NAT 4 Revision
Food webs are made up of many
food chains joined together. Eg in
the food web shown, two food
chains containing 4 organisms
would be;
Grass
grasshopper
sparrows
Grass
grasshopper
frogs
hawks
snakes
All organisms in a food web depend on each other, for example, although foxes don’t
eat grass, they depend on it because they do eat rabbits and mice, and both of them
eat grass.
In a food web, If one population of organisms dies, it will affect others. For example,
if all the sparrows were to die of disease, what would happen to the mice?
Three possible correct answers would be;
1. If sparrows die there will be more grasshoppers for mice to eat so mice numbers will
increase.
2. If sparrows don’t eat grasshoppers, there will be more grasshoppers eating the grass, so
less grass for mice , therefore mice numbers go down.
3. A combination of both ie. More grasshoppers to eat but less grass to eat , so mice
numbers stay the same.
Humans affect biodiversity in a number of ways. Two examples
are;
Habitat destruction
Organisms in the habitat are displaced or destroyed
Deforestation
Loss of trees + the animals which use them
Plants obtain nitrogen from the soil through their roots in the form of nitrates. If fields
have very little nitrogen, then crops would either not grow, or grow very slowly. Farmers
can increase the nitrogen in their soil by adding manure or growing clover, which they
then plough back into the soil. Animals obtain nitrogen by eating plant protein, and then
use the nitrogen to build up their own protein.
Animals have many adaptations to survive in their environment
Eg. Bats have skin between their fingers to form a wing
Camels have broad feet to stop them sinking into the sand
Lions have good eyesight for hunting prey
Rabbits have good hearing to hear predators coming.
Even animals of similar type can be adapted in quite different ways eg birds.
Eagles have strong, sharp beaks to kill prey such as rabbits, but finches have smaller,
pointed beaks to pick small seeds off of shrubs and pelicans have a pouch- like beak to
scoop up fish.
Adaptations can be behavioural as well, for example, If an animal is subjected to the
same stimulus over a prolonged period of time but is not harmed by it, then the animal
will eventually stop responding to the stimulus. This is an advantage to the animal
because it stops wasting energy reacting to a stimulus which is not actually harmful to it.
Problem Solving
Ratios
10
15
20
25
30
30
35
40
50
60
30 seconds : 60 seconds
answer
Ratio = 1 : 2
Calculate the ratio of time taken to travel
10 cm to the time taken to travel 30 cm
Divide both sides by a number that does
not leave a remainder
In this case 30