Orcas patrol a river while alligators bask on a beach. Wolves haunt a coastline, sea otters claim an estuary and mountain lions prowl a prairie. What's going on?

In all these examples, relatively big predators are thriving outside of their typical habitats. Lots of predators are prone to wander, but these aren't just outliers. Sightings of large predators in places where they "shouldn't be" have increased in recent years, a trend made possible by decades of hard-fought conservation.

As certain predators rebound, some researchers have suggested they're expanding their range, colonizing new areas as they search for food. The authors of a new study, however, offer a different theory: Predators are reclaiming ancestral habitats they hadn't occupied since long before scientists began to study them.

"We can no longer chock up a large alligator on a beach or coral reef as an aberrant sighting," says lead author Brian Silliman, a professor of marine conservation biology at Duke University, in a statement. "It's not an outlier or short-term blip. It's the old norm, the way it used to be before we pushed these species onto their last legs in hard-to-reach refuges. Now, they are returning."

orcas, aka killer whales, in a freshwater river Orcas, also known as killer whales, swim through British Columbia's Campbell River in July 2017. (Photo: Yutaka Seki/Flickr)

To be clear, this isn't happening everywhere. Large predators are still fading from many ecosystems around the world, often due to habitat loss and fragmentation, combined with the legacy of more direct persecution by humans.

But where conservation efforts have had the time and resources to work, many predators are bouncing back with surprising gusto. The idea of expansionist predators may seem scary, but these animals still have much more to fear from us than we do from them. Not only do they rarely pose a threat to people, but they actually benefit their surrounding ecosystems — humans included.

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Using data from recent scientific studies and government reports, Silliman and his colleagues found that large predators — including alligators, bald eagles, sea otters, river otters, gray whales, gray wolves and mountain lions — may now be as abundant or more abundant in "novel" habitats compared with traditional ones.

This challenges some widely held assumptions in large-animal ecology, Silliman says. After generations of people rarely saw alligators outside of swamps, or sea otters outside of saltwater kelp forests, it became conventional wisdom that these species live where they live because they're habitat specialists.

"But this is based on studies and observations made while these populations were in sharp decline," he says. "Now that they are rebounding, they're surprising us by demonstrating how adaptable and cosmopolitan they really are."

sleeping mountain lion, aka puma or cougar A wild mountain lion naps in a bathroom at Chatsworth Nature Preserve in California. (Photo: U.S. National Park Service/Flickr)

Alligators, for example, have made a "remarkable recovery" since the 1960s, the study's authors write, with more than 1 million now inhabiting Florida alone. Long typecast as swamp things, the resurgent reptiles have recently been flexing their flexibility — and not just with the occasional swim 20 miles out to sea. Marine animals like sting rays, sharks, shrimps, horseshoe crabs and manatees now make up 90 percent of alligators' diet when they're in seagrass or mangrove ecosystems, the researchers note, showing how smoothly they can adapt to a saltwater lifestyle.

Such flexibility isn't universal, and shouldn't overshadow the many endangered species whose fates really are linked to narrow ecological niches. But for certain predators, these findings offer hope in the face of rampant habitat loss. "It tells us these species can thrive in a much greater variety of habitats," Silliman says. "Sea otters, for instance, can adapt and thrive if we introduce them into estuaries that don't have kelp forests. So even if kelp forests disappear because of climate change, the otters won't. Maybe they can even live in rivers. We will find out soon enough."

Predator perks

Wolves chase an elk in Yellowstone National Park Wolves chase an elk in Yellowstone National Park. (Photo: Doug Smith/U.S. National Park Service)

The decline and return of a predator can illustrate the species' previously unappreciated value to its ecosystem. A famous example occurred in Yellowstone National Park, where gray wolves were wiped out in the mid-20th century, then reintroduced by scientists in the 1990s. The wolves' absence had boosted and emboldened deer and elk populations, which began overgrazing the park's woody plants. When wolves returned, however, so did the vegetation.

The presence of predators can also save human lives. Without mountain lions or wolves across much of the U.S., for example, deer have proliferated so much that vehicles strike them roughly 1.2 million times across the country per year. If mountain lions were allowed to reclaim their old stomping grounds in the Eastern U.S., a 2016 study estimated the cats would indirectly prevent 21,400 human injuries, 155 fatalities and $2.13 billion in costs within 30 years of establishment.

sea otter at Elkhorn Slough, California A sea otter eats a crab at Elkhorn Slough, a tidal slough and estuary on California's Monterey Bay. (Photo: Chase Dekker/Shutterstock)

Predators can save us money in other ways, too. Even predators as small as bats save U.S. corn farmers $1 billion a year, thanks to their appetite for corn earworms. And sea otters, thanks to their ability to thrive in estuarine seagrass beds, can even protect us from ourselves, Silliman says. They do this indirectly by eating Dungeness crabs, which would otherwise prey on too many algae-eating sea slugs. Those slugs help prevent the bed from being smothered by epiphytic algae, which feed on excess nutrients carried there by runoff from inland farms and cities.

"It would cost tens of millions of dollars to protect these beds by re-constructing upstream watersheds with proper nutrient buffers," Silliman says, "but sea otters are achieving a similar result on their own, at little or no cost to taxpayers."

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.