Four small chain-link fences have saved about 100 turtles a year from being crushed to death on Minnesota roads.
The fences, installed over the past three years, have cut mortalities by as much as 85% at popular turtle crossings, showing they may be a relatively cheap and effective tool for bringing the state's turtle populations back.
"The ones that are still getting hit are the really small hatchlings that can fit through these half-inch gaps," said Tricia Markle, a wildlife conservation specialist for the Minnesota Zoo. "So we're adding a finer mesh screen and think we can even keep out the really small turtles."
Minnesota's turtles have been declining for years, in part due to habitat loss and more frequent spring floods that destroy nests. Predators, such as raccoons that eat eggs and young hatchlings, have also been on the rise. But traffic has long been one of the biggest threats for the state's turtles.
The Minnesota Zoo and the state Department of Transportation have counted about 700 turtles a year killed by cars at just 30 sites each of the past three years.
"Across the state, we're looking at thousands, if not tens of thousands of dead turtles every year on roads," Markle said. "A lot of those are nesting females moving around in June."
No population studies have been done, but state and federal wildlife officials say it's clear that too many adults are dying before they can breed replacements.
Two of the state's nine native species — wood turtles and Blanding's turtles — have recently been listed by the Minnesota Department of Natural Resources (DNR) as threatened and are on their way to becoming endangered, or even wiped out of the state.
While not endangered, other turtle species face threats as well. State lawmakers are considering banning commercial harvests of snapping and painted turtles as those populations also fall.
The problem is it takes the animals more than a decade to reach maturity and reproduce. Young turtles are extremely vulnerable and the vast majority die off in their first year. As they grow, they become harder to catch but can still be swallowed whole, and their shells can still be cracked by natural predators. But, eventually, a few survive to reach their teen years with such hard shells that almost nothing in nature can harm them.
"They evolved to be indestructible at that point," Markle said.
Populations depend on those teenage turtles surviving for the next 40 to 50 years, laying hundreds of eggs each spring just to produce a handful that will live long enough to replace them and lay eggs for the next generation.
Traffic is so damaging because it kills mature egg-laying females in their prime, often well before they've produced the next generation of survivors.
"So when you're losing dozens of turtles at a single site, it doesn't take long to diminish those populations," Markle said.
When turtles are lost, the ecosystem suffers.
They clean up wetlands, acting almost like janitors, eating dead and dying plants and fish before they rot and spread disease, Markle said.
The Minnesota Zoo and the Department of Transportation started projects in 2018 and 2019 to install fences to cut road mortalities, funded with money raised from the state's Environmental and Natural Resources Trust Fund. At the same time, the zoo has been raising about 20 turtles a year and releasing them when they're old enough to have a high chance a survival. The zoo is also working the DNR to use GPS monitors to track the turtles and protect their nests from raccoons and skunks.
The fences alone, which turn the turtles around or direct them to culverts that safely run under the road, won't be enough to save them, Markle said.
But it's a promising start.
"What we need now is to do more research and figure out where the problems are and what we can do," she said.
Greg Stanley • 612-673-4882