Taxonomy and the Kingdoms of Life
Walk through a forest in Oregon and you might see a banana slug, a fern, a mushroom, a Douglas fir, and a woodpecker, all within a few steps. Millions of species live on Earth. How can scientists talk about all of them without getting lost? They sort them into groups, like a giant filing system for life. By the end of this page you will be able to place any organism into that system.
The Levels of Classification
Taxonomy is the science of naming and grouping organisms based on their similarities and differences. The groups are nested, like boxes inside boxes. The biggest box holds the most organisms. Each smaller box holds fewer organisms that share more traits.
There are eight levels, from broadest to most specific:
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Domain
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Kingdom
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Phylum
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Class
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Order
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Family
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Genus
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Species
A classic memory trick: Dear King Philip Came Over For Good Soup.
Going down the levels, the groups get smaller and the organisms in each group look and behave more alike. A species is the most specific level: it includes the fewest organisms.
List the levels of the taxonomic classification system from the broadest to the most specific: domain, kingdom, phylum, class, order, family, genus, and species. Write…
List the levels of the taxonomic classification system from the broadest to the most specific: domain, kingdom, phylum, class, order, family, genus, and species. Write them in order, and say which level includes the fewest organisms.
Scientific Names
Common names can be confusing. A "cougar" is also called a mountain lion, puma, or panther, depending on where you live. To avoid mix-ups, scientists use a two-part name called binomial nomenclature. It uses the genus and the species.
The gray wolf is Canis lupus. Canis is the genus. lupus is the species.
Rules for writing a scientific name:
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The genus is capitalized. The species is lowercase.
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The whole name is in italics (or underlined if you handwrite it).
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After the first use, the genus can be shortened: C. lupus.
Organisms in the same genus are very closely related. The domestic dog is Canis familiaris and the coyote is Canis latrans. Their shared genus tells you they are close relatives of the wolf.
The scientific name of the gray wolf is Canis lupus*. Which part is the genus and which part is the species? Describe one rule for writing scientific names.
The scientific name of the gray wolf is Canis lupus. Which part is the genus and which part is the species? Describe one rule for writing scientific names.
Reading a Classification
Two organisms are more closely related if they share more levels. Find the first level where they differ. The later that happens, the closer the relationship.
Two organisms have these classifications: Organism X: Animalia, Chordata, Mammalia, Carnivora, Felidae, Panthera*, Panthera leo* Organism Y: Animalia, Chordata,…
Two organisms have these classifications:
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Organism X: Animalia, Chordata, Mammalia, Carnivora, Felidae, Panthera, Panthera leo
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Organism Y: Animalia, Chordata, Mammalia, Carnivora, Felidae, Panthera, Panthera tigris
(a) At what level do they first differ? (b) Would they be more closely related to each other or to a gray wolf (Canis lupus, family Canidae)? Explain.
The Kingdoms of Life
The kingdom level sorts organisms using two big questions. Are the cells prokaryotic or eukaryotic? (A prokaryotic cell has no nucleus. A eukaryotic cell has a nucleus.) How does the organism get energy?
| Kingdom | Cell type | How it gets energy | Examples |
|---|---|---|---|
| Bacteria | Prokaryotic (one cell) | Varies: make food by photosynthesis, or absorb nutrients from the environment | E. coli, gut bacteria |
| Archaea | Prokaryotic (one cell) | Varies; many live in extreme places | Microbes in hot springs at Yellowstone |
| Protista | Eukaryotic (mostly one cell) | Varies: some make food, some eat other microbes | Paramecium, algae |
| Fungi | Eukaryotic | Absorb nutrients from dead or living material | Mushrooms, yeast, mold |
| Plantae | Eukaryotic (many cells) | Make food by photosynthesis | Oak tree, corn, ferns |
| Animalia | Eukaryotic (many cells) | Eat other organisms (consumers) | House cat, woodpecker, coral |
Some books combine Bacteria and Archaea into one kingdom, Monera. Both are prokaryotes.
Plant cells have cell walls and chloroplasts. Fungi have cell walls made of a different material (chitin) and no chloroplasts. Animal cells have no cell walls. Bacterial cells have cell walls but no nucleus. Photosynthetic bacteria make food without chloroplasts, which are found only in plant cells and some protists (algae).
Name the kingdom for each organism: (a) a mushroom, (b) an oak tree, (c) a Paramecium*, (d) a house cat, (e) E. coli* bacteria.
Name the kingdom for each organism: (a) a mushroom, (b) an oak tree, (c) a Paramecium, (d) a house cat, (e) E. coli bacteria.
Complete the table by describing each kingdom's cell type (prokaryotic or eukaryotic), how it gets energy, and one example. | Kingdom | Cell type | How it gets energy |…
Complete the table by describing each kingdom's cell type (prokaryotic or eukaryotic), how it gets energy, and one example.
| Kingdom | Cell type | How it gets energy | Example |
|---|---|---|---|
| Animalia | |||
| Plantae | |||
| Fungi |
Liam finds a new organism in a pond. It is made of one cell with a nucleus, it moves using hair-like structures, and it eats smaller microorganisms. Using these…
Liam finds a new organism in a pond. It is made of one cell with a nucleus, it moves using hair-like structures, and it eats smaller microorganisms. Using these observations, decide which kingdom it most likely belongs to, and explain why it does not belong to Bacteria, Plantae, Fungi, or Animalia. Describe one additional observation that would help confirm your choice.
Kingdoms in Ecosystems
Each kingdom plays a role in an ecosystem's energy flow:
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Producers (plants, algae, and some bacteria) make food from sunlight. They are the base of food chains.
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Consumers (animals and some protists) get energy by eating other organisms.
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Decomposers (fungi and many bacteria) break down dead organisms and waste. They return nutrients to the soil and water so producers can use them again.
In a Pacific Northwest forest, fungi and bacteria break down fallen logs and leaves. Without decomposers, dead material would pile up and nutrients would stay locked inside it. Plants would run short of nutrients, and the whole food web would suffer.
Gut bacteria are also helpful: they help animals digest food.
A coral reef off Florida shows all the roles together: algae living in coral and floating in the water make food, coral animals and fish are consumers, and bacteria and fungi break down dead material. The producers sit at the bottom of the energy pyramid, so losing them would cut off energy for every level above.
Fungi and bacteria are both decomposers in a forest in the Pacific Northwest. Explain what decomposers do for the ecosystem and describe why the ecosystem would suffer…
Fungi and bacteria are both decomposers in a forest in the Pacific Northwest. Explain what decomposers do for the ecosystem and describe why the ecosystem would suffer if they were missing. Also name one structural difference between a fungus cell and a bacterial cell.
A coral reef off the coast of Florida includes algae, coral animals, fungi, and bacteria. (a) Assign each group to a kingdom. (b) Explain the ecological role of each…
A coral reef off the coast of Florida includes algae, coral animals, fungi, and bacteria. (a) Assign each group to a kingdom. (b) Explain the ecological role of each group (producer, consumer, or decomposer). (c) Argue which group's loss would most seriously disrupt energy flow through the reef, and justify your choice with evidence from trophic levels.
Back to the start: a forest with a slug, fern, mushroom, fir, and woodpecker is easy to sort. Slug and woodpecker are in Animalia, fern and fir are in Plantae, and the mushroom is in Fungi.
Summary
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Taxonomy groups organisms in eight nested levels: domain, kingdom, phylum, class, order, family, genus, species. Species has the fewest organisms.
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A scientific name has two parts, genus and species. The genus is capitalized, the species is lowercase, and both are italicized (Canis lupus).
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The more levels two organisms share, the more closely related they are.
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Kingdoms are sorted by cell type (prokaryotic or eukaryotic) and how they get energy: Bacteria, Archaea, Protista, Fungi, Plantae, Animalia.
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Plants are producers, animals are consumers, and fungi and many bacteria are decomposers that recycle nutrients.