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In 2001, shortly after the September 11th attacks in the United States, something strange happened to a group of whales near eastern

Canada:. They relaxed. Specifically, researchers found lower levels of stress-related hormones in their poop, which is apparently how you measure these things. And this didn't seem to be a coincidence.

In fact, after more observations, there seemed to be an obvious cause:. Right after 9/11, the waters off the east coast of North America were quiet. There were fewer ships than normal, and the whales appeared to let out a collective sigh of relief.

At least, figuratively. This study was pretty famous when it finally came out in 2012, but it's not an isolated case. Across the world, scientists keep realizing that the noise we make is affecting even the farthest, most remote corners of the world, and is impacting everything from owls to dolphins to plants.

But thankfully, by learning what all this sound does, we can hopefully learn to keep it under control. Noise pollution is noise created by human activity. It's mostly associated with transportation, since vehicles can create a constant hum anywhere humans are doing their thing.

But technically, it can be any human-made noise that interferes with nature's normal activities, including outdoor concerts and the hum of air conditioners. Your A/C unit might not seem like a big deal to you, but in many areas, the level of noise we make is becoming excessive. And that comes with consequences.

For one, it's disrupting how animals use sound to communicate. Vocal communication, like chirps, grunts, and songs, is key between individuals within a species. And for many, it's the primary way to find a mate.

Like, I don't know if you've been outside in the spring, but it's kind of hard to not notice the eruption of flirty bird songs that hits once warm weather rolls around. Yes, we get it! You want to mate with somebody!

The problem is, if you want to use sound to communicate, you need to actually get the message. And for many animals, that's becoming harder. For instance, one 2016 study on birds played a recording of traffic sounds, along with the occasional lawn mower or airplane flyover noise.

Then, on top of that, they played a recording of a bird's alarm call. Normally, birds would chime in and respond to this. But in the study, the number of responses was 80 percent lower than expected.

And it wasn't just that the birds weren't responding:. They didn't adjust their foraging behavior in these areas, either. That's kind of significant, because if you can't hear a warning call or don't understand it, you might be the last to know about a predator, or an oncoming bus.

Meanwhile, another paper published that year looked at how noise pollution affects hunting. And it found equally concerning results. This team tracked the foraging activities of 78 short-eared and long-eared owls.

And they found that low levels of traffic noise impacted the birds' ability to detect prey more than 120 meters from roadways. At first, that might not seem so bad. Like, maybe owls just shouldn't look for dinner at the side of the road.

Except, it turns out, a lot of our land these days is “at the side of the road.” A 2003 study measured the proportion of land within reach of roadways, looking at more than 150 ecoregions and more than 2000 watersheds across the United States. It found that 20 percent of the total land area surveyed was within about 120 meters of a road. So, suddenly, a quiet hunting ground seems a lot harder to come by, especially if you figure the number of roads has only gone up since 2003.

And when you think of the delicate balance of predator-prey interactions, it's not like this sound just impacts hungry owls. It could impact their entire ecosystems. Now, you might think that at least some areas are immune to noise pollution.

Like, it couldn't be a problem, say, way out in the open ocean, right? Well, I've got some bad news for you. Even though there isn't as much traffic in the open seas, there's still a whole lot of noise.

The movement of ships creates enormous amounts of sound at low frequencies; frequencies that travel efficiently through water and cover great distances. And on top of that constant transportation, there are more specific sources of noise pollution, too, like resource extraction. Extracting oil and gas can actually be some seven orders of magnitude louder than the loudest ship noises.

They're so loud, in fact, that one researcher reported they can hear extraction happening near Brazil off the coast of Virginia. So even in areas that might seem peaceful, there's more than enough sound to be disruptive. And it is!

In 2016, researchers from the University of Maryland recorded a group of dolphins near coastal shipping lanes. And they discovered that, when ship activity elevated the ambient noise in the waters, the dolphins used shorter calls. Probably so they could actually get their message across.

One marine biologist on the project, Dr. Helen Bailey, compared the dolphins' situation to someone trying to order in a noisy bar. “I'll have fries with my burger, please.” “I said I'll have mine with fries, thanks.” “WITH. FRIES.” Although more research needs to be done to understand the impact of all this, it's possible that these simplified communications could impact social bonding.

Kind of like a noisy bar might. And since dolphins are such a social species, that could be significant. So, from land to sea, noise is changing the way animals behave.

But sometimes, animals just don't want to deal with all this. Sometimes, instead of adapting to shorter calls or different hunting spots, they decide to avoid noisy areas entirely. One study that got really into the weeds about this was conducted by researchers at Boise State University in 2012.

They knew that, when studying noise pollution, it can be really difficult to separate all the variables at play. Like, think about traffic. Unless you're careful about your study design, it can be hard to figure out if it's just vehicle noise that's disrupting animal behavior.

It could be a number of other things, too, like street lamps, vehicle collisions, or air quality. So, to try to separate things out and really figure out how noise disturbs species, these researchers set up a “Phantom Road.” They picked a stretch of wilderness far from actual roads, and set up 15 pairs of speakers along a half-kilometer path. Then, they played traffic noises.

But not like, sounds from a five-lane freeway. They played traffic sounds recorded in Montana's Glacier National Park, where only 12 cars go by every minute. And!

What do you know? This audio-only experiment still resulted in more than a 25 percent decline in migratory bird abundance in the area. A couple of species even avoided the Phantom Road entirely.

That's not an easy situation for migratory birds to be in, considering that it's already an incredibly dangerous time of year for them. Adequate migration stopovers are already disappearing for other reasons, too, and now, we're starting to understand how sound is further limiting the areas they can rest and feed. While I've been talking about these studies, you might have noticed that the amount of noise pollution has really varied from paper to paper.

Like, there's a big difference between the sound of 12 cars a minute and a roaring lawn mower. That's because the amount of noise that affects wildlife tends to vary from species to species. The amount of sound that bothers a migratory bird might not affect a dolphin as much.

And that's what makes it hard to draw a line in the sand and say, “Well, if we can only reduce noise pollution by this amount, animals will be okay.” Instead, scientists have to roll up their sleeves and look at things on a case-by-case basis. Because the effects of noise pollution can really vary. Like, in one study, scientists collected wood frogs from ponds around one to five kilometers from the nearest major road; relatively quiet areas that got only moderate amounts of traffic noise.

These frogs were then exposed to traffic noises in the lab and showed increased levels of stress hormones. Shifting from a quiet pond to noise pollution exposure affected their immune systems and impacted their ability to make antimicrobial compounds for their skin; compounds that are an important defense against pathogens. On the other hand, wood frogs raised in noisy ponds, those that were less than a kilometer from the nearest road, had a suppressed stress response to traffic noise exposure in the lab.

And in some ways, that's great! It means they can tolerate the noise pollution better. But it might also mean these frogs have a generally suppressed stress response, which could make them less wary about things like predators.

There are numerous other examples like this, for both animals on land and in the ocean. And it makes research on individual species really important. But even while scientists are honing in on those individual effects, they're also realizing there's a much bigger picture here: that this type of pollution might be changing how ecosystems function as a whole, too.

It might even affect plants. Take the results of a 2012 study, for example. It took place in an area that's noisy even by human standards.

It had a high density of noisy natural gas wells and compressors creating sounds over 95 decibels; enough to cause hearing damage in humans over long periods of time. This noise had driven birds called western scrub jays from their habitat. And while that was good for some species, like hummingbirds, whose nestlings are preyed upon by the scrub jays, it was bad for the area as a whole.

And it was because the scrub jays were important seed dispersers. Specifically, they helped spread the seedlings of the piñon pine. So without them, fewer pines.

In fact, the change was so dramatic that these trees were four times more abundant in quiet areas. That meant more shade, more oxygen, and more habitats for all the species that called them home. All because of a little quiet.

So, from dolphins in the ocean to owls in the sky to entire forest ecosystems, there's a lot happening here. But the good news is, there's also plenty of room to end on a hopeful note. Because there are a bunch of steps we can take to cut down on the amount of sound we're putting into the world.

For example, in some protected areas, shuttle services have already been implemented to cut down on traffic. There's also some effort being made to align aircraft flight patterns with roads, which would limit noise to more specific corridors and cut down on how far sound spreads. Navies around the world have also done research on how to make ships quieter, and research vessels today are much quieter than earlier ships.

So now, new tech is being developed to make oil and gas operations quieter, too. There are even changes we can make in our daily lives to cut down on the racket we're creating. Our modes of transportation, the industries we support, how we work and play in natural spaces, all these things have an impact on the sound we put out every day.

Again, this won't solve everything, because some species are impacted by even tiny amounts of artificial noise. But as we better understand these problems and their effects, we'll at least be able to make some compromises to bring back a little peace and quiet. Thanks for watching this episode of SciShow!

If you want to learn more about noise pollution, you're in luck, because we've got a whole separate video about how it affects humans. Just in case you thought we were spared from the perils of noise. If you watch it, let us know what you think! [♪ OUTRO].