The world's largest wildlife crossing

Summary

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The Documentary Podcast (BBC World Service)

Episode: The world's largest wildlife crossing
Date: August 12, 2026
Host: Molly Bentley

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Episode Overview

This episode explores the ambitious Wallace Annenberg Wildlife Crossing, currently under construction over Highway 101 on the outskirts of Los Angeles. The crossing aims to address the challenges of habitat fragmentation, primarily to save the iconic but endangered California mountain lion and other local species from extinction. Listeners are taken behind the scenes with the project's key figures—from landscape architects to wildlife biologists—who discuss the challenges, innovations, and broader impact of building what will become the world’s largest wildlife crossing.

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Key Discussion Points and Insights

1. The Need for Wildlife Crossings and Ecological Fragmentation

• Historical fragmentation: The Highway 101 has isolated the Santa Monica Mountains, trapping wildlife, especially mountain lions, and causing inbreeding and genetic decline.
  • "South of the 101, those cats were locked in. Cats from the north couldn't get in because they get hit. And these cats were inbreeding themselves out of existence."
    — Beth Pratt (03:25)
• Other species like western fence lizards and plants are also genetically isolated due to the freeway dividing their habitats.
  • (04:11) Beth Pratt notes the ecosystem-level effects.

2. The Global History and Variety of Wildlife Crossings

• Origin: First built in France in the 1950s, originally to aid hunters tracking game.
  • (04:34-05:02) Kylie Sones summarizes early origins and European leadership.
• Diverse designs: Crossings take many forms: overpasses, underpasses, rope ladders for arboreal animals, and small tunnels for amphibians.
  • "There's this enormous range of different designs that target pretty much every species you can ever have thought of. We are trying to help it get across roads."
    — Kylie Sones (05:34)

3. The Scale and Uniqueness of the Wallace Annenberg Crossing

• Size and setting: The crossing spans 10 lanes of busy highway in an urbanized, human-dominated landscape, challenging traditional conservation norms.
  • "This is a really exciting project for me because of its size... but also over an enormous road... in a landscape that most of us wouldn't picture when we picture a beautiful, enormous act for wildlife conservation."
    — Kylie Sones (06:35, 07:04)
• Moving conservation into cities is essential since that’s where people—and much wildlife—live.

4. Building the Impossible: Challenges and Creative Solutions

A. Project Skepticism and Visionaries

• Many doubted the project’s feasibility due to its scale and cost.
  • "I got told I was crazy 10 times a day back then... But yeah, we were told. I was told I was crazy. I'm still told I'm crazy."
    — Beth Pratt (08:50)
• The team consists of people keen on solving "impossible" puzzles.
  • "This project has always attracted people who are a little crazy, myself included... we are really building this impossible dream."
    — Beth Pratt (10:01)

B. Landscape and Soil Engineering (10:41–14:00)

• Soil recreation: The soil must mimic native habitats to entice animals and let native plants thrive, not just be “dirt on a bridge.”
  • "You want to restore the soil not only so that water will drain through it, but so their roots will grow through it, so that oxygen and carbon dioxide will exchange through it. So it literally gives the soil the opportunity to breathe, to drink, to basically be dynamic unto itself."
    — Ted Hartzig (10:53)
• Seismic/load engineering: Soil weight and structure had to be meticulously calculated—one chance to get it right.
  • "We only have one chance to do it right."
    — Ted Hartzig (13:05)
• Construction method: Soil was added in careful “lifts” to maintain natural structure and porosity.
  • (13:59) Ted Hartzig describes technique.

C. Sound and Light Mitigation (16:10–17:15)

• Acoustics: Sound-absorbing, scalloped concrete walls reduce noise from the highway.
  • "That variability of the texture gives us this slight amount of refraction to the sound every time a sound wave hits that surface..."
    — Robert Rock (16:10)
• Lighting: Using pigmented concrete for walls lowers headlight glare, creating a “darker, quieter” space.
  • (16:55) Robert Rock explains pigment use.

5. Will It Work? Science, Skepticism, and Hope

A. Early Signs of Success (19:14–19:54)

• Even before completion, butterflies, lizards, and bees are using the crossing.
  • "We just saw a monarch on the wildlife crossing. That's the first monarch we've seen up here. That is amazing."
    — Beth Pratt (19:19)

B. The Limits and Known Benefits of Crossings (20:03–22:04)

• Global data: 98% of wildlife crossings are used by their target species.
  • "We found that in 98% of cases where a wildlife crossing structure had been installed, it had been used by the animals that it was designed for."
    — Kylie Sones (20:03)
• Caveat: Few studies track genetic or long-term “population level” improvements—this remains a major research need.

C. Open Questions and Caution (22:04–23:27)

• Just because a structure is used doesn’t guarantee population recovery.
  • "We don't know if that's one animal who loves to cross the crossing structure a thousand times or a thousand different animals. And that makes a big difference."
    — Kylie Sones (22:04)

6. The Next Steps: Monitoring and Broader Impact

• Rigorous science: The National Park Service will study if genetic diversity improves and populations rebound once the crossing opens.
  • "For me, it's that first genetic test that comes back and shows how the diversity's gone up. And yes, that is very measurable."
    — Beth Pratt (23:37)
• Setting precedent: While not every crossing needs to be massive, big, bold projects are sometimes necessary for large-scale connectivity.
  • "We can't shy away from big."
    — Beth Pratt (24:21)

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Notable Quotes and Memorable Moments

• "Cats from the north couldn't get in because they get hit. And these cats were inbreeding themselves out of existence."
  — Beth Pratt (03:25)
• "We can't keep pretending that nature isn't all around us all the time anymore. It's making itself very loudly heard."
  — Kylie Sones (07:04)
• "I was told I was crazy. I'm still told I'm crazy. But we had some early people who were crazy with us."
  — Beth Pratt (08:50)
• "You only have one chance to do it right."
  — Ted Hartzig (13:05)
• "We just saw a monarch on the wildlife crossing. That's the first monarch we've seen up here. That is amazing."
  — Beth Pratt (19:19)
• "In 98% of cases where a wildlife crossing structure had been installed, it had been used by the animals that it was designed for."
  — Kylie Sones (20:03)
• "We can't shy away from these big ones... these are the ones that really are going to make such a heavy lift for preserving biodiversity."
  — Beth Pratt (24:21)

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Timestamps for Key Segments

| Time | Segment | |-----------|--------------------------------------------------------------------------------------------------------| | 01:19 | Introduction to the crossing and California native plants | | 03:11 | Beth Pratt discusses mountain lion isolation and genetic decline | | 04:34 | Kylie Sones on the origins and types of wildlife crossings | | 06:35 | The Wallace Annenberg project uniqueness and scale | | 08:43 | Skepticism faced by the project | | 10:41 | Soil engineering and ecological restoration challenges | | 13:20 | Seismic requirements and soil loading on bridge | | 16:10 | Acoustic and lighting solutions | | 19:14 | First wildlife sightings on the crossing | | 20:03 | Effectiveness of wildlife crossings worldwide | | 22:04 | The importance of measuring population-level genetic and demographic impacts | | 23:37 | Planned monitoring for genetic diversity | | 24:21 | The necessity of “big” conservation projects |

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Conclusion

This episode provides a hopeful and detailed view into the intersection of wildlife conservation, engineering, and urban planning. The Wallace Annenberg Wildlife Crossing stands as both an engineering feat and a symbol of what’s possible when visionaries refuse to accept “impossible” as an answer—showing that bold, science-backed solutions can make a tangible difference for biodiversity, even in the midst of bustling cities.

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Transcript

[Podcast Narrator]

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[Molly Bentley]

So we're walking through the over the mulch, around the plants, around the chaparral, there's some sage, two California coffee, California

[Robert Rock]

Poppy, Santa Barbara milk vetch.

[Molly Bentley]

I'm Molly Bentley and you're listening to the documentary from the BBC World Service.

[Robert Rock]

Well, I mean, I think what you see right now is that there's a handful of colors that pop up here on top of the structure right now. There's obvious ones like the California poppy. You've got that kind of beautiful orange yellow cupped flower and these delicate little stems, you know, a little kind of carrot esque foliage underneath.

[Molly Bentley]

Now it may sound like landscape architect Robert Rock is giving me a colorful tour of native California plants, but there's something special about the flowers we're seeing.

[Robert Rock]

I think the thing that I'm most surprised by right now is just how well the sage has taken to the site. It's got this kind of dusty green color and these delicate and obviously the smell of sage is something that I think everyone knows quite well.

[Molly Bentley]

These plants are growing over 10 lanes of freeway traffic on the Wallace Annenberg Wildlife Crossing on the outskirts of Los Angeles, California. When it officially opens in December, it will be the largest wildlife crossing in the. 3 to 400,000 cars travel this stretch of Highway 101 every day, and that has proven to be a problem for local wildlife, especially the California mountain lion.

[Beth Pratt]

So the 101 goes in decades ago, cuts off an entire mountain range from the rest of the world.

[Molly Bentley]

Beth Pratt is the California executive director of the National Wildlife Federation and head of the Wildlife Crossing Fund.

[Beth Pratt]

The time we put the road in, we didn't know what that did. But science is showing us it wasn't just the obvious that a mountain lion gets hit by a car. It's that south of the 101, those cats were locked in. Cats from the north couldn't get in because they get hit. And these cats were inbreeding themselves out of existence.

[Molly Bentley]

These charismatic cats are iconic in the state with a long evolutionary history.

[Beth Pratt]

Mountain lions are a holdover. They have an ancient lineage of a couple million years. But you know, mountain lions coexisted with saber toothed cats here. And when all the megafauna died out, you know, the big stuff, they survived.

[Molly Bentley]

But it wasn't just mountain lions suffering from genetic isolation.

[Beth Pratt]

The National Park Service also started studying and doing genetic samples on rin tits and western fence lizards. Same thing's happening. So we know that this fragmentation has vast effects on whole ecosystems. Plants as well start becoming genetically separated.

[Molly Bentley]

But there may be a solution.

[Kylie Sones]

The origin of the very first wildlife crossing structure is a little bit murky. The general consensus is that it was somewhere in France in the 50s and Europe has absolutely been sort of the place where this technology, this approach has emerged from.

[Molly Bentley]

Kylie Sones is an urban biologist and Rhodecologist at the University of Melbourne and who has studied the effectiveness of wildlife crossings around the world.

[Kylie Sones]

So they were built because people realized that as we fragment the landscape with roads, the animals that they were hunting couldn't move freely across the landscape. So if they wanted to go on a big game hunt, they were also being restricted by roads and couldn't follow animals through the landscape. So the first wildlife crossing structures were probably largely to facilitate hunting.

[Molly Bentley]

Can you give us an overview? An overview and an underview of the different kinds of wildlife crossings that have been designed?

[Kylie Sones]

It is so incredible, the amount of different designs and the amount of different animals that these structures have been targeted for, Right. So we have the overpasses or the underpasses that people might be quite familiar with, but then we also have some that are rope ladders that help monkeys and possums and all of these incredible tree dwelling creatures get from one side of the road to the other. Some of the tunnels are very small because they're specifically to attract salamanders and toads. So it's no thicker than your hand, this very dark, gloomy tunnel that I would never want to travel through. But it's perfect for helping these amphibians cross Roads. So there's this enormous range of different designs that target pretty much every species you can ever have thought of. We are trying to help it get a crossroads.

[Molly Bentley]

While the Wallace Annenberg Crossing is just one example of the many types of animal crossings in the world, it is also in a class of its own.

[Kylie Sones]

This is a really exciting project for me because of its size. For starters, it is a simply enormous structure, but also over an enormous road. There was a time in rhodicology in particular where we were there has to be a limit.

[Molly Bentley]

Can I ask what seemed to be the limiting factors with the Annenberg Wildlife crossing? Was it 10 lanes of highway? Is it that simple? Or is it 10 lanes of highway in Los Angeles?

[Kylie Sones]

I think it's a combination of the 10 lanes of highway and the landscape that it's in. That were those initial hesitancy points, the points that make people go, no, that's not, that's not going to be possible. Because we also have this very strong aversion to doing big conservation actions in a human dominated landscape. We have for a very long time put urban landscapes in the not important for conservation pile because fine, you'll build this wildlife crossing structure, but it will still be in the city and everything will be bad and nothing will ever use it. That's a really old way of thinking. We know that that's not true anymore. There are so many wonderful conservation solutions that we can apply and urban landscapes are really important places to apply them because that's where people live and can enjoy the nature and that's where nature already lives. We can't keep pretending that nature isn't all around us all the time anymore. It's making itself very loudly heard. So I think this is positive story for me on both of those fronts because, you know, not only is it going to be the biggest in the world, but it's being done in a landscape that most of us wouldn't picture. When we picture a beautiful, enormous act for wildlife conservation. I secretly find that very exciting.

[Molly Bentley]

Beth Pratt says that the scale of the project meant it was met with skepticism from potential backers.

[Beth Pratt]

I got told I was crazy 10 times a day back then. I had to go to donors, I had to go to elected officials and I had to say, hey, we got to connect this landscape or else the mountain lions will die out. I don't know how much going to cost. I, you know, I don't know what it's going to look like, but could you help? So I think there really was a pushback a on just the bigness of this two for some wildlife doesn't belong here. And we got pushed back on that. But yeah, we were told. I was told I was crazy. I'm still told I'm crazy. But we had some early people who were crazy with us. And my heart always, I have a special place in my heart for them because they made it happen.

[Molly Bentley]

One person Beth didn't have to convince, though, was Robert Rock, the lead architect on the project.

[Robert Rock]

I like to think that's why this is a project that I really gravitated towards is because of the challenge, because of the scale and the number of questions that go along with it, because I appreciate that degree of complexity.

[Molly Bentley]

And Beth, is it true that he took it in stride, the way Robert's telling the story, as though this is a big challenge. He felt that he could do it. He wasn't perturbed. Is that how it went down?

[Beth Pratt]

I think that this project has always attracted people who are a little crazy, myself included. I mean, I have to say, we have our late nights where, oh my God, how are we going to do this? But what is amazing about this team is here we are really building this impossible dream, visionary project. And you'd think we'd be tearing our hair out every day, but we, you know, Robert has taken it in stride. I mean, for him, it is a puzzle to be solved. And he does that so well now. Yeah, there might be a few late nights where I'm not sure. He's so low key about it.

[Molly Bentley]

That's understandable. The challenges of building a structure like this are extensive.

[Ted Hartzig]

My name is Ted Hartzig. I'm a senior soil scientist with an engineering firm, Olson Inc. Ted and two

[Molly Bentley]

other scientists worked on the soil for this project. A mycologist and a soil biologist.

[Ted Hartzig]

I remember one detractor said, oh, you just throw dirt on the bridge. No, you don't. You want to restore the soil not only so that water will drain through it, but so their roots will grow through it, so that oxygen and carbon dioxide will exchange through it. So it literally gives the soil the opportunity to breathe, to drink, to. To basically be dynamic unto itself.

[Molly Bentley]

Without the right soil chemistry, water won't flow through it correctly. Native plants may not take root, and the crossing won't match the adjacent habitat. That means animals may be wary of using the structure. So healthy soil is important. Creating it is a meticulous endeavor.

[Ted Hartzig]

You just don't go build a landscape without understanding the surrounding environment. So I will do a soil assessment. I will collect samples. What is the chemistry of that soil? Like, what is the physics of that soil? Like, what is the biology of that soil? Like, so if we look at a successful ecosystem environment, what type of vegetation is there? If we take samples of the soil that we can have analyzed, evaluated for the microbial content, then we could learn what is the balance, what are the types of microbes that are in that soil. Oftentimes it's very similar between areas or regions. Other times they're very specific ones and it's really hard to reproduce those. Most of the soil on the crossing deck itself, we had to go look at sources all over the whole northern LA region. What type of soils were being harvested from varying areas that we could get that would mimic the types of soils that are there in the Santa Monica and the Simi Mountains. So we have this conglomeration of soils and this is what we wanted that actually represent that natural environment. That's easier said to find than done. It took us quite a while to make sure that we had the right soils in place and what we needed. And I really appreciate one of the leaders at the Caltrans engineering department who told his contractors, you guys better get this right, because we only have one chance to do it right.

[Molly Bentley]

Boy, no, no pressure there. When someone says you only have one

[Ted Hartzig]

chance, we only have one chance. And so we, we wanted to make sure we were right.

[Molly Bentley]

But there's another consideration when adding a heavy load to a suspended structure in Los Angeles.

[Ted Hartzig]

We have a pretty highly seismic area here, and so you don't want to overload the bridge. And this is where the engineers and we all talk. What can the load bearing capacity of this bridge be? And so then I went back to and this is how I determined what is the bulk density of the soil we could use? How deep can it be? What type of materials can we put into it? How do we calculate almost to a small fraction what will be the weight of soil we have on this crossing?

[Molly Bentley]

Another challenge, how to lay heavy soil on a bridge over a busy highway? The answer carefully.

[Ted Hartzig]

So as we place the soil, we have to do it in lifts, and we have to do it carefully so that we keep that porosity or that soil structure consistent and pretty much alive. So we'll do the first lift, we'll make sure it's even, we'll scarify it to make sure it has a rough surface. We'll do the second lift, we'll do the same thing.

[Kylie Sones]

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[Molly Bentley]

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[Beth Pratt]

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[Podcast Narrator]

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[Molly Bentley]

I'm Molly Bentley and you're listening to the documentary from the BBC World Service. The Wallace Annenberg Wildlife Crossing is a tricky project. It needs to hold a lot of soil and it needs to be quiet. 3 to 400,000 cars a day make a lot of noise. Acoustical engineers were tasked with the challenge of designing sound dampening walls on either side of the crossing.

[Robert Rock]

The surface texture has a varied character that's been done with an internal form within the concrete pour, kind of giving it a little bit of an undulating texture left to right across the face of the surface and kind of scalloped from the bottom to the top. That variability of the texture gives us this slight amount of refraction to the sound every time a sound wave hits that surface and breaks things up a little bit more. And I mean, I won't get to the level of geeky detail we did with our acoustical engineer about talking about phononic crystals and how that's like layered into technology like that as well. But we did have those discussions. It got really nerdy.

[Molly Bentley]

The glare of hundreds of thousands of headlights required adjusting the color of the walls too.

[Robert Rock]

What we were attempting to do with this is instead of using a conventional concrete color that would be really reflective, we integrated a pigment into the concrete to change the color of it to reduce the amount of surface reflectivity of this concrete. So we create a darker, quieter space.

[Molly Bentley]

The end result, though, is remarkable. Relatively quiet. The highway below virtually disappears, and with the rugged terrain and an abundance of shrubs and flowering plants, the bridge really feels like a part of the California Habitat. For these reasons, says Kiley, the crossing could set a critical precedent.

[Kylie Sones]

A question we have thought about in road ecology for a very long time is, is there a road that is too big to fix? And people, people don't want to try something if they're being told that it's probably never going to work. And so I think for a really long time we have assumed that there was an upper limit to how big a crossing structure could be. And I think that's been to our detriment. I think we were just a little bit. A little bit more hesitant than we should have been because. Because there are enormous roads. And this project shows that with the combination of science and engineering and community and government, you really can think of a solution to this problem rather than let it lie in the too hard basket for all eternity.

[Molly Bentley]

The Wallace Annenberg Wildlife Crossing has shown that we can build ambitious structures that had long been considered impossible. But now the big questions are, will it work? Will animals use it? Will it repair the fragmented habitat and help save the California mountain lion from extinction? I visited the structure in June, six months before its official opening. At that point, the platform over the highway was almost complete. But the slopes connecting it to the land on either side where were still under construction. Yet animals that could fly or climb had already discovered it. I saw lizards, birds, bees and even.

[Beth Pratt]

Oh, oh, is that a monarch again?

[Molly Bentley]

Beth Pratt.

[Beth Pratt]

We just saw a monarch on the wildlife crossing. That's the first monarch we've seen up here. That is amazing. As we said, we have been designing this crossing from everything, the mountain lions to monarch butterflies. This is a very important overwintering area for monarch butterflies. Again, the 101 is a wall that they cannot navigate across. And we just saw the first monarch up here. What a moment. That is really amazing.

[Molly Bentley]

I hope you have some emotional support when the mountain.

[Beth Pratt]

Yes, I'm gonna need it because that

[Molly Bentley]

was a lot of enthusiasm for the butterfly.

[Beth Pratt]

For a butterfly, yeah, for that beautiful butterfly.

[Molly Bentley]

But for animals larger than butterflies, how much do we know about whether wildlife crossings work? Here's Kylie Sones again.

[Kylie Sones]

We are very lucky that we have an extraordinary amount of studies from around the world on wildlife crossing structures and whether or not animals use them. That's the very first question we have to ask themselves, is we've built this remarkable thing. Is it doing what we thought? Do animals see it the way that we hoped? And the answer is yes. So we have conducted a really big review across the world of all these studies. And we found that in 98% of cases where a wildlife crossing structure had been installed, it had been used by the animals that it was designed for. And so that's really excellent first pass

[Molly Bentley]

at the knowledge, that's good news. But it comes with a caveat.

[Kylie Sones]

The next thing that we have to understand is what does that actually mean for the ability of those wildlife populations to persist in that landscape? And is the fact that they can get from A to B now, is that enough to conserve them in the long term? Are they breeding and intermixing again? Is it improving the genetic health of that population? Is their population size growing now because more of them can cross the road and breed and meet all of those needs that they have? And we don't know that in most cases. So there is hardly any studies across the world that look at what we call the population level impacts, where we've counted the number of animals and how long they live and how genetically diverse they are. Those studies are really, really rare. And every time they come up, I think I can count the number of studies that have done that on one hand. And that is something we need to get much better at, because that's the crux of the problem, right? That's what we want to know is are we actually saving nature when we put these wildlife crossing structures in?

[Molly Bentley]

But if we know that the animals are using these crossings, as you said, how could the populations still be in decline?

[Kylie Sones]

I mean, there's a few like one of them is the amount of movement might not be enough. So if we know we need 20 different mating occurrences from one side of the road to the other, but the wildlife crossing tractor is only allowing 5, that could still decline. If we need 50% of a reduction in roadkill, but the wildlife crossing structure is only providing a 20% reduction in roadkill, that balance is still out, right? We're still going to see that population decline. But sometimes in most cases, we know how many crossing events they are. We don't know if that's one animal who loves to cross the crossing structure a thousand times or a thousand different animals. And that makes a big difference. Because if it's just one lunatic animal, if it's just one very excited animal going backwards and forwards and backwards and forwards and backwards and forwards, you can see how that's not necessarily a good thing for the entire population. But if you have a large portion of that local population that is using the crossing structure, that's a much better indicator that you've restored the ecological health. For want of a better word, this is going back to how it should be where all the animals as part of that population can move freely instead of just a couple of animals in that population. Get very excited about a crossing structure.

[Molly Bentley]

After the crossing opens, the National Park Service will be conducting studies to see whether it is helping mitigate the effects of habitat fragmentation at the population level.

[Beth Pratt]

We're focusing a lot on that camera moment of the first mountain lion crossing. And there's no doubt I am going to be just so emotional over that. Again, you saw how emotional I was over a butterfly. But for me, it's that first genetic test that comes back and shows how the diversity's gone up. And yes, that is very measurable. The Park Service takes blood tests in all the animals they capture. They measure that specifically. And I'm even visualizing the graph they use right now when that graph, you know, diversity section goes up. That's really good. Going to be the moment for me where we're like, we did it. It worked.

[Molly Bentley]

For now, though, Beth hopes the crossing sets an example of what is possible for visionary conservation projects.

[Beth Pratt]

Crossings come in all shapes and sizes. There's one up in Washington state called the squirrel, the Nutter Highway. It's literally this little. This little almost like rope bridge for them. So, yes, not everything needs to be a Wallace Annenberg wildlife crossing. Some of these are just really simple solutions. But having said that, we can't shy away from these big ones. I mean, we. In some respects, it's these bigger crossings that are solving such monumental blockages to connectivity that we have to get better about accepting, you know, they. They are hard to do. They're big. They don't do cost money. But we have to be okay with that because these, these are the ones that really are going to make such a heavy lift for preserving biodiversity for the wild world. We can't shy away from big.

[Molly Bentley]

Sam.