Why Extinction HappensYou are living in era of extinction unlike anything the dinosaurs ever knew. Since humanity first wandered from its African birthplace over fifty million years ago, we have radically altered the environment everywhere we settled, often at the cost of the creatures that ruled the wild before our arrival. As our prehistoric ancestors spread throughout the globe, they began the most deadly epoch the planet's biodiversity has experienced since the demise of the dinosaurs. And particularly after the dawn of the age of exploration 500 years ago, the rate of extinction has accelerated ever more rapidly.
This modern series of animal and plant extinctions is the first of its kind because they are a period of nature-loss caused soley by the acts of one species-- people. And what's ironic is that only people can now save what's left of the wondrous diversity of life on Earth. Being human today then must equate to a willingness for both our compassion and action, to prevent losing any further pieces of wild nature.
(Text above taken from A Gap in Nature, by Tim Falannery and Peter Schouten, 2001.)The Extinction Crisis Today
Today, the human species, one of millions of life forms on this planet, is threatening the very existence of many other species. Biological diversity, or biodiversity, is the variety of life forms that interact to support and sustain the balance of nature. As the human population grows, more and more of the Earth's living space, food, and other resources are consumed. Explosive human population growth and consumption are causing loss of biological diversity at an ever-increasing pace, an extinction crisis that threatens to surpass the mass extinctions that have occurred periodically during the Earth's history. Never before have so many species been threatened with extinction in so short a period of time. In fact, some scientists estimate that species are disappearing at the rate of one every day, hundreds of times faster than the normal background rate of extinction.
What else is different about this mass extinction, in addition to how fast it is happening? Scientists believe that other extinction events were caused by phenomena such as climate change and collisions of asteroids or meteors with the Earth. After these events, new species evolved that were adapted to the changed conditions. In contrast, modern extinctions are being caused by human use of the Earth's resources. If humans continue to destroy, degrade, and fragment the habitat that for millions of years has supported other life forms, it will be difficult for new species to evolve. Only species that are adapted to human-altered landscapes will be able to survive or evolve.
Habitat Loss
The most common cause of endangerment is habitat loss. Plants and animals need space to live and energy provided by food, just as humans do. As human population and consumption increase, wildlife habitat is converted to houses and highways. Forests are cut down for building materials, fuel, and paper. Prairies and forest land are turned into crop land and grazing land for our livestock, and shopping malls stand where wetlands once existed. The damming of rivers to create hydropower has flooded river valleys, making it hard for ocean-going fish to migrate.
Even if habitat is not completely destroyed, it can be fragmented or degraded so much that it can no longer support the species it once did. Many species, particularly large mammals, need large areas of habitat to survive and reproduce. Patches of forest or grassland surrounded by farms or cities, or divided by roads, will not support these species.
A significant percentage of many habitats in North America that are important for wildlife have been destroyed or degraded since the time of European colonization. Over 50 percent of wetlands are gone, 90 percent of ancient forest in the Northwest has been logged, and millions of acres of grasslands have been obliterated.Overexploitation
Humans also deplete wildlife populations by capturing or killing individuals for their own use. Animals are killed for food, fur, feathers, oil, medicines, crafts, and a host of other uses. They are also shot to stop them from killing livestock, or simply for recreation.
Animal eggs are taken for food, and species are captured for pets or to use in medical experiments. Sophisticated technology allows ever-increasing numbers of animals to be captured *at once, depleting seemingly limitless species like ocean fish.
As the developing nations of the world accumulate more wealth, the demand for animal products grows. The international market for animals and animal parts is a huge and growing cause of wildlife endangerment.
Introduced Species
Humans often move species around, introducing species that are not native to an ecosystem and disrupting the delicate balance that evolved among species in that ecosystem. Species can be moved both accidentally and intentionally. The introduced species may compete with native species for food or nest sites, or they may prey on native species.
As humans penetrate into more remote places, we allow other species to do the same by using the roads we build. In addition, we transport species by sea. Ships take on water in one location for ballast, travel across the ocean, and then dump the ballast water, carrying new aquatic life forms to habitats already occupied by other species.
We travel from island to island for trade or recreation, taking foreign species with us. Islands are particularly vulnerable because they are isolated and native species have nowhere to go when other species move in.
Pollution
One of the ways habitat is degraded is by pollution. Creatures that depend on either freshwater or saltwater for all or part of their life cycles, like fish, frogs, marine mammals, and many invertebrates, are especially vulnerable to pollution.
Water is polluted by things like run-off of fertilizers and pesticides from farms, oil and other chemicals from roads, and human sewage that flows untreated into rivers, lakes, and oceans. In addition to polluting waterways, we divert fresh water from rivers and lakes for irrigation, drinking water, and industrial uses. There is less water left in the rivers and lakes to dilute the polluting chemicals.
Ships pollute saltwater by dumping waste. Oil spills, like the big spill from the oil tanker Exxon Valdez in Alaska in 1989, kill large numbers of animals. Many smaller spills and leaks go relatively undetected, but their cumulative effects over the years also can injure wildlife.
Water is not the only element that suffers from pollution. Factories and cars release chemicals into the air. The chemicals are deposited on land by rain, causing pollution, including what is known as acid rain. Acid rain weakens and kills plant life, decreasing the food supply for animals that eat the plants.
Pesticides are another source of pollution. Farmers use pesticides to keep insects from eating crops. Pesticides remain in crops and in wild plants eaten by herbivores (plant eaters). Insects also carry pesticides. Animals that eat herbivores (like predatory birds) and insects (like birds and amphibians) get high concentrations of these chemicals in their systems. The chemicals can disrupt physical functions like reproduction in these animals.
Other Factors
We know less about other factors that probably contribute to the decline of biological diversity. We know little about how changes in our atmosphere, such as global warming or ozone depletion, is affecting other life forms.
Disease and insect infestations, which are natural and nonthreatening phenomena in many ecosystems, can deal a death blow to populations weakened or depleted by other pressures.
We know very little about how all these factors interact to affect plant and animal populations. We do know, however, that these "natural" changes are becoming proportionately less significant as human impacts increase in magnitude, intensity, and duration.
Extinctions Long Ago
For every species that is alive today, perhaps a thousand more have lived previously and become extinct. Most of these extinctions occurred before humans evolved, and the species are known to us only through fossils. Extinctions are a natural part of evolutionary processes, but through most of the history of life on Earth, biological diversity has been increasing. Periodically, however, major changes in the conditions on Earth have caused the collapse of living systems, and large percentages of species a have become extinct. These species will never return. It takes millions of years for life forms to diversify again.The current extinction crisis is unique, in that the loss of biodiversity is occurring very rapidly, and the causes of the crisis are the activities of a single species: human beings. Some scientists believe the current crisis began when humans and their domestic animals first began to colonize the various parts of the globe. Others believe it began around 1600, when human population growth exploded, and the level of per capita resource consumption began to rise dramatically in some parts of the world. Of the species that are best known, the so-called "higher animals," more than one percent have become extinct in the last 400 years and the overwhelming majority of these extinctions are anthropogenic. Many more species are in danger of becoming extinct if we do not act quickly to conserve them.
Historic Extinction Rates
The background rate of extinction is the number of extinctions that would be occurring naturally in the absence of human influence. Estimates range from one to ten species per year for the past 600 million years. It is difficult to estimate this rate, in part because the number of species in existence is not known. The background rate of extinctions establishes a baseline from which the severity of the current extinctions crisis can be measured. The current rate of extinction appears to be hundreds, or perhaps even thousands, of times higher than the background rate. It is difficult to be precise because most of the disappearing species today have never been identified by scientists.
The background rate of extinction has been interrupted periodically in Earth's history by episodes of mass extinction, periods in which a large percentage of the existing species become extinct in a geologically short amount of time. Mass extinction episodes represent major collapses of biodiversity and ecosystems, and they lead to fundamental changes in the make-up and distribution of life on Earth. The species that are most likely
to survive mass extinctions are widespread generalists such as cockroaches and weeds.
There are five widely recognized major mass extinction episodes in the Earth's history, and many scientists believe that we have now entered the sixth. However, there is a fundamental difference. In the past, mass extinctions have been caused by climate change, extreme geological activity, huge meteors colliding with the Earth or other natural factors. These changes in the environment took tens of thousands or even millions of years to occur. The sixth great extinction episode has been precipitated by human activities, and it appears to be happening very quickly.
Types of Extinction
The word "extinction" can refer to several different phenomena. Most of the world's extinctions have been true extinctions, when a species completely dies out and leaves no descendants. A few have been pseudoextinctions, when the original or ancestral species has become transformed by evolution into another species. All species living today, including ourselves, evolved from another species.
True extinctions and pseudoextinctions are both a type of global extinction. Global extinction is the complete elimination of a particular species everywhere in the world. Many endemic species have a limited geographic range, such as a single island. No matter how small that area is, their disappearance from it is a global extinction if the species is not found anywhere else.
A local extinction is the extirpation of a species from a portion of its geographic range. Local extinctions mean the loss of the genetic diversity represented by that population and the removal of that species' contribution to the local ecosystem. Because members of the species still exist in other locations, local extinctions can be reversed if the original causes are addressed, and the species can recolonize or can be reintroduced into the area. Unfortunately, local extinction is often the precursor to global extinction.
Extinction is not limited to application at the species level. Extinctions in the ancient past frequently are described in terms of whole groups of related species, such as a genus or a family. The farther back in time, the more difficult it is to distinguish different individual species from one another on the basis of fossils, and sometimes scientists can only tell when all the members of a genus or a family disappear. In contrast, it is often useful categorize extinctions in the recent past by distinctions that are finer than the species level, such as subspecies and populations.
Another important type of extinction is extinction in the wild. Members of a species may exist in captive breeding programs in zoos, but if there are no individuals living in their natural habitat, that species has become extinct in the wild. Similarly, a species may be effectively extinct, if members of the species are still alive, but the species has no chance of reproducing. These cases include those in which all the remaining individuals are of a single sex.
Case Studies
The extinction case studies represent a broad geographical and temporal range of extinctions that were caused by humans or are believed to have been caused by humans. These past extinctions cannot be undone, but the future is unwritten. Human beings have created the factors that threaten so many species today, and human beings can create the solutions that will reverse the current extinctions crisis.
Species and Subspecies
If two groups or populations of a species do not interbreed, over time the genes of one group may become so different from those of the other that they are two distinct subspecies. Scientists suspect that this has happened to orangutans, for example. If there are two subspecies instead of one species, each subspecies is rarer than the species as a whole. Conservation money often goes to the rarest animals, so orangutans are more likely to get money for conservation if they are two subspecies than if they are one species.
There are many other species that actually may be two or more subspecies but are classified as a single species. New techniques for analyzing genes may resolve some of these questions.
Hawaiian Islands
The Pacific islands of Hawaii are more than 400 miles (644 km) from the nearest neighboring land. Today these islands are undergoing ecological disaster.
With more than 1 million residents and 6 million tourists visiting every year and crowding onto its lowlands, the coastal ecosystems have suffered heavy damage from human use.
Over millions of years, the Hawaiian islands have developed numerous plant and animal species found nowhere else. In addition to habitat destruction, Hawaii's native species have been severely preyed upon and out-competed by introduced species of both plants and animals. No land mammals, reptiles, or amphibians are native to Hawaii.Most of the food crops now grown in Hawaii were brought by humans. Since the first Europeans arrived in 1778, at least 104 species of native plants and 26 native bird species have been extirpated.
Human Population Growth and Consumption
Each year, about 90 million new people are added to the planet. This rate of growth will swell our population from today's 5.6 billion to about 8.5 billion by 2025. At present growth rates, global population will be 40 billion by 2100. The United Nations estimates, however, that growth rates will drop and world population will be 11.2 billion by 2100. This is still more than double today's total.
Asia and Africa are adding more people each year than are other continents, even though the percentage of people they add is not as high as in some countries. The huge number of people currently living on these continents translates into a large number of people added even with relatively low growth rates.
For example, China's annual growth rate is only 1.2 percent, but the country's population base of 1.2 billion people means that there is a net increase in China's population of 15 million people every year. Population growth in Asia and Africa puts tremendous pressure on wildlife habitat on those continents.
Population is not the only determinant of impact on the environment. The rate at which people consume resources is critical. The United States has a particularly heavy impact on resources because of its high consumption rate, even though its population is relatively low. For example, the environmental impact of a human baby born in the United States is 35 times that of one born in India, and 140 times that of a baby born in Bangladesh.
People in developing countries understandably aspire to the same level of consumption as that of developed countries. The combination of population growth and increased consumption rates, along with technology that allows us to exploit resources more effectively than ever before, will put intolerable pressure on the Earth's resources.
Question for Thought:
What do you think citizens in developed countries can do to lessen their impact on the environment?Why It Matters
Should it matter to humans that other life forms are disappearing? Many people think so. Human populations depend on plants and animals for much of their food, medicines, clothing, and shelter. Perhaps even more important, intact ecosystems perform many vital functions, like purifying the air, filtering harmful substances out of water, turning decayed matter into nutrients, preventing erosion and flooding, and moderating climate. It is not known how many species can be eliminated from an ecosystem without its functioning being impaired.
It is likely that an ecosystem with more species is more stable than one that has lost some species. For example, research has shown that grassland plots with a greater number of plant species are better able to withstand drought than those with less species diversity. This stability may well be important in the future, as changes in precipitation brought on by global warming stress ecosystems.
Some species are particularly important to the health of their ecosystems. These are called "keystone species", because like the center stone in an arch, their removal can greatly affect the entire system. A classic example of the consequences of removing a keystone species occurred when fur hunters eliminated sea otters from some Pacific kelp beds. Otters eat sea urchins, which eat kelp. With its major predator gone, urchin populations exploded and consumed most of the kelp. Fish and other animals associated with kelp beds disappeared.
In many cultures, humans value animals for reasons other than maintaining ecosystem health. Animals play a prominent role in the religions or belief systems of many cultures. Many people value other species for the enjoyment they give. Still others believe humans have a moral obligation to live in harmony with other life forms. Whatever their reasons, most people agree that it is important to try to prevent species extinction. Not all people agree on how to do this, however, or what to do when human needs conflict with needs of other species.
It is only recently that people have begun to be concerned about the decline of wildlife that has no commercial value to humans. Wildlife laws originally were passed to control exploitation of animals that people hunted. For example, international regulation of whaling started after hunting had depleted many whale populations.
Whalers were worried that soon no more whales would be left and their livelihood would disappear. In the United States, the first wildlife laws regulated hunting of game animals to preserve populations large enough for people to continue hunting.
Now, our concern has expanded to include animals that have little or no obvious economic value, like songbirds, as well as those we value for food or other uses.
Under the Endangered Species Act in the United States, a species or subspecies is regarded as "endangered" when it is in danger of extinction throughout all or a significant part of its normal range. A species is considered "threatened" when it is likely to become an endangered species in the foreseeable future.
A species does not have to be in danger of global extinction in order to be regarded as endangered. For example, many species that are endangered or threatened in the lower 48 states of the United States still thrive in Alaska, which is largely still wilderness. These include the grizzly bear, bald eagle, and gray wolf, among others.
It makes sense to protect a species before it has declined too far. When its populations have become small or isolated, it is harder and more expensive to help the species recover. In addition, many plants and animals play an important role in their ecosystems, such as that of a keystone species. It is important to conserve species in many parts of their range so ecosystems remain healthy.Why do we need animals and plants?
For one thing, the world would be less beautiful and wonderful without them. Every time an animal or a plant becomes extinct, some beauty and wonder goes out of this world-- and we are all the poorer for it. But even more important, all of the animals and plants on earth are linked together. Every single species of animal and plant has a special role to play in keeping the world of nature going. Every species depends on other species, and other species depend on it. So every time you take a species away, the natural world not work as well as it used to. Imagine that the world of nature is like a pyramid made of blocks. Each block has a role to play in building the pyramid up. Of you remove one block, it probably won'5 make much difference;. And even if you remove 25 blocks, if may not cause much damage. But if you remove hundreds and thousands of blocks, the pyramid will almost certainly fall down. That is the way it works with species. If we remove too many of them, we may find ourselves living one day in a world that doesn't work any more. (Text: From: Endangered Animals, Zoobooks, 1993, Wildlife Education Ltd.)Animals and plants need homes, just like people do.
They need places to live and places to find food. They need places to raise their young in safety. The home of an animal is called it's habitat-- and the loss of habitat is the most serious threat that many animals face today. People are destroying animal habitats at an alarming rate. The number of people on earth is growing very fast. As a result, we need more farms to frow food and room to build houses. So we are taking a lot of land away from animals. levery day, the homes of thousand of animals are destroyedl Most animals can'5 fins new homes. If their havitas are destroyed, they are destroyed. So if we want to save the animals shown on these pages--and many others as well--we must fins ways to save their habitats.
--Animals and plants that live in tropical forests are suffering the most. The trees are being cut down at the rate of 50 acres a minute.
--Meat-eating animlas like bald eagles, often need large areas in which to hunt their food. If too much of their hunting area is taken away, these animals may starve.
--All wild animals need protected places to raise their young. If there is not enough cover, or if people live too close by, some animals may stop having babies.All life needs food and water. But people often do things that make it hard for wild animals and plants to find the food and water they need. Some people bring farm animals to graze in places where wild animals need the grass. Some people take all the best food out of an area and leave very little for the animals. Others simply push the animals onto poor land, where food is hard to find, so that the people can have the good land. And most serious of all, some people spread chemicals in the air and water that poison the food that animals must eat.
--When people spray posons on vrops to kills insects, the poisons often stay on the soil for a long time. When it rains, the water running off into streams and rivers carries some of the poisons with it. And this often kills fish. Chemicals pumped into streams by factories kill many more fish.
--Sometimes, fish are not poisoned enough to kill them. But they carry the poison in their bodies. And when a meat eater, like a pelican, eats a lot of fish, the combined poison in all the fish is enough to kill the pelican.
--Grazing farm animals can quickly make land unfit for wildlife. Sheep cut the grass down so close to the ground that it often doen't grow back. Goats eat young trees, and can destroy forests in this way. When the old trees die, there are no new ones left to replace them.
--People sometimes hurt wildlife by bringing wild animals into places where they don't bnelong. The story of the rats and mongooses and the Hawaiian birds is a good example of this. Before people came to Hawaii, there were no animals that preyed on birds. For this reason, many types of birds could dafely build their nest onthe ground. But then, people came to Hawaii. Polynesians came in big canoes and white people can in tall sailing ships. There were rats on the ships, they quickly came ashore. They started eating the eggs of the ground-nesting birds. People thought they could help the birds by bringing in mongooses to eat the rats. But the people picked the wrong animals. The mongooses did not eat the rats, they ate the bird eggs. So thanks to people, the birds now had two deady new enemies.
--People reduced the amount of fire and flooding on the land, in hopes to protect people and their homes. But both fires and flooding were benficial to many types of plants and animals, bringing them their necessary kinds of food nutrients.
--A beautiful skin or an unusual pair of horns can be dangerous for the animal that has them. This is because there are some people who will pay a lot of money to get them. The best way for us to save animals shown on these pages--and many other animals and plants-- is to get people in rich countries to stop buying products made from wild animals.
--Some animals are made into fancy things for people who like to show off. Should pythons, alligators and other beautiful reptiles be made into belts and boots? Should rare zebras be made into wall decorations?
--Parts of animals are turned into useless jewlery and carvings. Do we want to lose more than 1,000 elephants a year so that people can have these things?
--Some animals are hunted because they are already very rare. This makes some people want them as trophies, to hang on their wall.
--Often, animals are put in danger to get products that can easily be taken from other sources. Do we really want to turn thousands of whales into fertilizer?
--A few animals are endangered because people think that parts of their bodies have magic powers. Some people grind the horms of rhinos into a powder that they say can cure high fevers. Rhino horn sells for $6,000 a pound.
--Some selfish people think it's more important to be "elegant" than it is to have beautiful living animals in this world. People like this will pay $200,000 for a snow leopard coat.
--People who want unsual pets may sometimes help push animals closer to extinction, without meaning to do it. They often don't realize that many of the most beautiful and unusual animlas in pet stores have been taken from the wild.
--Every year, more than 7 million wild birds are trapped and sold. Huge areas of tropical forests that were once filled eith biurds are now almost empty.
--People who trap animakls and sell tem are only doing it for the money. If everybody stopped buying eild animals the trappers wouldn't catch them anymore. The animals could stay where they belong-- in the wild.
--In general, wild animals do not make good pets. They are not tame enough, ans they may do things that annoy people. Macaws for example, like to get up early in the morning and make very loud noises.
--Pet stores sell many animals that have not been taken from the wild. These animals were born ans raised to be pets. If people buy only these animals, they will have wonderful pets-- and they won't be hurting wild animals.
--Parrots are very popular as pets. If people knew what wild parrots must go through on their way to the per store, the would probably never buy one. In the wild, the birds are caught in nets, often hundreds at a time. The shock of being caughy usually kills two out of every five birds. Then the parrots are jammed into tiny cages to be carried to marketl They may stay in these cages for days, without enoough food and without decent care. At least one out og every five birds dies from crowding and lack of food. Many parrots can be brought into country legally. But it is against the law to bring in some species--so they are usually smuggled across the border. To keep the parrots quuet while they are being smuggled, their beaks are often taped shut. Their feet may be tied together, and they may be stuffed into sacks or wrapped tightly in newspaper. Many of them die from lack of air and rough handling. In the end, for every beautiful parrot that sits in somebody's living room, at least four others may have been lost along the way.The growing number of people on earth is the main cause for all the problems shown in this book. As the human population continues to grow, there is a greater and greater demand for living space, food, lumber, minerals, and other things that must be taken from nature. Two hundred years ago, there were fewer than one billion people on earth. There was plenty of food and living space for both people and animals. Today, there are over 6 billion people on earth, and things are getting tight. In the future, if the number of humans continues to grow, there won't be any more food and room for all species of animals and plants that we now have.
(Text above-- "Why it Matters," "Why Save Plants and Animals," and "Animals an Plants need homes, just like people do,"-- is from: Endangered Animals, Zoobooks, 1993, Wildlife Education Ltd.)
Glossary
Acid Rain - Precipitation that becomes acidic due to acid-forming precursors put into the atmosphere by human activities.
Amphibian - Members of a class of cold-blooded vertebrates who are aquatic in the larval stage, and breathe air as adults. Frogs, toads, and newts are examples of amphibians.
Anthropogenic - Caused or influenced by human impact on natural systems.
Background rate of extinction - The natural rate of extinction in the absence of human influence. Estimates of the background rate of extinction range from one to ten species per year, which is 100 to 1,000 times lower than the current rate.
Biodiversity (also known as biological diversity) - The variety of living organisms. Biodiversity encompasses variation at all levels, from the genetic diversity within a species to the variation between higher level evolutionary groupings such as families and classes. It also includes the variety of ecosystems, habitats and the natural interactions of species in the wild.
Ecosystem - An integrated group of biological organisms located in a particular type of habitat, and the physical environment in which they live. The ecosystem includes the living organisms, habitat structure, factors (such as temperature, wind, elevation, etc.) and their interactions.
Endemic - Native to a particular, restricted geographic area.
Evolution - The change in organisms over generations that gradually results in changes in populations and species.
Exploitation - The killing, capturing or collecting of wild organisms for human use.
Extinction (Also see the introduction to Past Extinctions) -The state in which all members of a groups of organisms, such as a species, population, family or class, have disappeared from a given habitat, geographic area, or the entire world.
Extinction vortex - The interacting factors that serve to progressively reduce already small populations, drawing them into extinction like an inescapable whirlpool.
Extirpation - The complete removal of a particular type of organism from an area, usually a specified geographic area.
Food chain - A sequence of steps through which food and energy move through the environment from the primary source (plants), through the animals that consume plants, up to the animals which consume other animals.
Fragmentation (also see Spotlight on Island Biogeography [SP]) - The disruption of large areas of habitat into smaller, separate units. Fragmentation involves both a total loss of habitat area and the isolation of remaining habitat patches, which prevents interaction between some organisms located in the fragments, and renders them effectively separate populations.
Genetic diversity - Variation at the genetic level, within and between species, including the different forms of genes for particular traits.Geographic range - The geographic area within which the specified type of organism may be found.
Habitat - The physical and biological environment in which an organism lives, including the arrangement of food, water, shelter, and sites for rearing young.
Introduced species (also known as non-native, exotic, or alien species) - Species that humans transport to an area that was previously outside of that species' geographic range. Introductions may be intentional, such as with domestic animals like sheep and dogs, or unintentional, such as with rats and other pests that live on ships.
Invertebrate - The group of animals lacking a bony spinal column. Examples of invertebrates are insects, worms, starfish, sponges, squid, plankton, crustaceans, and mollusks.
IUCN (International Union for the Conservation of Nature) - The IUCN, also called World Conservation Union, is an independent body founded in 1948 that promotes scientifically based action for the conservation of wild living resources. A union of nations, government agencies, and non-governmental organizations, the IUCN links a global network of more than 4,000 scientists who share information and develop cooperative plans for conserving endangered plants, animals, and ecosystems.
Marsupials - The group of mammals whose young are born very undeveloped and must attach themselves after birth to the mother's milk glands, where they are usually protected by a pouch. Australia is known for its wide variety of marsupials, such as kangaroos, wombats, and bandicoots. The opossum, found in North and South America, is also a marsupial. Marsupials are known in Europe, Asia, and Africa only through ancient fossils.
Mass extinctions - Periods during which the rate of extinction is much higher than it is at other times, and a large percentage of the evolved biodiversity disappears in a geologically short amount of time. See the introduction to Past Extinctions for more information on episodes of mass extinction.
Metamorphosis - An extreme change occurring between the stages of life, such as from a tadpole to a frog, or from a caterpillar to a butterfly.
Niche - The unique set of resources used by a species within an ecosystem.
Ozone depletion - The reduction in the layer of ozone gas, found at the top of Earth's atmosphere, which absorbs most of the ultraviolet radiation (UVR) coming from the sun. Ozone depletion decreases the absorption of UVR, which allows more of the harmful rays to penetrate to Earth's surface.
Poaching - Illegal hunting, capture, or collecting of wildlife. Poachers may target organisms that are protected from all hunting, such as elephants, or they may target animals outside of the regulated hunting season or inside the boundaries of a protected area.
Population (of a species) - A subgroup of a species coexisting in the same time and area. Population may also be used in a different sense to refer to the number of individuals in a defined group.
Species - A group of related organisms that are capable of breeding with each other to produce fertile offspring but are not capable of breeding with members of other species.
Subspecies - A geographically isolated or physiologically distinct group within a species that is capable of interbreeding with other members of the subspecies but that usually does not.
Vertebrate - Any of the group of animals that have a backbone. These include amphibians, birds, fish, mammals, and reptiles.
Viable population or viability - A sufficient number of individuals of a species to make their continued existence possible. If the population dips below that number, the species will not be able to recover and will eventually become extinct. The number of individuals needed for a viable population will vary with the species, habitat conditions and other factors.
Wetland - A land area with high amounts of moisture in the soils and characterized by plant communities that prefer that moist environment. Examples of wetlands are tide flats and marshes.
Unless otherwise noted, above text is from: 1996-2000 Bagheera and ESBN(tm). All Rights Reserved. Visit www. bagheera.com