1868 beaver dam map.jpg
This 1868 map shows beaver dams and ponds near Ishpeming, Mi. It was drawn by anthropologist Lewis Henry Morgan.
ISHPEMING, MI -- On this International Beaver Day, April 7, 2016, take a moment to marvel at the industrious critter of toothy fame, which you may not realize is actually responsible for some of Michigan's oldest surviving infrastructure.
According to a fall 2015 study by ecologist Carol Johnston of South Dakota State University in Brookings, more than 70 percent of beaver dams mapped in the Upper Peninsula near Ishpeming in the 1860s are still around today.
Johnston, who published her findings in Wetlands scientific journal, compared an 1868 map of beaver ponds drawn by anthropologist Lewis Henry Morgan with aerial photos of the area in 2014 to analyze long-term endurance of the wild dams.
Of the 64 in Morgan's map, drawn in his 396-page book "The American Beaver and His Works," 72 percent of the beaver dams and ponds remain in some form.
Altered terrain through mining, residential development or channelizing-streams are the main sources of beaver pond loss, according to Johnston. By the time of Morgan's book, fur trapping had decimated the North American beaver population, but "this remarkable consistency in beaver pond placement over the last 150 years is evidence of the beaver's resilience."
According to Science Magazine, Johnston said that beavers, which generally live about 10 years, might not have continually occupied every dam in the last 150 years. There are studies that suggest beaver colonies can last 1,000 years.
A beaver in Horseshoe Lake at Denali National Park in Alaska.
In his 1868 book, Morgan reached the same conclusion.
"The evidence from these, and other sources, tends to show that these dams have existed in the same places for hundreds and thousands of years, and that they have been maintained by a system of continuous repairs."
That would likley make the beaver dams around Ishpeming older than Michigan's oldest surviving buildings -- the 1780s-era fort officers quarters, McGulpin and Biddle houses on Mackinac Island -- and perhaps even older that some of the old growth, 500-plus year-old white cedar trees on South Manitou Island.
Garret Ellison covers government, environment & the Great Lakes for MLive Media Group. Email him at [email protected] or follow on Twitter & Instagram