On the Significance of Species
Beginning in the mid-1980s with evolutionary biologist and writer Stephen J. Gould, the University of Minnesota has invited world-renowned speakers to give public addresses in a lecture series named for the university's longtime president and Graduate School dean, Guy Stanton Ford. In 1994, I had just started as assistant professor in the department of ecology, evolution and behavior when I was thrilled to discover that the speaker for that year would be Richard Dawkins, another famous evolutionary biologist and writer. I joined the hundreds in the packed auditorium, and I think all of us were surprised when he began by showing a slide of a familiar religious illustration: a lion and a little girl sitting peacefully beside one another. The illustration portrayed the familiar Christian, Jewish and Islamic construct of Edenic peace, in which all species — including predator and prey — live in harmony. Dawkins used the tableau as an emblem of the irrational thought he had struggled against for most of his career. I saw something different, something I had not seen before. I saw the illustration as an example of humanity's struggle to understand the significance of species, including our own. What is the significance of the millions and millions of species that cohabit our world? What roles do lions, leopards, wolves or, for that matter, worms, beetles and bacteria play in nature? What is our own significance as a species? Are we privileged in ways others are not? Is our role on Earth fundamentally different from the roles other species play?
The question of the significance of species may seem esoteric and subjective, but the root causes of our most pressing environmental problems — and others far worse than what we've yet experienced — stem from our inability to answer that question definitively. Climate warming, emerging diseases, invasive species, food and energy shortages and many other environmental issues are all caused, in part, by the massive loss of plant, animal and microbial species that has occurred in the face of human economic development. Theoretically, such loss of biodiversity could even lead to a complete collapse of Earth. Had we known, in the last few centuries, the true significance of biological diversity, we would not find ourselves where we are today, at the crossroads of tremendous economic, political and environmental upheaval caused by man's domestication of much of the Earth's surface, and even its oceans.
It is perhaps not surprising that I saw humanity's struggle to comprehend the significance of species clearly for the first time when Dawkins showed the religious picture of the little girl sitting peaceably with a lion. That same year, my colleagues and I had published a highly controversial study demonstrating that biodiversity plays a significant role in regulating our environment. Over the ensuing 15 years, hundreds of scientific studies like ours have been published, including a five-year global study done by some 1,300 social and natural scientists from around the world — all of the studies unequivocally illustrating that biodiversity is critical to our world and to our future. These studies also provide a new perspective on our own significance and role as a species, a perspective possibly more inspirational than any humans have heretofore entertained.
The Lion and the Little Girl
The drawing in Dawkins' presentation showed a rosy-cheeked, blonde little girl sitting peacefully beside an enormous lion. It was a relatively realistic rendering, so I imagine some in the audience were anxious at the thought of a little girl in danger. Others probably thought the illustration was from a tale of fantasy, perhaps a children's story like C.S. Lewis' The Lion, the Witch and the Wardrobe in TheChronicles of Narnia series. Still others, however, quickly and correctly recognized it as a variation on the biblical idea of the Peaceable Kingdom, made famous by the American painter Edward Hicks (1780-1849). To my mind, the English painter William Strutt (1825-1915) captures the idea better in his painting, A Little Child Shall Lead Them.
The Peaceable Kingdom refers to a time when all manner of beast will one day live in a harmony with humans, as they once did in Eden. Many in the audience probably even knew the specific biblical passages from which the Peaceable Kingdom derived; Isaiah 11:6: "The wolf also shall dwell with the lamb, and the leopard shall lie down with the kid; and the calf and the young lion and the fatling together; and a little child shall lead them." Or the more enigmatic verse, Isaiah 65:25: "The wolf and the lamb shall feed together, and the lion shall eat straw like the bullock: and dust shall be the serpent's meat."
Dawkins found the image preposterous. He suggested that if we reverse engineered a lion, we would discover that neither the Peaceable Kingdom nor the Garden of Eden is a rational construct.
Reverse engineering, of course, is a process by which one uncovers what a thing is and how it works by dismantling it, examining its parts, working out how the parts relate to one another and reconstructing a working version of the thing. It would not take much effort, Dawkins argued, to discern by reverse engineering that claws, teeth, massive jaws and a digestive tract that lacks any capacity to digest dried grass tells us that the lion was never, nor would ever be, an eater of straw. Rather, the lion's design clearly indicated that it is an eater of lambs, bullocks and, for that matter, little girls. One comes to similar conclusions about wolves and leopards. Such species could only have been designed with one purpose — to kill and eat other species. If we were to reverse engineer ourselves, our teeth, digestive tract and the dietary need for vitamin B12, linoleic and alpha linoleic acid, zinc and essential amino acids that are rare in plants suggests we too have some capacity for carnivory, which should make the lamb, kid, calf, bullock and certainly whichever one is the fatling a bit nervous about a human in their midst, even a little girl.
Eden was a place without evolution and without ecology. There was not only an absence of carnivory but no predation of any kind (insectivory, piscivory or even omnivory); neither was there parasitism, competition, disease, famine, pollination, reproduction or mortality. It seems strange, then, to imagine that predators, parasites and other malentities would have all peacefully populated Eden — until Adam and Eve ate the forbidden fruit. Equally strange is the idea that Eden would be re-created, at some future joyous time, in the form of a Peaceable Kingdom in which lions, leopards and wolves would give up meat and eat straw.
But of course, Eden and the Peaceable Kingdom are ideas that serve to illustrate the ultimate power of the Creator; they serve as heuristic, philosophical and pedagogical devices for the people of Judeo, Christian and Islamic faiths. They are not, and probably never were meant to be, scientifically rational, but they do suggest that humanity has a deep-rooted desire to understand its place among the species.
Though used by Dawkins to talk about evolution, the illustration of the lion and the little girl did not arise from a question about origins. Rather, it speaks to the sense of biological disharmony and discord in our world, what Alfred Tennyson, the 19th-century British poet, famously described as "nature, red in tooth and claw" in his epic poem, In Memoriam. The illustration allows us to imagine that a Creator did not originally design the world as it is today, that our sin brought forth ecological and evolutionary processes, and that one day, should we prove to deserve it, our world will be made right again by the elimination of those processes.
Evolutionary theory may provide a scientifically rational explanation for the origin of species, but it does not address the question that motivates Edenic ideas: What function do lions, wolves, leopards, parasites and other species that seem, to humans, to be nasty, noxious, toxic, inedible and ugly serve in a world where the only species that we seem to be able to live harmoniously with are domesticated, edible plants and animals, and beneficial microorganisms like the bacteria in yogurt?
Does Biodiversity Matter?
In spite of widespread awareness that life on Earth is diverse, few people could explain the significance of biodiversity or why it matters. Estimates of the number of species evolution has generated on Earth range from 10 million to 100 million species. (I like to use the modest figure of 30 million.) If asked why it is that we could not live with just 1 million species, or perhaps several thousand, or just a few dozen, even scientists — natural historians, conservation biologists, zoologists, botanists and microbiologists, many of whom could explain the evolution of biodiversity — often cannot tell you what function, if any, so much diversity serves. In fact, it was not until 1992 that scientists formally attempted to address the question, doing so at a conference in Bayreuth, Germany, organized by Ernst-Detlef Schulze of the Max Planck Institute and Harold (Hal) Mooney of Stanford University.
In 1994, when I began at the University of Minnesota, many of my colleagues, inspired by the conference in Germany, had just begun asking what the significance of prairie grassland diversity might have been. Like most Americans, my vision of the prairie included bison, badgers, prairie dogs, elk and wildflowers, but Minnesota seemed to be little more than miles and miles of farms, urban and suburban areas, towns and malls. Ponds, lakes, wetlands and forests were more prevalent, but less than 1 percent of the original prairie remained.
Most people know that the world is rich in species, understand that biodiversity is disappearing and care about species and their loss. Books, magazines, radio, cinema, television and now the Internet show endless pictures of pandas, whales, sharks, big cats and other exotic wildlife, including flowering plants, butterflies and less charismatic species. Natural history museums, gardens, zoos, wildlife parks and nature reserves generally enjoy widespread public and private support. The popularity of the National Geographic Society, cable channels like Animal Planet and Discovery, and well-funded nongovernmental organizations that support conservation, including Greenpeace, The Nature Conservancy, the Audubon Society, the Royal Society for the Protection of Birds, the Wildlife Conservation Society and Conservation International, are all testaments to an enormous interest in diversity. In fact, Edward O. Wilson, another famous evolutionary biologist and writer, has argued in his book Biophilia that our fondness and fascination for biological diversity is innately human. Even the many fungal, bacterial and viral species fascinate in their own way and with their own pathogenic qualities.
But if you asked even the avid ecotourist, birder or insect collector the significance of all this biodiversity, he or she likely would not know. For most people, biodiversity is just an enigmatic feature of life on Earth; it's due to some process, evolutionary or other, but of no particular significance.
Significance, of course, is both subjective and contextual. But it is also a relational concept, a comparison of one thing to others. In a scientific sense, if we remove a part in a system, yet the system functions flawlessly as a whole, then we can deduce that the part we removed has no significance. If we remove a part and system function alters, then that part is of intermediate significance. If we remove a part and the system stops functioning, then this part is, by scientific definition, highly significant.
For reasons that will become apparent later, consider a bomber plane whose function is to deliver payloads of bombs on enemy targets. Its functioning is dependent on an intricate web of interacting parts, some mechanical, some electrical and some biological (e.g., its human pilot, co-pilot, navigator, gunner and bomber). Pull out a tiny part, such as a single rivet, and there is often no detectable change in function. As one pulls out more and more rivets, however, the system is increasingly likely to be compromised. Pull out the pilot, and the plane can still fly under the direction of the co-pilot. Pull out the rather obviously valuable propellers, fuel pumps or engines, and the system ceases to function altogether. Pull out the navigator, bomb releasing switches or firing pins on the bombs, and even though the plane can fly, it is no longer capable of performing its intended function.
Because there may be more than 100,000 parts in a large bomber, it would take an enormous effort to assess the significance of all of them. If we wanted to copy the bomber, knowing the significance of all those parts would be invaluable because we could dispense with the insignificant parts. In the absence of such knowledge, it would be best to reverse engineer and copy the bomber piece for piece, under the conservative assumption that every piece matters.
Species are like the hundreds of thousands of parts in a bomber plane, only their system, the biosphere, was not designed to serve a particular purpose. One can study the design of something, however, without having to assume it was designed by a sentient being or intended to serve some specific purpose.
If a bomber plane dropped its payload, had its crew bail out, then crashed in a country whose residents did not know the plane's purpose, the residents might still study its design as a flying machine. One can likewise study the design of a bird without having to assume it was designed by a sentient being and has some specific function other than the one that interests us — the fact that it can fly. One can reverse engineer a bird and marvel at its hollow bones; its remarkable respiratory system, which allows near continuous exchange of gasses in its lungs; its intricate feathers, made up of hooklets and barbules that branch off the rachis; and its remarkable vision. Through this study, one could better understand how a bird successfully flies, lands and navigates the skies. The evolutionary biologist might argue that natural selection favored the most efficient designs; a religious person might assume that the bird reflects the wisdom of a divine engineer. Neither assumption is necessary, however, for understanding the function of a bird.
Thus, the significance of species can be determined in much the same way we went about assessing the significance of parts in a bomber, though we need to identify what function the collective activities of the species on Earth represent. This function is the generation of Earth's environment — the physical, chemical and biological properties of the space that surrounds all living things on Earth. All species contribute to this biospheric function, and the significance of each species can be readily determined in exactly the way we would go about determining function and significance for any part in any system.
Gaia, Mother Earth and Nature
On Christmas Eve 1968, William A. Anders, a crew member on Apollo 8, took two pictures of Earth rising over the lunar landscape. The image has since become an icon for contemporary environmentalism. Earthrise showed our planet residing in the vacuum of space, far from any other habitable place. Contrasted against the moon, the image of a distant Earth also reminds us of how different the two landscapes are. We know of few places on Earth as barren as the Moon. In fact, even in seemingly desolate places — parts of Chile's Atacama Desert and certain ocean abysses, for example — there is still microbial life.
Life is visible from a very long way off in space as a dynamic, oscillating green that maximally colors the Northern Hemisphere in July and August and maximally colors the Southern Hemisphere in January and February. But from far away it is difficult to know what the significance of this green stuff is. If there were another planet of exactly the same dimensions and history, traveling in the same orbit, but lacking life, would such a planet be noticeably different, aside from lacking the oscillating green color?
During the 1960s, in anticipation of NASA's Viking mission to Mars, scientists were charged with figuring out how best to determine if there was life on the Red Planet. James Lovelock, an independent British scientist, worked as a consultant on this question. The Viking mission objectives were primarily to obtain high-resolution pictures of the planet's surface, but it also analyzed the chemical makeup of the Martian surface and atmosphere. Along with others, Lovelock realized that one did not have to land on Mars to know whether there was life on it. The high abundance of carbon dioxide (95 percent), virtual absence of oxygen (0.13 percent), scarcity of nitrogen (2.7 percent) and the relatively steady state of the concentrations of these compounds in the atmosphere suggested to him that there was no life on Mars, at least not as we define life. (This did not exclude the possibility that there might be some trace amounts of living matter on or under the surface whose impacts on the atmosphere were not detectable, which is why data being collected by the Mars Exploration Rovers, Spirit and Odyssey, are still under intense scrutiny by those still thinking about whether life is possible on Mars.)
Lovelock also reversed the question: What would the chemical fingerprint for life on Earth be? Using a chemical model of Earth in which there were no living processes such as photosynthesis or respiration, he estimated that atmospheric concentration of carbon dioxide would be 98 percent (it is currently 0.03 percent), oxygen would be barely detectable (it is currently 21 percent) and nitrogen (currently the dominant gas at 79 percent) would make up less than 2 percent of the atmosphere. Temperatures for such a lifeless Earth would hover at 290 degrees Celsius (554 F), and its atmospheric pressure would be 60 times what exists today. These estimates were in line with observations for our neighboring lifeless planets, Venus and Mars, and thus highly plausible.
Clearly, that is not what we have here. The fingerprint of life on Earth is its anomalous atmosphere: The concentration of carbon dioxide is too low (it should be the dominant gas) while the concentrations of oxygen and nitrogen are too high for a nonliving planet. Oxygen in particular — a highly reactive, even explosive gas — should chemically bind to a wide array of elements and compounds on Earth, leaving virtually none in the atmosphere. Yet, more than one-fifth of our atmosphere is made up of this highly reactive element, and its concentration remains relatively constant year after year. In fact, its concentration has been reasonably constant for nearly 60 million years.
Lovelock came away with a sense that there was something truly remarkable about Earth, a sort of meta-life or gigantic global biological system in which the sum of the parts — all the plants, animals and microorganisms — made Earth the habitable planet that it was. He speculated even further that it was an autopoietic system, meaning (roughly) that all its species actively contribute to the functioning of the biosphere in such a way as to ensure their growth and regeneration, which, in turn, is what governs biospheric functioning. This is a complex idea, but essentially he felt that life actively holds the conditions of Earth's surface within a range conducive to the persistence and perpetration of life, a homeostasis similar to our bodies' regulation of core temperature to a constant of around 37 C (98.6 F). Considering just temperature, for example, life works like a giant, somewhat imprecise thermostat. If Earth ever got too hot, perhaps because of a buildup of greenhouse gasses, life processes would shift in such a way that the temperature would come down, perhaps by sequestering and storing greenhouse gasses until the Earth cooled. If it got too cool, however, life would again shift, only this time to induce warming, perhaps by the production of greenhouse gasses.
Interestingly, Lovelock referred to this idea of self-regulation as the "Gaia Hypothesis," named after the Greek goddess of Earth, who herself was the daughter of Chaos, or the void. There is a tendency to think of Gaia as a nurturing goddess, a maternal figure, but in Greek mythology she was the mother, grandmother and great-grandmother of many gods, good and evil. Gaia gave birth to the gods of the sky, mountains and the sea, but she also gave birth to the Titans, the Cyclopes and three monsters (the Hecatonchires). She even provided her son with the sickle he used to castrate and kill her husband, Uranus, although this murder was to protect her offspring from the murderous husband. Gaia was the Earth Mother, but she gave birth to both stability and turmoil.
Through three and a half of the four and a half billion years of Earth's history, it has had life on its surface (though it consisted mostly of microorganisms). Earth formed at just the right time, at just the right distance from the Sun, with just the right kind of axial tilt to generate seasons, with just the right kind of moon, and at just the right size to be geologically active, with volcanoes and drifting continents. Earth was also bombarded with just enough comets to have water and other materials important to life accumulate on its surface. In fact, how we got where we are today as a living planet requires so many singular, low-probability events that we should consider it miraculous we are here to think about it (which, of course, is what the anthropic principle says would have to be the view of a creature that could ponder its own existence).
Since the Earth became inhabited, it has had atmospheres of methane and no oxygen, periods in which it has been relatively ice free and periods when it was covered almost entirely with ice; it has been struck by giant asteroids, experienced enormous bouts of volcanic activity and seen vast swings in atmospheric greenhouse-gas concentrations and global temperature. Earth's environment has never run amok to produce an uninhabitable planet, but its history has been more dynamic than is often appreciated.
Through the lens of Lovelock's findings, the picture known as Earthrise suggests something that is not immediately apparent. The thin layer of atmosphere held to the planet by gravitational force shields us from harmful radiation, insulates us like a blanket and is relatively stable, yet it is also mutable. The small amount of carbon dioxide and other greenhouse gasses in the atmosphere is sufficient to warm the planet but has the potential to rise to levels that would ultimately sterilize Earth. The oxygen that serves the metabolic needs of most organisms, and is the basis of ozone production that shields us from harmful ultraviolet radiation, could completely transfer out of the atmosphere, suffocating most of life on the surface. Nitrogen gas —almost four-fifths of our atmosphere — is relatively inert, but it too could change, binding to other elements and dissolving into the seas. Earth is so massive, it is difficult to imagine any drastic change would occur quickly, but one is possible if biological processes were eliminated. Venus and Mars stand as visible reminders of sterile fates Earth could have.
The Gaia Hypothesis is frequently divided into two parts: The Weak Gaia Hypothesis states that life is critical to Earth's environment, and the Strong Gaia Hypothesis says that the biosphere is autopoietic. It would be convenient if Lovelock's Strong Gaia Hypothesis were right and the biosphere were self-regulating, self-healing and self-perpetuating. We would have a way of explaining three and a half billion years of a planet that has sustained an environment conducive to life.
Though the jury is still out, the bulk of the scientific evidence is against the Strong Gaia Hypothesis. One of its strongest critics is Dawkins, who sees no way that evolutionary or ecological processes can generate an autopoietic biosphere from a seemingly unstructured confederation of species whose fates are determined by their individual fitness and not the fitness or stability of the community, ecosystem or biosphere they reside in. Nevertheless, life is what makes Earth habitable, so the Weak Gaia Hypothesis is undeniable.
Reverse Engineering a Thousand-Billion-Ton Machine
Earth is a dynamic planet on which billions of tons of chemicals annually cycle between two basic states: complex molecules and the elemental constituents of those molecules. Geochemistry encompasses the composition of the gasses in our atmosphere, the chemical composition of the lithosphere or rocky portion of the Earth, and the chemistry of the water that covers much of the planet. Because biological processes influence this chemistry (as in the removal of carbon dioxide from the atmosphere by photosynthesis or the addition of carbon dioxide to the atmosphere by respiration), we refer to the dynamic chemical system of Earth as biogeochemical.
As Lovelock estimated, biological influences are clearly enormous, and life on Earth is massive, consisting of billions of tons of metabolically active organisms. Just how massive is not easy to quantify. Because most living things consist primarily of water, scientists prefer to estimate the mass of living things, or biomass, in terms of how much carbon they contain. If we were to weigh just the carbon of all the living material on Earth and ignore everything else, it would weigh about a thousand billion tons, divided roughly equally between microorganisms and plants. (The mass of animals on Earth is so small in proportion to the mass of plants and microorganisms that it is often ignored in estimating the mass of the biosphere.)
Aside from this number being very big, it is admittedly hard to relate to what we commonly think of as mass. When I lift my cat, for example, as far as I am concerned its mass is about 5.4 kilograms (12 pounds). I wouldn't find it terribly informative if someone told me instead that its mass is roughly 1 kg C (or 2.4 pounds of carbon), but there's a good reason we should stick with this carbon-centric approach. Even though most of us are not used to thinking about the mass of life in terms of its carbon content, the total mass of life on Earth is almost impossible to derive. Species vary in water content, from more than 97 percent in some marine invertebrates to 70 percent in terrestrial animals, such as my cat, and 40 to 45 percent in trees. The only real universal, standard estimate of mass that allows us to compare a 50 kg jellyfish to a 50 kg human to a 50 kg plant is to focus on carbon content. Carbon also helps us gauge the influence of each species on the Earth's carbon cycle.
Because every species influences Earth's chemistry — sometimes in barely detectable ways, sometimes in major ways — every species can be said to have a function (though not in the sense of purpose). As in our example of the bomber plane, the best way to deduce what function a part plays in the ecosystem is to remove it and see what happens. This is standard practice in ecology, with University of Washington zoologist Robert Paine's experiment in the 1960s being perhaps the best known example.
Paine removed a single species of starfish (Pisaster ochracues) from an intertidal community in Mukkaw Bay, Wash., and found that its absence allowed a prolific species of mussel (Mytilus californianus) to grow and displace most of the other species in the ecosystem. The starfish functioned as a regulator of mussel density, something that could only make sense in the context of the intact ecosystem
Note that I did not explain the starfish function in terms of biogeochemistry. To do that would require measuring how the distribution of elements in Mukkaw Bay changes in the presence or absence of the starfish. This would be difficult to do; it would require removing every single starfish, and keeping all of them out, for a long enough period to detect the resulting biogeochemical changes. One could, however, count up all the starfish in the region, determine their respiration rate and estimate how much carbon dioxide they release into the water and atmosphere over a year. One could also determine how much carbon they consumed in food and how much they excreted as waste, and do the same for nitrogen, oxygen, sulfur, phosphorous and so on, until all likely influences of the starfish species on the ecosystem's geochemistry were determined.
As this exercise shows, to determine the functional significance, in terms of biogeochemistry, of even a single species is a daunting task. For this reason, there are relatively few species whose biogeochemical impacts are experimentally known. In most cases, as we did for the starfish, one estimates what their function is based on size, abundance, growth rates and other biological properties.
When one reverse engineers a human-designed system, there is a sense that every part was deliberately put there for the system to function. For example, consider a case involving real bomber planes: In 1944, three U.S. B-29 Superfortress long-range bombers made emergency landings in Vladivostok, Russia. The Russians confiscated and then reverse engineered them. The Russians used one plane to learn how it functioned, dismantled the others and copied almost every part to produce the Tupolev Tu-4 Bull, which became an important weapon during the Cold War. U.S. B-29s dropped the atom bombs on Hiroshima and Nagasaki in Japan in 1945; by 1947, the Russians flew their first Tu-4, and by 1951, a Tu-4 dropped a Russian atomic bomb. With more than 100,000 parts, the B-29 made for a daunting reverse-engineering task, but the value of a long-range bomber, especially during the Cold War, made it worthwhile.
In much the same way that Russian engineers dismantled the B-29 to replicate its function, ecologists began to reverse engineer ecosystems in the 1990s, but to understand, rather than replicate, them. One of the first examples of this approach was an experiment conducted in 1992 and 1993 by Lindsey Thompson, Sharon Lawler, Richard Woodfin and me, all junior research scientists at Imperial College of London's Centre for Population Biology at Silwood Park in Berkshire, England. Under the leadership of Sir John Lawton, we conducted the largest experiment of its kind in a machine dubbed the Ecotron. The machine consisted of 16 integrated growth chambers in which every aspect of the environment could be tightly controlled by computers, from relative humidity and temperature to the timing and quantity of rain, generation of breezes and even the shift in red light that typically occurs at dawn and dusk.
The system we sought to replicate was a weedy meadow typical of Berkshire County, England. A meadow is vastly more complex than a B-29 bomber, but much simpler to replicate because its parts (its species) are readily available and, as reproductive entities, they did us the favor of making as many copies of themselves as we needed. To build an ecosystem, we started with sterile soil that we inoculated with microbial species extracted from a uniform muddy slurry taken from neighboring meadows. We then added more than 30 species of plants and animals to the containers. We used earthworms, soil invertebrates, snails to feed on the decomposing leaf litter, herbivores (aphids, whiteflies and slugs) to feed on living plant tissues, and small predatory insects to prey upon the aphids and whiteflies. Had the Ecotron been bigger (each chamber was a cube, two meters to a side), we might have added mice, rabbits, foxes, moles, sparrows, hawks and larger animals. But we knew that so long as we had soil, soil microorganisms, soil fauna, plants, herbivores and predators, we would have working models of real ecosystems.
Unlike the Russian engineers who replicated the B-29, we deliberately engineered our ecosystems to be different from a real ecosystem in one specific detail: All our meadows were identical except for the amount of biodiversity each had. Six of the chambers contained ecosystems with 31 species of plants and animals inside; four contained only 16 of the original 31 species; and another four chambers had just 10. Everything else was the same — the same volume of soil, same amount of light, same amount of water added each day, same breeze, same timing of dusk and dawn.
What we found was quite surprising. The amount of carbon dioxide absorbed by the communities, the amount of biomass they produced, the fertility of the soil and the amount of water retained by the ecosystems differed. Because everything was held constant among the ecosystems except for their biodiversity, the only conclusion we could come to was that our monkeying with the number of species was sufficient to drastically change the way the ecosystem functioned. Most important, there was a clear pattern that related how many species were in the ecosystem with how much carbon dioxide it absorbed: More diversity led to greater absorption of carbon dioxide.
In 1994, we published the Ecotron study in the journal Nature; it soon became one of the most cited papers in global change research. We were all surprised at how much attention our study garnered. There was no doubt that ecosystems were critical to processes such as the cycling of carbon dioxide between the atmosphere and biosphere and the cycling of nutrients between soil, water and the atmosphere, and that these processes were an integral part of global environmental processes. The Weak Gaia Hypothesis already told us this. There was also no doubt that some species had strong impacts on an ecosystem while others — such as Paine's starfish in Mukkow Bay — had weak impacts. But no one had experimentally tested the idea that simply reducing the number of species would change ecosystem function.
Since then, there have been numerous studies that have been variations on the same theme — hold as many factors constant as possible, vary biodiversity, then see what happens to the functioning of an ecosystem. Researchers have manipulated the diversity of plants planted in flower pots, the diversity of plants planted in the field, the diversity of microorganisms grown in Petri dishes, the diversity of marine invertebrates living on tiles suspended in sea water, the diversity of seaweed species in pools, the diversity of stream insects in artificial streams, and the diversity of pond species grown in artificial pools.
These experiments found that some species, when left out, had no detectable effect on biogeochemistry, while in others, if left out, had dramatic effects. But, on average, the removal of species caused changes in ecosystem functioning, and the more species one removed, on average, the stronger these changes became.
Though relatively straightforward in premise, these experiments have been incredibly difficult to conduct because of a mathematical reality: Even when the number of species in a re-created ecosystem is relatively small, the number of possible combinations of those species is enormous.
A concrete example will drive home the combinatorial nature of biodiversity. At the University of Minnesota, under the leadership of David Tilman and Peter Reich, dozens of researchers (including me) and hundreds of students have manipulated the composition of prairie grassland species in hundreds of replicate plots in a series of experiments that have been running for more than a decade. These studies have become some of the most influential of their kind, yet even the most ambitious used only 32 species of prairie plants, while other experiments typically limited themselves to just 16. Several hundred possible combinations of plant species were used in the experiments, but large though this may seem, there were 4,294,967,295 possible combinations, meaning that most were never explored. The area devoted to these experiments takes up roughly 600 by 300 meters (or about 38 acres), making them among the largest experimental series of their kind.
In spite of the limited number of species and tiny numbers of combinations involved, these studies have been stunningly successful at demonstrating that greater diversity means more biomass, more production, greater retention of nutrients, greater resistance to invasive species, greater resistance to the spread of plant pathogens and greater stability.
In 2006, Bradford Cardinale of the University of California, Santa Barbara and several colleagues reviewed all the reverse ecological engineering studies they could find. The studies included bacteria, fungi, plants and animals in ecosystems that ranged from lakes to streams to oceanic coastal habitat to temperate grasslands and even to forests. They found that 88 percent of the studies demonstrated that declining biodiversity generally led to a reduction in the amount of biomass in the ecosystem, not just of plants but of microorganisms and animals, too. Not surprisingly, if there is less biomass, there is usually (but not always) less in the way of biological processes to influence geochemical processes, and the majority (77 percent) of these studies found that, as one would predict, nutrient stocks were affected by biodiversity loss. That same year, Boris Worm of Dalhousie University in Halifax, Canada, and colleagues reviewed studies of the importance of biodiversity in aquatic ecosystems, particularly marine systems, and came to the same conclusion — the loss of biodiversity dramatically and adversely impacts marine biomass and marine ecosystem processes.
Biodiversity loss can affect ecosystem functioning for many reasons, but two keep emerging from the research. First, the more species one removes, the greater the probability that an extraordinarily important species will be lost. But there is a second reason that biodiversity loss reduces ecosystem function: complementarity. The more species you have, the more ways they make use of limited resources such as light, water, nutrients and space.
Complementarity effects can be significant. The classic example involves deep- and shallow-rooted plants; if you have only one of these forms, some of the nutrients and water in the soil are never used because no roots reach them. Likewise, an ecosystem with both drought-tolerant and flood-tolerant species will do better over many years of floods and drought than an ecosystem with only one or the other species.
In most studies, sampling and complementarity effects occur simultaneously, and it is very difficult to tease them apart. But the hundreds of ecological reverse engineering studies published in the last 15 years are rich in detail, involving all sorts of species in all sorts of ecosystems. How scientists have interpreted their findings varies enormously, and there is incessant discussion over the best explanation. In spite of these debates, the basic conclusion is that no matter what ecosystem you look at and no matter what organisms you look at, if biodiversity declines, there will be changes in the amount of biomass in an ecosystem and changes in how that biomass influences the geochemistry in the region.
This complex research, with its many interpretations, helped to shape the findings of the U.N.-commissioned Millennium Ecosystem Assessment, a five-year international endeavor to assess the state of the planet; the assessment's findings were published in 2005 and 2006. This assessment has become the standard reference for the state of the biosphere, much the way the Intergovernmental Panel on Climate Change has become the most widely accepted standard reference for climate change. The assessment places biodiversity squarely at the center of all the environmental processes that affect human wellbeing. Whether the environmental problem is the spread of emerging diseases, control of invasive species, food security or climate regulation, and whether we are talking about human health, poverty, education or even freedom itself, almost all aspects of human well-being and prosperity trace back to biodiversity for their foundation.
At the outset, it might have seemed that reverse engineering a thousand-billion-ton, three-and-a-half-billion-year-old biological system made up of some 30 million different parts distributed over 510 million square kilometers is simply impossible. But in 15 years, enough progress has been made that we have come to a revolutionary understanding of the significance of species. Every species influences the chemical composition of our seas, land surfaces and atmosphere. Collectively, the metabolic activities of species are responsible for the low carbon dioxide and high nitrogen and oxygen content of our atmosphere. Without their influence, Earth's surface conditions would likely be somewhere between that of Mars and Venus, and entirely inhospitable to life. The extent of this biological influence over our global environment can be estimated by the mass of life on Earth. But we now know through hundreds of scientific studies on reverse engineered ecosystems that the diversity of species making up this mass is equally important. If we lose biodiversity, ecosystems can change dramatically in the way they function, especially if we lose species that play significant roles in ecosystem function.
Biodiversity loss is the single most prevalent feature of our changing world, but this fact is often missed because it is conflated with the extinction of species or the global extinction of species. Species extinctions are serious, to be sure. The latest and most widely respected estimates of the number of species threatened with extinction are compiled by the International Union for the Conservation of Nature (the IUCN Red Lists) and include some 16,928 species, or more than a third of the species this watchdog nongovernmental organization has examined. The actual number of documented extinctions, however, is fairly small — about 700 since the 1600s. The number is small because most species are small, rare and yet to be cataloged; meanwhile, extinction, or the death of the last individual of a species, is usually a private, quiet event, witnessed by no one.
Conservation International, an NGO that aggressively seeks to save as many species as possible, estimates that one species goes extinct every nine to 44 minutes because of habitat destruction and climate change. The group's extinction clock is based on some rather limited scientific evidence but does draw attention to the issue. Estimating the magnitude of global extinction based on just the few hundred that have been observed over several centuries severely underestimates what is widely recognized as a fact: The Earth is undergoing the sixth major mass extinction of life it has faced in its long history.
Biodiversity loss, however, is less about global extinction than the homogenization of life on the Earth's surface. Imagine a square kilometer of Earth's surface, then count up how many species exist, from the top of the atmosphere down to the rocky surface of the lithosphere. In some places, such as the tops of tall mountains, you would encounter very few species; in others, such as the Amazon, you would encounter hundreds or possibly thousands of species. If you did this repeatedly, you would come up with an average for the total number of species per square kilometer. If we shoved all our wild species — not domesticated ones like wheat, corn, cattle, horses, pigs, chickens and the like — into wildlife refuges, national parks and other protected areas, the total number of species on Earth would not change. If you repeated the species census again, however, you would find that the average number of species per square mile had plummeted. Biotic impoverishment would be widespread, even though not one species was lost to global extinction.
Because a species cannot recover from global extinction (short of the fantasies of cloning extinct species from DNA), it is by far the major focus of conservation organizations. Even in the United States, the Endangered Species Act has enjoyed more than 30 years of support in spite of wide swings in administrative positions on the environment. The California condor, the whooping crane and the panda are widely known examples of tremendous efforts to stop global extinction.
While conservation efforts to prevent global extinction are laudable and should receive greater support than they currently do, they are a diversion from the much larger problem of biodiversity loss. The more we relegate wild species to parks, zoos, gardens and seed banks, and the more we place domestic species in their stead, the more homogenized the world becomes. Even without the loss of a single species, with increasing homogenization biodiversity declines. Consider that only 1 percent of the American prairie remains, most of its hundreds of species of plants, birds, mammals and reptiles and thousands of species of insects replaced by a few species of domesticated plants, such as corn, wheat and soybean, and domesticated animals, such as cattle, pigs and sheep. Even if every species seen by the settlers and American Indians lives in reserves, 99 percent of the land is much poorer in species than it was in the past
When it comes to the functioning of the ecosystems and the biosphere, it hardly matters if millions of species are hanging on by threads in protected areas while virtually the whole of the Earth is biotically homogenized. As the average number of species found in each square of Earth's surface declines, so too will its biomass, its biogeochemistry and its contribution to a stable, life-supporting biosphere.
The Paragon of Species and the Equitable Kingdom
I imagine that any species endowed with our psyche would ponder its own significance. Religion provides a variety of options for us to express that significance, sometimes to good effect (art and charity), sometimes in ways we come to regret (the Spanish Inquisition), but always with a sense that our existence and our actions matter in some profound way. Ideas of our significance derived from science frequently fail to inspire the general public. Physics tells us the universe will one day collapse upon itself or expand into nothingness; chemistry sees us as a collection of atoms; and evolution and ecology appear to marginalize humans by placing us in the context of millions of species that have roamed the Earth. Scientifically speaking, at home or in the universe at large, we seem small, insignificant, a blip among millions of blips on the evolutionary tree of life and aside from our intellect, less well endowed than many of our fellow species.
But we humans are not likely to accept any view that marginalizes us. A line from Hamlet sums up nicely humanity's self-perception:
What a piece of work is man! how noble in reason! how infinite in faculty! in form and moving how express and admirable! in action how like an angel! in apprehension how like a god! the beauty of the world — the paragon of animals!
Whether this perception is accurate hardly matters; it is what we want for ourselves. (Hamlet is not particularly impressed with humans, for he continues, "And yet, to me, what is this quintessence of dust? Man delights not me...") So in light of the knowledge science has conveyed to us, the question is: How do we identify our significance and shape our role so we become the paragon of animals — or the paragon of species — that we want to be?
I believe that the reverse engineering of ecosystems has revolutionized our scientific understanding of the natural world, suggesting not only a better way to understand the significance of species, but our own very human significance as well. Whether one sees the hand of a divine creator in the exquisite design of the biosphere or one simply marvels at its beauty independent of the question of creation, true understanding emerges from knowing the function of each species and how the various species relate to each other in the grand biogeochemical cycles of life that make Earth the wondrous place we see in Earthrise.
The appeal of the ancient idea of a Peaceable Kingdom is rooted in a fear of nature and a world that is "red in tooth and claw." The Peaceable Kingdom is also a world where humans have special status, one where we are not the accident of ecology and evolution. It is a tame world, a domesticated world, one where pet cats replace lions, pet dogs replace wolves, edible plants replace inedible or toxic ones, and where helpful microbes replace pathogens.
There are two ways of achieving such a Peaceable Kingdom without waiting for divine intervention. First, we could extirpate or exile all species that are harmful to ourselves or harmful to those species we value. Because many feel that extirpation or extinction is morally reprehensible, exiling species to zoos, gardens, seed banks and wildlife sanctuaries seems to solve this problem.
The second route is to domesticate every species, if not by breeding then by genetic engineering. We could convert all plants to edible species, all animals to herbivores and all microorganisms to beneficial species. Though it will be a long time before we could possibly breed or engineer lions such that they could live off of straw, our successes in plant and animal breeding and genetic engineering show much promise. If we follow one or both routes, we could completely transform our current world to a tame, domesticated one, a world shaped by neither the joint processes of evolution and ecology nor the hands of a divine creator.
This would not be the Peaceable Kingdom as described by Isaiah, but it would be a Domesticated Kingdom, one that achieves the same desired ends — nature no longer feared and the dominance or special status of humanity unequivocally established.
In fact, we have been following these two paths for thousands of years. Peter Kareiva, lead scientist for the Nature Conservancy, and several colleagues recently argued that the Domesticated Kingdom is already here. Humans have extirpated most predators from the wild, suppressed wildfires, built jetties and seawalls to prevent storm surges, and now store water for hydroelectric power, drinking, irrigation and flood control totaling six times what is contained in the world's rivers. They further argue that only 17 percent of the terrestrial surface is free of human influence, and 50 percent of land has been converted to agriculture and domestic livestock grazing. It is hard to measure human influence over the oceans, but here too it seems we have exerted dominion. Nearly a third of major marine fish stocks are down to 10 percent of their recorded maxima, and more than a quarter of the ocean's primary production is currently consumed by humans.
The Domestic Kingdom may have its appeal, but it lacks sensible integration into the biosphere. If we were to reverse engineer the Domesticated Kingdom the way we did the natural world, we would find a bizarre system. Compared to the original thousand-billion-ton, 30-million-species biosphere, the Domesticated Kingdom has less mass and continues to shrink (large stretches of rainforests have been replaced by agro-ecosystems and major fish stocks have collapsed); its biodiversity is less than half what it used to be (half of terrestrial ecosystems now have the low diversity associated with croplands, farms, plantations and heavily grazed lands); and its web of life, now dominated by domestic species consumed ultimately by a single consumer, has no particular structure. Grasslands once made up of largely inedible plant species have been replaced by agro-ecosystems made up of highly edible plants, and where farming proves intractable, we convert inedible plants by feeding them to livestock we then feed on. Forests — once filled with a variety of species, many of which supplied neither edible nor usable forest products — are similarly replaced by monoculture tree farms that supply us with food, lumber and other desired forest products. Oceans once fished heavily are seeing increasing amounts of aquaculture. The few species we would encounter in the most productive landscapes — rice, corn, wheat, soybean, sugar cane, cattle, pigs, sheep, goats and chickens, and in marine systems, farmed oysters, mussels, seaweed and salmon — are no longer part of an intricate web of millions of interacting species structured by ecological and evolutionary principles.
The Domesticated Kingdom is governed by economics and politics rather than ecology and evolution, and constitutes a relatively new and untested design for a thousand-billion-ton biosphere. Over the last 10,000 years, our world has shifted its function from one of sustaining life on Earth to channeling bio-materials and biofuels to a single species: Homo sapiens. The shift has been overwhelming and fundamental.
Consider: Terrestrial ecosystems remove an estimated 65.5 billion tons of carbon from the atmosphere each year, producing plant material that acts as biofuel for all of the biogeochemistry done by these ecosystems. Humans, however, appropriate about 24 percent of this biofuel, or almost 16 billion tons. Thus, one species out of 30 million appropriates almost of a quarter of plant production on land. The Domesticated Kingdom is strongly regulated by the cost of fertilizer, the availability of farm labor or machinery, the cost of transportation of goods, the state of infrastructure within and among nations, the regulation of trade, tariffs, war, famine and other social, political and economic forces. It is not entirely independent of the ecological and evolutionary processes — climate change is a contemporary example of the remaining dependence — but is predominantly a socially governed, rather than an ecologically or evolutionarily driven world.
Reverse engineering humans would not be best done by examining their teeth, jaws and digestive tract, as with the lion, but by looking at their impact on biogeochemical systems. And if we examine our species' significance as ecologists have done with plant, animal and microbial species over the last 15 years, we would find our functional significance to be enormous. Never in the long history of life has any species achieved the unprecedented, massive biogeochemical influence our species has.
Because the Domesticated Kingdom is designed to channel resources to humans, our kingdom is one in which all other species serve at the pleasure of ours. This is a biotic feudal kingdom in which we harbor and protect those species that serve us.
The biosphere was differently structured. It was an Equitable Realm, where every species functioned as part of a single system. It was not a kingdom because it had no single dominant species. It is endlessly fascinating to consider that this Equitable Realm was structured and governed solely by ecological and evolutionary principles and worked excellently as a thousand-billion-ton biogeochemical machine. There is no evidence or reason to believe, however, that it worked in any well-ordered fashion. It was not a super-organism shaped by evolution; it was not autopoietic; it had no rulers, no controllers, no stewards. It was robust, even to asteroid impacts, but imprecise. It wandered freely within environmental tolerances that sustain life, but it had no homeostatic mechanism that adheres strictly to one set point.
The pre-human Equitable Realm of the biosphere, however, is now the Domesticated Kingdom. The business of biogeochemical regulation of Earth's environment has been left to the vanishing remnants of the original biosphere. This is not sustainable; at some point the environmental functioning of the biosphere has to be either restored or assumed by the dominant species. Given that humanity has transformed half the land surface of the world and consumes a quarter of the Earth's biological production, restoring the original biosphere — returning to the Equitable Realm — is not an option most humans would accept.
But humans can assume responsibility for the regulation of the environment.
In assuming the responsibility for biogeochemical regulation, humans become the most significant species in a new Equitable Kingdom. As in the original biosphere, in the Equitable Kingdom each species fulfills its function in the complex web of life that governs ecosystem and biospheric functioning. But the Equitable Kingdom is different from the Domesticated Kingdom; it is structured and governed not just by human social processes such as politics and economics, but also by ecological and evolutionary requirements. Rather than functioning solely to feed and fuel humanity, the social, political and economic processes of the Equitable Kingdom are designed to ensure the proper biogeochemical functioning of the biosphere.
The Equitable Kingdom is admittedly a hard sell. It is not utopian in its vision, like the Peaceable or Domesticated kingdoms, but it is the one kingdom that ensures the long-term sustainability of our environment.
In the Equitable Kingdom, a child will not be safe in the immediate presence of a lion, leopard or wolf, but in the distance these predators will regulate the herbivores that regulate the vegetation that regulates the composition of our atmosphere that determines the state of our environment. In the Equitable Kingdom, if a nation has a forest that sequesters and stores carbon during a time when global warming threatens life on Earth, the international community recognizes this nation as rich and powerful. In the Equitable Kingdom, Brazil would not seek to emulate the American Midwest but would be rewarded for serving as the world's carbon bank. Similarly, in an Equitable Kingdom, the American Midwest would receive financial rewards for preserving and restoring the natural biogeochemical capacity of its landscape so it approximates that of its original prairies.
A New Environmentalism
For decades now, environmentalists have propounded a litany of humanity's crimes against nature, but the list has always been misguided, suggesting that the march toward a Domesticated Kingdom was wrong, but seldom offering an alternative that was realistic. The science of the last 15 years calls for a new environmentalism driven along a new path by the modern fields of sustainability science, environmental engineering, global change biology, restoration ecology, conservation biology, conservation medicine, biodiversity and ecosystem functioning, ecoinformatics and remote sensing. We have already started on this path with international treaties such as the Convention on Biological Diversity and the U.N. Convention on Climate Change and its Kyoto Accords, activities like the Millennium Ecosystem Assessment and new programs in sustainable development around the globe. Carbon trading, environmental certification of products such as wood, seafood and coffee, and new funding in American environmental science — including the U.S. commitment of more than $200 million for a National Earth Observatory Network that will gather real-time data on biodiversity and ecosystem processes and provide it to scientists, industry, businesses and citizens — are a few examples of first steps toward an Equitable Kingdom.
But the Equitable Kingdom is not just a new form of environmental activism; it is a new way of ordering the world, one that reveals the extraordinary significance of our species — a significance we always believed we had but couldn't envision clearly. We saw ourselves as the paragon species in Eden and the Peaceable Kingdom as a matter of divine ordination, but such beliefs are not scientifically tenable and swayed only those who subscribed to the faiths that promoted them. To become the paragon of species in an Equitable Kingdom — a kingdom in which biodiversity serves as the foundation for environmental sustainability — is not only an achievable goal but a critical one if humans are to reform and in many ways dismantle a Domesticated Kingdom that has no inherent ability to ensure environmental equanimity. The biosphere lacked any central organizing force, leadership or stewards. By assuming the responsibilities of the paragon position it has always yearned for as a species, humanity can become the steward the Earth has never had.
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