Page 35 - Science Coursebook

2.6 Populations

Food supply
Penguins eat squid and fish. If the

numbers of squid or fish fall, then
not all penguins will get enough food.
More penguins are likely to die. The
ones that survive may not be able to
have so many young.

Predators If adult penguins cannot
Penguins are killed by leopard seals get enough food to feed
and killer whales. If the number themselves and their chicks,
of these predators increases, then the death rate will increase.
more penguins will be killed. Their
population will fall. This leopard seal is
waiting for penguins
Disease to enter the water.
Some diseases can increase death
rate and decrease birth rate. For
example, in 2006, researchers in South Africa noticed that some penguins were
losing their feathers. They do not know why this is, but they think it may be caused
by a pathogen (a disease-causing organism). Although the feathers eventually regrow,
penguins without feathers are more likely to die.
Diseases caused by pathogens often become more common when a population is large,
because it is easier for the pathogens to spread when animals are crowded together.

Questions

Use the food web on page 28 to answer these questions.

A+I 4 Suggest how a decrease in the population of squid and fish could affect the
population of leopard seals. Explain your answer.

A+I 5 Suggest how a decrease in the population of squid and fish could affect the
population of krill. Explain your answer.
6 Food supply, predators and disease affect the sizes of animal populations.

Make a list of three factors that you think might aff ect the size of a
plant population.

Summary
• A population is all the organisms of one species living in the same
place at the same time.
• A population increases in size if the number of births per year is
greater than the number of deaths per year.
• A population decreases in size if the number of deaths per year is
greater than the number of births per year.
• A decrease in food supply, an increase in predators or an increase in
disease can reduce the size of a population.

2 Living things in their environment 33