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How Are Animals Classified?

By Jay W. Sharp

[How Are Plants Classified?]

Biological scientists estimate that collectively the earth’s 5 to 40 million species of organisms (depending on the estimate you choose to believe) make up a total of some two trillion tons of living matter, or biomass.  The plants comprise well over 90 percent of the biomass.  The animals, the focus of this article, comprise only a small percentage of the biomass, but they account for the majority of species. 

The prairie dog, which once numbered in the millions in our desert grasslands, is one of well over 100 species of mammals in the Southwest.

In accordance with the Linnaeus method, scientists classify the animals, as they do the plants, on the basis of shared physical characteristics.  They place them in a hierarchy of groupings, beginning with the kingdom animalia and proceeding through phyla, classes, orders, families, genera and species.  The animal kingdom, similar to the plant kingdom, comprises groups of phyla; a phylum (singular for phyla) includes groups of classes; a class, groups of orders; an order, groups of families; a family, groups of genera; and a genus (singular of genera), groups of species.  As established by Linnaeus, the scientists call an animal species, as they do a plant species, by the name of the genus, capitalized, and the species, uncapitalized.  So far, the scientists have classified and named something over a million animal species.  Without doubt, they have millions more to go.



Taxonomists, biological scientists who specialize in classifying and naming the living organisms, group the multicellular, independently mobile organisms that eat other organisms into the kingdom of animalia. The taxonomists recognize that the animals, unlike the plants, possess specialized tissues that may be organized into even more specialized organs, and they recognize that most animals, especially the more evolutionarily advanced species, have “bilateral symmetry,” which means that the right and left sides are essentially mirror images of each other. Critically, especially in the desert, animals, unlike plants, often utilize their mobility to seek refuge from environmental stresses such as intense heat and prolonged drought.

Animal Populations

Worldwide, the animal population consists of species numbering somewhere in the millions. The largest, the blue whale, may exceed 100 feet in length and 150 tons in weight. The smallest known animals, for instance, a parasitic wasp that taxonomists have named Dicopomorpha echmepterygis, measure no more than a few thousands of an inch in length.

The most abundant and diverse animal communities occupy earth’s most biologically productive regions, for example, the tropical rainforests, where the species of living organisms probably number in the millions. Conversely, the least abundant and diverse animal communities live in the least biologically productive regions, in particular, deserts like those of our Southwest, where the species of living organisms likely number in the tens to hundreds of thousands.

The biological richness of a tropical rainforest contrasts sharply with the biological impoverishment of our deserts. The net biological productivity of a typical area in a tropical rainforest may exceed that of a comparable area in our desert lands by a factor of 40 to 50 times, according to the Physical Internet site. Moreover, according to the Tropical Rainforest Biome Internet site, “Scientists believe that the tropical rainforests of the world might hold up to ninety percent of the plant and animal species on earth.” In a paper called “Tropical Biomes,” Professor Ralph E. Taggart, Michigan State University, said “The total biological diversity of only a few square kilometers of rich tropical rainforest can exceed that of entire regions in the temperate zone. Most of the plants and animals of the world are found in the complex mosaic of natural communities that make up this biome.” Nevertheless, our deserts host a diverse and highly adapted community of animals.

The Horned Lizard, an icon of the Southwest, is one of some four dozen species of reptiles in the Southwest.

The Animal Community

Taxonomists typically divide the animal kingdom into two “subkingdoms,” which include the invertebrates (animals without backbones) and vertebrates (animals with backbones). As with the plants, taxonomists turn the subsequent animal groupings and classifications, from phyla through genera, into a churning landscape that is simply a part of the scientific process. Depending on their academic roots and research, they divide and re-divide the animal community in many different ways, frequently regrouping, reclassifying and even re-naming species as they go. Some, called “lumpers,” identify species as belonging to the same group even though there may be small differences. Other scientists, called “splitters,” identify the same species as belonging in distinct groups because of the same small differences. The lumpers produce a relatively simple taxonomy, the splitters, a far more complex taxonomy.

The dragonfly serves as a good example of "bilateral symmetry," which means that the right and left sides are essentially mirror images of each other.

Classifying an Invertebrate

In our deserts, the invertebrate subkingdom includes phyla such as arthropods (insects, centipedes, spiders, scorpions, desert shrimp and many others), mollusks (snails) and annelids (earthworms). In the desert, as well as across the world, arthropods, measured in terms of abundance and diversity, rank at the top of all the animal phyla. An elegant insect, the monarch butterfly, serves as example of how the classification system works for the invertebrates.

At the phylum level, the monarch belongs to the arthropods, which share several physical characteristics. According to Barbara Terkanian, “A Vertebrate Looks At Arthropods,” A Natural History of the Sonoran Desert, the arthropods have jointed legs, and they have external skeletons, or exoskeletal material, that includes “eyes, mouthparts, antennae, body, legs, the fore and hind sections of the digestive tract, and some respiratory surfaces. Regions of flexible, unhardened exoskeleton serve as joints between neighboring segments.” The body cavity contains the digestive, circulatory, nervous and reproductive systems.

Monarch Butterfly

At the class level, the monarch has membership in the insect group, which comprises the overwhelming majority of the arthropods. The insects have several distinguishing physical characteristics, including three-part bodies, six legs (three pairs), compound eyes and two antennae. The class, called Insecta, includes three subclasses, according to Kendall Bioresearch Services Internet site. The first consists of insects that have never had wings throughout their evolutionary history. The young resemble the adults. The second subclass consists of insects that have wings at present or had them at some point during their evolutionary history. The nymphs resemble the adults. The third subclass consists of insects that have wings at present or had them at some point during their evolutionary history. The young take the form of larvae that change into adults during a non-feeding metamorphosis. The first subclass consists of four orders, including, for example, bristletails and springs tails. The second subclass has 16 orders, including, for instance, dragonflies; crickets, grasshoppers and locusts; termites and sucking lice. The third subclass has nine orders, comprising insects such as beetles; fleas; bees, wasps and ants; and the butterflies and moths.

At the order level, the monarch belongs to butterflies and moths, called Lepidoptera, which rank high among the most intriguing and conspicuous insect orders in the Southwest. They have two pairs of membranous, scaled and often brightly colored wings. Typically they have large eyes, long antennae and a long sucking tube (which the insect coils beneath its head when not feeding). The larvae, or caterpillars, all have silk glands that they use for spinning their cocoons. Their order contains well over 100 families.

At the family level, the monarch is the star of the milkweed butterflies, called Danaidae, which are among the best known in our deserts (as well as across the country). The milkweed butterflies usually have goldish wings trimmed in black, according to Donald J. Borror and Richard E. White, A Field Guide to the Insects of America North of Mexico. Their caterpillars feed on milkweed leaves, which invest both larvae and adults with a bitter and toxic taste that discourages predators.

At the genera level, the monarch is one of a mere handful of closely related species collectively called Dannaus. These species show apparently common evolutionary origins in their caterpillars, which share similar spots and smooth skin texture on their abdomens, according to David Munro, “The Biogeography of the monarch Butterfly,” San Francisco State University, Department of Geology, fall 1999.

At the species level, the monarch is called plexippus. It is, says Munro, “a medium sized butterfly, measuring about 3 inches from wingtip to wingtip. Its body is about one inch long. Its four wings are generally a field of yellow, orange or gold, with veins of black running through them. A band of black, thickest at the front, rings the wings, and the body is black as well. This black band is usually speckled with white spots, larger at the front and smaller at the back.”

The monarch, the aristocrat of the butterfly and moth world, bears the scientific name of Dannaus plexippus. In summary, it fits into the Linnaeus classification scheme as follows:

Classification Level

Scientific Name

















Common Name

Monarch Butterfly

Scientific Name

Dannaus plexippus

In one of the most compelling of nature’s mysteries, the monarchs, which have a nationwide distribution, begin annual epic migrations by the millions during the autumn. Those from our Southwestern states to the Northwest migrate to southern California. Those from the Midwest to the eastern seaboard migrate across the southern Chihuahuan Desert to the mountains of central Mexico, covering thousands of miles. Their journey is closely watched along the entire route. Their arrival in Mexico calls for celebration: “The butterflies have arrived in Saltillo (in the southeastern corner of the Chihuahuan Desert)!” one Mexican woman reported to the Monarch Butterfly Internet site in 1999. “Since early this morning (October 12), we have welcomed the arrival of the monarchs throughout the city, there have been hundreds of monarchs gliding through from 3 to 50 meters in height”

The butterfly, like the moth, is distinguished by its membraneous, scaled and often brightly colored wings.

Classifying a Vertebrate

The vertebrate subkingdom of our deserts includes phyla comprising a surprisingly diversity of mammals, birds, reptiles, amphibians and even fish.

“The Southwest contributes impressively to the continentwide diversity of mammals; native mammal species in southwestern states number about 120 in Texas, 138 in Arizona, 139 in New Mexico, and 163 in California. No other region in the country has so many mammal species,” says the United States Geological Survey in its Science for a Changing World Internet site.

The Western Bluebird is one of nearly 400 species of birds in the Southwest.

The Southwest contributes even more to the diversity of birds. Southeastern Arizona’s Chiricahua Mountains bird “checklist presently stands at 375 species [including the splendid Elegant Trogon], not including 18 species still considered hypothetical—about half of all the birds regular occurring on this continent north of Mexico,” according to Richard C. Taylor’s Location Checklist to the Birds of the Chiricahua Mountains. Central New Mexico’s Bosque del Apache’s checklist “contains 377 species which have been observed on the refuge since 1940,” according to the USGS Bird Checklists of the United States.

As for reptiles and amphibians, more than 150 species “occur in the Southwest; this is partly due to the region's diverse habitats,” says the USGS in Science for a Changing World. “Compared with the moister habitats of eastern North America, the southwestern amphibian fauna is relatively poor, with only 3 salamander species and about 30 frog and toad species. About 25% of the amphibians, though, are found nowhere else in the United States. About 50 lizard and 56 snake species account for more than 70% of the reptiles and amphibians inhabiting this region; most of these live in the arid and semiarid desert grasslands and shrublands that make up most of the Southwest.”

The nocturnal, smooth-scaled Glossy is one of nearly five dozen species of snakes in the Southwest.

The Southwest’s few water sources host a surprising variety of fish, although many of the species are imperiled by human assault on their habitat. According to the USGS, the Colorado River drainage system is home to numerous species including various chubs, the Colorado squawfish, the razorback sucker and the bonytail. The Rio Grande system of the Colorado River drainage system is home to chubs, the silvery minnow and shiners. Desert springs serve as habitat for pupfishes, springfishes and poolfishes.

An icon of the Southwest, the coyote, serves as a guide to how the classification system works for the vertebrates.

The Coyote is called Canis latrans, or barking dog.

At the phylum level, the coyote belongs to the Chordata, which is distinguished by a nerve chord that passes through the backbone. At the class level, the animal belongs to the Mammalia, the members of which possess fur and nurse their young. At the order level, the coyote, like his fellow Carnivora, shares “extreme night vision capabilities and various more broadly defined characteristics including: high intelligence, relatively long maturation period for the young, dental similarities, tendency for complex social organization, and generally bearing 2-3 young in a litter (but they may birth as many as 4-5 at one time),” according to the Internet site. At the family level, our guide and his close relatives, collectively called Canidae, typically have elongated faces. They have similar tooth arrangements, or “dental formulas.” Unlike, say, a bear, which belongs to another family of mammals, the Canidae walk on their toes. At the genus level, the coyote and the related species – including the gray wolf (northern Chihuahuan Desert and the Colorado Plateau), the red wolf (western Piney Woods) and your family dog – belong to the Canis. They all bear wolf-like characteristics, and they can interbreed, provided the male and female are of compatible size. (A male chihuahua would experience a difficult time mating with a female gray wolf.)

At the species level, the coyote is called Canis latrans, or barking dog, an allusion to the animal’s vocal talents. It is about the size and shape of “a medium-sized Collie dog, but its tail is round and bushy and is carried straight out below the level of its back,” according to ’s article on the animal. “Coyotes found in low deserts and valleys weigh about 20 pounds, less than half of their mountain kin, who can weigh up to 50 pounds. Desert coyotes are light gray or tan with a black tip on the tail.” William H. Burt and Richard P. Grossenheider point out in their A Field Guide to the Mammals of America North of Mexico, the coyote’s nose “is more pointed and [the] tail is bushier than normal in dogs; [its tail is] held down between [its] hind legs when running.” In summary, it fits into the Linnaeus classification scheme as follows:

Classification Level

Scientific Name

















Common Name


Scientific Name

Canis latrans

The coyote has long played a central part in the folk tales, myths and persona of the peoples of the Southwestern deserts, often taking on the role of clown or adventurer. In stories recorded by Charles F. Lummis in his Pueblo Indian Folk-Stories, foolish coyote falls for practical jokes played by ravens, blackbirds and a bear. Compassionate coyote saves an abandoned baby, bestowing it upon a mother antelope that has lost her fawn. Magical coyote jumps through a willow hoop and emerges as a handsome young man.

While fellow Canis Mexican gray wolf disappeared (until being reintroduced) from the Southwestern deserts in historic times, clever, adaptable and bold coyote has increased its population and expanded its range from its native desert basins well up into the mountains.

Thief coyote can make a nuisance of himself. “We always had a big watermelon patch,” said J. Frank Dobie in Some Part of Myself. “It had to be big if we were going to have any melons ourselves after the coyotes took their percentage. The patch was in a valley field only a few hundred yards away from the house. At night we could hear the coyotes laughing over how juicy the watermelons were.”

Related Pages:

How Are Plants Classified?/flora/plant-classified.html
Desert Animals & Wildlife
Desert Food Chain
Desert Plant and Wildflowers


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