How Scientists Classify
Plants and Animals

The Desert Food Chain

by Jay Sharp

For centuries, biological scientists have worked to classify earth’s organisms in a way that would help people understand how both the present and the extinct species relate to each other. What are the similarities? What are the differences? How, for instance, does a desert poppy relate to a desert willow? How does a ladybug relate to a mule deer? How does a modern desert rock squirrel relate to the extinct Ice Age wooly mammoth?

In 1753, a Swedish scientist, Carl Linnaeus, devised a system for grouping, classifying and naming organisms on the basis of shared physical characteristics, which would serve as a measure of biological relationships. (If they had many shared characteristics, he supposed they must be more closely related than organisms that had few shared characteristics.) Since then, his successors have refined and expanded his system many times. While some have devised other kinds of systems, many use the system derived from Linnaeus’ original idea for classifying organisms. They now recognize five kingdoms of living organisms:

Plantae (the plants)
Animalia (the animals)
Fungi (toadstools, mushrooms, yeasts, mold)
Monera (most bacteria and blue-green algae)
Protista (microscopic organisms called protozoa)

The first two kingdoms, the plants and the animal, form principal links in earth’s food chains, including those of the Southwestern deserts.

How the System Works

In the system founded by Linnaeus, the plant and animal kingdoms include categories of organisms known as:

Phyla
Classes
Orders
Families
Genera
Species

Each of the categories from the phyla through the genera includes groups with increasingly closely shared physical characteristics. The final category, species, includes organisms that all have very similar physical characteristics. In the custom established by Linnaeus, scientists still call a species by the name of the genus (singular of genera), capitalized, and the species, uncapitalized. For instance, they call the American black bear an Ursus americanus (the animal’s genus is Ursus and its species is americanus).

A Ladybug, an Invertebrate

As a game, imagine that two brothers each own jackets that have the shared physical characteristics of red color, zip-up fronts and waist-lengths, with each garment having a hood and three pockets. They both hang their virtually identical red jackets randomly in a closet that they share with their other brothers and sisters.

First, think of all the garments hanging in the closet as a kingdom of “Clothes.” How would the red jackets be classified and named?

First, the kingdom of Clothes would include phyla called Shirts, Pants, Dresses, Blouses, Skirts and Coats.

The phylum (singular of phyla) of the Coats could include classes called Blazers and Jackets.

The class called Jackets could include orders of Brown Jackets, Blue Jackets and Red Jackets.

The order of the Red Jackets might include families of Button-up Jackets and Zip-up Jackets.

Praying Mantis

Praying Mantis, an Invertebrate Predator

The family of Red Zip-up Jackets could include genera of virtually identical Long Jackets – with no hoods and no pockets – and virtually identical Waist-length Jackets – with hoods and three pockets.

The genera of Waist-length Jackets would include a species with the shared physical characteristics of red color, zip-up fronts and waist-lengths, with each garment having a hood and three pockets.

This species, of course, would be the jackets belonging to the two brothers. The species would be the Hooded zip-up (the genus and species) in accordance with the convention established by Linnaeus.

The identification and naming of the jackets would be more complex if another brother owned one virtually identical to Hooded zip-up except that it has one extra pocket and some blue trim. Would his rank as the same species with slight variations or a different species with distinctive differences? Scientists often find relatively small differences between otherwise similar species, raising significant problems in classifying and naming organisms.

Spider web, with spider, another invertebrate, waiting almost hidden in the hole.

Classifying Plants

What makes plants – the producers – distinctive from other organisms, specifically, the animals?

Most members of the kingdom of the plants, unlike the members of the kingdom of the animals, manufacture their own food through photosynthesis (See Part II: The Desert Food Chain: Photosynthesis). They have no ability to move independently. They lack specialized tissues such as muscles, nervous systems and digestive chambers. While biological scientists often disagree about just where to place the plants, leading to considerable confusion, they typically all use plant tissues and seed structures as criteria for classification.

For example, scientists may place plants into two broad categories, one for those with vascular tissues and another for those with non-vascular tissues. The vascular tissue plants, like desert poppies and desert willows, can conduct liquids, including water, from the roots through the plant stems and leaves. Non-vascular plants, specifically, mosses, which typically grow near desert springs or seeps, cannot conduct liquids, relying instead on the surrounding moisture to meet their needs for water.

Globemallow

Globemallow, a plant classified as vascular because it can conduct water from its roots up through its stems.

Scientists may also classify plants on the basis of seeds. Some plants, for example, pinyon pines that grow at some of the higher desert elevations produce uncovered seeds that develop within the plant’s cones. Other plants such as desert grasses and some flowering plants produce seeds each with a single embryonic (unborn) leaf. Still others, for instance, the prickly pear cacti, flower and produce fruit with seeds each with two embryonic leaves. Ferns and some allied plants such as horsetails, are among the more primitive plants, and reproduce from spores rather than seed.

Other scientists may rank plants, not only on the basis of tissue and seeds, but also on the basis of stature in the plant world, including in deserts, for instance, the cacti, yuccas, agaves, grasses, shrubs and the annuals.

Scientists’ range of approaches to classifying and ranking plants reflects the boggling complexity of the kingdom Plantae.

Fish, a vertebrate, portrayed on stone by a Native American of the desert long before the Spanish came to the Southwest.

Classifying Animals

What makes animals – the consumers – distinctive from plants?

Red-tailed Hawk

Red-tailed hawk, vertebrate and predator.

First, animals, unlike typical plants, eat other organisms to survive. Most animals, unlike plants, can move themselves from place to place. They can seek refuge from high heat and prolonged droughts. They have specialized tissues such as muscles, nervous systems and digestive systems. They tend to fit more neatly than plants into the classification system.

As another game, consider how you might classify and name (scientifically) a dog.

First, scientists have divided the kingdom of the animals into two main groups, or phyla: the invertebrates (those with no backbones) and vertebrates (those with backbones).

Mule Deer and Fawn

Mule deer, with a brand new fawn. The mother mule deer shows the hardships of living in the desert. A vertebrate mammal, she will nurse her fawn.

The phylum of invertebrates includes various classes with creatures such as insects (including the ladybug), spiders, centipedes, snails, worms and many more animal organisms that do not have spines.

The phylum of vertebrates include classes such as reptiles, amphibians, fish, birds and mammals (including the mule deer, rock squirrel and the extinct wooly mammoth). A dog, of course, would belong to the phylum of the vertebrates and the class of the mammals, which nurse their young and typically bear fur.

The class of the mammals includes, for a few examples, orders such as Proboscidea (elephants), Perissodactyla (horses and donkeys), Rodentia (rats and mice), Primates (monkeys) and Carnivora (dog-like animals, cat-like animals, bears, skunks and badgers).

Worldwide, the largely meat-eating Carnivora order consists of some 11 families. (University of Michigan’s Animal Diversity Web site) These include, as a few examples, the families of the Canidae, or the dog-like animals; the Felidae, or cat-like animals; and Ursidae, or the bears.

The family of the Canidae includes the genus of the Canis, which includes species of the dogs, wolves and coyotes.

Scientific names of a few representative species are, for the dog, Canis familiaris; for a red wolf, Canis rufus; for a gray wolf, Canis lupus; and for a coyote, Canis latrans.

The dog, therefore, would be classified in the kingdom of the animals, as a vertebrate, a mammal, a meat-eater, a wolf-and coyote relative, and a domesticated Canis species. Scientifically, you would name him as a Canis familiaris, or a domesticated dog.

Canis familiaris, a domesticated dog.

A Ways to Go

Since the 18th century, biological scientists have classified and named some 1.6 million organisms worldwide, including the representatives of all five kingdoms. Faced with mind-numbing complexity, they have been unable to reach a full agreement on classification systems or rankings within the existing systems. Meanwhile, they estimate that they may have another 10 to 30 million organisms to go.

Food Chain Introduction
How Do Green Plants Manufacture Their Own Food?
How Do Desert Plants Survive?
How Do Desert Animals Survive?
The Arithmetic of the Food Chain
The Classification of Desert Plants & Animals

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