Grouping Living Things and Using Keys

A garden holds many kinds of living things. You can make them easier to study by putting them into groups. To classify means to group things by what they share.

Pick one feature

A feature is something you can notice about a living thing. Wings, feathers, legs and flowers are features. Your sorting rule, or criterion, says which feature you will check. For example, ask, “Does it have wings?” Make two groups: “Has wings” and “Does not have wings.”

Check each animal against that same rule. A penguin has wings, so it belongs in “Has wings.” It cannot fly, but flying is a different rule. Changing your question halfway through gives you a muddled group.

Good rules give clear answers. You and a friend can count six legs and agree. You may disagree about whether an animal is pretty. “Is it big?” also needs a clear comparison. “Is its body longer than your hand?” gives you something to check.

Sort the same animals again

Try a robin, goldfish, frog, butterfly, bat and snake. First ask, “Can it fly?” The robin, butterfly and bat go together. The goldfish, frog and snake form the other group.

Now ask, “Does it have a backbone?” The robin, goldfish, frog, bat and snake go together. Only the butterfly goes in the group without a backbone. Both sorts work because each follows its own rule.

You can also group animals by their food, body covering or home. Scientists often choose lasting body features to show how animals are built. A bat has fur and feeds its babies milk. Those features place it with mammals, even though it flies.

Plants, animals and mushrooms

Plants and animals are two large groups of living things. Plants are usually green and make their own food using sunlight. Animals get food by eating. Most animals can move their whole body from place to place.

A mushroom grows from the ground, but it is a fungus. It cannot make food from sunlight like a green plant. It belongs in neither a plant group nor an animal group. Fungi are another group of living things.

Scientists have named about two million kinds of living things. Clear groups help people identify them and share what they learn.

Once you have sorted living things, show where each one belongs. Labels help another person understand your groups without guessing your rule.

Use a sorting table

For one rule, make a table with two columns. Ask, “Does it have wings?” A penguin and a butterfly have wings. A rabbit and an earthworm do not. Write each name once, under the correct heading. Then check that all four names appear.

Use two circles

A Venn diagram shows two rules with circles inside a box. The circles cross in the middle. That middle part is the overlap. Put an animal there when it fits both rules. An animal that fits neither rule stays inside the box, outside both circles.

Try the circles “Has wings” and “Has a backbone.” Sort a robin, bat, butterfly, goldfish, frog, snake and earthworm:

• Both circles: robin and bat.
• Wings only: butterfly.
• Backbone only: goldfish, frog and snake.
• Outside both: earthworm.

Count the names in all four places. There are seven names, matching the seven animals. Do not write the robin twice, once in each circle. Its single place in the overlap already shows both features.

Read a grid

A Carroll diagram uses rows and columns for two rules. Read a row label and a column label together. Their crossing box holds living things that match both labels.

Find “Lives in water,” then move across to “Has no backbone.” You reach the jellyfish. Reading backward from that box tells you two facts about it. A box can hold several names, one name or no names.

Check the meaning of a place

Imagine putting the bat in the wings-only part of your Venn diagram. That place says it has wings but no backbone. The bat does have a backbone, so move it to the overlap.

Choose a table for a single rule. Choose a Venn diagram to show shared features in overlapping groups. Choose a Carroll diagram to give every living thing one box for two rules.

You can sort every animal by asking one question: “Does it have a backbone?” This separates animals into vertebrates and invertebrates.

What a backbone is

Your backbone, or spine, runs down the middle of your back. It is a column of small parts called vertebrae. You can feel bumps along it through your skin. In most vertebrates, the vertebrae are bones. In sharks and rays, they are made of bendy cartilage.

A vertebrate has a backbone inside its body. Fish, amphibians, reptiles, birds and mammals are vertebrates. Humans are mammals, so you belong in this group too.

An invertebrate has no backbone. Most known kinds of animals are invertebrates: more than nine out of ten. Insects make up the biggest animal group.

Hard coverings can fool you

Some invertebrates have a hard outer covering called an exoskeleton. Beetles, spiders and crabs have this kind of protection. A snail has a shell. Earthworms, slugs and jellyfish have soft bodies.

A hard covering outside an animal is different from a backbone inside. A crab's hard body does not make it a vertebrate. A soft, furry mouse has a backbone hidden inside it.

Work through a group

Sort a cat, beetle, shark, octopus, frog and earthworm. Start with what you know about each animal's body.

The shark still belongs with vertebrates. Cartilage makes its backbone bendy; it does not remove the backbone.

Use body clues

You usually cannot see a backbone by looking at an animal. Fur or feathers help you recognize a vertebrate group. Six jointed legs suggest an insect; eight suggest an arachnid. Those are invertebrate groups.

Size is a poor clue. A giant squid is an invertebrate, even though it can grow longer than a car. Some frogs are smaller than a fingernail, yet they have backbones. Use the animal's body structure to decide.

A whale swims and a bat flies. Their movements do not tell you their animal group. Look at body covering, breathing and how they feed their young.

Match the body clues

Gills take oxygen from water. Fish use gills to breathe. Birds, reptiles and mammals breathe air with lungs. Young amphibians such as tadpoles usually use gills. Adult amphibians usually use lungs and also breathe through their skin. A toad's skin is bumpier and drier than a frog's.

Compare how young begin life

Most fish lay eggs in water. Most amphibians lay jelly-covered eggs there too. Most reptiles lay tough or leathery-shelled eggs on land. All birds lay hard-shelled eggs.

Most mammals give birth to live young. However, a platypus and an echidna lay eggs. They still have fur and feed their young milk, so they are mammals. Some snakes also give birth to live young. Their dry scales and other reptile features still place them with reptiles.

Solve the surprising cases

Check a whale one clue at a time. It lives in water, but comes up to breathe air. It feeds its baby milk and gives birth to live young. These clues place it with mammals. Whales and dolphins have very few hairs; some have them only as newborns.

A bat has fur and feeds its babies milk. It is a mammal with wings. A penguin has feathers and a beak. It is a bird that cannot fly. A snake has a backbone and dry scales. It is a reptile without legs.

Warm-blooded and cold-blooded

Birds and mammals are called warm-blooded. Their bodies keep a steady warm temperature. Most fish, amphibians and reptiles are called cold-blooded. Their body temperature changes with their surroundings. A lizard warms itself by lying in sunlight. “Cold-blooded” does not mean its body is always cold.

A small animal crawling under a leaf might be an insect. It might also be a spider, woodlouse or worm. Looking closely helps you name its group.

Insects and spiders

An adult insect has six legs and three main body parts. The head is at the front. The thorax is the middle part, where the legs attach. The abdomen is at the back. Insects have a pair of feelers called antennae. Many adults also have wings. Ants, bees, beetles and butterflies are insects.

A spider has eight legs and two main body parts. It has no antennae or wings. Spiders belong to the arachnids, along with scorpions, ticks and mites.

For a ladybug, count six legs, then find its three body parts and antennae. These clues identify an insect. A garden spider has eight legs, two body parts and no antennae. Small size and jointed legs do not make both animals insects.

Snails, slugs and earthworms

Snails and slugs are mollusks. They have soft bodies and glide on one muscular foot. A snail carries a shell it can hide inside. A slug has no shell to hide in. Octopuses and clams are mollusks too.

An earthworm has a long, soft body with many ring-shaped segments. It has no legs, eyes or bones. It stretches and squeezes its body to move.

Animals with more legs

Woodlice are also called pill bugs, sow bugs or roly-polies. Adult woodlice have fourteen legs and a hard, jointed covering. They need damp places and belong to the same group as crabs and shrimp.

Centipedes and millipedes have many body segments. On a centipede's leg-bearing segments, look for one pair of legs. A millipede has two pairs on most segments. Their names are not exact leg counts. Adult centipedes have from thirty to more than 350 legs, and none has exactly 100.

Use more than one clue

Start by counting legs, but also check body parts and coverings. A long animal without legs could be a worm or a slug. Ring-shaped sections point to an earthworm; a gliding muscular foot points to a slug. Water animals can be invertebrates too. Crabs, jellyfish and sea stars all lack backbones.

A flowering plant does not need to show bright petals today. Grass, an oak tree and an apple tree are flowering plants too. You need clues about what the plant grows over time.

Flowers, fruits and seeds

Flowering plants grow flowers and make seeds. After pollination, often by insects or wind, fruits develop from flowers. A fruit holds seeds. Apples, pea pods and acorns are fruits, even though they look very different.

Imagine an apple tree in the fall. Its spring flowers are gone, but apples hang from its branches. Cut-open apples show seeds inside. The fruits tell you that the tree is a flowering plant.

A pine tree makes seeds too, but carries them in cones. Seeds alone do not identify a flowering plant. Check whether the seeds form in fruits that develop from flowers.

Small flowers still count

Some flowers are large and colorful, like sunflower heads. Others are small, green or easy to miss. Grass and wheat have tiny flowers. Oak trees make hanging clusters of flowers called catkins in spring.

A lawn is usually cut before its grass can flower. That does not change its plant group. The question is whether the plant ever makes flowers, not whether you can see a flower on this visit.

Trees belong to different groups

“Tree” describes a plant's woody trunk and size. It does not tell you whether the plant flowers. Apple, cherry, maple and oak trees are flowering plants. Pine trees are non-flowering plants.

Look at these clues in order:

1. A rose bush has open flowers. It is a flowering plant.
2. An apple tree has fruits holding seeds. It is also a flowering plant.
3. Uncut grass has flower and seed heads. Its flowers are small, but it belongs in the same group.

Look at other features too

Once you know a plant's group, other features can help you identify it. Compare leaf shape, flower color and woody or soft stems. You can also check whether it loses its leaves in the fall. These features help you make smaller groups within flowering plants. A guide with pictures from different seasons can help when flowers are missing.

Some plants never grow flowers. Mosses, ferns and conifers are three groups of non-flowering plants. They make new plants in two different ways.

Mosses make spores

Mosses are small, soft plants that often form green carpets. Look for them in damp, shady places, such as stones and tree trunks. Tiny threads hold them in place; they have no true roots.

A moss has no flowers or seeds. It makes spores in little capsules on thin stalks. A spore is a tiny speck that can grow into a new plant. Spores are much smaller than seeds. Wind can carry them away.

Ferns make spores too

A fern has roots, stems and large leaves called fronds. Many fronds look feathery. New ones uncurl from tight coils. Ferns make no flowers or seeds.

On the underside of some fern fronds, you can see small brown dots. These are clusters of tiny cases that hold spores.

Conifers make seeds in cones

Pines, firs, spruces and redwoods are conifers. These trees and bushes carry seeds in cones. Most have needle-like or scaly leaves. Most keep their leaves all year.

A cone holds and protects seeds. It is neither a flower nor a fruit. A conifer therefore makes seeds without belonging to the flowering-plant group.

Try two sorting rules

Sort a daisy, pine, fern, moss, oak and grass. With “Grows flowers,” the daisy, oak and grass go together. The pine, fern and moss never flower.

Now use “Makes seeds.” Move the pine beside the daisy, oak and grass. The fern and moss stay together because they make spores.

Keeping green leaves in winter is not enough to identify a conifer. Holly stays green but grows flowers. Larch is a conifer that drops its needles in the fall.

You find a small animal under a leaf. A classification key helps you identify it using questions about its features. You work from what you can see toward a name.

Read the branches

A branching key starts at the top and splits into paths. Answer its first question. Follow the branch labeled with your answer. You reach either another question or a name.

A key that splits into two at each question is called dichotomous. That word means “cut in two.” Yes-or-no questions are a useful way to make these two paths.

A key for six garden animals

This key names a snail, earthworm, butterfly, beetle, spider and woodlouse.

• Ask, “Does it have legs?”
  • No: ask, “Does it have a shell?” Yes → snail. No → earthworm.
  • Yes: ask, “Does it have six legs?”
    • Yes: ask, “Does it have large, colorful wings?” Yes → butterfly. No → beetle.
    • No: ask, “Does it have eight legs?” Yes → spider. No → woodlouse.

Read only the questions on your path. You do not need to answer every question in the key.

Work through a spider

First, look at the animal's legs. It has legs, so follow “Yes” at the first split. Count eight legs. At “Does it have six legs?” follow “No.” Then answer “Yes” to “Does it have eight legs?” You reach spider.

You answered three questions. You skipped the questions about a shell and large wings because they were on other paths.

Check an unexpected name

If the name does not match the animal, return to the start. Check each answer carefully. A leg hidden behind the body can make your count wrong. A magnifying glass helps you see small features.

Also check which animals the key can name. Try a centipede in this six-animal key. It has legs, but not six or eight. That path ends at “woodlouse,” even though it is a centipede. The centipede was never included in the key.

Choose a key that includes the animal or group you are studying. The questions narrow down that key's list; they cannot name every living thing in the world.

A key can be a numbered list instead of a branching drawing. Each answer tells you a name or says “go to” another number. The questions do the same job in both layouts.

Read a plant key

Use this key with pictures showing five plants. The dandelion and white-petaled daisy are in flower. The pine picture shows cones. The fern shows its fronds, and the moss forms a low green patch.

1. “Does it have flowers?” Yes: go to 2. No: go to 3.
2. “Are its petals yellow?” Yes: dandelion. No: daisy.
3. “Does it have cones?” Yes: pine. No: go to 4.
4. “Does it have large, feathery fronds?” Yes: fern. No: moss.

Trace the fern's path

Start at question 1. The fern has no flowers. The instruction sends you to question 3, so skip question 2.

At question 3, check for cones. The fern has none. Follow “No: go to 4.” At question 4, its large, feathery fronds give “Yes.” You reach the name fern. Your path is 1 → 3 → 4 → fern.

Question 2 never helps identify this fern. It separates only the two plants already known to have flowers.

Keep your place

“Go to 3” means find question 3. It does not mean take three steps forward. Keep a finger on the question you are answering. After choosing an answer, move to its next number. Writing your path makes it easier to check a mistake.

For the pine picture, question 1 gives “No.” Question 3 gives “Yes,” and you stop at pine. Do not continue to question 4 after reaching a name.

Match the key to what you see

This picture key depends on flowers and cones being visible. An actual dandelion without flowers would take the wrong path. For plants out of season, choose a key that asks about features you can see, such as leaves.

You can turn this list into a branching drawing. Give each question two labeled paths, ending in the same numbers or names. Another correct key might ask about cones first. Test every plant to see whether its truthful answers reach its own name.

You can make a key for a set of living things. Start by looking at the whole set. Choose features that divide it into smaller groups until each name stands alone.

Build the first split

Make a key for a ladybug, spider, earthworm and snail. Ask, “Does it have legs?” The ladybug and spider answer “Yes.” The earthworm and snail answer “No.” Both branches still hold two animals, so both need another question.

On the legged branch ask, “Does it have six legs?” Yes → ladybug. No → spider. On the legless branch ask, “Does it have a shell?” Yes → snail. No → earthworm.

You now have three questions and four names. Each question splits a group into two. You stop splitting when only one animal remains on a branch.

Choose questions people can answer

A useful question checks one clear feature. Legs, shells and leaf shapes are often helpful. Avoid asking about what an animal is doing at that moment. A resting beetle is still the same beetle when it starts walking.

“Is it cute?” is an opinion. “Is it big?” needs a clear size. “Is its body longer than 5 centimeters, about 2 inches?” can be checked with a ruler. Use size only when it separates the living things in your set reliably.

Ask about one feature at a time. “Does it have six legs and wings?” mixes two checks. Separate them so you can see which feature makes the groups different.

Count the questions

When every question splits a group into two nonempty parts, you need one fewer question than names. Four living things need three questions. Six need five, and ten need nine. This counts all the questions in the complete key, not how many you answer on one path.

Test and repair

Start every animal at the first question. Follow its answers to the end. Each must reach its own name.

Suppose you add an ant beside your ladybug. Both have six legs, so both now reach “ladybug.” Add a question that separates those two animals, such as “Does it have spots on its back?” Check the pictured ant and ladybug before choosing it.

Finally, ask a friend to use your key without help. If a question confuses them, rewrite it. Then test every living thing again, including the ones that worked before.

A habitat is where a living thing lives and finds what it needs. A park holds smaller homes too: leaf litter, tree bark and the damp space under a log.

Look with care

Search several spots to find different living things. A magnifying glass or bug viewer helps you see small bodies. A soft paintbrush or spoon can gently move a small animal for a closer look. Use a key or picture guide to identify it.

Handle animals briefly and gently, then return them to their exact spot. Put logs and stones back in place. Look at wild plants where they grow.

Work with an adult, especially near water. Never touch or taste unknown mushrooms, berries or plants. Use a tool for animals that could sting or bite. Wash your hands when you finish.

Record animals and kinds

Make a tally chart with a row for each kind. Add one mark whenever you find another animal. Groups of five tally marks make counting easier.

A class searches two places for ten minutes each. Under a damp log they find twelve woodlice, four earthworms, three slugs and one centipede. There are twenty animals altogether, but only four kinds. On a sunny, dry path they find one woodlouse and five ants: six animals of two kinds.

“More animals” and “more kinds” mean different things. Twelve woodlice count as twelve animals but one kind. Count rows with finds to work out how many kinds you found.

Make a fair comparison

Search both habitats for the same time, over the same area and with the same tools. A twenty-minute search can turn up more animals than a two-minute search just because it lasts longer. Different methods make the habitats hard to compare.

The damp log gave shelter to more animals and more kinds in this search. Woodlice need damp places because they dry out in hot sun. A pond offers a different home, including water for animals with gills.

Say what your search showed

A short visit can miss hidden or night-active animals. Some living things appear only in certain seasons. Write, “We did not find any earthworms on the path.” That reports your finding clearly. Saying, “Earthworms never live there,” goes beyond what you saw.

The environment is everything around a living thing. It includes air, water, soil, light, temperature and nearby living things. Its habitat is part of that environment.

Changes happen in different ways

Some changes are natural. Day becomes night, seasons turn, and a hot spell can dry a pond. Storms, floods, droughts and wildfires can change habitats too. A drought is a long period with little rain.

People also change environments. They build roads and homes, cut down forests and drain ponds. They can bring in animals or plants from other places. These changes may remove food, shelter or space from living things already there.

Some changes happen quickly. A wood might be cleared in a week. Others build up slowly over many years. Earth's warming climate melts sea ice and warms oceans.

Look before and after

Imagine a field with long grass, wildflowers and a small pond. Later, the field becomes a parking lot. Compare each part:

• Grass and flowers have been removed.
• The pond has been filled in.
• Pavement covers the soil, so rain cannot soak into it there.

Now you can explain the change using clear observations. “The flowers and pond are gone” tells you more than “The place looks different.”

Spot pollution

Pollution happens when harmful things get into air, water or land. Animals can swallow plastic or become tangled in it. Oil spills can foul seawater. Farm or factory chemicals can wash into rivers. Smoke and fumes can pollute the air.

Litter is one kind of pollution, but pollution is not always a visible pile of trash. Harmful chemicals can be mixed into water.

Changes can help too

People plant trees and hedges, create ponds and grow wildflowers. They protect places as nature reserves and clean litter from rivers. These actions change environments by providing or protecting what living things need.

A change is not always dangerous. Many animals cope with seasonal changes by growing thicker fur, moving to warmer places or sleeping through winter. To judge a change, look at which needs it affects. A rapid change over a large area can leave little time or space to find another suitable home.

A changed habitat can take away food, water, shelter or space. To explain the danger, connect the change to a need and then to what might happen.

Build a chain of reasons

Animals need food, water, shelter and safe places for their young. Plants need light to make food, plus water, air, nutrients and space to grow.

Use three steps: change → need lost → possible result. For example, “The pond was filled in. Frogs lost a place to lay eggs. Fewer young frogs may grow up there.”

The effect can spread. Herons and snakes that eat frogs lose part of their food too. Many living things cannot simply leave. Plants are rooted in place. Small animals may not travel far, and a suitable home may be hard to reach.

A polar bear hunts seals from sea ice. When the ice melts earlier in spring, it has less hunting time. Losing that hunting place means less food.

Endangered and extinct

An endangered kind of living thing faces a serious danger of dying out. Very few may remain, or threats may make survival difficult. Tigers and blue whales are endangered. Some plants, frogs, bees and butterflies are endangered too.

Extinct means none of that kind remain alive anywhere. An endangered kind still has living members that people can help. An extinct kind has none left to have young. Losing an animal from one pond is different from losing its entire kind worldwide.

The dodo

The dodo was a flightless bird found only on Mauritius. Dutch sailors arrived in 1598. People hunted dodos, and introduced animals such as pigs and rats harmed the birds and their eggs. By the end of the 1600s, the dodo was extinct. Less than a hundred years had passed.

The passenger pigeon

Billions of passenger pigeons once flew across North America. People hunted huge numbers and cut down forests the birds needed. The last known passenger pigeon, Martha, died at Cincinnati Zoo in 1914. The kind became extinct.

Large numbers once did not keep these birds safe forever. Hunting removed birds while forest loss removed their homes. Connecting both changes to survival helps explain how such a common bird disappeared.

You can help living things by protecting what they need where they live. Choose an action for the place and the living things you hope to help.

Protect homes and food

Nature reserves and national parks protect habitats. Trees and hedges provide homes and shelter. Wildflowers offer food for visiting bees and butterflies. Laws can stop people from hunting or harming endangered animals.

A bird box needs to be near birds that can use it and food they can find. A new pond helps frogs only if they can reach it. A good plan connects the action to the need.

Plan a schoolyard change

Suppose a schoolyard has few insects and little shelter. Plant wildflowers for food. Leave a pile of logs in a shady corner for damp shelter. The flowers help bees and butterflies. The logs provide a home for woodlice and beetles.

Before making the change, count what you find. Afterward, repeat the search over the same area, for the same time and using the same method. Compare both the animal counts and the kinds found. Repeat visits help you see whether a change lasts.

Scientists and volunteers use repeated counts to notice when living things become rare. Correct identification matters: mixing spiders into an insect count would change what that count means.

A recovery story

The bald eagle became rare partly because of a farm chemical called DDT. DDT pollution caused thin eggshells that broke easily. Fewer young eagles survived.

In 1972, the United States banned most uses of DDT. People also protected eagles and their habitats. Eagle numbers rose. In 2007, bald eagles were removed from the endangered and threatened list for the lower forty-eight states. They still had legal protection afterward.

Help without bringing harm

Put litter in the trash or recycling. Leave wild animals and plants in their homes. Grow flowers that local bees and butterflies visit, or make a small log pile.

Never release pets or garden plants into the wild. A newcomer can eat local living things or take their food and space. Helping the animals already living in a place starts with keeping their habitat suitable.