for their fur pushed them to the brink of extinction in just a few centuries.

Legal protections finally helped beavers claw back last century, and they're now abundant in North America again (albeit with only about 10 percent of their historical population). Castor fiber has made a similar comeback, more so in Europe than Asia, and both species are now listed as "Least Concern" on the IUCN Red List.

It's unclear exactly how Morgan's beavers fared as more humans moved in, but the new study suggests they weren't unscathed. Although most of their dams still exist, the 18 that don't were in places where humans have radically changed the landscape since 1868 — presumably too much for beavers to change it back. "Land use changes that altered the terrain (mining, residential development) or stream paths (channelization) were the main sources of beaver pond loss," Johnston writes.

Taking a Lesson From Rodents

Still, it's encouraging that so many beaver homes survived the 19th and 20th centuries, a particularly turbulent time for wildlife across North America. Any averted extinction is good news, but beavers are keystone species whose DIY wetlands boost all kinds of biodiversity, so their comeback is especially welcome.

Beavers only live for 10 to 20 years, and since they're often parents by age 3, dozens of generations could have inhabited Morgan's ponds since he mapped them. The aforementioned California dam could have even spanned 400 generations, about the number humans have had since our ancestors began farming. Yet despite all our species' success, we have a knack for destroying ecosystems in the process. Beavers, on the other hand, use local resources to enrich themselves and their habitats.

That doesn't mean beavers have all the answers. But the industrious rodents are a useful reminder that we're all defined by what we leave for our descendants, whether it's an unpolluted atmosphere, a biodiverse bog or just a "dammed" place to live.