New York City is very old, about 400 years old. Over those 400 years, the animals and plants that live in New York City have had to adapt to things like fast food trash, pollution from cars and factories, and being separated from other natural areas.
There is a secret spot in a big park in Manhattan. It is a small, damp hill with some young maple, oak, and cherry trees giving it shade. A busy road borders it on one side, and there is a school right across the street.
Even though it is surrounded by the city, there are animals living on this little hill. They roam the meadows with moss and wildflowers, and rest among the fallen logs and leaves. These animals that hardly anyone knows about are some of the most hidden residents of New York City.
This hill is home to a rare killer. It is an animal that most people in New York City have never seen.
These animals will eat anything they can fit in their mouths. They will even eat each other.
Ellen Pehek is a retired scientist. She used to work for the city’s parks department. She says these hunters have strong jaws for crushing their prey. They have thick legs and muscular bodies, almost like little dragons.
Pehek says if these animals were 6 feet long, people would run away from them.
But the hunters she is talking about are just small salamanders. These northern dusky salamanders are so little they can fit in your hand.
Most people, even park workers, do not know about them. But Pehek thinks they have lived on this hill for almost 100 years.
As New York City grew bigger, the little salamanders kept living on this small hill.
The hill is much smaller than the huge skyscrapers nearby.
Erik Baard is with an environmental group called HarborLAB. They give free kayak tours in NYC.
Baard says the hill is only about 75 yards across. And it is maybe 40 yards high.
New York is known for its city birds and rats. Like the famous Pizza Rat video of a rat carrying a pizza slice.
But there are other animals in NYC that most people don’t know about. Animals that lived there before the city was built.
Coyotes sometimes visit Central Park. Beautiful orchid flowers also grow in the park.
Comb jellyfish swim in the East River under the Manhattan Bridge.
And every spring, horseshoe crabs come up onto beaches in Brooklyn and Queens to mate and lay eggs.
Some animals in New York City still live like normal. But it is hard for them to stay the same. Over 400 years, about one million buildings went up in the city.
The city is very busy and crowded. New York’s wildlife is changing quickly.
Some animal species have already changed a lot. Their bodies and genes are now different and unique.
An island of salamanders
In 1945, Carl Gans was walking in Manhattan. He was walking down a hill when he found a group of northern dusky salamanders. This type of salamander needs very clean water, shade, and to be left alone by humans. But there they were, living in a park in a big city. And they were doing well.
Soon, Gans published a scientific paper about his discovery. He wrote the exact location where he found the salamanders, including the street and details about their habitat. He said they should be protected. But nothing was done. After a while, no one saw the salamanders anymore. Most scientists thought they had disappeared from the area forever.
Then in 2005, Pehek wondered if maybe the salamanders were still there, even after over 50 years of more development. She thought it was unlikely, but she took some people and went to look. They searched the area Gans had described.
The salamanders like to live in a special type of small wetland. This wetland is on a gentle slope. Groundwater flows over the surface, keeping it moist.
The Manhattan hill had a little stream with rocks around it. The ground was covered with a thin layer of fallen leaves and mud.
Pehek says, “It was so delicate. If you stepped on it, you would slide down the slope.”
She doesn’t remember finding the first salamander that day. There were so many. “It was exciting. We kept finding more and more,” she says. They even found a mother with her newly hatched babies. The babies had just come out of little white eggs.
At one time, this type of salamander was very common in New York City. But now they are extremely rare. Pehek has only found a few small groups.
Pehek keeps the locations a secret. She does not want curious people to accidentally step on the salamanders’ homes.
Pehek carefully searched for more salamander homes. She found another group in the same Manhattan park. But it was about a mile away from the first area. Two bridges with fourteen lanes of traffic were between the two groups.
She also found other salamander groups. They were living in streams inside public parks on Staten Island.
Living in these isolated areas has affected the salamanders’ genetics. In 2013, Pehek worked with scientists from City University New York. One was Jason Munshi-South, who now leads a lab at Fordham University. They studied how inbred the urban salamanders had become.
The two groups in Manhattan had very low genetic variation. This is called a genetic bottleneck. It happens when a population gets very small.
These bottlenecks can occur from big events like climate change. It’s thought that our own species was once reduced to just 1,000 people. For the salamanders, their low genetics is probably caused by man-made roads and bridges separating their homes.
The Washington and Hamilton bridges are impossible barriers for the salamanders to cross. The two groups in Manhattan are completely separated. It’s like being divided by mountains or oceans.
One Manhattan group has become very inbred. They only have one version of most of their genes. This is called “allele fixation” and it is unhealthy.
Even the Staten Island group has low genetic diversity compared to salamanders in rural areas.
By living in these isolated habitats, the Manhattan salamanders could eventually become a new species. Pehek says it’s unlikely, but “Desmognathus Manhattani” could become real someday if they survive.
Other animals in New York City are also becoming very different from countryside animals of the same species. You can see evidence of this without even leaving the salamanders’ park.
A parallel existence
The white-footed mouse has lived in the New York City area for over 18,000 years. As the glaciers melted at the end of the last Ice Age, the species returned to its old home. It has been there ever since.
Every night, these small brown mice with big eyes come out of their burrows. They live in the small forest areas that are left in the city.
One evening in the 1990s, Stephen Harris was walking in Central Park West. He had just left the Natural History Museum. He saw white-footed mice scurrying on the rocky areas at the park’s edge.
Then he looked across the street at the sidewalk. He saw more mice, but they looked different. These were house mice digging through trash cans. House mice are not native to the US. They originated in Southeast Asia.
It’s thought house mice first came to the United States in the 1600s. Now they live alongside humans all over the world. They survive by eating human food scraps, seeds and grains.
As Harris looked at the two types of mice, he was struck by their parallel lives. They were both surviving in the busy city, just meters apart.
“There’s an idea that when an area becomes urban, some animals avoid it,” says Harris. “Native animals like large predators and bears move away as humans move in.”
Then there are the urban exploiters. These are animals that rely on humans – like pigeons, cockroaches, house mice and rats.
“And then there are the urban adapters. That’s the really interesting group,” he says.
Harris explains that white-footed mice are urban adapters. “They were in Manhattan before European settlers. And they’re still there now. But they’re living in spaces we forced on them. So what is that doing to them?”
Harris views New York City’s parks like the Galapagos Islands – places where we can watch in real-time as organisms adapt to their environment and become different.
For many years, Harris thought about the white-footed mice he saw at the edge of Central Park. Eventually, he decided to study if they were different from rural mice. Did the city mice have new mutations?
In 2017, Harris worked with Munshi-South on a study. They analyzed the genomes (full set of genes) of white-footed mice. The city mice came from Central Park, the Botanical Gardens, and Flushing Meadows. They compared them to mice from rural areas about 62 miles away.
The researchers found evidence that the urban white-footed mice were evolving. The city mice had mutations in genes related to metabolizing fats and carbohydrates. These mutations were not found in the rural mice.
“It makes sense,” says Harris. “They’re in parks with fewer native seeds and fruits they would normally eat. But there is lots of human food waste.” He calls it the cheeseburger hypothesis – these once-wild mice now eat human takeout food.
In a study not yet published, Harris analyzed DNA from mouse droppings in Central Park to see what they ate. He found surprising things: tomato and dog!
There are no tomato plants in Central Park. Harris thinks the tomato is from pizza, like the rats eating pizza. But the dog was a surprise.
“The mouse isn’t eating dog,” Harris reassures. “But there are dogs in the park with good diets. The mice might be digging through dog poop and eating fruit from that.”
Other research found urban life is changing the mice’s skull shapes. With their new diets of soft foods like cheesy chips, mice in Manhattan and the Bronx tend to have shorter jaws. This is from chewing less as they develop.
The history of New York City is literally written into the white-footed mouse’s DNA
Interestingly, this happens in humans too. Modern diets need much less chewing than our ancestors’ diets. As a result, our jaws have become smaller and our teeth more crowded.
Like the salamanders, New York’s white-footed mice are also extremely isolated. This is another factor making them diverge. In urban areas, the mouse populations live in park forests separated by impassable roads and buildings. So they tend to have lower genetic diversity.
The impact of this separation is very clear. One study found New York City’s mouse population became genetically isolated from rural mice around 400 years ago – when the city was founded. Today, each city park has mice with their own distinct genetic signature.
However, Harris explains some genetic mixing likely still occurs when outside mice come in. So they probably won’t become a new species. “Even if one mouse comes down from the Catskill Mountains, that’s enough gene flow to stop speciation,” he says. But they are diverging. “It’s very feasible they could become a unique urban population needing conservation status someday.”
A remarkable survival
Living near a city brings another big challenge: pollution. The Hudson River had extreme pollution between 1947 and 1976. During those years, General Electric facilities dumped large amounts of harmful chemicals called PCBs into the river.
The pollution from that time is still there. About 200 miles of the Hudson River is called a “Superfund” site. This means it is contaminated with toxic materials that are still being cleaned up today. Many pollutants are lurking in the river sediment at the bottom. The Hudson also has some unusual fish living in it.
Atlantic tomcod are small grey fish with white bellies and big googly eyes. They look like tiny versions of the cod fish that humans love to eat. Tomcod spend most of their time at the bottom of the water, eating small crustaceans and other foods from the sediment. Unfortunately, this also exposes them to high levels of toxic chemicals in the sediment.
In the 1980s, there was a lot of concern about bottom-dwelling fish getting tumors at polluted sites, says Isaac Wirgin from New York University. Around 90% of two-year-old tomcod had liver cancer. But at clean sites, almost none had tumors.
However, despite the tumors, the tomcod were doing surprisingly well in the Hudson River’s PCB-polluted waters. While other species died out, the tomcod survived. Their livers had the highest known levels of PCBs in nature, yet they were alive. How?
In 2011, Wirgin and colleagues from other institutions analyzed the genomes of Hudson tomcod. They compared them to tomcod from other places. “It turned out the key was in a single gene,” he says. Regular tomcod have a receptor that strongly binds to PCBs, causing toxic effects. But the New York tomcod had mutant receptors that didn’t bind PCBs as well, allowing them to survive the pollution.
This was very fast evolution, creating a unique New York population. But despite adapting, the tomcod face other dangers. The Hudson has the southernmost U.S. tomcod population. As the climate warms, the summer heat may become too much for them to handle. “People worry the tomcod may disappear, exposed to both toxic PCBs and high heat,” says Wirgin.
Other city animals may also struggle. On a recent visit, Pehek found the salamanders’ habitat devastated by flooding and trash. “Obviously a torrential stream washed over, bringing sediment and plastic bags,” she says. “It was very disturbing.” She could find very few salamanders.
But Pehek has a hard time getting people to care. She thinks the salamanders benefited from neglect – their steep habitat was left wild and unnoticed. But this also means they’re ignored. She still worries about them.
New York’s native wildlife survived centuries of change. Today’s parks are man-made landscapes from the late 1800s, with planned trees, hills and rocks. Before that it was swamps and farmland. Yet local species never fully disappeared. After 400 years alongside humans, many evolved new paths. But while adapting, they could still be at risk.