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CHAPTER ONE
This Web of Life
Overleaf
Ecologists are starting to probe how plants and animals help to generate vital ecological services such as productivity and nutrient cycling.
Our oldest faiths and deepest symbols reflect a primal connection to the natural world, to a living planet that long ago imprinted on the human consciousness a cyclic sense of death and decay, rebirth and renewal. We do not question that flesh and bone and leaf litter will decay to dust, that seeds will sprout season after season and find renewed nourishment in the soil, that rivers can flow endlessly without running dry, that we can breathe for a lifetime without depleting the air of oxygen. Despite our fascination with other worlds and our hopeful probing of outer space, we’ve found no other planet where any of these things are true. What humans have not fully appreciated until recently is that these services are the work of nature, performed by the rich diversity of microbes, plants, and animals on the earth.
It is this lavish array of organisms that we call “biodiversity,” an intricately linked web of living things whose activities work in concert to make the earth a uniquely habitable planet. But today, as never before, the species in this web are under siege, threatened by human activities that encroach on their habitats. At the same time, ecologists are increasingly aware that the impoverishment of species—the planet’s work force—threatens to erode the basic life-support services that render the earth hospitable for humanity. Indeed, we are approaching a crossroads in time, when the survival and extinction of other species may well delimit the future of Homo sapiens.
Consider what life has done and continues to do for the earth.
Some 4 billion years ago, the primordial atmosphere was a ghastly brew, devoid of oxygen and unable to shield the earth’s surface from the scorching, molecule-cleaving ultraviolet radiation of the young sun. Eventually life changed all that. Over billions of years, photosynthetic organisms in the sea released enough oxygen to create a protective ozone shield and a reservoir of free oxygen that allowed the first plants to venture onto the land. Through the alchemy of enzymes and solar energy, green plants from plankton to redwoods still carry on photosynthesis, turning water and carbon dioxide into free oxygen and also the carbon-based sugars needed to build all living tissues. These are the raw materials that underpin the earth’s food webs and generate the food, fiber, timber, and fuel that sustain human societies.
Together, plants, animals, and microbes perform an array of vital services. They generate and preserve fertile soils. They break down organic wastes, from leaf litter to feces and flesh, recycling the mineral nutrients, carbon, and nitrogen needed for new plant growth. They absorb and break down pollutants; help maintain a benign mix of gases in the atmosphere; regulate the amount of solar energy the earth absorbs; moderate regional weather and rainfall; modulate the water cycle, minimizing floods and drought and purifying waters; blunt the impact of the seas that batter the land margins; pollinate crops; and control the vast majority of potential crop pests and carriers of human disease.
In addition, this rich abundance of organisms serves as a “genetic library,” a catalog of solutions to the problems of living on the earth. This catalog is written in the language of DNA, and from it human societies have derived crops, livestock, medicines, and many other commodities.1
On a larger scale, the earth’s various species form populations that are aligned into communities and ecological systems—ecosystems, for short—which deliver such subsidies as clean air, pure water, and lush landscapes. Ecosystems, flexibly defined, are living communities interacting with the physical environment in a specific geographical place. An ecosystem may be as small as a rotting log or a pond or as large as a spruce forest or a vast lake. Of course, species are the critical components, the cogs and wheels of functioning ecosystems. Lose too many species from a forest—the trees, the truffle-forming fungi on their roots, the insects that prey on tree-destroying pests, the beavers that create ponds and meadows amid the woods—and at some point the assemblage ceases to work like a forest.
One question increasingly on the minds of ecologists is, how many species can the earth’s communities lose before the ecological systems that nurture life begin to falter? To take the extreme, if nature had to run with a skeleton crew, what organisms would be absolutely vital to maintain the earth as a living planet? Probably the only truly indispensable groups of organisms are the plants that capture carbon and solar energy and the ranks of decomposers that release the nutrients and energy in dead plant litter for reuse.2 But a conservation agenda based on this extreme would ignore the elaborate tangle of loops and flourishes in the food webs, the intricate array of consumers that eat plants and predators that eat consumers, the symbionts, parasites, and other hangers-on who have claimed places for themselves in the earth’s myriad communities. Such an agenda would overlook virtually all the charismatic creatures on today’s conservation hot lists: pandas, wolves, elephants, bald eagles. Unfortunately, that agenda would also sacrifice civilization, which is supported by many of those loops and flourishes in the web of life.
If we are realistic about our dreams for tomorrow, our goal is not really “saving the planet” in some minimalist form, but perpetuating its atmosphere, climate, landscapes, and living services in a state that allows human civilizations to prosper. For that to occur, we need to preserve natural systems that are rich, healthy, and resilient enough to continue to support human welfare and economic activity for the next decade, the next century, and beyond. Some twenty-five years ago, as the space-age metaphor of “Spaceship Earth” took hold, ecologists Eugene P. Odum of the University of Georgia and his brother, Howard T. Odum, of the University of Florida first used the engineering term “life-support systems” to describe the earth’s self-renewing, life-giving natural ecosystems. 3,4 It is these systems, not a mere skeleton crew, that human societies must seek to maintain.
Thus, the real questions facing ecologists today center on how much biological diversity any particular ecosystem needs to remain functional, self-sustaining, and life supporting. How many species must humanity protect, and which ones and where, to assure pure water supplies from an alpine watershed; to preserve the fertility of tropical soils; to prevent cactus and shrubs from taking over productive grasslands; to maintain local rainfall patterns; to nurture coastal shrimp and fish populations; or to assure the integrity of pristine wildlands we value for recreation, tourism, or cultural traditions?
Conservationists have long asserted that every species counts to some degree in keeping the earth’s life-support systems working. Some play crucial roles day to day; others step into the breach only in times of stress or disturbance. Until recently, however, researchers have made little concerted effort to define the functional roles played by specific plants, animals, or microbes. In truth, function itself is a human concept. Organisms were not designed by natural selection to fill slots on an assembly line; each organism strives to make a living and reproduce itself. But as it eats, grows, excretes waste, and moves about, disturbing the physical environment, it unwittingly plays a part in generating grander processes that alter the flow of water, the recycling of energy and materials, the renewal of the atmosphere.
An Emerging Science
It may seem surprising that scientists know so little about species’ roles in generating ecological services. Yet unraveling links between the feeding behavior of an animal or the nutrient-cycling traits of a plant and subsequent changes in the character of a landscape or the chemistry of the soil may require years of observations, experiments, and analysis. Sometimes this work spans professional lifetimes and thus extends well beyond the practical scope of graduate student projects and the longevity of most research grants.
The questions also cross the lines of professional disciplines. Until recently, there were specialists who studied ecosystems and others who studied species and populations, and there was minimal discourse between them. Ecologists concerned with ecosystem processes such as nutrient cycles and energy flows, for instance, have traditionally focused on quantifying the work that gets done in a given system, not who does it. These ecologists might chart the flow of energy through a food web, filling in numbers atop arrows that run from plant producers to herbivore consumers, predators, and, finally, decomposers. Few have attempted to fill in the names and numbers of the organisms doing the work at each level, much less to determine how many of the species could be lost before the cycles faltered and the arrows had to be erased.
Population biologists, for their part, ask what determines the abundance of plants and animals, why certain creatures live where they do, how they interact with their neighbors, and why one species is part of the community while another is not. They also look at food webs and study herbivory and predation, but traditionally they have not translated into quantified flows of carbon the dynamics, say, of moose browsing on birches or wolves feasting on elk.
Scientists now recognize, however, that the two research traditions must come together if they are to understand how biological losses impair ecological services. As part of a broader movement to integrate these traditions, the Scientific Committee on Problems of the Environment (SCOPE) launched a project in 1991 that would lay the foundation for an emerging scientific discipline linking ecosystem processes with biological diversity.
SCOPE is a committee of the International Council of Scientific Unions (ICSU), a nongovernmental body established in 1931 to promote international science and its application for the benefit of humanity. SCOPE was created by ICSU in 1969 to identify gaps in environmental knowledge, assess research needs, and provide syntheses of existing scientific information on emerging environmental issues. Its projects are funded by a wide variety of sources, from United Nations agencies, the World Bank, and the European Communities to various governmental agencies and private foundations. The projects are carried out by scientists who donate their time and expertise.
During the three-year SCOPE program on ecosystem functioning, hundreds of scientists from around the world came together at a series of meetings and workshops to synthesize what is known so far about the functional role of biodiversity at all levels. Biodiversity is most often talked about in terms of numbers of species, yet the SCOPE teams expanded that perspective to include the complexity, richness, and abundance of nature at all levels, from the genes carried by each local population of a species to the layout of communities and ecosystems across the landscape.
Although much of the information came from the literature of ecology, the SCOPE team also took a much broader view, drawing on relevant studies from a wide array of other sciences: forestry, soil science, agriculture, plant pathology, range management, fisheries, microbiology, limnology, oceanography, marine biology, hydrology, meteorology, and atmospheric sciences. Results of the team’s synthesis have been published in a number of technical volumes, many of them dealing with specific biotic regions of the world such as savannas, tropical forests, islands, and arctic and alpine ecosystems.5
The Work of Nature relies heavily on the ideas and examples assembled during the SCOPE project. It is intended to introduce students, policymakers, and concerned citizens to the risks our own species incurs when we impoverish the rest of the life of the planet. This book is not meant to be an exhaustive account of species’ roles in providing life-support services but rather a suggestive and cautionary tale from the frontiers of a new science. The SCOPE assessment showed that most of the information scientists have accumulated so far about ecosystem functioning and biodiversity deals with species rather than diversity at other levels, from genes to landscapes, and so this book focuses predominantly on the work of species.
The SCOPE assessment makes it clear that species are not ecologically equal; some are more valuable in terms of service rendered than others. Yet so little is known about the prodigious array of organisms on the earth, and so few names have been entered into the flow charts of energy and materials, that it would be both premature and irresponsible to declare any species, however humble, expendable.
How should scientists begin, then, to identify those species and natural communities whose loss will cause the greatest impacts on ecological processes? One approach the SCOPE teams took was to look for clues indicating which species play indispensable roles—either as dominant species or as so-called keystones, which are less imposing creatures from a human point of view, but ones that hold the key to the integrity of their communities. The Work of Nature explores the progress ecologists have made so far in learning how to spot such creatures, and also in determining whether most communities have alternates—backup workers—that can step in and compensate for the loss of a dominant or keystone species.
Another approach taken by the SCOPE researchers was to examine ecosystems directly to determine how the traits or activities of individual species might influence ecological processes. The Work of Nature highlights examples where shifts in species have affected
- the persistence and stability of natural communities;
- water quality and flow and the health of aquatic habitats;
- soil fertility and its relation to healthy crops and forests;
- the productivity or lushness of both our wild lands and agricultural lands;
- the look and functioning of the landscape and the frequency of disturbances, such as fires; and
- local rainfall patterns and weather, as well as the state of...
