Ecosystems and Interactions
5. Population Dynamics
Learning outcomes
- I can define population and population size.
- I can explain factors that influence population growth.
- I can describe carrying capacity and limiting factors.
- I can interpret population data and trends.
- I can predict how environmental changes affect populations.
Introduction
Populations of living organisms are constantly changing. Some populations grow rapidly when food is plentiful and conditions are favourable, while others decline because of disease, predators, or habitat loss. Ecologists study these changes to understand how ecosystems function and to help protect wildlife.
The study of how and why populations change over time is called population dynamics. By understanding the factors that influence population size, scientists can predict future changes, manage endangered species, and monitor the health of ecosystems.
What Is a Population?
A population is a group of individuals of the same species living in the same area at the same time.
Examples include:
- All the deer living in a forest.
- All the clownfish living on a coral reef.
- All the oak trees growing in a woodland.
- All the frogs living in a pond.
Populations are one part of a larger ecosystem and interact with other populations.
Population Size
Population size is simply the number of individuals in a population.
For example:
- 250 rabbits in a grassland.
- 12,000 penguins in a breeding colony.
- 75 oak trees in a park.
Population size is always changing as individuals are born, die, enter, or leave a population.
Figure 1. A population is a group of individuals of the same species living in the same area.
Factors That Influence Population Growth
Population size changes because of four main processes.
Births (Natality)
When organisms reproduce, new individuals are added to the population.
More births generally increase population size.
Deaths (Mortality)
Deaths remove individuals from the population.
Disease, starvation, old age, predation, and accidents all contribute to mortality.
Immigration
Immigration occurs when individuals move into a population.
This increases population size.
Example:
A group of birds moves into a nearby forest.
Emigration
Emigration occurs when individuals leave a population.
This decreases population size.
Example:
Young wolves leave their pack to establish new territories.
Population Growth
A population grows when:
- Births are greater than deaths.
- Immigration is greater than emigration.
A population declines when:
- Deaths exceed births.
- Emigration exceeds immigration.
Limiting Factors
Although populations can grow, they cannot increase forever.
Limiting factors are environmental conditions that restrict population growth.
Some limiting factors include:
Food Availability
Less food means:
- Increased competition
- Slower growth
- Lower survival
Water Availability
Water shortages can reduce plant growth and affect every level of the food chain.
Space
Animals require territory, nesting sites, or shelter.
Limited space increases competition.
Predation
Predators remove individuals from prey populations.
Disease
Diseases spread more easily in crowded populations.
Climate
Extreme temperatures, droughts, floods, and storms may reduce population sizes.
Figure 2. Population growth is limited by environmental factors such as food, water, disease, and available space.
Carrying Capacity
Every ecosystem has a maximum population size that it can support.
This is called the carrying capacity.
Carrying capacity is the largest population that an environment can support over a long period without damaging its resources.
If a population exceeds the carrying capacity:
- Food becomes scarce.
- Competition increases.
- Disease spreads more easily.
- Death rates rise.
- Population growth slows or declines.
Eventually, the population usually stabilises near the carrying capacity.
Population Growth Curves
Scientists often represent population growth using graphs.
Exponential Growth
When resources are abundant, populations may grow rapidly.
Characteristics:
- Unlimited resources
- Rapid increase
- J-shaped curve
Examples:
- Bacteria in a laboratory
- Invasive species entering a new habitat
Logistic Growth
As resources become limited, growth slows and levels off near the carrying capacity.
Characteristics:
- Growth slows over time
- Population stabilises
- S-shaped curve
Most natural populations follow this pattern.
The logistic growth curve shows how a population grows quickly at first but slows as it approaches the environment's carrying capacity. This pattern is common in natural ecosystems where food, space, and other resources are limited.
Figure 3. Exponential growth occurs when resources are abundant, while logistic growth levels off as populations reach carrying capacity.
Interpreting Population Trends
Ecologists collect data over many years to understand how populations change.
A graph may show:
- Rapid population growth
- Stable populations
- Seasonal fluctuations
- Population decline
- Population recovery
Scientists use this information to:
- Protect endangered species.
- Manage fisheries.
- Monitor forests.
- Predict environmental changes.
How Environmental Changes Affect Populations
Changes to the environment often affect population size.
Habitat Loss
Building cities or clearing forests reduces available habitat.
Result:
- Population decreases.
Drought
Reduced rainfall means less water and less food.
Result:
- Smaller populations.
Introduction of a New Predator
A new predator may reduce prey populations.
Result:
- Population decline.
Conservation Efforts
Protecting habitats and reducing hunting can help endangered species recover.
Result:
- Population increases.
Figure 4. Environmental changes such as habitat destruction can cause populations to decline dramatically.
Worked Example
Question
A rabbit population changes over one year:
- Initial population: 120 rabbits
- Births: 35
- Deaths: 18
- Immigration: 12
- Emigration: 9
What is the population at the end of the year?
Solution
Population = Initial population + Births − Deaths + Immigration − Emigration
= 120 + 35 − 18 + 12 − 9
= 140 rabbits
The population increased because births and immigration were greater than deaths and emigration.
Real-World Connection
Wildlife biologists regularly monitor populations of endangered species such as sea turtles, pandas, and orangutans. By tracking population size and identifying limiting factors like habitat loss or illegal hunting, conservationists can develop strategies to help these species recover. Population studies are also used to manage fisheries, control invasive species, and protect biodiversity.
Did You Know?
The reintroduction of wolves to Yellowstone National Park in the United States changed much more than just the wolf population. By reducing elk numbers, wolves allowed young trees and shrubs to recover. This created new habitats for birds, beavers, insects, and many other species, demonstrating how changes in one population can affect an entire ecosystem.
Key Terms
Carrying capacity – The maximum population size an environment can support over a long period.
Emigration – The movement of individuals out of a population.
Immigration – The movement of individuals into a population.
Limiting factor – A factor that restricts population growth.
Mortality – The death rate within a population.
Natality – The birth rate within a population.
Population – A group of individuals of the same species living in the same area.
Population dynamics – The study of how and why populations change over time.
Population size – The number of individuals in a population.
Key Takeaways
- A population consists of members of the same species living in one area.
- Population size changes through births, deaths, immigration, and emigration.
- Limiting factors such as food, water, disease, and predators prevent unlimited population growth.
- Carrying capacity is the maximum population an environment can sustainably support.
- Environmental changes can cause populations to grow, decline, or stabilise over time.