The Intricate Web of Life
“The variety of life is our insurance policy. Our own lives and livelihood depend on it.”—UNITED NATIONS ENVIRONMENT PROGRAMME.
LIFE on earth is abundant and immensely diverse. The term “biological diversity,” or “biodiversity” for short, designates all the world’s species, ranging from the smallest bacteria to the giant sequoias; from earthworms to eagles.
All this life on earth is part of one great, interdependent web that also includes nonliving elements. Life depends on nonliving components such as earth’s atmosphere, oceans, fresh water, rocks, and soils. This community of life is called the biosphere, and humans are an integral part of it.
Biodiversity embraces all the bacteria and other microbes. Many of these are known to perform vital chemical functions that keep ecosystems operating. Biodiversity, or the web of life, also includes the green plants that produce oxygen through photosynthesis, trapping solar energy and storing it in the form of sugars, which are the base of energy resources for most other forms of life.
Sadly, despite the beauty and variety of life-forms, a number of researchers say that man is pushing species to extinction at an alarming rate. In what ways?
▪ Habitat destruction. This ranks as the leading cause of extinction. It includes logging, mining, clearing trees for cattle, and building dams and highways where wilderness once existed. As ecosystems shrink, species lose the resources they need to survive. Natural environments are fragmented, degraded, and eliminated. Migration routes are disrupted. Genetic diversity diminishes. Local populations of living things cannot rebound from disease and other stresses. Hence, one by one, species gradually die out.
The extinction of certain species can even trigger a chain reaction of extinctions, for when one part of the web of life is eliminated, others can be affected. Extinction of keystone species—such as pollinators—can affect a myriad of other species.
▪ Introduced species. When humans introduce a foreign species into an ecosystem, that species may take over niches that other species have occupied. The foreign species might also indirectly change the ecosystem enough to force out native species, or it might bring with it diseases to which the natives have no immunity. Especially on islands, where species have long existed in isolation and have not dealt with newcomers, the original inhabitants may be unable to adapt and survive.
A typical example is a “killer” alga, Caulerpa taxifolia, which is destroying other marine species in the Mediterranean Sea. Introduced accidentally off the coast of Monaco, it has now begun to spread on the seabed. It is toxic, and it has no known predators. “We could be seeing the beginning of an ecological catastrophe,” says Alexandre Meinesz, professor of marine biology at the University of Nice, France.
▪ Overexploitation. This has led several species to extinction. A classic case is that of the passenger pigeon. In the early 19th century, it was the most abundant bird in North America. When it migrated—in flocks of a billion or more—it darkened the skies for days at a time. However, by the end of the 19th century, it had been hunted to the brink of extinction, and in September of 1914, in a Cincinnati zoo, the last remaining passenger pigeon died. Similarly, the American bison, or buffalo, of the Great Plains was nearly hunted out of existence.
▪ Human population growth. The human family in the mid-19th century had a population of one billion. One and a half centuries and five billion people later, humans are beginning to wonder whether they are in danger of exceeding the limits of their resources. Each year, as the human population continues to grow, species are becoming extinct at an alarming rate.
▪ Threat of global warming. According to the Intergovernmental Panel on Climate Change, it is possible that temperatures could rise by as much as 6.3 degrees [3.5°C] Fahrenheit within this century. This may be too fast a jump for some species to survive. According to researchers, it appears that a contributing cause of the death of coral reefs (anchors of much of marine biodiversity) is warming water.
Scientists say that a three-foot [1 m] rise in sea levels could eliminate a large portion of the world’s coastal wetlands, the home of abundant biodiversity. It is believed by some that global warming may be affecting the Greenland and Antarctic ice sheets. If these were to melt, environmental catastrophe could follow.
An Epidemic of Extinctions
How fast are species being lost? Answers to that question are very inexact. Most of what is being lost is still a mystery to scientists. First they have to establish how many species exist. According to John Harte, ecological scientist at the University of California, Berkeley, “there are about one and a half million named species on earth, but we know that many unnamed species exist, and the total number is probably between 5 and 15 million.” Some raise the estimate to 50 million species or more. Determining the exact number is all but impossible because “most extinctions will occur before the species have even been named and described,” according to scientist Anthony C. Janetos.
Modern science has barely begun to decipher the intricate ecological mechanisms that keep natural communities running smoothly. If humans do not know how many species there are, how can they understand the complex web of life and how it is affected by extinctions? How can they tell what the species’ disappearance might mean for the planet’s life-support system?
When scientists try to determine the rate of extinction, their estimates, although varied, are often disheartening. “Some 50 percent of the world’s flora and fauna could be on a path to extinction within a hundred years,” states one writer. Harte’s prognosis is even more grim: “Biologists estimate that tropical deforestation will result in the loss of half or more of the existing species on earth during the next 75 years.”
Based on the calculations of scientist Stuart Pimm of the University of Tennessee, National Geographic states that “11 percent of birds, or 1,100 species out of the world’s nearly 10,000, are on the edge of extinction; it’s doubtful that the majority of these 1,100 will live much beyond the end of the [21st century].” The same magazine stated: “A team of respected botanists recently reported that one in eight plants is at risk of becoming extinct. ‘It’s not just species on islands or in rain forests or just birds or big charismatic mammals,’ says Pimm. ‘It’s everything and it’s everywhere. . . . It is a worldwide epidemic of extinctions.’”
Do We Need All These Species?
Is there reason to be concerned about the loss of life’s diversity? Do we really need such a variety of species? Many respected experts insist that the answer is yes. Earth’s endowment of species provides humans with food, useful chemicals, and many other products and services. Think, too, about the potential benefits that undiscovered species may hold for mankind. For example, it has been estimated that 120 of the top 150 prescription drugs used in the United States come from natural compounds. Thus, in losing the flora of the world, mankind also loses the opportunity to find new drugs and chemicals. “Every time we lose a species, we lose an option for the future,” says Sir Ghillean Prance, director of Kew Gardens in London. “We lose a potential cure for AIDS or a virus-resistant crop. So we must somehow stop losing species, not just for the sake of our planet but for our own . . . needs and uses.”
We also need natural ecosystems to provide essential services on which all living things depend. The production of oxygen, the purification of water, the filtering of pollutants, and the prevention of soil erosion are all vital functions performed by healthy ecosystems.
Insects provide pollination services. Frogs, fish, and birds control pests; mussels and other aquatic organisms cleanse our water supplies; plants and microorganisms create our soils. The economic value of all these services is immense. A conservative estimate of the monetary benefits of biodiversity worldwide is around 3,000 billion dollars per year, at 1995 prices.
Despite our dependence on the diversity of life, however, the world seems to be in the midst of an extinction crisis that has threatened the intricate web of life. Now at a time when we are beginning to understand the vital role of biodiversity, humans are causing more extinctions than ever before! Is man, though, in a position to solve the problem? What does the future hold for the variety of life on earth?
[Box/Picture on page 6]
What Is Life Worth?
All the discussion about the value of biodiversity may seem to indicate that we should care about other life-forms only as long as they serve our needs. Some feel that such thinking is narrow-minded. Paleontologist Niles Eldredge points to the inherent value life has: “We humans also value life around us—beautiful, eye-catching species, gorgeous intact wild places—for its intrinsic worth. Something within us recognizes that we are connected to this natural world and that we gain peace and pleasure from being in it whenever we can.”
[Box/Pictures on page 7]
The Red List
The “Red List” is published by the International Union for Conservation of Nature and Natural Resources, an organization that evaluates the condition of endangered species. A few of the threatened species included in the “Red List” for the year 2000 are featured on this page:
Wandering albatross (Diomedea exulans)
This is one of 16 albatross species identified as globally threatened. It is said that significant numbers drown after being accidentally caught on baited hooks set by longline fishing boats.
Photo by Tony Palliser
Red-shanked douc langur (Pygathrix nemaeus)
This handsome Asian colobine monkey is found in south-central Vietnam and parts of Laos. It is threatened by habitat destruction and hunting. It is hunted for both food and body parts, which are used as ingredients for traditional medicines.
Monkey on pages 7 and 32: Photo by Bill Konstant
Corsican snail (Helix ceratina)
The habitat of this critically endangered snail is only 20 acres [7 ha] in the suburbs of Ajaccio, on the southwest coast of Corsica. Its survival may be in jeopardy because of development that includes the construction of an airport and of access roads to the beach.
Photo by G. Falkner
Golden pagoda (Mimetes chrysanthus)
This beautiful flower was discovered in 1987 in Western Cape, South Africa. Frequent wildfires and invasive species that are alien to the golden pagoda’s habitat pose a constant threat to this plant.
Photo by Craig Hilton-Taylor
Freshwater sawfish (Pristis microdon)
This endangered fish is found in the Indian and West Pacific oceans, as well as in adjoining bays, estuaries, and rivers. It is extremely vulnerable to fishing and has experienced serious decline. It is also threatened by habitat loss and degradation.
Photo courtesy of Sun International Resorts, Inc.
[Box/Picture on page 8]
Decimating Sea Life
The wealth of the oceans, once deemed inexhaustible, has proved finite. Paleontologist Niles Eldredge, writing in Natural History magazine, described the extent of the overexploitation of the oceans: “Modern technology has made marine fishing so efficient that vast tracts of ocean bottom are being denuded in the marine equivalent of forest clear-cutting. This very same technology, however, is hideously wasteful; marine turtles and seals, along with many unmarketable species of fish and invertebrates, die with every pull of the net or passage of the trawler.”
Commenting on what it called “the wasteful wake of shrimping,” National Geographic magazine explained that “along the Gulf Coast [off Texas, U.S.A.] a dozen pounds of sea life—much of it juvenile fish—may be sacrificed for a single pound of shrimp.” These unwanted fish and shellfish are called bycatch. A federal biologist lamented: “The average bycatch ratio is about four to one.” No wonder our oceans have become the killing fields of many an endangered species!
[Box/Picture on page 9]
Life Hiding in the Forests
The forests of our planet are teeming with life, including species that have yet to be discovered by man. Ecological scientist John Harte observed: “Tropical rainforests cover less than two percent of the planet and yet are the only home of at least 50 percent and possibly as many as 90 percent of all species on earth. The higher estimate is based on the assumption that a large share of the to-be-discovered species will be tropical because biological exploration of the tropics is so fragmentary. Other habitats are also poorly explored, though, and undoubtedly contain numerous species unknown to science today. Among these are the soils of temperate forests, such as the moist old-growth forests in the Pacific Northwest of the United States.”
Who can tell what surprises await man if he ever gets a chance to explore the life hidden in the forests?
[Picture on page 5]
The passenger pigeon, now extinct
U.S. Fish & Wildlife Service, Washington, D.C./Luther C. Goldman