Life in the Treetops – Exploration of the World’s Canopies
Our ancestors were tree dwellers. Throughout human history, people have taken to the trees as safe havens, sites of special spiritual connection, and a cornucopia for food, medicines, materials, and productivity. With their billions of green leaves that produce sugars from sunlight, tree canopies are epicenters of life and food chains throughout the planet. In an evolutionary sense, humans descended from ancestors in the treetops. Anyone who pauses at the zoo to watch a monkey cavorting in the branches is amused, inspired, and subconsciously reminded of some aerial sensation that tugs on our evolutionary memory banks.
In Papua New Guinea, a tribe called the Korowai still lives in the treetops, erecting amazing aerial houses accessible by twig ladders. It is speculated that their unusual habit of community tree houses evolved as a mechanism to escape enemies on the forest floor, and provide a healthy environment above the dank, dark understory. Tree houses remain a recreational vestige of children and adults alike that inspire links between humans and the natural world. Many famous people have escaped to childhood tree houses – John Lennon (of the Beatles), Winston Churchill, the Roman Emperor Caligula, and Queen Victoria when she was a young princess. Recent medical findings indicate that children who play outdoors and learn about nature have better health and well-being.
Why do the treetops hold such a spiritual, as well as scientific, importance for cultures throughout the world? And why have scientists only recently explored these heights for scientific discovery? Relatively few unknown frontiers of exploration remain in the twenty-first century, but the treetops are considered the “eighth continent,” since they are so unexplored. Similar to astronauts, scientists who explore the canopy are called “arbornauts!”
The treetops reputably house almost 50% of the land-based biodiversity of Planet Earth. The combination of sun, fruits, flowers, and year-round productivity of the foliage in tropical rainforests provide ideal conditions for an enormous diversity of inhabitants. Thousands of species of trees and vines produce a veritable salad bar for millions of insects that are in turn eaten by myriad reptiles, amphibians, and mammals; and in turn those primary consumers are eaten by secondary consumers such as harpy eagles, jaguars, and other carnivores. Finally, the cycle of life is completed when soil decomposers break down and recycle all matter back into the canopy.
In addition to classic food chains circling from sunlight to leaves to herbivores to carnivores/omnivores to decomposers, forest canopies house extra niches for other unique forms of life. Bromeliad tanks, tree crotches, leaf surfaces, and epiphyte communities host extra layers of miniature life in forest canopies. For example, bromeliad tanks house virtual swimming pools in the sky that are home to an entire microcosm of tiny organisms. Mosquito larvae, nematodes, tarantulas, katydids, shovel-tailed lizards, and monkeys live in and/or drink from these aerial watering holes. Some poison dart frogs trek all the way from the forest floor into emergent trees to deposit their eggs in phytotelms (canopy pools of water). Other unique canopy niches include the crotches of trees which provide germination sites for strangler figs, and soil repositories that house millipedes, nematodes and other critters usually associated with the forest floor. Strangler figs are the only tree known to start life “at the top” and send their aerial roots extending downward, eventually penetrating the soil below to expand and strangle their unwitting host plants. Air plants, technically called epiphytes, add an extra layer of biodiversity in the moist, sun-flecked branches about two thirds of the way toward the treetops. Even more amazing, the surfaces of canopy leaves provide homes for “epiphlly,” another extra layer of miniature plants including lichens, mosses, and fungi, many of which grow exclusively on leaf surfaces. Within the “canopies” of these epiphylls live an entire microcosm of tiny invertebrates and other undiscovered critters visible only with a microscope. Nothing rivals the forest canopy in terms of biodiversity – layers of life upon life, all nurtured by sunlight, moisture, and warmth in a unique combination that fosters an extraordinary diversity of species.
This world of canopy plants, insects, birds, mammals, and their interactions remained relatively unknown and out-of-reach to scientists until as recently as thirty years ago. Field ecologists first used slingshots to propel ropes into the treetops in the late 1970s, but even then no one fully understood that this green umbrella overhead was a critical component of global health. Exploration of both undersea and outer space was relatively commonplace prior to the exploration of forest canopies. But with the recent escalation of climate change and landscape degradation, canopies have become a proverbial “canary in the coal mine” since their declining health is a harbinger of environmental changes on a global scale. Currently, forest canopy scientists – along with reef ecologists, ice physicists, soil biologists, water chemists, and others– are taking on the role of planetary physicians, working against a near-impossible timeline in hopes of unraveling the critical mysteries of how our planet functions. With access into forest canopies, our knowledge of the machinery of forest ecosystems has greatly expanded. And perhaps less appreciated in a technical sense, forest canopies enhance our sense of wonder and appreciation of the natural world, leading to more ambitious conservation agendas.
Biologists in the nineteenth and twentieth centuries traditionally based their ideas about forests on observations made at ground level. These ground-based perceptions are summarized in a comment by Alfred R. Wallace in 1878:
Overhead, at a height, perhaps of a hundred feet, is an almost unbroken canopy of foliage formed by the meeting together of these great trees and their interlacing branches; and this canopy is usually so dense that but an indistinct glimmer of the sky is to be seen, and even the intense tropical sunlight only penetrates to the ground subdued and broken up into scattered fragments…it is a world in which man seems an intruder, and where he feels overwhelmed.
Binoculars and telescopes were probably the first tools for canopy exploration. Charles Darwin, in the nineteenth century, looked into the tropical rainforest foliage with his scope, but did not sample except for those canopy denizens that fell to the ground. He was nonetheless enthusiastic about the diversity of tropical forests. Ideas about forest canopies changed very little for almost a hundred years from Darwin’s day until the 1950s, when a steel tower was constructed in Mpanga Forest Reserve in Uganda to study gradients from the forest floor to the canopy. Towers provided access to monitor insect vectors of human diseases, representing one of the first applied biological studies conducted in the forest canopy.
The 1970s represented the golden age of canopy access with development of SRT (single rope techniques) for long-term canopy research, not just occasional collections (Figure 1). Whereas SCUBA equipment heralded the age of exploration for coral reefs, ropes and harnesses inspired the “race to the top” (of trees). This versatile method enabled scientists to reach the mid-canopy with ease, and dangle from a rope to observe pollinators, epiphytes, herbivores, birds, monkeys, and even sloths. Portable and relatively inexpensive, SRT allowed even graduate students with a modest budget to survey life in the treetops. This method was developed by Lowman in Australia in 1979, and at almost the same time independently by Don Perry in Costa Rica. Ropes were ineffective, however, to reach the leafy perimeters of tree crowns, since the ropes had to be looped over sturdy branches usually close to the tree trunk. To access the leafy outer foliage of canopy trees, new devices were invented to overcome this limitation. For example, botanists in Indonesia devised the canopy boom, a horizontal bar with a bosun’s chair at one end, which could be swung around the leafy canopy away from the woody trunks. In Pasoh, Malaysia, a combination of ladders, ropes, and booms launched research that solved the mystery of the pollination of dipterocarp flowers.
Creative engineers and canopy biologists partnered to construct the first canopy bridges and platforms for public access in the 1980s. The first two such canopy walkways were constructed simultaneously: one in Malaysia anchored in tree crowns, and another in Queensland, Australia supported by telephone poles. North America’s first canopy walkway was built as recently as 1992, suspended between oak trees in Massachusetts; but America’s first public canopy walkway was opened among oak-palm hammocks in Myakka River State Park in Florida in 2000 (Figure 2). Canopy walkways were replicated throughout the world in the 1990s, using a modular construction design developed by Canopy Construction Associates and others. Since then, canopy walkways and ladders used in conjunction with climbing ropes, zip lines, and other tools have become popular for ecotouris as well as for research. Subsequently, canopy walkways provide sustainable economy to local people in tropical forests (e.g., www.treefoundation.org). The world’s longest walkway, supported by trees, exists in the Peruvian Amazon and provides income for over 100 local families (Figure 3.). The world’s newest canopy walkway is opening in 2014 in Taiwan in She-ih National Park.
Perhaps one of the most creative canopy access tools is the French-designed hot-air balloon, called Radeau des Cimes (translation: raft on the rooftop of the world). It flies independently, but also operates in conjunction with an inflatable raft (27 meters in diameter) that serves as a base camp or platform atop the uppermost branches of tall trees. In 1991, the Radeau des Cimes expedition team pioneered a new technique called the sled, or skimmer. This small (5 meter across) equilateral, triangular mini-raft was towed across the canopy by the dirigible, similar to a boat with a trawling apparatus in the water column. The sled allowed rapid exploration between trees to compare pollinators, photosynthesis, herbivores, and relative diversity and abundance of canopy life.
Construction cranes represent the most recent tool for safe access into the forest canopy. In 1990, the Smithsonian Tropical Research Institute erected a 40-m-long crane in a Panamanian seasonally dry forest; since then, ten other crane operations have commenced operation in countries such as Australia, Switzerland, Germany, Japan, Indonesia, the United States, and Venezuela. Cranes are expensive to install and operate (usually ranging from US $1 to $5 million), but offer unparalleled, repeated access to the uppermost canopy as well as to any section of the understory within reach of the crane arm.
In the 1800s, naturalist Charles Darwin estimated that approximately eight hundred thousand species inhabited the Earth. (One can only imagine that the Queen of England was most impressed by his scientific prowess in calculating this apparently enormous number!) Access to forest canopies has led to the discoveries of millions of species inhabiting this above-ground world. One hundred years after Darwin, Dr. Terry Erwin of the Smithsonian Institution first quantified the abundance of life in the treetops. Erwin sprayed several tree canopies in the tropics with a mild insecticide, and all of the arthropod residents fell to the ground in a heap, enabling him to count and catalogue them. From his initial harvest of insects in Panamanian rainforest trees in 1982, Erwin extrapolated that there may be 30 million species on our planet, not 1-2 million as previously estimated. Since then, studies by other canopy biologists confirmed Erwin’s predictions that millions of insects inhabit forest canopies. My own work in Australia, using the same fogging techniques, provided similar data to Erwin, with millions of species (mostly insects) in the canopy. Professor Edward O. Wilson, eminent biologist at Harvard University, speculated that as many as 100 million species may inhabit our Earth based upon initial surveys of the canopy, soil, and oceanic biodiversity combined. The jury is still out as to exactly how many species inhabit Earth; however, estimates range from as low as 10 million to as high as 100 million!
Field biologists who focus on biodiversity seek to catalog, identify, and understand the role of all creatures on Earth. This is not simply a naming game: its ultimate purpose is to understand the structure and function of an ecosystem, almost the same way that we seek to know how the components of a car engine operate together to create an efficient machine. The challenge to discover and identify species throughout the world is not easy. Finding a new beetle in the treetops is akin to locating the proverbial needle in the haystack –ninety percent perspiration and ten percent luck. All organisms collectively – orchids, beetles, birds, vines, frogs, and many others – constitute biodiversity, otherwise known as the variety of species on Earth. The word “biodiversity” has become politically and scientifically important over the past two decades, as human activities have accelerated ecosystem degradation and subsequent loss of species throughout the world.
Many canopy species provide essential ecosystem services that human beings depend upon for survival. In the Amazon, plants produce chemical defenses against insect attack; and these chemicals, in turn, are used by indigenous cultures for medicinal purposes. The shaman (or medicine man) is a highly respected community leader who has inherited generations of knowledge about the practice of using plants for medicinal purposes. Canopy leaves, barks, and fruits provide a veritable apothecary in the sky, all of which have evolved over time due to the unique interactions of plants with their herbivores (mostly insects!).
As a result of this tumultuous history of developing creative methods for exploration and asking questions in a three-dimensional and complex habitat, the role of canopy biologists has changed. No longer can scientists dangle leisurely from the trees and simply contemplate the beauty of harpy eagles and katydids; instead, they are caught up in an urgent race against time whereby answers are needed before the chainsaws win. In short, canopy scientists cannot afford to sleep! To date, only 1.8 million of an estimated 10-30 (or perhaps 100?) million species have been identified. At their current rate, taxonomists classify approximately 7,000 new species per year, so the pace is agonizingly slow as compared to the carnage of deforestation. The notion of sorting, counting, and naming as many as one hundred million species is daunting. The ecological task of determining which ones are most important to forest health is even more challenging. As Stewart Udall, former U.S. Secretary of the Interior once said: “Over the long haul of life on this planet, it is the ecologists, and not the bookkeepers of business, who are the ultimate accountants.”
How many is one hundred million species? Is there a way to make that enormous number more meaningful for those of us who are not mathematicians? If 200 scientists discovered and identified one new species every day for the rest of their lives, they would need almost 1,500 years including weekends and holidays to complete their task of identifying the estimated biodiversity on Earth. That is a lot of time and effort! What a lot of species! But even more urgent than names alone, biologists need to determine benchmarks for forest canopy health. Is biodiversity important? And do we care if some species become extinct? How much forest and which tree species are critical to maintain this global machinery that we call a forest ecosystem? Can forests function in fragments, and will they remain healthy if replanted in single-species plantations? Unfortunately, no one has answers to these important questions. Biologists have not studied forest canopies long enough to understand the processes that are critical to their health. In the wisdom of Sand County Almanac (Oxford University Press, 1949), renowned ecologist Aldo Leopold said: “To save every cog and wheel is the first precaution of intelligent tinkering.” Another famous scientist, Paul Ehrlich, considered biodiversity analogous to the mechanical parts of an airplane. He speculated that if an airplane mechanic continued to remove nuts and bolts from a plane, the machine will eventually cease to fly. Similarly, as species disappear, we may find that our ecosystems can no longer “fly”, i.e., support life. The critical question asked most often still remains: Will such extinctions reach a critical threshold beyond which humans cannot survive?
Sixty years later, Leopold’s words hold true today. We need to preserve all the pieces of ecosystems (i.e., species) until we identify which ones are essential to the operation of their machinery. Forest canopies represent “home” to a disproportionately large number of species. Because forests are such efficient machines producing energy, medicines, materials, fibers, foods, nutrient cycling, and atmospheric gases critical to all life on Earth, the continued health of forests is directly intertwined with human health.
Since the construction of the first canopy walkway over 25 years ago, millions of hectares of tropical rainforest have disappeared, along with thousands of as-yet-unnamed new species. Losses of forest canopies, as well as other ecosystems, are critical issues for our children and their children, as they grow up to inherit the stewardship of our planet. Earth is full of exciting discoveries relating to forest canopies –new medicines, exotic perfumes, ecosystem services such as cleaning the air and storing carbon dioxide, keystone predators, and other food chain dynamics, and important economic products such as chocolate, corn, oranges, coffee, and rubber, to name but a few. The next decade is critical. Forest canopies are essential to healthy ecosystems that translate into sound economic policies. As my children summarized in our recent book, It’s a Jungle Up There (Yale University Press, 2006), “Conservation over conversation is critical to the next generation.” Simply changing the order of the letters “s” and “v” creates action that will likely determine the fate of forest canopies. The world’s forests are in the hands of the next generation of scientists; forest canopies may inspire them to seek sustainable solutions (Figure 4).