In 2003, an army of 350 scientists and volunteers swept out across Central Park. Their mission, called a BioBlitz, was to find as many species as possible over the course of 24 hours. At the end of the day, they had compiled a catalog of 836 species of plants and animals.
It’s impressive that Central Park–an 843-acre island in an ocean of Manhattan concrete–can play host to so many species. But that’s hardly a complete inventory of the biodiversity of the place. Along with its plants and animals, Central Park is home to invisible wildlife too.
The ground swarms with invertebrates, fungi, and a wealth of microbes. This underground diversity–especially the microbes–has been very hard to explore, not just in Central Park but around the world. For one thing, you have to dig. For another, you can’t usually can’t tell the species apart with the naked eye. It’s possible to distinguish between the five species of turtles in the Central Park’s Turtle Pond just by looking at them. But if you dig up a patch of dirt by the pond and look at the bacteria it contains, they might well look like just a bunch of rods and spheres. The diversity of microbes is instead a matter of chemistry. They have evolved a staggering range of ways to break down molecules and grow on them.
In recent years, scientists have developed powerful new tools for measuring that diversity. Rather than looking at feathers or stripes, they look at DNA.
A team of researchers has now used this approach to carry out a sort of MicroBioBlitz in Central Park. They marched their way systematically through the park, and every fifty 50 yards or so, they stopped, bent down, and scooped up some dirt. All told, they gathered 596 scoops. Back at their lab, they threw out everything from those scoops except for the DNA. And then they plucked out just one particular stretch of that DNA. To be more precise, they plucked out different versions of that stretch, each carried by a different species.
The scientists then looked at the sequence of each of those versions. In some cases, the DNA turned out to be identical to a known species, or nearly so. In other cases, the sequence was very different. A peculiar sequence of DNA told the scientists that it came from a species that’s new to science.
The results were, to be blunt, pretty insane. All told, the scientists identified over 167,000 species.* That’s about thirty times more species that all the mammals on Earth–everything from fruit bats to walruses, from musk ox to marmosets.
This chart shows just how ignorant we are of the life even in Central Park. In each chart, the blue bar shows number of species in the park that are already known to science. The gray bar shows the ones that don’t match anything we know of. The top chart shows bacteria and another group of microbes called archaea. (Archaea are single-celled microbes that, like bacteria, keep their DNA floating loose inside their cell.) The bottom chart shows our own branch of the tree of life, the eukaryotes. Eukaryotes include not just animals and plants, but fungi, amoebae, and other protozoans. In both cases, the unknown dwarfs the known.
The more you drill down into these numbers, the more insane they get. Even the “known” species in these charts aren’t very well known at all. For the most part, scientists have never seen the organisms from which they come. They’ve only fished out the same DNA segment from another sample.
The geography of Central Park’s microbes is also mind-boggling. It’s not as if all 167,000 species were present in every sample of the soil. Instead, different species showed up in different places. On average, each sample had about 7000 bacteria and archaea, and 1250 eukaryote species. When the scientists compared the diversity in the samples, they found that any pair picked at random shared only 19.3% of their bacterial and archaeal species, and just 13.5% of their eukaryote species. Even neighboring sites were no more similar to each other than ones on opposite ends of the park. And as the scientists scooped up more dirt, they kept finding more species. So the true number of species in Central Park is probably far higher. (It’s also worth noting that there are probably a lot of other microbes living on the trees, in the ponds, and in other places in the park the scientists didn’t even touch.)
Now, it might have turned out that many of the species in Central Park were closely related to each other. There are many, many species of beetles, for example. Perhaps Central Park only had microbial versions of beetle, and no microbial ants or termites.
That’s not how things turned out. Central Park has the microbial ants and the termites, too. In fact, its microbes span much of the tree of life. You’d get a similar span of species if you took the same number of soil samples from around the world, from jungles to deserts.
Some of the results of the study may be the result of Central Park’s own peculiar history. Its soil contains a high level of species that are considered potential human pathogens. In their report, the scientists hasten to note that this doesn’t mean you’re especially at risk of getting sick in the park. But it may be a sign of the presence of lots of people nearby.
For the most part, though, Central Park may just be a typical plot of microbial habitat. Soil, as a rule, is just rife with microbial richness on a scale we can barely understand. It’s loaded with dying plant matter and the remains of dead animals. Its particles and tunnels and other features make soil an incredibly complex environment, where microbes can specialize in all sorts of ways of making a living. It may turn out that the only thing that makes Central Park unusual is the many holes that scientists have dug there.
*I’m going to use the term species here for convenience. Determining exactly what a species is can be tricky for microbes, and the scientists only presented distinct lineages that might or might not be separate species–what they call phylotypes. Here’s a piece I wrote on why this whole problem is so challenging.