Lesson 23 — Activity 1: What Determines the Size of a Population?
Completion requirements
Lesson 23 — Activity 1: What Determines the Size of a Population?
Explore
A population is a group of inhabitants (people or animals) of the same species that lives together in the same place at the same time. Each individual person or animal is one member of the population.
In this activity, you will learn why populations change and what factors determine the size of a population.
Look at the image below.
The image above shows a population. It includes individuals (a teacher and students), of the same species (humans) in the same place (a classroom in Edmonton) at the same time (2015).
Below are other examples of populations.
The image above is a bird population, and the people on the right make up a Canadian population.
Populations can change in size over time. Scientists can find out how healthy a population is by calculating its rate of increase or decrease. If a population is decreasing quickly, that population may be considered at risk.
Swift Fox
Swift foxes are the smallest North American wild dogs. When fully grown, they are about the size of a large house cat. Swift foxes live in the short, mixed grasslands of southern Alberta. When these grasslands are used and cultivated by farmers, their habitats are decreased. As well, weather conditions such as severe winters and droughts and the trapping and poisoning of these animals pose risks to this species and have led to decreases in their population.
Factors that Determine the Size of a Population
There are four factors that determine the size of a population at any given time: birth rate, death rate, immigration, and emigration.
Birth Rate and Death Rate:
The birth rate involves the number of babies born each year, and the death rate involves the number of inhabitants that die in a year. When the birth rate is higher than the death rate, the population increases.
On average, every 20 minutes, the world adds another 3,500 humans — that's 10,500 every hour. The world loses approximately 1,800 people every 20 minutes — about 5,400 per hour. Because the birth rate is much higher than the death rate, the world's population increases rather than decreases.
Birth rates are affected by:
- environmental conditions: fewer babies are born when conditions are poor
- nutrition: mothers who are well fed give birth to more and healthier babies
- where inhabitants live: fewer babies are born in crowded areas
Immigration and Emigration
Immigration refers to those coming into an area. Immigration increases the size of a population in an area. Emigration refers to leaving an area. Emigration decreases the size of a population in an area.
Scientists who study the populations of species look at the four
factors that determine the size of populations.
For example, some scientists may study the populations of
mountain whitefish, a native fish of Alberta, that are found throughout
the province in most lakes and streams.
Here is what they would look at:
1. Emigration: Mountain whitefish leave their river habitat and swim up smaller streams to spawn. This leaves a smaller population in the river habitat.
2. Birth rate: They spawn between October and February, and the eggs hatch in the early spring. Then new members are added to the whitefish population.
3. Death rate: Mountain whitefish are the prey of other fish, most notably, the Alberta bull trout. This, along with competition for food and disease, contributes to the death rate of the whitefish population.
4. Immigration: The mountain whitefish immigrate to a different area because they need to find new sources of food, such as larvae, eggs, and small fish.
Scientists would note that birth rate and immigration increase the population size and death rate and emigration decrease the population size.
As you read in the information on the swift fox, the increase in the human population in Alberta has changed the populations of other living things. Human activities such as farming, logging, draining of wetlands, and building cities and towns also cause the loss of natural habitats for other organisms. Loss of habitat threatens many species of animals and plants.
By Sasata - Own work, CC BY-SA 3.0,
By Rbreidbrown - Own work, CC BY-SA 4.0
These are some of the endangered or threatened species in Alberta.
Self-Check
Try This!