Animal Crossing

A cougar prowls the brush-covered hills surrounding the bustling city of Los Angeles, California. The big cat’s ears twitch as it spots its favorite prey, a mule deer, walking along a nearby ridge. But there’s a deadly obstacle between the cougar and its target: a highway filled with speeding cars. Should the cougar risk its life and brave the crossing?

This dilemma is a common one for animals that live in sprawling urban areas like Los Angeles. Busy roads often cut through natural habitats, separating wildlife from food, mates, and territory. This is known as habitat fragmentation. “Being hemmed in by freeways makes it difficult for animals to maintain a sustainable population,” explains Miguel Ordeñana. He’s a wildlife biologist at the Natural History Museum of Los Angeles and a board member of the National Wildlife Federation, the nation’s largest conservation group.

A cougar hunts near Los Angeles, California. It prowls the brush-covered hills around the busy city. The big cat’s ears twitch as it spots its favorite prey. A mule deer is walking along a nearby ridge. But a deadly obstacle lies between the cougar and its target. It’s a highway filled with speeding cars. Should the cougar risk its life and cross the road?

This is a common problem for animals in large urban areas like Los Angeles. Busy roads often cut through natural habitats. The roads separate wildlife from food, mates, and territory. This is known as habitat fragmentation. “Being hemmed in by freeways makes it difficult for animals to maintain a sustainable population,” explains Miguel Ordeñana. He’s a wildlife biologist at the Natural History Museum of Los Angeles and a board member of the National Wildlife Federation. That’s the nation’s largest conservation group.

When desperation forces an animal to cross a highway, the results are often tragic. An estimated 1 million to 2 million collisions occur between vehicles and wildlife each year in the United States. And it’s not just animals that are affected. These accidents result in more than 26,000 injuries to people and 200 human deaths annually.

To make highways safer for people and wildlife, cities and towns are building wildlife crossings to help animals navigate around busy roads. Last April, the National Wildlife Federation began construction on the Wallis Annenberg Wildlife Crossing in Southern California. The overpass will span one of the state’s busiest roads: Highway 101. The new route will provide cougars (also known as mountain lions), coyotes, deer, foxes, and many other species safe passage across the 10-lane freeway. Once completed in 2025, the crossing will be the largest in the world.

When a desperate animal must cross a highway, tragedy often results. Around 1 million to 2 million strikes occur between vehicles and wildlife each year in the United States. And not just animals are affected. These accidents cause more than 26,000 human injuries and 200 human deaths each year.

Cities and towns are trying to make highways safer for people and wildlife. So they’re building wildlife crossings. These structures help animals travel around busy roads. Last April, the National Wildlife Federation began building the Wallis Annenberg Wildlife Crossing in Southern California. The overpass will span Highway 101. That’s one of the state’s busiest roads. The new route will allow cougars (also known as mountain lions) to safely cross the 10-lane freeway. Coyotes, deer, foxes, and many other species will also use it. This will be the world’s largest crossing when it’s finished in 2025.

Highway 101 runs along the border of the Santa Monica mountains (see Roadblock, above). North of the freeway lie thousands of square miles of wilderness, including Los Padres National Forest. But with more than 300,000 cars traveling along Highway 101 each day, “the freeway acts like an invisible fence,” says Ordeñana. The road separates the mountains from the wild lands to the north. Animals south of the road, including a population of about a dozen cougars, are limited to an area that’s roughly 520 square kilometers (200 square miles).

Highway 101 runs along the border of the Santa Monica mountains (see Roadblock). Thousands of square miles of wilderness lie north of the freeway. That includes Los Padres National Forest. But more than 300,000 cars travel along Highway 101 each day. As a result, “the freeway acts like an invisible fence,” says Ordeñana. The road separates the mountains from the wild lands to the north. Animals south of the road, including about a dozen cougars, are cut off. They’re limited to an area of about 520 square kilometers (200 square miles).

Urban development and habitat fragmentation have been particularly hard on the region’s cougars. They are naturally solitary animals that require space to roam. Without human-built structures limiting a wild cougar’s range, a single male might patrol a territory the same size as the one shared by the Santa Monica population. Male cougars can be very territorial and may kill each other if forced to share too small an area, says Ordeñana. Restricted territory has also led to inbreeding within the local cougar population. Mating between closely related individuals results in some kittens being born with health problems.

Los Angeles’s cougars need the chance to expand their range and access the wilderness north of the freeway. Otherwise, scientists say, the local cougar population could completely disappear within 50 years. “It’s a race against time,” says Ordeñana.

Urban development and habitat fragmentation have hit the area’s cougars hard. They are natural loners that need space to roam. Because human-built structures limit their range, the entire Santa Monica population shares the area. But normally, one male might patrol a territory that size. Male cougars can be very territorial. They may kill each other if they share too small an area, says Ordeñana. And the limited territory has led to inbreeding within the local population. Cougars that are closely related are mating. As a result, some kittens are born with health problems.

Los Angeles’s cougars need to expand their range and reach the wilderness north of the freeway. If not, the local cougar population could completely disappear. Scientists say this could happen within 50 years. “It’s a race against time,” says Ordeñana.

Conservationists have already built more than 1,000 wildlife crossings throughout the U.S. and are planning many more. In some places, larger strips of natural habitat known as wildlife corridors are being created. These often provide access to seasonal migration routes, allowing animals to freely move between breeding and feeding grounds. To determine where corridors and crossings are most needed, scientists examine vehicle collision statistics as well as movement data from animals tagged with GPS locators.

Patty Garvey-Darda, a wildlife biologist with the U.S. Forest Service, oversaw an ambitious project in Washington State that included multiple highway crossings. Several years ago, workers began constructing routes to help animals bypass Interstate 90, a highway that cuts through Snoqualmie (snoh-KWAHL-mee) Pass. The road divides the habitats of hundreds of animal species, including black bears, elk, fish, mountain goats, salamanders, and toads. When it came to designing each crossing, “the first step was identifying what habitats we were trying to restore and what areas needed to be connected,” says Garvey-Darda. She worked with a team of biologists, engineers, and hydrologists—scientists who study Earth’s waters—to determine where the landscape was best suited for wildlife crossings.

Conservationists have already built more than 1,000 wildlife crossings across the U.S. They’re planning many more. In some places, they’re creating wildlife corridors. These are larger strips of natural habitat. The corridors often allow animals to use seasonal migration routes. The animals can move freely between breeding and feeding grounds. But where are corridors and crossings most needed? To find out, scientists look at records of vehicle strikes. They also tag animals with GPS locators and study their movements.

Patty Garvey-Darda is a wildlife biologist with the U.S. Forest Service. She ran a large project with multiple highway crossings in Washington State. Several years ago, workers began building routes to help animals get past Interstate 90. This highway cuts through Snoqualmie (snoh-KWAHL-mee) Pass. The road divides the habitats of hundreds of animal species. They include black bears, elk, fish, mountain goats, salamanders, and toads. To design each crossing, “the first step was identifying what habitats we were trying to restore and what areas needed to be connected,” says Garvey-Darda. She worked with a team of biologists, engineers, and hydrologists—scientists who study Earth’s waters. They found the best places in the landscape for wildlife crossings.

The project included both overpasses and underpasses for different species. Overpasses, which cross over roadways, were covered with local soils, fungi, and plants. Underpasses mimicked streams, with water, logs, and rocks. Scientists have found that animals are more likely to use pathways that blend into the surrounding ecosystem—the community of organisms interacting with their nonliving environment.

The new overpass in Southern California will follow a similar design. It will be planted with local trees and flowers to attract animals. It will also have sound-blocking walls to dampen roadway noise and block headlights.

The project included both overpasses and underpasses for different species. Overpasses cross over roadways. They were covered with local soils, fungi, and plants. The underpasses looked like streams. They had water, logs, and rocks. Scientists have found that animals are more likely to use pathways that blend into the local ecosystem. That’s the community of organisms along with their nonliving environment.

The new overpass in Southern California will be similar. It will include local trees and flowers to attract animals. It will also have sound-blocking walls. They’ll reduce roadway noise and block headlights.

Once a crossing is finished, scientists rely on GPS tracking data and motion-sensor cameras to study how animals use it. At Snoqualmie Pass, cameras have captured black bears, bobcats, otters, and other species safely making their way across the highway. “We saw our first cougar last year, which was very exciting,” says Garvey-Darda. In 2019, fencing was installed along the roadway to help funnel animals toward the crossings. Since then, not one large animal has been struck by a car.

Wildlife crossings around the world are seeing similar success. They have nearly eliminated elk collisions in Canada’s Banff National Park and allowed crabs in Australia to safely access the sea from their homes on land (see Over and Under, below).

Biologists hope that the Wallis Annenberg Wildlife Crossing will bring positive results for the threatened cougar population and other animals in the area. “This new corridor shows that the people of Los Angeles are not giving up on their urban wildlife,” says Ordeñana. But he also believes that the project should serve as an important lesson when it comes to future development. “If we had thought about wildlife before expanding the freeways, we wouldn’t be facing the kind of habitat fragmentation problems we have now,” he says.

When a crossing is finished, scientists study how animals use it. Scientists rely on GPS tracking data and motion-sensor cameras. At Snoqualmie Pass, cameras have captured many species safely crossing the highway. They include black bears, bobcats, and otters. “We saw our first cougar last year, which was very exciting,” says Garvey-Darda. In 2019, fencing was installed along the roadway. It helps funnel animals toward the crossings. Since then, no large animal has been struck by a car.

Wildlife crossings around the world are also having success. They have nearly ended elk strikes in Canada’s Banff National Park. And they’ve helped crabs in Australia. Now the crabs can safely reach the sea from their homes on land (see Over and Under).

Biologists hope that the Wallis Annenberg Wildlife Crossing will help the threatened cougar population and other local animals. “This new corridor shows that the people of Los Angeles are not giving up on their urban wildlife,” says Ordeñana. But he also believes that the project carries an important lesson for future development. “If we had thought about wildlife before expanding the freeways, we wouldn’t be facing the kind of habitat fragmentation problems we have now,” he says.