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One of the challenges that faces a student of the biological sciences is the seemingly endless array of unfamiliar terms that one must learn. It is a relief to come across a relatively familiar one, such as species. Although it has a scientific sound to it, the word has entered everyday language, such that when people use it, most everyone understands what is meant. Or do they? As it turns out, there is no hard and fast definition for the word. Nonetheless, it is easy enough to find examples of species, since there are many millions of them in five kingdoms of living thingsa product of a phenomenon know as speciation, whereby evolutionary lines of descent diverge and new species are created. In the world today, there are many interesting groups of species, distinguished neither by evolutionary line nor by taxonomy but instead by the ways in which they interact with their environments. Among these groups are endangered species, of whose existence most people are aware, owing to the spread of the environmentalist message through media and entertainment outlets since the early 1970s. Less familiar is another broad group that in many cases threaten humans: introduced or invasive species.


Taxonomy in Brief

The concept of species falls under the heading of taxonomy, the area of the biological sciences devoted to the identification, naming, and classification of living things according to apparent common characteristics. Taxonomy is discussed in detail within the essay on that subject, but to appreciate the topic of species in context, it is helpful to have at least some knowledge of the larger subject. At one time taxonomists were concerned most with the morphological characteristics (i.e., the structure or form) of organisms as a basis for classifying many species within a larger grouping. Today, however, shared evolutionary lineage is much more important than morphological features in determining whether taxa (plural of taxon, meaning a taxonomic group or entity) can be classed together. Organisms may be linked closely in terms of evolutionary lines of descent but differ in a particular morphological aspect as a result of the adaptive changes that accompany natural selection. The latter, a key concept in the theory of evolution put forward by the English naturalist Charles Darwin (1809-1882), is a process whereby some organisms thrive and others perish, depending on their degree of adaptation to a particular environment.

It is therefore possible for organisms in a particular environment to develop a common adaptive mechanism through generations of natural selection, even though those organisms themselves are not related to fish closely in terms of evolutionary line of descent. Thus, whales and dolphins, mammals that live underwater, evolved the ability to swim just as well as fish, but that does not mean they are connected closely. Conversely, organisms may be close, or relatively close, in terms of evolutionary lines of descent yet differ in significant morphological features. To use the whale and dolphin example again, these creatures are classified as mammals owing to certain particulars (discussed later), but they differ from the vast majority of mammals in that they have no legs. They do, however, have four appendages, just like the rest of the mammalian class; as a result of natural selection, however, theirs ceased to operate as legs (an encumbrance for life in the water) a long time ago, and today they function instead as fins.

Obligatory Ranks

The classification system used today is an outgrowth of a system developed by the Swedish botanist Carolus Linnaeus (1707-1778) in the 1730s. The realms of zoology and botany, areas of biology devoted to the study of animal and plant life, respectively, differ somewhat with regard to their classification systems, but both use international codes of nomenclature with roots in the Linnaean system. There are many possible ranks of classification, but only seven are obligatory, meaning that all species must be assigned a place in these groupings. The obligatory ranks are listed here. The entire list of rankings, including versions of obligatory ranks with such prefixes as sub-, super-, and infra-, as well as such additional ranks as cohort or tribe, are given in Taxonomy. Note the difference between the zoological and botanical names for the second rank.

Obligatory Taxonomic Ranks

  • Kingdom
  • Phylum (Division in botany)
  • Class
  • Order
  • Family
  • Genus
  • Species

As discussed in Taxonomy, this book uses a system of five kingdoms, whose characteristics are defined in that essay. Even at the level of kingdom, not everything is delineated precisely (see the discussion in Taxonomy), and there are significant areas of dispute. For example, some taxonomic systems include viruses. Because viruses are not cellular in structure and are not universally regarded as true organisms, however, they are not included in the five-kingdom system used here.

Below the level of kingdom, definitions become even more difficult. Organisms are grouped into phyla on the basis of body plan or organization, but there is no regular pattern for grouping within the smaller categories. For example (as noted later herein), humans are placed within their particular phylum and sub-phylum on the basis of their spinal columns and overall internal bone structure, but those specifics play no significant role in categorizing them within any of the more specific groupings to which they belong. Furthermore, the generic definitions of the categoriesfor example, class as opposed to class Mammalia, class Insecta, or some other class in the taxonomic systemare purely relative. In other words, class is simply the obligatory rank that is more specific than phylum but more general than order.


When preparing an outline for a paper, students are taught that no topic should have only one subheading; instead, that solitary subheading should be moved up one level. Such rules do not apply in taxonomy, and it is not necessary that there be more than one subgroup within a larger group. For example, there might be only one class in a phylum. Taxonomists use detailed definitions to single out particular groups, such as class Mammalia. The following list shows the placement of humans within the larger taxonomic universe, along with brief explanations of a few (though far from all) characteristics that define each group.

  • Kingdom Animalia: Multicell eukaryotic (that is, possessing cells with a nucleus and specialized compartments called organelles) organisms that obtain their nutrition solely by feeding on other organisms. (Other defining characteristics of Animalia are discussed in Taxonomy.)
  • Phylum Chordata: Animals whose bodies, at least at some point in their life cycles, include a rudimentary internal skeleton with a stiff supporting rod known as a notochord. All chordates at some point also breathe through gills (in the case of a human, while still in the womb). Other characteristics set apart chordates, including a tail or the remnants of one. Humans belong to the subphylum Vertebrata, or chordates with a spinal column.
  • Class Mammalia: Vertebrates that feed their young from special milk-secreting glands, known as mammae, located on the mother's body. Mammals have other distinguishing characteristics, such as a hinged lower jaw attached to the skull.
  • Order Primates: A group of mammals whose characteristics may include some version of an opposable digit (e.g., the human thumb) and other features that, while they are prevalent among most primates, are not universal to them. Not every one of these traits is exclusive to primates, a group that includes prosimians (e.g., lemurs), monkeys, apes, and humans.
  • Family Hominidae: Primates noted for their erect posture, large brains, rounded skulls, small teeth, bipedal locomotion (i.e., they walk on two legs), and tendency to use language for communication. Humans are the only surviving species in the family, but extinct hominids include Homo habilis (about 1.6 million years ago) and H. erectus (about two million years ago) as well as the more distant Australopithecus (about eight million years ago).
  • Genus Homo : Hominids with especially large skulls as well as the features that characterize family Hominidae. Members of this genus, which included H. erectus and H. habilis as well as H. sapiens, also are known for their ability to fashion precise tools.
  • Species Homo sapiens : Members of the genus Homo ("man") noted for, among other things, the ability to use symbols and writing. This category includes modern humans and the extinct Cro-Magnon and Neanderthal man.

Note that the proper name of any ranking more general than species is capitalized (e.g., phylum Chordata), with species (and subspecies) names in lowercase. Genus, species, and sub-species names are rendered in italics (e.g., Homo sapiens, or "man the wise"), whereas proper names of the more general groupings are presented in ordinary type (e.g., class Mammalia). If the same name appears a second time in the same article, the genus name usually is abbreviated: thus, H. sapiens. Another important abbreviation is spp., implying several species within a genusfor example, Quercus spp. refers to more than one species of oak.

Taxonomy makes use of a system called binomial nomenclature, in which each species is identified by a two-word name, designating genus and species proper. Beyond the species name, there may be subspecies names: humans are subspecies sapiens, so our full species name with subspecies is Homo sapiens sapiens. Additional rules govern the inclusion of a name or an abbreviation at the end of the species or sub-species name, to recognize the person who first identified itin this case, Linnaeus. Hence the proper full name of our species is Homo sapiens sapiens Linneaus, 1758.

The Mystery of Species

If one studies the delineation of humans' place in the overall taxonomic structure, one may notice that for several groupings, the defining characteristics are a bit "fuzzy around the edges." This is true even of the animal kingdom, as noted in Taxonomy: mobility and locomotion, seemingly so integral to the definition of animal, are not prevalent among all animal species. Given the many gray areas and areas of dispute in the larger taxonomic categories, it should come as no surprise that the smallest of the obligatory rankings, that of species, lacks a precise definition.

The most widely accepted definition of species is the one put forward by the Germanborn American evolutionary biologist Ernst Mayr (1904-) in the 1940s. Mayr's idea, known as the biological species concept, defines a species as a population of individual organisms capable of mating with one another and producing fertile offspring in a natural setting. Members of two different, but closely related species in some cases can mate with one another to produce infertile offspring, the most well-known example being the mule, a sterile hybrid produced by the union of a male donkey and a female horse.

The definition offered by the biological species concept requires qualification. While many plants and animals reproduce sexually, many more do not; no single-cell life-forms reproduce in this way, yet there are certainly many different and distinct species of bacteria and protozoa. Thus, a further qualification typically is added to the definition of species : members of the same species share a gene pool, or a total sum of genes. Genes carry information about heritable traits, which are passed from parent to offspring. Whereas the gene pool is shared by members of a species, nonmembers of that species have genes that do not belong to that gene pool. To use a rudimentary example, let's say that there is a gene pool containing genes x, y, and z. Individuals that have these genes fit within the gene pool, but an individual with gene w does not.

The definition of species remains challenging, with special problems raised in the area of botany. It is also sometimes possible to confuse species and race, a grouping that applies not only in the world of humans but also that of other animal and even plant species. Race is different from species inasmuch as races are not isolated genetically from one another; in other words, there are no biological barriers to interbreeding between races. (See Speciation for a discussion of the process whereby single species develop over time into more than one reproductively isolated species.)


Endangered Species

An endangered species is any plant, animal, or microorganism that is at risk of becoming extinct or at least of disappearing from a particular local habitat. Over the course of Earth's geological history, species have become extinct naturallysometimes in large proportions, as discussed in the context of mass extinction in Paleontology. In modern times, however, species and their natural communities are threatened mostly by human activities.

The number of endangered species worldwide is not known. In the United Statesa country that, unlike most, expends considerable effort on keeping track of its endangered speciesthere were more than 750 species and subspecies listed by the late 1990s as endangered under the federal Endangered Species Act. Additional endangered species are being added at a rate of about 50 per year, and there is a "waiting list" of an estimated 3,500 candidate species.

Efforts at monitoring endangered species in the United States have directed a disproportionate amount of attention toward larger organisms; consequently, smaller endangered species from such groups as arthropods, mosses, and lichens have received insufficient attention. The regions of the United States with the largest numbers of endangered species are in the humid southeast and the arid southwest. These areas tend to have the unfortunate combination of unique ecological communities alongside runaway urbanization and resource development.

Overdevelopment and destruction of habitats is perhaps the most well-known ways that humans endanger the survival of species. For example, the habitat of the northern spotted owl is under threat from loggers in the Pacific North-west (see Succession and Climax). Another threat is the introduction of new species, particularly predators, to an area that is not their natural habitata topic we discuss in more depth later in this essay.


Another way humans threaten species is by excessive hunting. An example of a species thus threatened is the Eskimo curlew (Numenius borealis ), a sandpiper (a type of bird) that was still abundant in North America during the nineteenth century. A large, friendly creature, it was hunted in vast numbers during its seasonal migrations over the prairies and coasts of Canada and the United States and during its winter seasons in South America. (See Migration and Navigation for more about birds' winter migrations.) The Eskimo curlew became very rare by the end of the nineteenth century, and the last time an Eskimo curlew nest was seen (1866), the guns of the Civil War were practically still smoking. The last time a scientific team collected an Eskimo curlew specimen was in 1922. It might seem that the bird is extinct, but this is not the case. Although it is extremely rare, there have been a few reliable sightings of individuals and small flocks of this species, mostly during migration in Texas and elsewhere but also in its breeding habitat in the Canadian Arctic. Once abundant, the Eskimo curlew now hangs on by a thread.


More familiar is the endangerment of whales, a cause made popular by many a "Save the Whales" bumper sticker. Among endangered animals of this group are the blue whale (Balaenoptera musculus ) and various species from the genus Balaena, or right whale. The latter species gained its common name because whalers of the nineteenth century considered it the "right" whale to hunt: it swims slowly and close to shore and so could be found and slaughtered easily. In addition, it yields a large amount of oil, used for lighting lamps in the era when Herman Melville's Moby Dick (1851) was written. The estimated world population of right whales is currently about 2,000 individuals, much depleted from the historical high numbers; though it is now protected from whaling, it suffers an excessive mortality rate from ship collisions.

As for the blue whale, it occurs virtually worldwide, and with a typical weight of 150 tons (136 tonnes) and a length of 100 ft. (30 m), it is the largest animal ever to have lived on Earth. Because it is such a fast swimmer, it could not be hunted effectively by whalers in sailing ships. Once steam-powered ships were invented, however, these whales were taken in tremendous numbers and became endangered. Because of its precarious status, this species has not been hunted for several decades, but it remains rare and endangered.

The Fate of the Dodo

When a species becomes extinct, it is gone forever. It is like a family whose last member has died without leaving an heir, but in this case the impact is potentially much more profound. Several thousand species have become extinct as the result of human activities, mostly hunting, in the past few hundred years, and of these species perhaps none is more well known than Raphus cucullatus, or the dodo.

Long before the application of the term clueless in the 1990s, a person out of touch or out of step was called a dodo. How did the bird's name come to be a synonym for stupidity? Perhaps it is just the funny sound of the name, or perhaps it is the fact that the dodo looked a bit like a turkey, another bird name used for someone of less than exemplary capabilities. Or perhaps the application of the name dodo in this way carries a hint of blaming the victimthe implication that the dodo somehow played a part in its own extinction.

In fact, the dodo's only shortcoming was its inability to overcome the threat posed by an extremely dangerous predator: the human. A member of the dove or pigeon family, the dodo was flightless and lacked natural enemies until humans discovered its homeland, the Indian Ocean island of Mauritius, in the early sixteenth century. First came the Portuguese and then, in 1598, the Dutch, who made the island a colony in 1644. By 1681 the dodo had ceased to exist. Not only did sailors collect the birds for food, but introduced species, including dogs, cats, pigs, monkeys, and rats, also preyed on dodos. They were subjected to regular slaughter by sailors, but the species managed to breed and survive on the remote areas of the island for a time. After the establishment of their colony, however, Dutch settlers launched what amounted to an extermination campaign.

No part of Earth's living environment can be removed without repercussions, and the destruction of the dodo illustrates the ripple effect that occurs when one species is eliminated. As it turned out, the bird had a symbiotic relationship (see Symbiosis) with the dodo tree, or Calvaria major, whose fruit it ate, thus releasing the seeds to germinate. With the dodo gone, the dodo tree stopped being able to reproduce. Fortunately, it is a species with a long life, and some specimens of C. major continue to survive after some 300 years; when those die, however, this species, too, will be extinct.

Exotic, Introduced, and Invasive Species

An introduced species is one that has been spread to a new environment or habitat as a result of human activity. An invasive species may or may not have been spread by humans (the ones we discuss were), but as the name suggests, it threatens an aspect of the habitat to which it has been introduced. Both introduced and invasive species are examples of exotic species, or species that have been introduced to a region or continent, usually but not always through human activity.

In the case of species introduced by humans, some were introduced deliberately and were intended to improve conditions for some human activity (for example, in agriculture) or to achieve desired aesthetic resultsfor example, when colonists wanted to plant a flower or tree that reminded them of home. Other introductions have been accidental, as when plants were brought with soil transported as ballast in ships or insects were conveyed with timber or food.


Some introduced species have been wildly successful. In fact, most agricultural plants and animals are introduced species: for example, wheat (Triticum aestivum ) was originally native only to a small region of the Middle East, but it now grows virtually anywhere conditions are suitable for its cultivation. Likewise, corn, or maize (Zea mays ), has spread far beyond its home in Central America. The domestic cow (Bos taurus ) once lived only in Eurasia and the turkey (Meleagris gallopavo ) only in North America, but today these species can be found throughout the world. If all introduced species were like cows and corn, or turkeys and wheat, there would not be much cause for alarm. Many introduced species are invasive, however, and pose a wide variety of threatsthreats to their environments or, in some cases, to human well-being. All manner of weeds and pests are among the nefarious roll-call of invasive species, a broad grouping that ranges from nuisances to serious dangers.


There are more than 30,000 introduced species in the United States, and most of them enhance rather than diminish the quality of life. For example, there are the many species introduced by colonists to make them feel more comfortable in their new homes, among them, the Norway maple (Acer platanoides ), linden (Tilia cordata ), horse chestnut (Aesculus hippocastanum ), and other trees as well as many exotic species of shrubs and herbaceous plants. The European settlers also introduced some species of birds and other animals with which they were familiar, such as the starling (Sturnus vulgaris ), house sparrow (Passer domesticus ), and pigeon, or rock dove (Columba livia ).

These are all deliberate introductions; on the other hand, accidental introductions are more likely to be undesirable. When cargo ships from Europe did not have a full load of goods, they had to carry other heavy material as ballast, to help the vessel maintain its stability on the ocean. Early ships to the New World often used soil as ballast, and upon arriving, sailors dumped this soil near the port. In this way, many European weeds and other soil-dwelling organisms arrived in the Americas. In addition, ships have used water as ballast since the late nineteenth century, and many aquatic species have become widely distributed by this practice. This is how two major pests, the zebra mussel (Dreissena polymorpha, discussed later) and the spiny water flea (Bythothrepes cederstroemii ) were introduced to the Great Lakes from European waters.

Several European weeds are toxic to cattle if eaten in large quantities, and when these plants become abundant in pastures, they represent a significant potential problem. Some examples of toxic introduced weeds in the pastures of North America include common Saint-John's-wort (Hypericum perforatum ), ragwort (Senecio jacobaea ), and common milkweed (Asclepias syriaca ). Several introduced insects have become troublesome pests in forests, as is the case with the gypsy moth (Lymantria dispar ), which has defoliated many trees since its introduction to North America from Europe in 1869.

Similarly, the introduced elm bark beetle (Scolytus multistriatus ) has helped spread Dutch elm disease, itself caused by an introduced fungus, Ceratocystis ulmi. It would be interesting to note the irony inherent in this affliction, which at first glance seems to involve another apparently introduced species, the "Dutch elm." There is no such tree, however; the name refers to the fact that the disease arrived in America from Holland, probably some time after World War I. Its principal victim is the American elm, or Ulmus americana.


Not all harmful introduced species were introduced accidentally. Settlers from Europe deliberately brought pets, such as the domestic dog (Canis familiaris ) and cat (Felis catus ); while these pets may add greatly to the quality of human life, they can cause problems, because they are wide-feeding predators. Such creatures threaten vulnerable animals in many places, especially isolated oceanic islands. Among other predators are mongooses (family Viverridae), often introduced to get rid of snakes, as well as omnivores, such as pigs (Sus scrofa ) and rats (Rattus spp.) Meat-eating animals are not the only threat: herbivores such as sheep (Ovis aries ) and goats (Capra hircus ) also endanger plant life in some areas as a result of overgrazing.

A particularly striking example of harmful, deliberate species introduction is the Nile perch (Lates niloticus ). First introduced to Africa's Lake Victoria in the 1950s, it has proved an economically important food source, with a large worldwide market. The problem is that the Nile perch is an extraordinarily active predator and has brought about a tragic mass extinction of native fishes. Until the 1980s, Lake Victoria supported an extremely diverse community of more than 400 species of fish, mostly cichlids (family Cichlidae), with 90% of those species being endemic, meaning that they exist only in one area. About one-half of the endemic species are now extinct in Lake Victoria because of predation by the Nile perch, although some species survive in captivity, and a few are still in the lake.


Three notable examples of invasive species in America are Africanized honeybees (Apis mellifera scutellata ), better known as "killer bees"; the zebra mussel (Dreissena polymorpha ); and kudzu (Pueraria lobata ). The first "killer" bees were released accidentally by a Brazilian bee breeder in 1957. These aggressive insects have no more venom than domesticated honeybees (another A. mellifera subspecies, which is also an Old World import), but they attack more quickly and in great numbers. Interbreeding with resident bees and sometimes traveling with cargo shipments, Africanized bees have spread at a rate of up to 200 mi. (320 km) a year and now threaten humans, fruit orchards, and domestic bees throughout much of South and Central America and north to Texas and California.

The zebra mussel was introduced to the Great Lakes in about 1985 in ballast water dumped by a ship or ships arriving from Europe. It colonizes any hard surface, including rocks, wharves, industrial water-intake pipes, and the shells of native bivalve mollusks. A bean-sized female zebra mussel can produce 50,000 larvae (an immature form of an animal) in a single year. Growing in masses with up to 70,000 individuals per square foot, zebra mussels clog pipes, suffocate native clams, and destroy the breeding habitats of other aquatic animals. These invaders have placed a great burden not only on the environment but also on the economy of the Great Lakes region: area industries spend hundreds of millions of dollars annually to unclog pipes and equipment.

Kudzu is an integral part of culture in the southern United States, but it originated in Japan and did not arrive on American shores until 1876. In that year, numerous foreign governments sent exhibits to the Centennial Exposition, held in Philadelphia to honor the country's 100th birthday. Two generations later, during the Great Depression, the U.S. Soil Conservation Service began promoting the use of kudzu for erosion control.

At a time when work was scarce, young men in the government-sponsored Civilian Conservation Corps (CCC) earned a living by planting kudzu throughout the South. The federal government paid farmers as much as $8.00 an acrea fabulous sum at the timeto plant kudzu fields. Before another generation had passed, in 1953, the federal government stopped promoting the use of kudzu. In 1972, just four years shy of a century after its first introduction, kudzu was officially declared a weed by the U.S. Department of Agriculture.

Obviously, something had gone wrong. The problem was that kudzu grew too wellso fast, in fact, that in the minds of many southerners, it began to possess some sort of mystical significance. This preoccupation with kudzu is reflected in the work of several Georgians, whose state has been particularly afflicted with the vine. There is the poem "Kudzu" by James Dickey as well as the cover of Murmur, the music group R.E.M.'s 1983 debut, which features a photograph of a kudzu-covered railroad trestle near the group's hometown of Athens.

Kudzu covered more than railroad tracks, and in the mid-twentieth century, it began to seem as though it would cover the entire South with its tangled vines. The plant is capable of growing by as much as 1 ft. (0.3 m) per day during the summer and can cover virtually anything that is not moving. Over the course of a good year, kudzu can grow by as much as 60 ft. (20 m), and it has proved impervious to many herbicides. One herbicide used in Auburn, Alabama, actuallymade it grow better! Thanks to the developmentof better chemical treatments, and the use of grazing animals, such as goats, kudzu no longer isperceived as such a great threat. Additionally, various entrepreneurs and scientists have set out tomake use of the vine in weaving baskets or inpreparing foods and medicines. Ground kudzu root, called kuru, has long been used in foods and medications in China and Japan.

One might wonder why Japan is not covered in kudzu and why kudzu is not crawling up the Great Wall of China. The answer is more than a little interesting from a biological standpoint. When kudzu was transplanted to America, it was taken out of its native environment and thus away from the local insects that threatened its growth. In its new home there were no threats to its spread, and with no obstacles in its way, it began to take over the South. (For more about the development of species, see Speciation. See also the discussion of keystone and indicator species in Food Webs.)


All Species Foundation (Web site). <>.

"Endangered Species on EE-Link." EE-Link (Environmental Education Link), North American Association for Environmental Education (Web site). <>.

Integrated Taxonomic Information System (ITIS), United States Department of Agriculture (Web site). <>.

Invasive Species, National Agricultural Library, U.S.Department of Agriculture (Web site). <>.

Levy, Charles K. Evolutionary Wars: A Three-Billion-Year Arms RaceThe Battle of Species on Land, at Sea, and in the Air. New York: W. H. Freeman, 1999.

Schilthuizen, Menno. Frogs, Flies, and Dandelions: SpeciationThe Evolution of New Species. New York: Oxford University Press, 2001.

Schwartz, Jeffrey H. Sudden Origins: Fossils, Genes, and the Emergence of Species. New York: John Wiley and Sons, 1999.

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Vergoth, Karin, and Christopher Lampton. Endangered Species. New York: F. Watts, 1999.



A system of nomenclature in biological taxonomy whereby each type of plant or animal is given a two-word name, with the first name identifying the genus and the second the species. The genus name is always capitalized and abbreviated after the first use, and the species name is lower-cased. Both are always shown in italicsthus, Homo sapiens and, later in the same document, H. sapiens.


The third most general of the obligatory taxonomic classification ranks, after phylum but before order.


Deoxyribonucleic acid, a molecule in all cells, and many viruses, containing genetic codes for inheritance.


Anyplant, animal, or microorganism that is at risk of becoming extinct or at least of disappearing from a particular local habitat.


Species that exist in only one geographic region.


A cell that has a nucleus as well as organelles (sections of the cell that perform specific functions) bound bymembranes.


Species that have been introduced to a region or continent, usually but not always through humanactivity. See also introduced species and invasive species.


A condition in which all members of a taxon have ceased to exist.


The third most specific of the seven obligatory ranks in taxonomy, after order but before genus.


A unit of information about a particular heritable (capable of being inherited) trait that is passed from parent to offspring, stored in DNA molecules called chromosomes.


The sum of all the genesshared by a population, such as that of aspecies.


The second most specific of the obligatory ranks in taxonomy, after family but before species.


The product of sexual union between members of two species or other smaller and less genetically separate groups, such as two races. In the case of species hybrids, the process of hybridization involves genetic abnormalities that>lead in most cases to sterility.


A species that has been spread to a new environment or habitat, whether deliberately or accidentally, as a result of human activity. Introduced species, like invasive species, are considered exotic species.


An exotic species that threatens some aspect of the habit at to which it has been introduced.


The highest or most general ranking in the obligatory taxonomic system. In the system used in this book there are five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia.


Structure or form, or the study thereof.


The process whereby some organisms thrive and others perish, depending on their degree of adaptation to a particular environment.


The act or process of naming or a system of namesparticularly one used in a specific science or discipline. See also binomial nomenclature.


The middle of the seven obligatory ranks in taxonomy, more specific than class but more general than family.


The second most general of the obligatory taxonomic classificationranks, after kingdom and before class.


A cell without a nucleus.


Ribonucleic acid, the molecule translated from DNA in the cell nucleus, the control center of the cell, that directs protein synthesis in the cytoplasm, or the space between cells.


The divergence of evolutionary lineages and creation of new species.


The most specific of the seven obligatory ranks in taxonomy. Species often are defined as a population of individual organisms capable of mating with one another and producing fertile offspring in a natural setting. Also, members of the same species share a gene pool.


A taxonomic group or entity.


The area of the biological sciences devoted to the identification, nomenclature, and classification of organisms according to apparent common characteristics.

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Habitat Loss

Habitat Loss

Many biologists consider habitat loss, habitat degradation, and habitat fragmentation the primary threats to species survival. Habitat is the place or kind of place where an organism or a community of organisms lives and thrives. Habitat loss occurs when habitat is converted to other uses, such as when a wetland is filled or a prairie is covered by housing developments. Habitat degradation occurs when the habitat is so diminished in quality that species are no longer able to survive. Urban development can degrade a habitat because plants and soil are replaced with asphalt and concrete. Water runs off instead of soaking in. Average temperature goes up because the asphalt and concrete absorbs more solar energy. Fragmentation occurs when terrestrial habitats are separated into small, isolated fragments. Even when the total acreage of habitat appears to be sufficient, the fragmentation prevents species from surviving.

Some species have a very limited habitat. For example, as its name implies, the habitat of the creosote bush grasshopper, Bootettix argentatus, is the creosote bush, Larrea tridentata. It is found nowhere else. Most animals avoid the creosote bush and find the leaves distasteful or even toxic, whereas this small grasshopper thrives on the leaves. If the creosote bush were to become extinct, so would the little grasshopper.

Fortunately for Bootettix argentatus, there are plenty of creosote bushes and neither the plant nor the grasshopper is in danger. This is not true for many other animals. The Texas horned lizard, Phrynosoma cornatum, has become so rare in much of its original range that it has been listed as a threatened species. Many people who grew up in Texas and Oklahoma in the 1940s and 1950s remember playing with this docile and easily caught lizard commonly called "horny toad" or "horned frog." The lizard's habitat is open dry country with loose soil (for burrows), supporting grass, mesquite, cactus, and plenty of the large ants they prefer to eat.

Several factors have caused the decline in population of Texas horned lizards (and the other horned lizard species as well). Many are taken for the pet trade. This is unfortunate, as they do not make good pets and most starve to death within a few months. Another factor contributing to the decline is the displacement of native ants by imported fire ants, Solenopsis invicta. Texas horned lizards feed almost exclusively on a few species of large red ants, such as harvester ants. Imported fire ants drive harvester ants and other ants out of their range, thus depriving the horned lizards of their primary food supply.

Many biologists believe that the major cause of the decline of the Texas horned lizard is the loss of habitat. The open areas with prickly pear, sparse grass, and mesquite are being converted into farmland and housing subdivisions. Of course, the first thing many homeowners do when moving into a new subdivision is to start eradicating the ants. People do require living space, clean water, food, and a safe environment, but there are many things that humans can do to reduce or prevent loss of habitat.


Many human activities can cause habitat loss, degradation, or fragmentation. In addition to urbanization, industrial agriculture, improper forest management, overgrazing, poorly managed mining, water development projects, pollution, the introduction of non-native species, and fire suppression all degrade habitat.


Industrial agriculture is the main cause of habitat loss while urbanization is the major hindrance to species recovery. Urbanization is a complex process that involves a progressive increase of the percentage of a population that lives in an urban area and a corresponding decrease in the percentage of people living in rural areas. Urbanization is often accompanied by urban sprawl as the city expands to accommodate an ever-increasing population. Managing urban sprawl is an enormously difficult and political process.

Many urban and suburban developments try to preserve "green belts" for aesthetic and other reasons, but habitat fragmentation still occurs. For example, by the early 1990s urban sprawl in California had reduced the indigenous coastal sage scrub ecosystem by more than 90 percent. Coastal sage is the habitat of the threatened California gnatcatcher. The remaining 10 percent of sage scrub is broken up into small fragments. Instead of four or five large patches containing thousands of hectares, there are now hundreds of widely separated tiny patches of a few hectares each. These patches are too small to support healthy populations of California gnatcatchers.

Industrial agriculture.

Nearly half of the land area in the United States is devoted to agriculture. There are 472 million acres in cropland and 587 million acres in range or pasture. According to ecologist Curtis Flather and others, massive single crop industrial agriculture is the leading cause of habitat destruction in the United States, substantially affecting our forests, rangelands, and wetlands. Nearly 90 percent of recent wetland losses are due to agricultural practices.


In many parts of the world, logging, grazing, and mining are the major threats to endangered ecosystems and species. Deforestation occurs when trees are removed at a rate faster than they can be replanted. Habitat degradation occurs when the largest and oldest trees are removed, leaving behind scrubby stands of small and immature trees. Worldwide, deforestation is decimating tropical rain forests with enormous habitat loss. Logging activities can have devastating impacts on habitats and the wildlife in those habitats. In the United States 260 threatened and endangered species live in our national forests. Poorly planned clear-cutting and building of logging roads degrade habitat by removing large stands of trees and fragmenting the remainder. Logging and the construction of timber roads also cause erosion that can clog streams with silt.


Livestock grazing is the most widespread of the federally subsidized, private commercial practices operating on public lands. Commercial livestock grazing is allowed on 270 million acres of land managed by the federal government. Poorly managed livestock grazing (including overgrazing) can severely damage wildlife habitat by changing the species composition of native ecological communities. In addition to directly destroying habitat, overgrazing has a number of indirect impacts. For example, land users often try to kill predators or species that may compete with livestock for food.


Mining is the extraction of useful materials from the ground. Surface mining strips away overlying soil and rock, removing the useful material (usually coal) and then replacing the rock and soil. Properly done, surface mining can leave some habitats in good condition. Improperly managed mining significantly degrades ecosystems by degrading habitat and by polluting and degrading streams and waterways. Even well-managed mining increases road building. Poorly managed surface mining can destroy the surface ecosystem. In addition, mining requires a large amount of underground material to be brought to the surface. These materials, when exposed to rain, can create runoff that is highly acidic or has high concentrations of metal ore, both of which are highly toxic to aquatic species.

Water development projects.

Water development projects include dams, dredging, stream channelization, flood control structures, and canals. These projects adversely affect species in a number of ways. The natural flow of rivers and streams may be disrupted. Riparian (stream bank) habitat may be destroyed, fragmented, or degraded. Because riparian habitat is often unique to a region, water development projects have the potential to destroy a habitat entirely. Water projects also alter water flow, which may change wetlands, marshes, and other downstream habitat. For example, in the portion of the Colorado River that flows through the Grand Canyon, the river habitat has been completely changed by the construction of Lake Powell and the Glen Canyon dam. The red, silt-laden Colorado River with its frequent floods has been replaced by a cold, clear river that never floods. Native species of fish cannot tolerate the cold water, although imported species such as rainbow trout do well. The riparian habitat has been completely changed as well. When the Colorado River flooded, it stripped vegetation from the banks and built large sandbars. Now the banks are covered with another imported species, tamarisk or salt cedar, and the sandbars are disappearing.

Introduction of non-native species.

After outright habitat destruction, many biologists consider the introduction of exotic species to be the primary threat to rare and native species and even to complete ecosystems. Non-native species change the vegetation, compete with native species, and prey on native species. Hawaii, California, and Florida face particularly severe problems with exotic species. In Hawaii introduced species are now considered to be the single greatest cause of extinction of the state's native fauna and flora. For example, the introduction of cattle to the state has destroyed many plant communities. Many species in Hawaii, such as the hau hele 'ula (Hawaiian tree cotton), have been placed on the threatened or endangered species lists.


Pollution damages and degrades ecosystems in many ways. Airborne pollutants such as acid precipitation often affect natural communities miles away from the source. Acid rain and acid fog destroy northern forests, lakes, and streams hundreds or thousands of kilometers from the source of the pollutants. Acid precipitation can lower the pH of streams and lakes to the point that some fish are unable to reproduce and some die. Acid rain can cause chemical reactions in the soil that release metallic elements, such as aluminum. These elements can enter water supplies, damaging fish or other organisms. More than a billion pounds of toxic chemicals, including mercury and lead, were discharged directly into America's waters between 1990 and 1994, according to the Environmental Protection Agency. Thirty million pounds of these chemicals were known to cause cancer.

Fire suppression.

For decades in the United States, we have assumed that suppression of fire was a good thing. Fire kills wildlife, destroys trees and grasslands, and damages property. Now we realize that many ecosystems depend on fire for their survival. Fire suppression allows other species to flourish, changing the species composition. For example, in Central Texas the hills are covered with Ashe juniper, a small native tree commonly called cedar. When Europeans first saw these hills, they were covered by thick, tall grass with only a few stands of cedar. Frequent wildfires swept over the hills, burning both grass and trees. However, the grass quickly recovered after the fire, whereas many trees were permanently removed. Thus the fire helped to maintain a balance between grass and trees. The suppression of fire and overgrazing disrupted that balance. Now the trees dominate and open grassy areas are rare. The distribution of animal species also changed.

Fire is an integral part of many ecosystems, maintaining the ecosystem's natural vegetation. There are many plant species that require fire to trigger the release of their seeds. Fire also clears out the underbrush in forests, and the prevention of all forest fires actually leads to fires that burn hotter and longer because of the accumulation of underbrush. Thus the suppression of all fires leads to habitat destruction and degradation. If handled judiciously to protect life and property, fires can restore an ecosystem's natural balance.


Sometimes we love our natural areas to death. Recreation takes a great toll on wildlife and habitats, especially inappropriate recreational uses of open land. Probably the most destructive form of outdoor recreation is the improper use of off-road vehicles. These vehicles can provide access to remote areas otherwise unreachable. Improper operation of these vehicles can result in the harassment and inadvertent killing of wildlife. For example, vehicles can crush desert tortoises or the eggs of sea turtles and piping plover on beaches. These vehicles can also cause acceleration of soil compaction and erosion, pollution of water and air, and destruction of vegetation. Other forms of recreation, such as hiking and backpacking, are generally less harmful, but all recreational activities involve some harm to the environment.

Effects of Habitat Loss on Animal Species

Ecologists Curtis Flather, Linda Joyce, and Carol Bloomgarden studied the pattern of endangered species in the United States for the National Forest Service. In a report published in 1994, they concluded that habitat destruction was the leading cause of species endangerment, threatening 80 percent or more of federally listed species. They also found that habitat destruction and degradation was at least part of the reason why more than 95 percent of species listed as endangered or threatened were imperiled. In a different study of taxpayer-subsidized resource extraction, researchers found that logging affects approximately 14 to 17 percent of listed species, grazing affects 19 to 22 percent, water development affects 29 to 33 percent, recreation affects 23 to 26 percent, and mining affects 14 to 21 percent.

Habitat loss and degradation is a factor in the decline of every category of species. The decline of nearly 40 percent of migrant bird populations is directly linked to habitat destruction. For amphibians, declining populations are linked to habitat destruction, introduction of exotic species, water pollution, and ozone depletion.

Habitat Protection

The 1973 Endangered Species Act (ESA) has been a great success. Many species, such as the American alligator, have been brought back from the brink of extinction to healthy populations. However, many biologists question the species focus of the ESA. Rather, a focus on preserving extensive habitats is thought to be the best way to prevent the loss of wild species. Preservation can be achieved through a worldwide system of reserves, parks, and other protected areas. The plan put forward by biologists is ambitious, with a goal of setting aside 10 percent of Earth's land area. These preserves would conserve and manage entire ecosystems. This approach would be cheaper and more cost-effective than managing species one by one and would require less human intervention to prevent extinction. Activities in the preserves could include research and education as well as limited commercial activities such as ecotourism.

Elliot Richmond


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Brazil, a country that is 8,511,960 square kilometers (3,286,969 square miles), originally was home to 2,860,000 square kilometers of rainforest. In 2001, the coverage had been reduced to 1,800,000 square kilometers. The annual rate of deforestation, then, works out to be 2.3%, or 50,000 square kilometers. This is the equivalent of 1000 football fields every year in Brazil alone.

Worldwide, scientists estimate that nearly all the tropical rainforest ecosystems will be destroyed by the year 2030 if the current rate of deforestation continues.

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