Grade by Grade
B. Life Systems · Biodiversity

Invasive Species: Unwanted Guests

B2.6 A1.3 A1.4 A1.5About 50 minutes
Learning goals
  • I can explain what makes a species invasive.
  • I can explain how invasive species reduce biodiversity in local environments, using Ontario examples.
  • I can describe ways people help stop the spread of invasive species.
  • I can design, build and test a device that removes seeds and mud from footwear.

Words to know

native species
A species that has lived naturally in an area for a very long time.
non-native species
A species brought by people, on purpose or by accident, to an area where it did not live before.
invasive species
A non-native species that spreads quickly and harms the environment, the economy or people's health.
ballast water
Water that ships take in to stay balanced, then release at another port, sometimes with living things in it.
biological control
Using a natural enemy, such as an insect that eats a certain plant, to control a pest.

A stowaway in the Great Lakes

In the late 1980s, a tiny striped shellfish turned up in the Great Lakes: the zebra mussel. It came from Europe and western Asia, probably hidden in the ballast water of a ship. Within a few years, zebra mussels were crusted on rocks, boats, docks and water pipes, sometimes so thick that pipes clogged.

The zebra mussel is an invasive species. Today you will find out why some newcomers cause so much harm.

A shoreline rock covered with many small striped zebra mussels, with a lake behind.
Zebra mussels crowd together so thickly that they cover rocks, docks and pipes, and they push out native mussels. AI-generated image

Remember from Grade 4

In Grade 4 you learned that the emerald ash borer is an invasive beetle that has killed huge numbers of ash trees, and that moving firewood can spread it. Now you will look more closely at how invasive species reduce biodiversity.

Native, non-native and invasive

  • A native species has lived in an area for a very long time, along with the other species around it.
  • A non-native species was brought to a new area by people. Many are not harmful. Wheat, apples and many garden flowers are non-native and do not take over.
  • An invasive species is a non-native species that spreads quickly and causes harm.

Why invasive species succeed

  • No natural enemies: the predators, diseases and parasites that kept them in check at home are not in the new place.
  • They reproduce fast and spread easily.
  • Native species are not adapted to them: native trees may have no defences against a new beetle, and native fish may not recognize a new predator.
In its home rangeIn a new placepredatordiseaseenemies keep its numbers in checkno natural enemies: it spreads fast
At home, predators and diseases keep a species in check. Moved to a new place without those natural enemies, it can spread quickly. Each oval stands for one member of the species.

How invasive species reduce biodiversity

  • Competing: they take food, light, water or space from native species. Phragmites (an invasive reed from Europe) grows in tall, thick stands in wetlands and crowds out native plants.
  • Eating native species: the round goby, a fish that arrived in ballast water, eats the eggs of some native fish.
  • Harming as parasites: the sea lamprey came into the upper Great Lakes through canals. It attaches to fish such as lake trout and feeds on their blood and body fluids, and many of those fish die.
  • Killing a key species: the emerald ash borer, a beetle from Asia found in Ontario in 2002, kills ash trees. Animals that relied on ash lose food and shelter.
  • Changing the habitat: zebra mussels filter huge amounts of water and attach to native mussels, smothering them. Garlic mustard, a plant from Europe, spreads across forest floors and crowds out native spring wildflowers like trilliums.
  • Tricking native species: monarch butterflies sometimes lay eggs on dog-strangling vine, an invasive plant related to milkweed, but the caterpillars usually cannot survive on it.

When native species decline or disappear, biodiversity goes down: the community has fewer species, and often one invader takes over large areas.

Invasive speciesWhere it came fromHow it reduces biodiversity
zebra musselEurope and western Asia, in ballast watersmothers native mussels; changes lake food webs
emerald ash borerAsia, likely in wooden packing materialkills ash trees
round gobyEurope and western Asia, in ballast watereats eggs of native fish; competes for food
sea lampreyAtlantic Ocean, through canalsfeeds on lake trout and other fish, often killing them
Phragmites (invasive common reed)Europecrowds out native wetland plants; poor habitat for many animals
garlic mustardEuropecrowds out native forest wildflowers
Some invasive species in Ontario.
Worked example

Let's follow the chain of effects

Someone empties an aquarium into a pond, releasing a few goldfish. Goldfish are not native to Ontario. Several years later the pond has hundreds of large goldfish. Explain how this could reduce the pond's biodiversity.

  1. Why they spread: goldfish reproduce quickly and there may be few predators in the pond to eat them.
  2. Competition: goldfish eat many of the same foods (insects, small animals, plants) as native fish and frogs.
  3. Changing the habitat: goldfish stir up mud from the bottom, which makes the water cloudy. Less sunlight reaches underwater plants.
  4. Chain reaction: with fewer underwater plants, insects and young fish that hide in them lose shelter and food.
  5. Result: native species decline, and goldfish make up more and more of the pond's life.

Never release pets or aquarium plants into the wild. One choice can reduce a whole pond's biodiversity.

goldfishmultiplythey stir upmudcloudy water:less sunlightfewer under-water plantsinsects and youngfish lose shelternative speciesdecline
Follow the chain from the goldfish example: one change leads to the next, until native species decline.
Just for fun

Zebra mussels are the world's clingiest houseguests: they arrived without an invitation, stuck to everything in sight, and never, ever left.

How people fight back

  • Prevention is best. Boaters clean, drain and dry their boats and gear before moving to another lake. People buy firewood where they burn it instead of carrying it to new places. Hikers brush off boots to remove seeds.
  • Never release pets, bait or aquarium plants into the wild.
  • Plant native species in gardens.
  • Early detection: people report sightings so experts can act while an invader is still rare.
  • Control: volunteers pull garlic mustard; workers cut or treat Phragmites; barriers and treatments in streams reduce sea lamprey. For purple loosestrife, an invasive wetland plant, scientists tested and then released small leaf-eating beetles from its home range in Europe. This is called biological control, and it is only used after careful testing to be sure the new insect will not harm native plants.
Hands-on activity

Design challenge: the boot-cleaning station

Problem: hikers carry invasive plant seeds on muddy boots. Design a station at a trailhead that removes seeds and mud from shoe soles.

You need: an old pair of shoes with treads, a tray or shallow box, materials like an old scrub brush, bristly doormat pieces, plastic forks, bottle caps, cardboard, tape; damp soil and a spoonful of uncooked rice (the "seeds"); a timer.

  1. Criteria: your station must remove as many "seeds" as possible, be quick (under 30 seconds per shoe), work without hands touching the sole, and use cheap materials.
  2. Plan: sketch two ideas and choose one. Label materials.
  3. Build your station in the tray.
  4. Test: press the shoe sole into the damp soil and rice mixture. Count the rice grains stuck to the sole (or estimate). Use your station for 30 seconds. Count again.
  5. Calculate how many grains were removed. Try 3 tests and find the average.
  6. Improve: change one part of your design and test again. Did it work better?

Safety: do this outside or on newspaper. Wash hands after handling soil. Keep uncooked rice away from little children (choking hazard). If you use scissors to cut materials, a grown-up should help.

What to look for: a labelled sketch, fair repeated tests, counts before and after, and one improvement based on results. Real trailheads in many parks have boot-brush stations for this reason.

Your turn

1

What is an invasive species?

2

Are all non-native species invasive? Give an example.

3

Give two reasons invasive species can spread so quickly.

4

How do zebra mussels harm native mussels?

5

What does the emerald ash borer do?

6

What does "clean, drain, dry" mean for boaters?

7

Why should you never release a pet goldfish into a pond?

More practice: warm-up

1

What is a native species?

2

How did zebra mussels probably arrive in the Great Lakes?

3

Why should you buy firewood where you burn it?

4

Name an invasive plant in Ontario.

5

Does an invasive species usually increase or decrease biodiversity?

More practice: on your own

1

Explain how garlic mustard reduces biodiversity on a forest floor.

2

Why might a native tree have no defence against a beetle from another continent?

3

How does dog-strangling vine harm monarch butterflies?

4

What is biological control? Why must it be tested carefully first?

5

In your boot-station test, 40 grains were stuck before cleaning and 6 after. How many were removed? What fraction is that?

Challenge

1

Why is prevention usually easier and cheaper than control once an invasive species is established? Use an example.

2

An invasive species takes over a wetland, but the total number of plants there goes up. Explain how biodiversity can still go down.

3

Three boot-station designs removed an average of 21, 33 and 28 grains out of 40. Which design worked best? Before choosing it for a real park, what else should you consider?

Check your understanding

Quiz

1.

An invasive species is...

2.

Zebra mussels most likely arrived in the Great Lakes...

3.

Which action helps prevent the spread of invasive species?

4.

Invasive species often spread quickly because...

5.

The emerald ash borer reduces biodiversity mainly by...

6.

Which is a non-native species that is not considered invasive?

For parents and teachers

Big idea: invasive species are non-native species that spread quickly and cause harm. They reduce biodiversity by competing with, eating, or harming native species and by changing habitats. Prevention is the best defence.

How to help at home:

  • Practise good habits as a family: clean boots after hikes, clean, drain and dry boats and canoes, buy firewood where you burn it, and never release pets or aquarium plants.
  • Look up which invasive plants grow in your area, and choose native plants for your garden.
  • Help with the boot-station design challenge and encourage repeated tests and one improvement.

Safety: soil can contain germs, so wash hands; keep rice away from small children; supervise any cutting.

Common difficulties:

  • Thinking every non-native species is invasive.
  • Thinking more plants always means more biodiversity. One invader can increase numbers while reducing variety.