WHAT’S AN ECOSYSTEM?
CORSICA, the granite island in the middle of the western Mediterranean, is one of my favorite places on Earth. When I first went to conduct research there as part of my Ph.D. work, in 1993, it was, as my doctoral adviser Charles-François Boudouresque warned me, “like traveling back to the Mediterranean of 500 years ago.”
I had spent my childhood summers in coastal towns with crowded beaches and concrete walls. Even my favorite coves, where I did my first observations of marine life, were surrounded by villas, hotels, and apartment blocks. But Corsica was different. It was just before sunrise when the ferry that took me from mainland France approached the shoreline of Ajaccio, in the southwest of the island. I stood on deck, sleepy but in awe. The tall and proud Corsica was wild, with very few signs of human habitation, in contrast with the mainland I was used to, where patches of green poked through concrete and asphalt. As the sun peeked over the mountains, a pocket of warm air delivered an aroma from the island that filled my eyes with tears. I can still remember it: juniper, laurel, rosemary, myrtle, sage, mint, thyme, and lavender—the essence of the wild maquis of Corsica. That was the beginning of a love affair that soon became central to my scientific endeavors.
I am extremely privileged to have been to Corsica many times with a handful of dear friends and colleagues, to conduct scientific research at the Scandola Marine Reserve, on the northwest side of the island. Many people have joined us over the years, but initially we were a tight group of friends, people who also were my mentors and colleagues: Kike Ballesteros, who taught me about algae and natural history; Mikel Zabala, an amazing naturalist and professor of ecology at the University of Barcelona who co-directed my Ph.D. thesis; and Joaquim Garrabou, also working on his Ph.D. at the time, studying how the dynamics of ecological communities change with depth. What brought us together was that we were all fanatic divers fascinated by nature, and all of us were unable to stay idle. We all wore green wet suits for diving and, bouncing off the nickname for the famous U.S. basketball team that won the Olympic title in Barcelona in 1992, we called ourselves the “green team.”
Our fieldwork in Corsica typically took place in October, after the few tourists were gone and the reserve manager could dedicate his attention to our work. October in Corsica is a crapshoot. You never know what weather you will get. Some years we had sun and calm seas, but other years we had strong winds or rough seas that prevented us from reaching our diving spots. But we never stood idle, and when the sea did not want us, we explored the old oak forests in search of wild mushrooms—mostly the delicious cèpes, chanterelles, and Caesar’s mushrooms. Or we simply walked the elegant pine forests along the desert beaches, or hiked the spectacular granite mountains that stretch up to their summit at Monte Cintu, 2,710 meters (8,891 ft) above sea level.
IF WE PUT TOGETHER all the dives and hikes in one transect from depths to heights, it would reveal a clear distribution of Corsica’s plants and animals. Sixty meters (197 ft) below the surface are forests of white and red sea fans and yellow sponges like organ pipe cactus. At 50 meters (164 ft) they give way to a forest of old brown algae that look like miniature olive trees, with gnarled trunks and a tuft of branches growing from what look like olive pits. As we move toward the surface, a different species of brown alga appears at about 30 meters (98.5 ft) depth, this one with a brown trunk as thick as a thumb, crowned by a palm tree–like canopy. Different algal species become more dominant closer to the surface, forming forests of different height and age. The animals follow similar patterns, with sea fans living deeper and sea urchins closer to the surface. Some fish, such as the salema porgy, move through different depths, but most species are found within a predictable range.
As we exit the water, we climb red volcanic rocks sprinkled with deep-green bushes and the wild aromatic herbs that brought me to tears when I first smelled them—and still fill me with sweet nostalgia every time I recall them. Or we can turn left and walk across a sandy beach bordered with stone pines, cork oak, and evergreen oak, and meet an undammed river, home to freshwater turtles and fringed by a riparian forest. As we climb up, we encounter maritime pine interspersed with mixed forests of downy oak, sessile oak, Italian alder, and sweet chestnut, with a rich diversity of the wild mushrooms that we gathered and enjoyed when the weather was too rough for diving. Higher up on the mountain, these broadleaf deciduous forests are replaced by forests of Corsican pine on the slopes facing south, and silver fir and European beech on the slopes facing north. Above the forest line, at about 2,000 meters (6,560 ft), we find shrublands of green alder, juniper, sycamore, maple, and silver birch. Continuing up, eventually it becomes too cold for large plants, and all you can see are lichens growing stoically on granite. The very top of Monte Cintu is bare rock—and a lot of snow in the winter.
If we drew the borders between the different types of plant and animal associations we saw, they would look like a series of belts, roughly parallel to each other. Each of these unique groupings of plants and animals can also be defined as different ecological systems—or ecosystems.
AN ECOSYSTEM is simply the community of living organisms (microbes, plants, and animals) and the physical environment (the habitat) they occupy. The organisms and their relationships are what ecologists call a “food web”—a collage of overlapping food chains where a predator eats a predator eats a prey, and where species compete for space, light, and other resources. But living beings don’t just occupy their habitat, be it granite or volcanic rock, sandy beaches or inland plains; they can actually create their own habitat (for example, coral reefs) and provide room and food for many creatures. If life on Earth is a miracle, what life does is still an even more wondrous miracle.
Ecosystems grow and shrink and senesce, and parts of them regress to a young state that allows dormant species to have a day in the sun. Ecosystems are never static. They self-regulate through feedback loops within the biological community but also between living organisms and their habitat. They create rain and regulate the weather. They fill the atmosphere with a mix of gases that allows us to breathe and survive. They filter the clean water we drink. They protect us from floods. They have been saving us from catastrophic climate change for more than a century. But few of us have noticed.
Ecosystems have had billions of years to experiment and, through trial and error, self-organize into the most efficient machines in the universe. They are always changing, and until recently they always fluctuated within reasonable bounds, following predictable pathways. We cannot really re-create much of what ecosystems do for us. Yet dead ecosystems have allowed humans to be the masters of life on our planet—and its destroyers too. But we will park all these stories for later.
Not only forests and wetlands and rivers are ecosystems. Our cities are too. For instance, New York City’s habitat is primarily a built environment made of asphalt, concrete, glass, and steel, interspersed by some greenery. When thinking of wildlife in New York City, most may think of rats, Central Park squirrels, or the odd peregrine falcon nesting on the roof of an office building and making headlines. But New York is also home to thousands of plant and animal species that coexist with the city’s almost nine million people. This wildlife includes coyotes, squirrels, bats, skunks, opossums, red foxes, white-tailed deer, snapping turtles, eastern box turtles, salamanders, and more than 200 species of birds. Strikingly, in the waters surrounding New York City, and in the Hudson River, live 80 species of fish. Even humpback whales and fin whales have been observed. In the most claustrophobic concrete jungle, life hangs on.
If humans suddenly abandoned New York City, the built habitat would collapse. New York City is like Emmentaler cheese belowground, with dozens of tunnels, 245 miles of subway routes, and 6,600 miles of sewer mains and pipes. Without the 290 pump rooms working 24/7 that the city currently uses to drain more than 16,000 gallons per minute of water from the Hudson River, the East River, and the Upper Bay, the metro routes and the tunnels would be flooded. That would turn the holes in the cheese even larger and eventually cause the collapse of buildings. It would not take long for dust to accumulate in holes and crevices on the surface, and for plants to colonize the rubble. Wildlife would start to overtake the ramshackle surroundings.
Life—and the ecosystems it forms—has an extraordinary capacity to regenerate and self-assemble, even in the most unlikely places. Everyone in my generation can remember the explosion of the Chernobyl nuclear reactor in 1986. Despite the heroic efforts of Soviet scientists, soldiers, and miners to contain the radiation, it became so pervasive that people were evacuated from the neighboring town of Pripyat—permanently. Even pets had to be killed to prevent them from spreading radiation. And then nature took over. Now the buildings are crumbling, conquered by shrubs and trees, and the city is the territory of wolves. Apparently the built habitat cannot survive without its builders. In a few thousand years, Pripyat might look like Maya cities in the jungle when first rediscovered under a thick canopy of green.
IF WE ZOOMED OUT from the Corsican forests, we would see the divide between the land and the sea. Zooming farther out, we’d notice that Corsica is an island ecosystem surrounded by the Mediterranean. Zooming farther out still, the Mediterranean itself would appear as a distinct ecosystem with clear boundaries north—the Alps and the Carpathian Mountains—and south—the Sahara. Astronauts on the International Space Station, who have zoomed out even farther, recognize that the entire planet is an ecosystem, with no visible borders except for those between land and sea, desert and vegetation, cities and farms. No wonder. Ecosystem comes from the ancient Greek word oikos, meaning “family,” and also “house.”
Full circle.
But how does this living miracle work and sustain itself? How can nine million species of creatures we can see and a trillion types of microbes we cannot see interact in a way that provides stability to the entire planet? To answer these questions, we need to start from the beginning. Let’s put two species together and see what happens.