Climate change has overtaken other threats such as deforestation and pollution as a primary danger to the survival of plant and animal species.

Although the endangerment and extinction of species has taken place continuously since life on Earth began, human activities have intensified the process. In the past century, an increasing human population has built more cities, towns, and roadways; sacrificed forests for agricultural land; increased pollution; and produced other trends that disrupt stable ecosystems. In addition, symptoms of the Earth's gradual warming have been evident since the mid-1970's. An number of plant and animal species unprecedented on the human timescale faces conditions that threaten their survival.

The U.S. Endangered Species Act was enacted in 1973 in an effort to protect wildlife and counter those processes that endanger the survival of various animal and plant species. The World Conservation Union issues its Red List of species deemed to be at risk. Its 2008 report classified 16,928 species as threatened, out of a total of 44,838 surveyed. Half of all the mammal species surveyed were in decline. The report listed one in eight bird species, one in three amphibians, and 70 percent of all plant species as likely to perish in the foreseeable future.

Because so many anthropogenic hazards threaten the planet's life-forms, proof of the prime role of climate change has been difficult to establish. As time goes on, though, the effects of warming temperatures and related weather phenomena become more clear. Startling events in several different biomes bear this out.

Climate change leading to extinction probably has taken place many times, but out of the view of biologists or of any human observer. Zoologist Tim Flannery recounts a series of events in the Monteverde Cloud Forest Preserve in Costa Rica which, in retrospect, demonstrate a species suddenly dying out as a result of climate change.

The preserve is a mountain area whose forests are typically cloaked in mist. It is the home of an intriguing variety of rain-forest species. Among these was the golden toad, whose bright orange-gold males were so spectacular they became a symbol for happiness in tribal legends. These toads were seen gathered in their forest pools to mate in April, 1987. Then, the pools rapidly dried up, along with the golden toad eggs just deposited there. It was an El Nino year. The dry conditions were the culmination of a decade when, each year, the cloud cover drifted higher up the mountainside. As a result of this drift, the toads lost the mist in which they thrived.

The higher cloud levels were later traced to a rise in sea surface temperatures of the central Pacific over the preceding decade. In May, 1987, more toad eggs were laid, perhaps an instinctual effort to replace the dried-up ones. Few tadpoles hatched or survived, however. During the next two years (1988 and 1989), the researcher found only one lone toad where many had lived before. Since that time, no toads have been seen. Because the golden toads live only in this region and have been meticulously studied, their disappearance became the first documented case of extinction from climate change.

Golden toads were not the only victims of this quiet catastrophe. The Cloud Forest Preserve, a relatively accessible rain-forest area, was home to some fifty species of frogs and toads. At the end of the disastrously dry 1987 season, thirty of the fifty species had vanished. Among these was the Monteverde harlequin frog. Its demise was also connected to shifting climatic conditions, via the outbreak of a fungus that thrived only as the surrounding weather became drier and warmer. In fact, the continual decline of amphibians has been a worldwide occurrence. It seems likely that these animals' sensitivity to changes in temperature and humidity has been a significant factor in their decline. Their loss is an early warning of the ravages of a warming world.

Other animal life in the preserve also suffered. Several species of lizards living there have disappeared or become rarer. The keel-billed toucan, a lowland bird, has moved onto the mountainside, where it threatens the eggs of the quetzal, a spectacular bird famous in Mayan folklore. Altogether, this one small area, crowded with species adapted to its unique climatic conditions, has been an object lesson in what happens when these conditions abruptly change.

Among Earth's most rapidly changing habitats are those in the Arctic. For some time, each Arctic winter has been milder than the winter before. Sea ice in Hudson and Baffin Bays, for example, typically broke up in the early twenty-first century some three weeks earlier than it did in the 1970's. This trend disrupts the polar bears' annual trek onto ice to find their main food, the ringed seals that live there. There are now malnourished adult polar bears, and fewer cubs are being born.

Large stretches of the sea lack ice chunks big enough to support a bear. There have been reports of polar bears marooned on ice 640 kilometers away from any land or food source. The higher winter temperatures bring rains, which collapse the bears' birthing dens, killing the mothers and cubs inside. In short, each season of the polar bears' life cycle is threatened by a warmer Arctic, and this region has been warming almost twice as rapidly as most of the world.

The ice is home to some four species of seals that are equally threatened. The Gulf of Saint Lawrence has had several years when, because of the scarcity of ice, no harp seal pups were born. In fact, the whole Arctic biosphere is at risk. Walruses, caribou, and other animals all are threatened as winters average four to five degrees warmer than in generations past. Caribou herds, for instance, have drastically decreased in size. The main factor seems to be newly occurring autumn rains, which freeze their lichen supply so it is hard to browse. The rains also create swollen rivers which are fatal to many caribou cubs.

Coral reefs flourish in shallow tropical seas. Known for their intricate, many-colored forms and their role in island formation, the reefs also serve as a sort of nursery for fish and other marine organisms. A typical reef in the Indian Ocean may contain over five hundred coral species and provide food and shelter for more than two thousand different fish species. Coral itself is a phylum of invertebrate animals. The coral formations are an exoskeleton built out of calcium carbonate which supports a tiny animal living inside, called a polyp. Corals live in symbiosis with algae, the zooxanthellae. The algae give the reefs their spectacular color. They provide the polyp with food produced from photosynthesis.

Coral reefs require a delicate balance of temperature, water chemistry, and sunlight to stay healthy. When the surrounding sea's temperature rises above a certain level, the algae-polyp partnership breaks up. Extended warm spells make the algae disappear, and the coral polyps starve. The reef becomes bleached and dead. In the two El Nino years of 1998 and 2002, this happened on Australia's Great Barrier Reef, damaging a total of 60 percent of the reef's area in whole or in part. Many coral reefs on the outskirts of civilization have been damaged by surface runoff of silt, sewage, and trash and by harvesting of the coral. The added destruction from warming tropical waters means the loss not only of the reef structures themselves, but also of the habitat and breeding grounds of much marine life. Fish, crustaceans, anemones, and other creatures, some not even known to science, lose their home. Coral reefs are a complex ecosystem in delicate balance, susceptible to ruin from small changes in the planet's weather patterns.

The foregoing scenarios are notable for their exotic settings and fauna, and for the rapid pace with which global warming took a toll on the species involved. However, no part of the Earth is unaffected. Even nonscientists living in temperate regions have noticed such symptoms as the scarcity of butterflies, which in the earlier twentieth century were abundant.

One of the markers of ecosystem threat is the earlier occurrence of spring. University of Texas biologist Camille Parmesan examined nearly two thousand studies showing this happening on six continents and in all of the world's oceans. In the majority of cases, life cycles were disrupted. Insects and animals in a given ecosystem regulate their dormancy, reproduction, and growth in tandem with the growth and blossoming of plants. Temperature and day length appear to be the two signals which key their life cycle events. When the two become de-synchronized, the web of interactions involving food availability, pollination, seed dispersal, and other daily events does not stay intact.

When this happens, animals tend to migrate when they can. Shifts of habitat northward have been documented for multiple species of birds and butterflies since 1960, in both Europe and America. Red foxes have expanded their range northward, driving the arctic fox further toward the Arctic Ocean. This can be a successful survival strategy, but it also disrupts existing ecosystems and may displace native species that filled the same ecological niche. Eventually a species may run out of spaces to migrate to, or exceed its own ability to adapt.

Migrating to higher elevations is a variant of this strategy, and it is subject to the same risks. The Edith's Checkerspot butterfly in California has shifted its range upward some 100 meters in the Sierra Nevada. A closely related species, the Quino Checkerspot, has become endangered because it is unable to cross the Los Angeles metropolis and establish itself in cooler, wetter environment.

Past experience in reversing the plight of endangered species is not always relevant to the global warming situation. The Endangered Species Act emphasizes using law and human efforts to counter damage threats from hunting, disruption of habitats by urbanization or agriculture, and pollution. It has had notable successes in the recovery or reestablishment of species such as the gray wolf, the peregrine falcon, and the humpback whale. But there are still eighteen hundred species on its list, and thousands of other threatened species needing protection. Moreover, the act was not designed to counter threats to an entire biome, much less those of planet-wide scope.

There have been five great extinctions in Earth's geologic past in which most existing species vanished. Their causes appear to be varied, but rapid climatic change is implicated in most. The most recent, culminating with the dinosaurs' extinction and the slow ascendancy of the age of mammals, happened 55 million years ago. The immediate cause was probably Earth's collision with an asteroid, but it was the aftereffects that affected the climate so drastically as to overturn all existing ecosystems. By colliding with limestone-rich rock, the asteroid created an explosion that put enough carbon dioxide into the air to warm the planet by an estimated 4 to 10 degrees Celsius. No creatures that weighed over 35 kilograms survived; there were also major changes in the vegetation.

A case can be made that ever since human activity began to alter the planet, Earth's other fauna and flora have been threatened. In the past decades, however technologies have been developed that can document and measure the planet's warming trend, and these technologies may prove sufficient to convince humanity to alter its behavior. Still, unless humans find ways systematically to counter the trend toward anthropogenic climate change and environmental degradation, the survival of threatened species can be accomplished only on a piecemeal basis.

Bibliography:

1)         Flannery, Tim. We Are the Weather Makers: The Story of Global Warming. Rev. ed. London: Penguin, 2007.

2)         McGavin, George C. Endangered: Wildlife on the Brink of Extinction. Buffalo, N.Y.: Firefly Books, 2006.

3)         Spicer, John. Biodiversity: A Beginner's Guide. Oxford, England: Oneworld, 1996.

4)         Ward, Peter D. Under a Green Sky. New York: Harper-Collins, 2007.

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