How can an invasive species change an ecosystem?

1. In this pond, every living thing is kept in check by another. Snails graze the pondweed, minnows eat the snails and a heron eats the minnows. Nothing takes over.
2. One day somebody lets a pet fish go in the pond. It is a purple puffback, a made-up fish from a faraway country. It does not belong here.
3. Back home, bigger fish ate puffbacks. Here, nothing does: the heron finds it far too spiky. With no enemies, almost every baby puffback survives.
4. Puffbacks eat snails, the same food the minnows need. The two are competing, and there are more and more puffbacks. The minnows go hungry and their numbers fall.
5. The change spreads. With few minnows, the heron leaves to find food elsewhere. With few snails to graze it, the pondweed grows out of control and the water turns green.
6. People can help by catching the invaders early, before there are too many. Best of all is stopping them from arriving: never release pets, and clean boats and boots.

Tap each living thing, then the two special spots, to see how the puffback fits in.

• Pondweed. Pondweed makes its own food from sunlight. Snails graze on it, which stops it from filling the whole pond.
• Pond snails. Snails eat pondweed and are eaten by minnows. Now puffbacks eat them too, so the snails have twice as many hunters.
• Minnows. Minnows are small native fish that eat snails. They have always lived here, and the heron depends on them.
• Heron. The heron wades in the shallows and spears minnows with its long beak. It cannot eat the spiky puffback.
• Purple puffback. The puffback is a made-up fish from far away. It eats snails, lays lots of eggs and is too spiky for anything here to eat.
• Competition. Minnows and puffbacks both eat snails. When two kinds of living thing need the same food, they compete, and there may not be enough for both.
• No predator. This arrow is missing. In the pond, nothing eats the puffback, so nothing keeps its numbers down.

Check the five meters. Then tap the bucket to release the puffbacks and watch which meters change, and in what order. Tap a puffback to net it.

Experiment: What happens to the minnows after the puffbacks arrive?

1. Before you start, look at the meters: everything is steady.
2. Tap the bucket. Watch the purple meter grow.
3. Which meter drops first? Which ones follow?

What we found: The puffbacks multiply because nothing eats them. They eat the snails, so the minnows run short of food and get fewer. Then the heron loses its food too, and the pondweed spreads because few snails are left to eat it.

Turn on JavaScript to try it yourself.

Puffbacks have just arrived, and they breed every year. Tap them to net them all before your 14 nets run out.

Plants and animals live in the places where they have lived for a very long time. Those are their native homes. But people move living things around the world, sometimes by accident, in ships, on boats or in soil, and sometimes on purpose, as pets, crops and garden plants.

Most newcomers cause no trouble, and many cannot survive in a new place at all. But a few spread quickly and harm the living things that were already there. These are called invasive species. The purple puffback in this topic is made up, but it behaves the way real invaders do.

Why do invaders win? Back home, something ate them or made them sick, and that kept their numbers down. In the new place those enemies are missing. If the newcomer also has lots of babies and eats a common food, it can grow and grow. It competes with native animals for food, and the natives lose out.

The harm then spreads along the food web. Animals that ate the natives go hungry, and plants that were kept short may grow wild. The best cure is to stop invaders from arriving: never let pets go in the wild, never empty a fish tank into a pond, and clean boots and boats. If an invader does arrive, catching it early, while there are only a few, works far better than waiting. Like the picnic guest, the invader means no harm. It is just in the wrong place.

Key words

Native species

A living thing that has lived in a place naturally for a very long time.

Invasive species

A newcomer that spreads fast and harms the living things already there.

Competition

When two living things need the same food or space and there is not enough for both.

Enemy release

Leaving behind the animals and germs that kept your numbers down.

Indirect effect

A change that reaches a living thing through others, like dominoes falling.

Prevention

Stopping a problem before it starts: here, stopping new species from being moved and released.

Check your thinking

1. Why can a newcomer spread so fast in a new place?

   Show the answer

   Nothing there eats it, so most of its babies survive. At home, enemies kept its numbers down. In the new place they are missing.

2. Minnows and puffbacks both eat snails. What is that called?

   Show the answer

   Competition. When two kinds of living thing need the same food, they compete for it.

3. The heron does not eat puffbacks or snails. Why does it leave the pond?

   Show the answer

   The minnows it eats have become scarce. The puffbacks eat the minnows' food, so there are fewer minnows for the heron: a change passed along the food web.

4. When is the best time to remove an invader?

   Show the answer

   As early as possible, while there are only a few. A few are easy to catch. Wait, and they breed faster than anyone can catch them.

5. In the model, a native predator that can eat the puffback is added. What changes?

   Show the answer

   The puffback stays rare and the natives are much less affected. A predator raises the invader's death rate, which removes its main advantage, enemy release.

6. What is the best way to protect ponds from invaders?

   Show the answer

   Never release pets or plants, and clean boats and boots. Stopping invaders from arriving is far easier than getting them out again.