As species recover, some threaten others in more dire shape

Concealed behind trees near Lake Michigan, two scientists remotely manipulated a robotic owl on the forest floor. As the intruder flapped its wings and hooted, a merlin guarding its nest in a nearby pine sounded distress calls.

The small falcon dove toward the enemy — and into a net that Smithsonian interns Tim Baerwald and Zachary Bordner had stretched between steel poles. They disentangled the merlin, then attached a leg band and transmission unit to trace its movements.

The mission will enhance knowledge of a species still recovering from a significant drop-off caused by pesticides including DDT, banned in 1972 after harming many birds of prey. It's also helping Sleeping Bear Dunes National Lakeshore managers protect the piping plover, an endangered shorebird that merlins eat.

“Merlins are a big threat to their recovery,” said Nathan Cooper, research ecologist with Smithsonian’s National Zoo and Conservation Biology Institute.

The situation is ironic: A troubled species rebounds thanks to restoration efforts, only to make things worse for others in peril by preying on them or outcompeting them for food and living space. Similar circumstances have turned up elsewhere, challenging wildlife experts who want them all to thrive in balanced, healthy environments.

The iconic bald eagle's comeback has pressured rare water birds. Resurgent peregrine falcons menace endangered California least terns and Western snowy plovers near San Diego. Off the California coast, attacks from protected white sharks hinder recovery of threatened sea otters.

Gray seals previously on the brink of extirpation in New England waters now occupy some Massachusetts beaches by the hundreds. The 800-pound mammal's return has raised worries about vulnerable fish stocks.

Such unintended consequences don't necessarily reveal flaws in the U.S. Endangered Species Act or conservation programs, experts say. Rather, they illustrate nature’s complexity and the importance of protecting biological communities, not just individual species.

“Clearly there are occasions when we get these conflicts between species that we're trying to protect,” said Stuart Pimm, a Duke University extinction specialist. “But is it a major worry in conservation? No.”

Species recoveries can produce tradeoffs, as some animals are more adaptable than others to changes in the climate or landscape, said Bruce Stein, chief scientist with the National Wildlife Federation.

“A lot of ecosystems where these things are occurring are a little out of whack to begin with because we've altered them in some way,” Stein said.

The Great Lakes region has an estimated 65 to 70 pairs of piping plovers. They're among three remaining North American populations, their decline caused primarily by habitat loss and predation.

Meanwhile, merlin numbers in the region have jumped. They're suspected of killing at least 57 adult plovers, Cooper said.

Data from the transmitters might help determine whether relocating them is worth trying, said Vince Cavalieri, a biologist with the national lakeshore.

Recovery of America's national bird, the bald eagle, is a triumph. But in one area of coastal Maine, it poses a problem for the only U.S. breeding population of great cormorants.

“When they’re disturbed by eagles, the adult cormorants will flush and leave their nests,” said Don Lyons, a conservation scientist at the National Audubon Society’s Seabird Institute.

Gulls, ravens and crows gobble abandoned cormorant eggs and chicks. “If this happens repeatedly, an entire colony can fail,” said Lyons, whose team organizes volunteers to scare away eagles.

In Southern California, least terns and snowy plovers are no match for attacking peregrine falcons, which like eagles bounced back after the DDT ban. Such pesticides cause large birds to produce eggs with thin shells, which females crush when trying to incubate them.

The San Diego Zoo and Wildlife Alliance tries to protect the endangered birds by hiring a falconer to capture problem peregrines for release elsewhere, said Nacho Vilchis, a conservation ecologist.

Hunting and bounties devastated New England's gray seals. Legally protected since 1972, the population has rebounded to tens of thousands.

Fishing groups contend the seals could threaten cod stocks that regulators are struggling to rebuild from overfishing.

“Gray seals are certainly this case where recovery has both been cause for celebration and cause for concern," said Kristina Cammen, a University of Maine marine mammal scientist who says they're less of a hazard to fish populations than humans are.

Some reviving species may be more a nuisance to people than a threat to other wildlife.

Fish farmers in the South and anglers in the Great Lakes region and Pacific Northwest complain about the double-crested cormorant, a diving bird that gorges on catfish, perch, salmon and other prized species.

Cormorants have done so well since the DDT ban that agencies have tried limiting them in some locations with egg oiling, nest destruction and even shooting.

Wild turkeys, spread across North America before European settlement, had dwindled to tens of thousands by the 1930s, disappearing from many states. Now they're hunted in 49 states and so common in New England they cause traffic tie-ups.

Some hunters say hungry turkeys outcompete ruffed grouse, which are decreasing in parts of their range, including the Upper Midwest. Scientists blame habitat loss and climate change.

The National Wild Turkey Federation is helping move turkeys from states with plenty — such as North Carolina, Maine and West Virginia — to Texas and others needing more, said Mark Hatfield, national director of conservation services.

Conflicts between recovering species and ones still in trouble could reflect a return to how things were before humans got in the way, scientists say.

“When a population gets back to where it's having the same interactions with other organisms as before it went down, that's nature at work,” said John Fitzpatrick, emeritus director of Cornell University's Laboratory of Ornithology.

The bald eagle is "challenging our preconceived notions about what’s normal” for prey such as great cormorants in New England and common murres on the West Coast, which might have been less abundant before eagles declined, said Lyons of the Audubon Society.

The eagle's recovery “complicates the conservation of certain other species,” Lyons said. ”But their recovery is such a wonderful outcome ... that’s a welcome complication."

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Larson reported from Washington, D.C., and Whittle reported from Portland, Maine.

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On Twitter follow Flesher: @JohnFlesher; Larson @LarsonChristina and Whittle: @pxwhittle