The ANTS of Manhattan

When Henry Hudson sailed past Manhattan in 1609, his crew described the island as teeming with fish and wildlife “as beautiful a land as one can tread upon.” During the next four centuries, forests gave way to farms, and farms to concrete and skyscrapers. As the city grew, most people assumed the wildlife had largely disappeared, leaving behind only pigeons, rats and a few pest insects able to tolerate urban life.

When my former adviser Rob Dunn first searched for ants in Manhattan, he expected to find very little: mostly common pest species like pavement ants and odorous house ants, with perhaps a few others hanging on in small patches of grass or near planted trees. But that assumption turned out to be wrong.

HIDDEN BUT PLENTIFUL. Hidden among skyscrapers and hot dog carts, Manhattan’s ants have thrived despite centuries of change. Within a 10-mile radius of the Empire State Building, we have now documented more than 40 ant species — more than surveys of the nearest protected natural habitat, which recorded only 33 species. Not only are they diverse, but they are also abundant. New York City hosts ~8 million people but at least 16 billion ants. In other words, ants outnumber New Yorkers by 2,000-to-one.

Even though more than 40 ant species live in Manhattan, most people will never see them. About 85% of species occur only in parks, while only a handful regularly inhabit the most heavily built parts of the city. These are the ants that show up on sidewalks, in apartments or in traffic islands. They include familiar pests, such as pavement ants (Tetramorium immigrans), odorous house ants (Tapinoma sessile) and black carpenter ants (Camponotus pennsylvannicus).

But some highly urban species can be easy to overlook. One example is Hypoponera punctatissima, a tiny, cryptic ant with mostly blind workers that form colonies of only a few dozen individuals. They live in small pockets of soil around buildings and often go unnoticed despite being common.

SURVIVAL OF THE FITTEST. What allows some ants to thrive in cities while others disappear? That question is what drives my own work in New York City. As one of the oldest and largest urban areas in the United States, New York City provides an ideal testing ground for understanding which species succeed in urban environments and which do not.

One key factor is food — specifically, our trash. The average New Yorker produces about half a ton of garbage each year, and roughly 15% of that is food waste. With eight million residents, that adds up to more than 600,000 tons of discarded food every year. Much of this waste will eventually end up in landfills, but before it gets there, it sits in garbage bags on the sidewalk or accumulates as litter, creating a massive resource for urban ants.

The species that dominate Manhattan are typically those that are best able to exploit our trash. These species often form large colonies with many foragers capable of scanning wide areas for food. Once food is located, they use pheromone-based mass recruitment, rapidly bringing additional workers to collect the resource and defend it from competitors. Many also maintain multiple satellite nests, creating a distributed network that allows workers to respond quickly when food appears.

So how much food do ants remove from our city streets? My colleague Elsa Youngsteadt estimated from her studies that ants consume the equivalent of 60,000 hot dogs per year along Broadway alone. Her work showed that ants even consume more discarded food than rats in highly urban areas, suggesting that they may be competitors. So even as ants help us clean up our litter, they may also be reducing the food available to rats — a bonus service.

Abundant food helps colonies grow, but ants also need heat. Cities are typically warmer than surrounding landscapes because concrete and asphalt absorb heat during the day and release it slowly at night. This “urban heat island” effect can be stressful for some species, but for others it is beneficial. Pavement ants, for example, often nest beneath warm sidewalks and roads. The heat from pavement can accelerate brood development and allow colonies to grow faster and stay active over a longer season.

The combination of abundant food and warm nesting sites allows urban colonies to reach impressive sizes. In some cases, ants form much larger colonies in cities than they do in natural habitats. The odorous house ant, one of the most common household pests, is a good example. In urban environments it can form enormous supercolonies, with hundreds of queens and hundreds of thousands of workers spread across loosely connected nests that span multiple city blocks.

Outside cities, however, this same species usually forms small colonies with a single queen and only a few hundred workers — often small enough to fit inside an acorn or hickory nut. What drives this dramatic shift in colony structure remains unclear, but abundant food, urban heat and reduced competition from other species may all play a role.

MANHATTANT. One of the biggest surprises during our work in Manhattan came during our second year of sampling. My colleague Amy Savage collected an ant that did not match any known North American species. When news of the discovery spread, the New York Post dubbed it the “ManhattAnt,” and the name stuck.

Within a few years, the ManhattAnt became the second most common ant on Manhattan sidewalks. This ant also holds a very unique distinction: It is the only ant we have collected in the middle of Times Square — possibly the most urban ant colony that has ever been documented in the United States.

Eventually we decided the Manhatt-Ant needed a formal scientific name. My graduate student Samantha Kennett used DNA barcoding to compare the ants with species from around the world. The results showed it belonged to a group of ants in the genus Lasius found across Europe and the Mediterranean.

To identify the exact species, we sent specimens to Bernard Seifert at the Senckenberg Museum of Natural History in Germany, the world’s leading experts on this genus. By measuring 15 common anatomical traits — ranging from head width to the number of hairs on each cheek — he confirmed that the ManhattAnt is an introduced population of the European bicolored ant, Lasius emarginatus.

In Europe, Lasius emarginatus is not considered a major pest, and it rarely reaches the densities we see in New York. In Manhattan, however, the story appears to be different. As the ManhattAnt spreads across the tristate area, pest management professionals are beginning to report infestations, including large worker populations in attic spaces of apartment buildings. We have collected workers as high as the sixth story of Manhattan townhomes, and they may climb even higher by nesting in cavities and planter boxes on balconies.

Interestingly, the ManhattAnt behaves differently from most urban pest ants. It does not form supercolonies like the odorous house ant, and it does not primarily forage on human food. Instead, it appears to feed mainly on honeydew produced by plant-sucking insects. While native ants fill this role in forest ecosystems, the ManhattAnt seems uniquely capable of doing so in highly urban environments. Exactly why it thrives there remains an open question.

Cities may seem like some of the least wild places on Earth, yet in many ways they are less studied than nearby natural landscapes. They are also places where millions of people encounter wildlife every day — often without realizing it.

To help track the spread of the Manhatt-Ant, we launched the ManhattAnt Project on iNaturalist, allowing residents to submit photos of ants they encounter. These observations have already helped map the species’ current range and track how quickly it is spreading. Based on its native distribution in Europe, the species could eventually extend as far north as Boston and as far south as Atlanta. Because it can also establish in forested parks, it may eventually affect native ant communities outside cities as well.

UNEXPLORED BEAUTY. More than four centuries after Hudson’s crew first encountered Manhattan, the island remains “as beautiful a land as one can tread upon.” Today that beauty often hides in places we rarely notice — small park fragments, roadside plantings, and even the narrow seams of soil between sidewalk slabs where ants build their colonies.

My colleague Sarah Diamond has described cities as potential “bioarks,” places where remnant habitats and green spaces can help protect biodiversity even as surrounding landscapes change. In Manhattan, these pockets of habitat support dozens of ant species living alongside millions of people. As natural areas outside cities face increasing pressure, these small urban refuges may become even more important.

Most ants in New York City are not pests. They spend their lives in parks, gardens and bits of soil tucked into the built environment, rarely interacting with us at all. Yet they remain part of a larger urban ecosystem — one shaped by the heat, food and constant movement that define city life.

Four hundred years after Hudson’s crew first admired Manhattan’s wildlife, it is still a land worth treading upon — so long as we remember to tread lightly and take care of the species that share our cities.