Frogs' mating call is music to Lake County biologists

  • Wood frogs appear to be making a comeback in Lake County.

    Wood frogs appear to be making a comeback in Lake County. Courtesy of Lincoln Park Zoo

Posted4/15/2014 5:30 AM

They are not quite ready to call it a complete success, but the mating calls of wood frogs in a Lake County Forest Preserve represents a tentative victory for scientists whose efforts can take years to assess.

The croaks were music to the ears of Gary Glowacki, a wildlife biologist with the Lake County Forest Preserve District, who heard them by chance last week during routine monitoring.


"Wood frogs only call for a short period of time," he said. "I got extremely lucky."

According to historical notes and records, wood frog calls were last heard in Lake County in the late 1980s. Since then, this is the first time their reproduction has been documented.

"That was huge for us," explained Allison Sacerdote, reintroduction biologist in the department of conservation at Lincoln Park Zoo. "That told us we had adults that survived to breeding age."

As part of her dissertation in 2004, Sacerdote studied the feasibility of reintroducing the wood frog at the site, as the forest preserve district was well into wetland habitat restoration there.

Once an area of shallow temporary pools, the landscape changed decades ago with the installation of drain tiles. Invasive buckthorn crowded out native vegetation and changed the soil chemistry, making it harder for the frogs to disperse.

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Busy roads also hampered migration, and over time the population of wood frogs became isolated and disappeared.

Even after the flood plain areas had been restored, a change in the forest canopy from oaks and hickory to maple trees posed a problem. Maple leaves break down quickly and use oxygen needed for frog eggs to hatch, Sacerdote said.

The maples were thinned to improve conditions. From 2008 to 2010, thousands of wood frog eggs were put in mesh laundry hampers and dropped in ponds at the site.

Then, everyone waited. It can take up to three years for males and females to reach reproductive maturity and the window of opportunity is narrow.

"They're what we call explosive breeders," Sacerdote said. The mating call can last a week, but usually only three or four days. "If you're not out there at the right time, they're easy to miss."

About 50 egg masses containing 300 to 1,000 individual eggs also were documented in two ponds at the site.

"The biggest thing is that our restoration (efforts) are paying off and that the reintroduction took," he said. "It's a relatively new science. There's not a manual on how to do this."


Sacerdote said the hope is a population of wood frogs will become established.

"They're clearly breeding at the site. Now we have to see if they hatch," she said. "This is really the first time we can say this seems to be working with some certainty."

Frogs, she added, play a major role in the ecosystem by moving nutrients and controlling aquatic insects, for example.

"You can consider them a sentinel for environmental quality," she said.

Call: Masses of eggs a good sign of restoration, too

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