Organisms and Populations

Lesson

Part 1 — a population has features that no single individual has.

In this lesson you will

Define a population and list its attributes
Calculate per capita birth and death rates
Interpret age pyramids
Explain how population density is measured

Concept

Attributes of a population

An individual is born and dies; a population has birth rates and death rates. An individual is male or female; a population has a sex ratio. A population also has an age distribution and a population density.

Table of population attributes with meanings and examples
Attribute	Meaning	Example
Birth rate (natality)	Births per individual per unit time	0.4 offspring per lotus plant per year
Death rate (mortality)	Deaths per individual per unit time	0.1 deaths per fruit fly per week
Sex ratio	Proportion of females and males	60 percent females, 40 percent males
Age distribution	Proportion of individuals in each age group	Plotted as an age pyramid
Population density (N)	Population size in a given area	Trees per hectare

Worked example

Worked example 1: per capita rates

A pond had 60 frogs last year, and 15 new frogs were added through reproduction. In a laboratory, 5 of 50 fruit flies died during a week. Calculate the birth rate and the death rate.

Birth rate $= \dfrac{15}{60} = 0.25$ offspring per frog per year.
Death rate $= \dfrac{5}{50} = 0.1$ deaths per fruit fly per week.
Always state the unit: per individual per unit time.

Worked table: birth rate 0.25 per frog per year, death rate 0.1 per fly per week
Population	Size	Events	Per capita rate
Frogs (births, 1 year)	60	15	0.25
Fruit flies (deaths, 1 week)	50	5	0.1

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