Births and deaths

The idea

The rates of birth and death in a population determine its size.

A village, a year

In a village, some babies are born and some people die during a year.

What decides whether the number of people in the population goes up or down?

• The rates of birth and death
• The colour of the houses
• The age of the oldest person

Three populations

Open each tab. The numbers are only examples. Each starts with 10 individuals.

Births in, deaths out

Reproduction is how organisms increase their populations. Births add to a population and deaths take away from it. So the rates of birth and death decide its size.

The rates of birth and death in a population determine its size.

Notes

The rates of birth and death in a population determine its size.

Check yourself

What determines the size of a population, according to the book?

In an example, a population of 1000 has 40 births and 15 deaths in a year. How many individuals are there after the year?

Answer: 1025 individuals

If, in a year, there are fewer births than deaths in a population, its size will …

• Only the rate of birth. The book says the rates of birth and death determine the size.
• The rates of birth and death — correct. Yes!
• The colour of its members. The book says the rates of birth and death in a population determine its size.
• become bigger. Fewer births than deaths means more individuals leave the population than join it.
• stay the same. Equal births and deaths would keep the size the same, but here there are fewer births than deaths.
• become smaller — correct. Yes!