Biodiversity

Introduction

Biodiversity is the number of distinct varieties or types within a group of living systems: distinct genes in a species, species in an ecosystem, or ecosystems in a biome. The term is often used to mean the total number of species living in a given ecosystem or on Earth as a whole. Biodiversity is created by speciation, that is, the evolutionary appearance of new species; it is diminished by extinction. Only evolution can create new species, and this occurs only over geologic time (usually millions of years). On human or historical timescales, extinction decreases biodiversity irreversibly.

Human beings have impacted biodiversity for thousands of years through activities such as hunting, pollution, and habitat destruction (swamp-draining, deforestation, plowing of prairies, and the like). Nor is human-caused extinction entirely a recent affair. There is evidence that a number of animal species in North America, Australia, and New Zealand became extinct tens of thousands of years ago after the arrival of the first human settlers. In the last century or so, however, extinction due to habitat destruction for expanding human populations and industrial activity has increased manyfold. Moreover, a new human-caused foe of biodiversity has begun to manifest, namely global climate change.

Climate change threatens biodiversity mostly by shifting the boundaries of ecosystems, changing the timing of seasonal events such as hatching and budding, and altering the temperature and chemical characteristics of lakes, rivers, and oceans. Atmospheric carbon dioxide (CO₂), which is the main cause of global warming, also has direct affects on ecosystems, such as acidifying the oceans and encouraging the growth of some plants more than others. Such stresses inevitably cause extinctions, that is, loss of biodiversity. The rate of extinctions caused by climate change is predicted to be greater later in the twenty-first century than today.

Historical Background and Scientific Foundation

Context

The modern systematic classification of species was invented by Swedish naturalist Carl Linnaeus (1707–1788) in the eighteenth century. In following decades, European naturalists scoured Earth looking for and classifying new species of plant and animal, greatly expanding scientific knowledge of just how diverse life on Earth is. Such knowledge was crucial to the development of evolutionary biology by Charles Darwin (1809–1882) and others in the mid-nineteenth century. Today, biologists estimate that there are more than 280,000 species of plants and over 1,250,000 species of animals (including insects). Nineteenth- and twentieth-century paleontology has shown that extinction is a normal process: over 99% of all species that have ever lived are now extinct. The typical lifespan of a species is between 1 and 10 million years.

Awareness that species biodiversity is a key feature of ecosystems was not common until the 1970s, however, when the term “biological diversity” first came into frequent use. The word “biodiversity,” a shortening of “biological diversity,” first appeared in print in 1988. In the 1990s and beyond, most biologists have agreed that human beings are causing the first mass extinction in 65 million years. This modern pulse of extinctions is sometimes called the Holocene extinction event. (The Holocene is the geological term for the era from 11,500 years ago to the present.)

Humans are causing extinctions through land development, destruction of rain forests, over-hunting and over-fishing, pollution, and other activities. Some of the more famous extinctions of recent times include the dodo bird, the passenger pigeon, and (announced in 2007) the baiji, a white river dolphin of China. Hunting has caused about 23% of known animal extinctions since 1600; the introduction of invasive species, 39%; habitat destruction, 36%. These numbers are approximate because many of the species that are being destroyed are uncatalogued insects and plants living in rain forests. Estimates of how many extinctions have already occurred due to human activities range from several tens of thousands to over a million.

Although impacts by large asteroids have caused or at least contributed to some ancient mass extinctions, climate change has been the most common cause of natural mass extinction throughout Earth’s history. Ancient climate changes have been brought about by shifts in Earth’s orbit, continental drift, volcanism, and other processes. Today’s episode of climate change is unique in being caused by a single species, humans. Moreover, today’s ongoing mass extinction is unique in that human beings, by taking action to mitigate (reduce the severity of) climate change and to reduce the other impacts of human activity on biodiversity, can influence the overall severity of the event.

Case Study: How Climate Change Affects Biodiversity

As warming continues, other forms of human pressure on biodiversity will continue and will be, in most cases, amplified by the effects of climate change. Although effects may vary from region to region, the overall effect of global warming is to cause the cooler zones of the world—the regions around the poles (especially the North Pole) and on mountains—to shrink. Shrinkage of habitat puts species at risk because smaller habitats support smaller populations, and smaller populations are always at higher risk of extinction.

Climate change also has many other effects on ecosystems. Some are not immediately obvious. For example, Lake Tanganyika in Africa, the world’s second-largest and second-deepest lake, harbors at least 350 species of fish, most unique to its own waters. Lake Tanganyika is not typical of ecosystems most vulnerable to climate change. Being near the equator it is likely to see less drastic warming than, say, the Arctic, while its large thermal mass (4,526 cubic mi of water; 18,900 cubic km) will resist temperature shifts and thus might be expected to moderate climate-change impacts on the lake ecosystem.

However, regional climate warming by 1.08°F (0.6°C), along with lessened wind speeds, has had rapid effects on Tanganyika’s ecosystem. Warming of surface waters more than deep waters has decreased mixing between the two: Warm water is less dense than cool water, so the bigger the temperature difference between

WORDS TO KNOW

BIOME: A well-defined terrestrial environment (e.g., desert, tundra, or tropical forest) and the complex of living organisms found in that region.

DEFORESTATION: A reduction in the area of a forest resulting from human activity.

GEOLOGIC TIME: The period of time extending from the formation of the earth to the present.

PALEONTOLOGY: The study of life in past geologic time.

pH: The measure of the amount of dissolved hydrogen ions in solution.

PLANKTON: Floating animal and plant life.

the layers, the more stably the warm upper water floats on top of the cool deeper water. Since the deeper waters are more nutrient-rich, reduced mixing has meant that fewer planktonic organisms (tiny, floating organisms, both plants and animals) can thrive in the upper water, where energy from the sun is abundant but nutrients are poor. As of 2003, plankton density in Lake Tanganyika had declined to less than one third of what it was 25 years before; algae density had declined by 30% from values 80 years before. Since plankton are the basis of the marine food chain, fish stocks declined along with the plankton: Fish stocks in the lake were 30% smaller than they were 80 years earlier.

The reduction of water mixing due to climate warming has made other changes in the lake’s chemistry. For one, oxygen dissolved from the air no longer mixes as well in deeper waters. As a result, the habitat has shrunk for some of the lake’s endemic species, such as the snail Tiphobia horei, which in 1890 lived at depths down to 1,000 ft (300 m) but as of 2003 lived only down to 330 ft (100 m). The snail’s habitat has thus shrunk by about two thirds, even though the lake itself has not shrunk and its bulk average temperature has changed only slightly.

Tanganyika surface plankton loss has been reflected in declining food-fish harvests from the lake (about 400,000 tons per year in 2003). Since Tanganyika supplies 25-40% of the protein needs of the four nations bounding the lake, such declines can have direct impacts on human populations as well as on biodiversity. Losses in biodiversity have not yet been measured directly in Tanganyika, but smaller populations will put some species at risk of extinction, especially as warming in the region continues to about 2.7°F (1.5°C), possibly higher, with even more drastic stabilization of the lake’s waters and consequent effects on its ecosystem.

IN CONTEXT: WILDLIFE POPULATION DECLINE AND BIODIVERSITY LOSS, 1970–2005

In May 2008, the Zoological Society of London and World Wildlife Fund (WWF) released the results of a study indicating a dramatic decline in global wildlife populations during the last four decades. The data showed that reductions in populations of land-based species fell by 25%, marine by 28%, and freshwater species by 29% between 1970 and 2005. Biodiversity loss, in the form of extinction of about 1% of the world’s species each year, was also reported in what the foundation asserted was one of the “great extinction episodes” in Earth’s history.

The losses were attributed to pollution, climate change, urbanization, over-fishing, and hunting. The foundation and WWF based their conclusion on population estimates and other data related to 1,400 species tracked as part of its Living Planet index. Data were obtained from published sources in scientific journals.

Slight changes in climate can lead to pressures on biodiversity by other mechanisms. For example, a 2006 study by J. Alan Pounds and colleagues found that global warming has almost certainly caused the recent extinction of about 67% of the 110 or so species of the Monteverde harlequin tree frog of the mountains of Costa Rica. The scientists saw the extinctions as validating the climate-linked epidemic hypothesis, according to which shifts in temperature, rainfall, and other climate variables make populations more vulnerable to disease and therefore to extinction. In the case of the Monteverde frogs, more frequent warm years shifted conditions toward the growth optimum of the Bactrachochytrium fungus, which infects the frogs. The researchers found that extinctions of the frogs consistently followed temperature peaks that were favorable to growth of the fungal disease.

Other effects are not strictly changes in climate, in the sense of temperature or precipitation, but chemical changes to air and water. Increased carbon dioxide (CO₂) in the atmosphere has two major effects that are likely to decrease biodiversity: 1) Heightened atmospheric CO₂ causes increased levels of dissolved CO₂ in the ocean. When CO₂ dissolves in water, it produces a weak acid, carbonic acid. Rising atmospheric CO₂ thus acidifies the oceans. 2) Green plants extract carbon from the air by breaking up CO₂, constructing their tissues using the carbon, and releasing the oxygen. CO₂ is plant food. Thus, increasing atmospheric CO₂ tends to cause more rapid growth in most plant species, an effect called CO₂ enrichment.

Acidification of the oceans by dissolved excess CO₂ will impact biodiversity by making survival more difficult for organisms that form shells of calcium carbonate. This includes bivalves such as clams, mollusks such as periwinkles and conches, microscopic plankton species, and corals. Corals, which are also subject to bleaching in excessively warm waters, form large, shallow communitities in tropical waters that have been compared to rain forests because of their high level of biodiversity. A typical large reef may support on the order of a million species of plants and animals.

Over the last two centuries, the average pH of the oceans has fallen by 0.1, corresponding to a rise in acidity and a 30% reduction in the number of carbonate ions (CO₃^2-) available to shell-making organisms as building material. When carbonate ions fall below a certain level, corals have difficulty making their skeletons. This threshold may be reached if the atmospheric CO₂ level, today about 375 parts per million, rises to over 500 parts per million, as may occur by the end of the twenty-first century.

Increasing atmospheric CO₂ will also affect plant growth. Farmers today often add CO₂ to the air inside greenhouses, because under indoor conditions, extra CO₂ speeds plant growth and increases crop yields. Under outdoor conditions, however, the gain in yield is about half as much and the foods produced are significantly lower in protein and minerals. In the wild, rising CO₂will favor some species over others, depending on rooting depth, woodiness, and photosynthetic chemistry; this will impact biodiversity by altering competitive balances. The CO₂ fertilization effect will be strongest in biomes where plant growth is limited by water availability, such as grasslands, savanna, and desert. The biodiversity impact of a 2.5-fold increase in CO₂ would likely be only about a third as great on a boreal (northern pine) forest as on a savanna or grassland, and half as great as on a desert.

Impacts and Issues

Scientific studies estimate by that 2050 there will about 9 billion human beings on Earth. Industrial activity, pollution, hunting, settlement, introducing species to environments where they have not hitherto existed, and other human activities that decrease biodiversity will grow at least in proportion to population, maybe more. Biodiversity is already decreasing and will continue to do so, though the amount of decrease may be affected by human choices about behavior.

Although the future is always inherently uncertain to some extent, scientists have estimated the likely range of extinctions that will be seen by 2050. According to studies published in 2003, about 3.5% of the world’s bird species will be extinct by that time, about 350species; mammals and freshwater fish are likely to experience a somewhat higher rate of extinction. Destruction of tropical forest for wood products or for conversion to agricultural land will be one of the primary causes of extinction. Continued overfishing of the world’s fresh and ocean waters will cause top predator species to become rare or extinct.

The five main drivers of biodiversity change between now and 2100, ranked from most severe impact to least severe, are, apart from climate change, 1) land-use changes (including deforestation), 2) climate change, 3) nitrogen deposition (from fertilizer use), 4) biotic exchange (the introduction of invasive species), and 5) direct effects of increasing atmospheric CO₂.

During this century and beyond, however, climate change is likely to play an increasing and perhaps eventually dominant role in reducing biodiversity. Most plants require a specific range of temperature, moisture, and seasonal change to thrive; most animals require certain plants or other animals to thrive, and also have a limited range of tolerance for temperature and moisture. As climate warms, a typical ecosystem will tend to migrate away from areas where it was at the warm edge of its tolerance range and toward places where it was formerly at the cool edge. The most general effect of global climate is thus to move ecological zones toward the poles and toward higher altitudes. For each 1.8°F (1°C) of warming, terrestrial (on-land) ecosystems typically shift pole-ward by 100 mi (160 km): For example, if climate warms

by 5.4°F (3°C) by 2100, plant and animal communities in the Northern Hemisphere will migrate an average of 300 mi (480 km) northward—if they can—to stay in a suitable climate zone. This effect is observed, not only predicted.

In the Northern Hemisphere, terrestrial animal and plant ranges have been observed to shift northward, on average, by 3.8 mi (6.1 km) per decade over the last 50 years. In mountainous terrain, plant and animal ranges have shifted upward by 20 ft (6.1 m) over the same time period. Fragmentation of landscapes by human activity such as agriculture and city-building makes ecosystem migration more difficult today than during past climatic shifts, such as glacial periods. Species that fail to colonize new areas as the climate changes may go extinct.

Although climate change has so far been most intense in the Arctic and the West Antarctic Peninsula, where warming has been about twice the global average and dramatic effects such as retreating sea ice and melting tundra are readily visible, biodiversity is low in these regions compared to the tropics, where rain forests, coral reefs, and other particularly diverse communities are found. Thus, a smaller climate shift can have a greater impact on biodiversity in the tropics than a larger shift in boreal or temperate regions.

For marine ecosystems, changes in circulation patterns, ocean temperature, and ocean chemistry all influence biodiversity. For example, over the last 40 years or so, warm-water plankton species have shifted about 620 mi (1,000 km) in the North Atlantic due to warming.

Not all observed climate effects on biological systems are consistent with climate warming; a few are consistent with cooling. Also, some observed effects are consistent with natural climate shifts rather than those attributed to human-caused (anthropogenic) global warming. However, mathematical analysis shows that the very great majority of changes are consistent with warming trends, and that a combination of natural and anthropogenic climate changes describe observed changes in physical and biological systems better than either natural or anthropogenic changes alone. Anthropogenic changes have been added to or laid over those caused by natural processes, and are gradually becoming more dominant.

Future Impacts

Irreversible loss of biodiversity in many parts of the world is likely in the near future and possibly beyond. However, many uncertainties remain. Ecologists do not understand the relationship between ecosystem structure and rapid climate change well enough to predict the exact effects of current climate changes on biomes. It is also unknown whether efforts to mitigate climate change and other human impacts on biodiversity will occur or succeed, and if so, to what extent. Parks, endangered-species laws, hunting bans, and similar conservation efforts have slowed the loss of biodiversity in some regions, especially for certain species of larger, more-easily-noticed animals.

Scientists have estimated the likely impact that climate change will have on biodiversity. In 2004, Chris Thomas and colleagues published their study of an unbiased or representative sample of 1,103 animal and plant species. They found that climate change was likely to commit 15–37% of all species examined to extinction by 2050. “Committed to extinction” does not mean that a species would necessarily be extinct by that time, but that the population of each species would be so reduced that its species’ extinction becomes highly likely. In many or most ecological regions, climate change will become the greatest threat to biodiversity by 2050. There are 5 to 15 million species of creatures on Earth (the large range arises from the difficulty of counting insect, bacterial, and fungal species). If only 15% of all species are committed to extinction by climate change—the lower end of the range given by Thomas and colleagues—then 750,000 to 2,250,000 million species will eventually become extinct as a result of global climate change.

Primary Source Connection

The following news article recognizes the United Nations’ involvement with the issue of climate change as gaining strength with international heads of government. The article acknowledges that recent U.N. meetings have made it hard for government officials to look away from the issue, provoking more countries to take the issue of global warming more seriously. Although obstacles such as convincing developing countries like India, China, and Brazil of the importance of decreasing CO₂ emissions do exist, on the whole, going green is developing into an international agenda.

U.N. REVS UP OVER GLOBAL WARMING

The annual summertime retreat of the Arctic icecap, greater this year than perhaps at any time during the 20th century. The nightmare of intensifying storms in some areas and extended drought in others, already taking place in developing countries of South Asia and sub-Saharan Africa.

It is against this backdrop of almost daily news of what scientists describe as signs of advancing global warming that the United Nations holds Monday what may be the largest high-level international meeting ever on climate change.

The conference, called by UN Secretary-General Ban Ki Moon, kicks off what many experts and officials say will be the high week of a turning-point year in the global political response to the challenge of a warming planet.

More than 80 heads of state or government are expected among the representatives of better than 150 countries attending the UN session. Then on Thursday, President Bush will convene at the White House a gathering of leaders from the world’s top emitters of greenhouse gases.

In addition, the Clinton Global Initiative will host a forum in New York Wednesday, drawing business and international political leaders to promote grass-roots responses to global warming.

Over the past weekend, at a UN conference in Montreal, the governments of about 200 countries agreed to accelerate a treaty to phase out hydrochlorofluorocarbons.

Together, the meetings put climate change at the center of the global stage this week—and they will make it harder for leaders to drop the issue in the future, experts say. That may be especially true of Mr. Bush: He may be known internationally as the foot-dragging leader of a top emitter of fossil-fuel pollutants, but by endorsing Mr. Ban’s meeting and then calling for his own at the White House, he will be seen as committing to a path of no return on climate-change action.

The Monday UN meeting “is looking to be quite an extraordinary event in what is turning out to be a remarkable year in the international response to climate change,” says Richard Kinley, deputy executive director of the UN Framework Convention on Climate Change.

Typically, such international sessions draw environmental ministers, who may or may not attend with the full-fledged backing of their government. This time, many heads of state are involved, with the meeting coinciding with the opening of the UN General Assembly. Such high-level involvement suggests that the world has turned a corner in perception of the seriousness of the climate-change challenge, Mr. Kinley says.

“Increasingly, leaders are seeing this as an issue of national interest, and not just a question of responsibility. And the fact is that states are more apt to act when they see their interests at stake,” he says.

Other factors this year, he says, include what many experts describe as a “conclusive” report from an international group of scientists and officials finding evidence of global warming to be “unequivocal.” The Intergovernmental Panel on Climate Change concluded with what it termed near certainty that the warming taking place is the result of human activities.

“That doesn’t mean we won’t hear some unorthodox views” at the meeting, Kinley says. Czech Republic President Vaclav Klaus, for example, intends to portray the idea of man-made global warming as a “myth” that does not justify constrictive measures.

Yet other experts cite factors beyond scientific findings in global warming’s rise to the international spotlight. David Sandalow, a specialist in the political dimensions of climate issues at the Brookings Institution in Washington, spoke at a forum earlier this year on the “new political climate” around climate change and listed factors such as: the business community’s “astounding” rallying to the issue, the Democrats’ taking control of Congress, increased Republican support for action—starting with California Gov. Arnold Schwarzenegger (R)—and even the documentary featuring Al Gore, “An Inconvenient Truth.”

What UN experts hope for—and what Ban wants coming out of Monday’s meeting—is fresh momentum for action at the UN climate-change conference set for Bali, Indonesia, in December. The Bali conference will take up the crucial question of what kind of agreement should replace the Kyoto Protocol on reducing greenhouse-gas emissions, which expires in 2012.

“What I want… is a strong political commitment at the leaders’ level for the meeting in Bali,” Ban told reporters last week. “There must be no vacuum” after expiration of the Kyoto accord, “so we must address the challenge boldly.” That means reaching a replacement agreement by 2009 so national governments can act and implementation can begin in 2012, he says.

None of this means that the debates on climate change are over, or that global accord on measures for action will be easy to reach.

Many eyes will be focused on this week’s White House meeting to gauge just how committed the United States is to action—and what kind of action the Bush administration will accept. Some experts worry that the administration will stick to a preference for voluntary goals in emissions cuts rather than reduction commitments, or that the Washington meeting signals a willingness on the part of the US to go outside the UN framework for addressing climate change.

Another potential stumbling block is the continuing disagreement between industrialized and developing countries over who should make the most substantial commitments to cutting greenhouse-gas emissions. It was the exclusion of developing countries from the Kyoto Protocol that in part led Bush to reject the agreement.

Major developing powers like China, India, and Brazil are still wary of any accord that would place the same burden on them as on developed economies, which they say have much higher per capita emission rates.

But these developing countries are also beginning to signal a seriousness about global warming and to address pollution and sustainable-development issues on their own, leading some experts to find hope that the divide can be narrowed. “Among a number of developing countries, we’re seeing a growing realization that they have to take a new approach for their own well-being. And the leading example of that is China,” says Kinley of the UN.

“That is not to say they have changed their position that the industrialized world should take the lead,” he adds. But he says a door once shut tight has now been opened.

“What we hear more now is talk of incentives, of the right investment and technology flows into the developing countries, and that is bringing the two sides closer together,” he says. “That’s crucial, because it’s clearly only through international cooperation that this challenge can be addressed.”

Howard LaFranchi

LAFRANCHI, HOWARD. “UN REVS UP OVER GLOBAL WARMING.” CHRISTIAN SCIENCE MONITOR (SEPTEMBER 24, 2007).

See Also Climate Change; Ecosystem Diversity; Endangered Species; Extinction and Extirpation; Genetic Diversity; Global Warming; Human Impacts

BIBLIOGRAPHY

Books

Parry, M. L., et al, eds. Climate Change 2007: Impacts, Adaptation and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Periodicals

Araúo, Miguel B., and Carsten Rahbek. “How Does Climate Change Affect Biodiversity?” Science 313 (2006): 1396–1397.

Higgins, Paul A. T. “Biodiversity Loss Under Existing Land Use and Climate Change: An Illustration Using Northern South America.” Global Ecology and Biogeography 16 (2007): 197–204.

Jenkins, Martin. “Prospects for Biodiversity.” Science 302 (2003): 1175–1177.

Livingstone, Daniel A. “Global Climate Change Strikes a Tropical Lake.” Science 301 (2003): 468-469.

Pounds, J. Alan, et al. “Widespread Amphibian Extinctions from Epidemic Disease Driven by Global Warming.” Nature 439 (2006): 161-167.

Sala, Enric, and Nancy Knowlton. “Global Marine Biodiversity Trends.” Annual Review of Energy and the Environment 231 (2006): 93-122.

Sala, Osvaldo E., et al. “Global Biodiversity Scenarios for the Year 2100.” Science 287 (2000): 1770–1774.

Thomas, Chris D. “Extinction Risk from Climate Change.” Nature 427 (2004): 145-148.

Willis, K. J., and H. J. B. Birks. “What Is Natural? The Need for a Long-Term Perspective in Biodiversity Conservation.” Science 314 (2006): 1261–1265.

Zimmer, Carl. “Predicting Oblivions: Are Existing Models Up to the Task?” Science 317 (2007): 892-893.

Web Sites

Intergovernmental Panel on Climate Change (United Nations). “Climate Change and Biodiversity.” April 2002. http://www.ipcc.ch/pdf/technical-papers/climate-changes-biodversity-en.pdf (accessed June 25,2008).

Larry Gilman