Within two decades, more than 3,000 wind turbines could be turning atop Pennsylvania's ridgetops.

And more than 60,000 Marcellus Shale natural-gas wells might be drilled.

This level of energy development could alter up to 40 percent of the state's most ecologically valuable large forest blocks, endangering habitats of species from songbirds to trout, according to a Nature Conservancy report released Monday.

"Pennsylvania is at the epicenter of a major worldwide transformation of how we get our energy," said Bill Kunze, executive director of the conservancy's Pennsylvania chapter.

"So far, the impacts of this wave of new energy development on our most important natural habitats have been relatively limited," he said. "But that wave is going to expand . . . The cumulative impacts could be enormous."

Kathryn Klaber, executive director of the Marcellus Shale Coalition, an industry group, termed the report timely and useful and said she looked forward to reading it more closely.

She added that some of the recommended mitigation measures are already being used by an industry that has made "major leaps" in lessening its impact.

For instance, drilling companies already put more wells on a single well pad, as the report recommends. And they are sharing access roads rather than building separate ones because it also makes economic sense.

The conservancy, a nonprofit conservation organization that works worldwide to protect habitats, has not taken a formal stand on either natural-gas extraction or wind power, said spokeswoman Misty Edgecomb.

"All forms of energy have downsides," the conservancy's Kunze said. What the report is trying to address "is the lack of this information being in the conversation."

Already, about 3,500 acres of forest have been cleared, and 8,500 acres of habitat have been degraded due to energy development, the conservancy says.

In researching the report, the conservancy's lead scientist for energy analysis, Nels Johnson, said aerial photos and GIS mapping were used to analyze current development. Then, researchers factored in existing projections for the growth of each industry, and finally used computer modeling to predict where the growth might occur.

Besides direct clearing of 90,000 acres of forest, Johnson said, new "edges" would be created in larger forest patches. These would allow in more light, noise, predators, and invasive plants and insects, challenging species that rely on deep forest interiors.

Development also is likely in some of the last remaining strongholds for the eastern brook trout, an iconic species for clean, cold water, he said.

Brook trout have been pushed into remote mountain watersheds, and "as much as 80 percent of those overlap with areas that are likely to see Marcellus Shale and, to some extent, wind development," he said.

Johnson said he hoped landowners, industry, and regulators would use the report to find ways to avoid impacts.

A subsequent report will look at potential effects of electric and gas transmission lines, plus energy produced from "wood biomass."

Contact staff writer Sandy Bauers at 215-854-5147 or sbauers@phillynews.com.