BROOKLYN, New York — When people imagine what nature looks like, this probably wouldn’t be it. On an overcast afternoon in August, I stood next to a strip of plants between the sidewalk and the street in Central Brooklyn, no more than a block from a six-lane highway. An ambulance wailed in the distance. It smelled of exhaust. This was New York City after all.

But this narrow patch of green was full of life — of what you might call nature. Furry bumblebees hovered around clusters of shaggy white flowers. Iridescent flies appeared and then disappeared, like flecks of glitter briefly catching the light. And on the underside of a few leaves were the unmistakable pinhead-sized eggs of a monarch butterfly, which look like tiny lemon candies.

Cities like New York are obviously not known for their wildlife. You won’t find wolves or jaguars or other charismatic megafauna strolling the streets or hunting in big city parks. But if you know what to look for and take a moment to observe your surroundings, you can find interesting and even rare animal species everywhere. I recently learned, for example, that NYC has more than 200 species of native bees, including the Gotham sweat bee — a species that scientists first discovered in the city.

In the summer and early fall, NYC is also home to a large number of monarch butterflies, America’s most iconic bug. Nationwide, these Halloween-colored insects are imperiled. Their population has declined so much in recent decades that the Biden administration proposed listing them late last year under the Endangered Species Act, a powerful environmental law that’s considered a last resort for species facing extinction. Yet in NYC, you can still find them all over — even in tiny patches of plants near a highway. This is a pretty strange situation: A species that may be federally protected in the same category as animals like sea turtles and manatees is fluttering around the largest and most densely populated city in the country.

How are monarchs holding on in New York when they seem to be in such steep declines nationwide?

Over a few weeks in August, I traveled to urban ecosystems across the city to try to answer this question. And along the way, I learned something valuable — that helping wildlife is a lot easier than you might think.

Why monarchs need help in the first place

Monarchs aren’t just nice to look at. They also lead miraculous, almost improbable, lives. Like many birds, whales, and caribou, monarchs migrate. Each fall, nearly all the butterflies that live east of the Rocky Mountains — including those in New York City — fly to the same grove of fir trees in the mountains of Central Mexico, often traveling some 2,000 miles. They ride out winter clumped together on the trees, often in such great numbers that they cause the branches to droop.

Their springtime behavior is even more remarkable: The butterflies migrate back north for the summer, but it takes them two to three generations to get there. The adults in Mexico will fly to the southern US, lay eggs, and die. Their offspring will complete the next leg, flying a bit further north. That happens again and again until the butterflies reach the northern US and parts of southern Canada, where they breed and their offspring start the process all over.

All kinds of mysteries surround this process — including how tiny-brained insects coordinate an intergenerational relay race — but what’s clear is that fewer butterflies are making it to Mexico. Each winter, scientists measure the number of acres occupied by monarchs in those fir trees. Between 1993 and 2002, the first 10 years of monitoring, butterflies were clumped on trees across an average of about 21 acres. That’s an area roughly equal to 16 American football fields. During this past winter, however, monarchs occupied just 4.4 acres.

Scientists blame these declines largely on the loss of milkweed, the only plant that monarch caterpillars can eat. Milkweed once grew abundantly throughout the Midwest in places like Iowa and Kansas, the core breeding range for monarchs. Yet in recent decades, herbicides sprayed by farmers on corn and soybean fields, which blanket the region, destroyed an enormous number of milkweed plants. Researchers estimate that between 1999 and 2014, herbicides and the destruction of grasslands for farmland, homes, and other infrastructure killed more than 860 million stems of milkweed in the Midwest. These chemicals — which farmers still use — also kill native wildflowers that provide food for adult monarchs, fueling their long migrations.

It’s no surprise, then, that conserving monarchs........

© Vox