Keeping a pond in balance
Watch the food chain in a village pond: water plants, insects, frogs and snakes. Catch frogs, or let soap or pesticide wash in, and see how the numbers of the others change.
Core concept
In short: Change a pond's food chain and see how every living thing depends on the others.
In a food chain every living thing depends on the others; taking one away upsets the balance of the whole pond
Topic: Ecosystems
A village pond is home to many living things that depend on each other. Water plants make their own food using sunlight. Insects such as mosquito larvae eat tiny plants, frogs eat the insects, and snakes eat the frogs. This is a food chain: Who eats whom: grass is eaten by a goat, and the goat by a tiger. All key words: water plants → insects → frogs → snakes. In a healthy pond the numbers stay fairly steady, because each animal keeps the next one down the chain in check. This steady state is called the balance of nature. People can upset it, for example by catching too many frogs, or by letting soapy water from washing, or pesticides sprayed on fields, run into the pond.
What you will learn
- Draw the food chain of a pond: water plants → insects → frogs → snakes.
- Explain how removing frogs changes the numbers of insects and snakes.
- Explain how pollution from soap and pesticides upsets a pond's balance.
- Suggest ways people can keep a pond in balance.
Before you start
- Know that animals eat plants or other animals for food.
Materials
- A village pond (watched, not touched)
Equipment
- A notebook for counting what you see
Procedure
- Record the healthy pond, with no frogs caught and clean water.
- Catch half the frogs and record.
- Catch all the frogs and record.
- With no frogs caught, let soapy water from washing into the pond and record.
- Let pesticide from the fields into the pond and record.
How this simulation is simplified
- The pond is followed for one season; the numbers are what it settles to.
- Each frog eats about 15 insects that would otherwise grow up, and each snake needs about 10 frogs to live.
- Soap kills about half the frogs, and pesticide kills them all; the insects are counted as surviving both.