1Pesticides in the water
Farmers spray pesticides to kill insects and other pests on crops. Rain can wash some into rivers and lakes. A few pesticides, such as DDT, can't be broken down by living things.
pesticides
Lesson 25.4 notes
Toxins and sampling
Poisons that build up along food chains, how living things change their environment and are changed by it, and counting with quadrats.
2Building up the chain
Plankton take in the pesticide. Small fish eat lots of plankton, and big fish eat lots of small fish, keeping the poison each time. So it builds up along the chain: bioaccumulation.
bioaccumulation
3DDT and birds of prey
In the 1950s and 60s, pesticides such as DDT built up in birds of prey like peregrine falcons. They laid thin-shelled eggs that broke, and numbers crashed. Since the bans, they have recovered.
4Changing the environment
Living things change their environment. Beavers dam streams to make ponds, earthworms mix the soil and let air in, and humans clear forests, build towns and pollute rivers.
5Changed by the environment
The environment changes living things too. A drought can dry up a pond, killing the tadpoles. A hard winter kills many insects, so the birds that eat them go hungry.
6Sampling with quadrats
You can't count every daisy in a field. Instead, place a square frame called a quadrat at random, count the daisies inside it, and repeat many times.
quadrat
7Estimating a population
Find the mean number per quadrat, then multiply by the number of quadrats that would fit in the field. More quadrats, placed at random, give a better estimate.
mean
Explore: Quadrat lab
Estimate the daisies in a field: place quadrats at random, count the daisies in each, and compare your estimate with the true number. Then see what happens if you pick the spots with the most daisies.
Common mistake: Tiny amounts of poison can build up to a harmful dose
A pesticide may be too dilute in the water to harm plankton. But it isn't broken down, so it builds up along the food chain until the top predator gets a dangerous dose.
Remember
- Some poisons, such as the pesticide DDT, can't be broken down by living things. They build up along a food chain, so the top predator gets the biggest dose.
- Living things change their environment, and the environment changes them: a beaver's dam makes a pond; a drought dries it up.
- To estimate a population, place quadrats at random, count in each, find the mean and multiply by the number of quadrats that would fit in the area.
Going further at GCSE