“We did a little investigating to see if it was OK to send them through the mail, and from what we found out it was OK,” he says. “They are not officially pests, they are not on the quarantine list of the FDA, and they are not a protected species. They can contribute to asthma for some people and they can track things into food like a fly, but they don’t carry specific blood-borne diseases. So it seemed an OK way to do it. We put it out on Facebook and talked to everyone we knew to see if we could get other people to collect cockroaches.”
Soon Mark had a couple hundred cockroaches in his laboratory freezer, most of which came from the public, and more are arriving all the time. But he didn’t want any old cockroach. Only the American cockroach would do.
The American cockroach isn’t New York’s most abundant roach. That title is held by the German cockroach. They are the ones most inclined to live in homes, where they lurk behind refrigerators and stoves.
Despite the name these roaches aren’t German. In fact the Germans call it the Russian cockroach. But that name is wrong too, for these light-brown bugs are thought to have started out in Southeast Asia. Not that their point of origin matters much as they are so widespread now that asking where they came from is almost meaningless.
The American roach is the next most common species in New York and it is the biggest cockroach in the world. It’s more of a sewer dweller than the German roach, although it also lives in prisons, hospitals, and other large institutions. Sometimes they visit apartments by coming up the drains, but this is rare.
The other five New York roaches are minor players, their numbers dwarfed by the hordes of German and American cockroaches.
There’s the Oriental cockroach. It likes damp basements and the sticky insides of discarded soda cans. While the Oriental species likes it wet, the brown-banded roach is like a camel. It doesn’t need constant access to water to get by, and so it can survive in drier parts of our homes, like bedrooms.
The Surinam roach is a digger and often lives in the soil of potted plants in Manhattan offices. In contrast, the speedy Australian roach prefers the outdoor life and regularly hangs around building perimeters.
The last member of the New York roaches is a newcomer. The Japanese cockroach was first discovered there in summer 2012, munching on poisoned bait inside a High Line rat trap. It took entomologists months to work out which of the world’s four-thousand-plus cockroaches it was. The Japanese cockroach’s ability to endure the cold should serve it well, but whether it can survive long enough to make it in New York remains to be seen.
Despite the options available, the American cockroach was the obvious choice for Mark’s study. Not only had the previous study hinted that there was something to learn about the DNA of these roaches, but they were common enough to make them easy to find.
So it didn’t take long before Mark had enough specimens to run the DNA tests. “It turned out that there are very distinct genetic types of American cockroach and these types are different enough that they are probably separated by a million or two million years, which means they probably came here from different parts of the world,” he says.
“The biggest difference in the DNA of the types is 4 percent, so it’s similar in magnitude to the difference between humans and chimps. There are certainly different species of warblers in North America that are less different than these cockroaches are.”
What’s more, these roaches don’t mix. Each keeps to its own neighborhood. So the Upper West Side roaches are distinct from those scurrying around the Upper East Side, who are again different from those on Roosevelt Island. “To us they all look the same, but they are different and so they have to be staying in the neighborhood they were born in. Otherwise their DNA would be like a candy jar with everything mixed up in it. But it’s not like that; it is like a series of candy jars.”
Although Mark’s tests show that the different ethnic groups of roaches don’t mix, working out why is more difficult. One possibility is behavioral. Roaches are family oriented creatures. They recognize their own family members and prefer to stick together, using chemical communication to agree on where to rest and what to eat. They even get ill when alone.
“Cockroaches don’t have the same complicated society that ants do, but they live in groups,” says Mark. “That’s a strange idea to me, to think of these cockroaches as these underground societies.” It could be that these social bonds are keeping the cockroaches apart as they divide into the insect equivalent of the Jets and Sharks of West Side Story.
Another possibility is that the sewers and tunnels the different groups use are not connected, so they spend their lives completely unaware that other groups of roaches live just a couple of blocks away.
The genetic gulf between the New York roach tribes might be more attributable to where they first came from, but some cockroaches are having their DNA rewired by life around people. One of the most dramatic examples of this has been uncovered by biologists at North Carolina State University. After noticing that German cockroaches had, over a twenty-year period, stopped being attracted to the sugar baits of pest controllers, the university set out to discover why.
After unpicking the neural pathways of the roaches, they found that the insects had evolved an aversion to sugar. “What had happened is, essentially, that the taste receptors for sugar had been rewired so that sweet was now perceived as bitter,” says Rob Dunn, head of the Dunn laboratory at the university.
Sugar-phobic roaches are not the only creatures exhibiting signs of evolutionary responses to urban environments. European blackbirds born in cities produce fewer stress hormones than those born in the forest, and this seems to have a genetic dimension, suggesting that being a chilled-out bird is helpful when living amid the hustle and bustle.
Then there are the subterranean river crabs of Rome. Italian scientists found the crabs in 1998 living in Cloaca Maxima, the stream that was turned into the main sewer of ancient Rome in the sixth century. Today, just three hundred feet of its length is exposed to the air and the water is no more than a few inches deep.
That the crabs were there at all was a surprise since the next nearest group of river crabs lives twenty miles away. More surprising still was that the underground crabs were not like their nonurbanized kin. The Cloaca Maxima crabs grow more slowly but live longer, eventually ending up 50 percent fatter than those found in streams and rivers elsewhere in the Mediterranean. These underground crabs also breed at a different time of year, a behavioral shift that suggests they could be on the long path to becoming a distinct species.
More familiar urban animals have changed too, says Rob. “Rats have undergone a lot of changes. It’s not been super-well studied, but there have been a lot of behavioral shifts relative to their ancestors. So in cities they do this thing where they run along the walls. Their closest relatives, they don’t do that.”
Cities encourage the covert. For many urban animals, and especially pests like rats, the biggest danger they face is being discovered and then killed by us. “Historically, our approach to cities has been to kill stuff we don’t like, and what that does is favor sneakier versions of those same species,” says Rob. “So with rats it favors rats that run along the walls, and with roaches and bedbugs it favors ones that are resistant to insecticides and are then really hard to get rid of.
“So, on one hand, we’ve clearly experienced huge health benefits from trying to kill some of these things, and I am grateful to not be at much risk of the plague, but, at the same time, we’ve expended no energy on trying to figure out how to garden beneficial species.
“We have constructed a biome by default, and one of the questions that interests me is how could you garden a city for species that filter our air or that we find lovely or calming?”
The concept of biomes dates back to the formative years of the science of ecology when the botanist Frederic Clements floated the idea that communities of plants were “an organic unit” at a 1916 meeting of the Ecological Society of A
merica.
The University of Illinois botanist took the idea further in 1939 when he and fellow ecology pioneer Victor Shelford published Bio-Ecology. In the book, Clements made the case that animals as well as vegetation were part of these organic units, which he was now calling biomes, and that these biomes were “superorganisms” with distinct characteristics.
The world’s great landscapes are biomes, he argued. The tundra, the desert, the steppe, and the coniferous forest. They were all biomes, each with their own characteristic flora and fauna, and each containing myriad ecosystems.
Clements’s claim that biomes were abstract superorganisms spanning hundreds, often thousands of miles, overstated the case, but he had hit on something profound. The biome concept stuck and became a central concept in ecology. Over the years other ecologists built on Clements’s definition, adding environmental factors like climate and soil characteristics to the definition, but for the most part his original vision of biomes as collections of interrelated plants and animals endured.
For ecologists the biome was a useful concept, even if defining them precisely sometimes felt like hammering a square peg into a triangular hole. As well as being a convenient way to describe the world’s wildlife, biomes help explain why similar species evolve in different parts of the world.
One example of this is the pronghorns of the Great Plains. Pronghorns are much like the antelopes of the Old World, but are actually more closely related to giraffes. The reason the pronghorn evolved to be much like the antelopes is that, despite residing on separate continents, they both live in a grassland biome and so face similar evolutionary pressures.
Yet fundamental to ecology as biomes are, the traditional definition only refers to natural habitats, acting as if people and cities do not exist. This oversight is causing some to question whether the traditional definitions still apply.
Leading the push for reframing biomes in light of human development is Erle Ellis, an environmental scientist at the University of Maryland. “The concept behind the classic view of biomes is that these global patterns are shaped by climate, terrain, and soils, that sort of thing, but mostly climate,” he tells me over the phone from his office in Baltimore. “But when you look at the patterns now, they are not just shaped by climate. There’s a huge amount of shaping going on from human activity. The most extensive one is agriculture: crops and pastures and range lands.”
The idea that human influence was being overlooked came to him in the early 1990s when he went to rural China to study how the move from traditional to industrial farming altered the environment. “Place likes that, they are places that, since the last glacial, have not had a natural history. Humans have been using those landscapes for thousands of years and managing the ecology of the whole landscape. So it became obvious there was a lot of ecology that was essentially a human ecology, and my big question was how much of the Earth’s ecology has been transformed by our activity, and I set out to answer that question.”
The answer he arrived at was “almost all of it” and that—on land at least—natural landscapes have largely ceased to exist. In light of this he drew a new map of the world’s biomes, one that replaced the traditional grasslands, tundra, and forests with eighteen “anthropogenic” biomes that represented the degree to which humans have altered the landscape. At one end there were the few remaining wildernesses, places like Antarctica. At the other end of the scale, the modern metropolis.
The city may be the most dramatic example of how we have changed the world, but agriculture is more significant, says Erle. “Urban areas do represent a very distinctive form of human transformed and sustained ecology, but it’s important to understand that the urban areas are just a very, very small part of that global transformation. Urban areas are definitely less than 2 percent of ice-free land surface; they are not that extensive.”
But can cities really be biomes? To fit that definition cities would need to have similar environmental processes and be home to similar communities of plants and animals regardless of where they are. It would mean that the ecology of Atlanta, Singapore, and Lagos have more in common with each other than the rural areas surrounding them.
That sounds far-fetched, but the evidence that this could be the case is growing. Take the example of Baltimore and Phoenix. On the face of it these cities sit in very different environments—Baltimore in the humid East, Phoenix in the arid West. But urbanization has altered their climates. Baltimore has become hotter than the surrounding countryside because of the urban heat island effect, while the construction of waterways has made Phoenix cooler. The net result is that air temperatures in Baltimore and Phoenix have become more similar.
Temperature is not the only thing that makes the two cities a closer match than their locations would suggest. The residents of Phoenix and Baltimore have similar gardening tastes, so they opt for the kind of lawns that can be seen in almost every American suburb. The result? Phoenix and Baltimore share similar green spaces with similar plant species.
Water systems also bring cities into line with each other. To build Miami, wetlands were drained, while the development of Phoenix saw the construction of lakes and canals. Now, when it comes to water, Miami and Phoenix are more like each other than the Everglades or Arizona desert.
The similarities between cities extend to fauna too. As we’ve seen, the animals that thrive in the urban world share common traits. The ability to keep a low profile is one, but urban animals also tend to be fast breeders with flexible behavior and diets. Just think of the stone martens of Berlin breeding faster than they can be killed, or the coyotes working out how to cross Chicago freeways or the red foxes learning to forage on Brighton Pier.
These animals are the garden weeds of the animal kingdom: adaptable, sneaky generalists that can overcome death by making lots of babies. This isn’t just true of mammals. The most successful city birds are those with the most flexible behavior and an “I’ll eat anything” attitude to life. Birds like pigeons and crows.
Size matters too. Larger animals like mountain lions, elk, and bears are more likely to be spotted and removed if they go too far into the city, while opossums and rats are small enough to slip past unnoticed. Bigger animals also have a harder time finding all the food they need in the city, and this principle applies as much at the level of tiny phorid flies as it does to cougars.
“The flies found in the center of the city tend to be small,” says Brian Brown, the phorid fly expert who heads the entomology department at the Natural History Museum of Los Angeles. “You expect that with mammals. You expect that there are going to be deer and bears in the mountains but downtown not so much, because there is a limited resource base available for them. But it’s also the case in the tiny one-to-three-millimeter-long flies.”
Ecologists divide animals into two broad groups: K-selected and r-selected species. K-selected species are those whose numbers are limited by the amount of resources in the environment, animals like elephants and horses. In contrast, r-selected animals are limited by how much they can breed, and most of the successful urban animals fall into this group.
“The K-selected species are adapted to stable environments as it takes a few years for them to get big, whereas things that are small don’t grow as much and can reproduce more,” says Brian.
“The same principle operates at the level of small flies. So what we have living in downtown Los Angeles are fungus feeders largely and they are small species. The fungi they feed on are in the very perturbed environments like lawns and gardens that are constantly being disturbed, whereas the flies that live on gopher burrow and oak-associated fungi are going to be found in the mountains.”
Add to this our tendency to spread species like the European starling or spitting spider around the world, and the animal life of far apart cities becomes increasingly similar. More evidence is needed, especially from cities outside the most developed nations, but what already exists suggests that urban areas are a biome and, uniquely, one tha
t is almost entirely manmade.
As Rob suggests, that raises an interesting question. Since we control, shape, and design the urban biome, can we mold cities into something that fosters the wildlife we want rather than just a gathering spot for animals that are sneaky, smart, and sex-crazed enough to make it their home?
It’s an idea some people are already experimenting with.
It’s an unexpectedly sunny October day in Chicago, a final hurrah of summer before the cold sets in, and Seth Magle is giving me a tour of the Nature Boardwalk at Lincoln Park Zoo.
The boardwalk’s path snakes around the edge of a b-shaped pond within the grounds of the zoo. On either side of the path are clumps of tall, thin grasses and, among them, a smattering of delicate flowers. The calm water offers a clear reflection of the city skyline and, as we walk, we catch glimpses of the Lake Michigan shoreline that lies just across the road from the zoo.
“This used to be just a concrete-lined pond where they did paddle boats,” says Seth. “But then the zoo, some years ago, decided they wanted to renovate it as a native urban pond prairie ecosystem. They reseeded the whole thing and threw out all the concrete. All of these plants are native Illinois prairie plants that attract certain arthropods and birds.”
As we pass under a small bridge that arches over the narrow of the pond, Seth points to the small ledges above us. “These are for cliff swallows,” he says.
Elsewhere along the path are strategically placed birdhouses. “Those are for black-capped chickadees because they are cavity nesting birds and we don’t have a lot of cavities out here. We’ve built them in such a way that the aperture size excludes house sparrows but allows black-capped chickadees. That’s been very successful.”
The pond also teems with life. Fish can be seen moving beneath the surface, and shiny dragonflies flit around near the bulrushes lining the water’s edge.
Feral Cities Page 21