Population Ecology
Important Terminology:
Individual: A single organism that forms part of a species
Species: A group of organisms with similar characteristics, which
produce fertile offspring.
Population: Organisms of the same species living in the same area at
the same time.
Community: Different populations living and interacting in the same
area.
Ecosystem: A community interacting with its non-living environment,
such as soil, water and sunlight.
Ecology: The study of interactions between organisms and their
environment.
Population Ecology: The study of changes in population size and what causes
them.
Changes in Population Size
Factor Meaning Effect on population
Natality Birth rate Increases
Mortality Death rate Decreases
Immigration Individuals move into a population and stay Increases
Emigration Individuals leave a population and don’t return Decreases
Population change = (births + immigration) − (deaths + emigration)
Example:
20 births + 5 immigrants − 10 deaths − 3 emigrants = 12 more individuals.
A population grows when additions are greater than losses.
A population decreases when losses are greater than additions.
A closed population has no immigration or emigration.
Population size can change seasonally or annually as resources change.
,Example:
More rain → more grass → more food for zebras → zebra numbers may increase
Limiting Factors
Factors that limit the growth of a population.
Type Meaning Examples
Density- Limit the population size when Competition, predation,
dependent the density is high. spread of disease.
Density- The population size is limited Floods, droughts, fires,
independent due to a natural event. extreme temperatures.
Population density: The number of individuals per unit of area or volume.
Example: Disease spreads more easily in a crowded population.
Environmental resistance: The combined effect of limiting factors that prevent a
population from growing at its maximum rate.
As population size increases, resources may become scarce and
environmental resistance increases.
Carrying Capacity
Carrying capacity (CC) is the maximum population an environment can
support sustainably with its resources.
It can change when environmental conditions or resources change.
When the population exceeds carrying capacity:
Resources become scarce
Competition increases
Natality and immigration may decrease
Mortality and emigration may increase.
Population size falls.
When the population falls below carrying capacity:
More resources are available per individual
Environmental resistance decreases
Population size may increase again.
Stable Populations
A stable population fluctuates around carrying capacity.
1. Population size rises above carrying capacity.
2. Resources per individual decrease.
3. Environmental resistance increases.
, 4. Natality and immigration may decrease, while mortality and emigration may
increase.
5. Population size falls below carrying capacity.
6. Resources per individual increase and environmental resistance decreases.
7. Population size can increase again.
This cycle may occur seasonally or annually.
Unstable populations
Population size rapidly exceeds carrying capacity.
Resources are used up.
Many individuals die or leave.
A population crash occurs.
Very unstable populations
Population size greatly exceeds carrying capacity.
Overuse of resources damages the habitat.
The population crashes.
Habitat damage lowers carrying capacity, so fewer organisms can survive in
future.
Example: Overgrazing removes vegetation and damages soil, reducing future food
production.
Population Growth Graphs
population growth curves can tell as the size of the population AS WELL As the
type of environment the species is in
Graphs show how population size changes over time.
X-axis: Time.
Y-axis: Population size.
Carrying capacity: The maximum population the environment can
support.
Exponential growth — J-shaped curve
Occurs when conditions are favourable and resources are plentiful.
At first: Growth is slow while organisms establish themselves.
Then: Growth becomes increasingly rapid as more organisms reproduce.
There are few limiting factors and little environmental resistance.
Remember: Plenty of resources → rapid growth → J-shaped curve.
Important Terminology:
Individual: A single organism that forms part of a species
Species: A group of organisms with similar characteristics, which
produce fertile offspring.
Population: Organisms of the same species living in the same area at
the same time.
Community: Different populations living and interacting in the same
area.
Ecosystem: A community interacting with its non-living environment,
such as soil, water and sunlight.
Ecology: The study of interactions between organisms and their
environment.
Population Ecology: The study of changes in population size and what causes
them.
Changes in Population Size
Factor Meaning Effect on population
Natality Birth rate Increases
Mortality Death rate Decreases
Immigration Individuals move into a population and stay Increases
Emigration Individuals leave a population and don’t return Decreases
Population change = (births + immigration) − (deaths + emigration)
Example:
20 births + 5 immigrants − 10 deaths − 3 emigrants = 12 more individuals.
A population grows when additions are greater than losses.
A population decreases when losses are greater than additions.
A closed population has no immigration or emigration.
Population size can change seasonally or annually as resources change.
,Example:
More rain → more grass → more food for zebras → zebra numbers may increase
Limiting Factors
Factors that limit the growth of a population.
Type Meaning Examples
Density- Limit the population size when Competition, predation,
dependent the density is high. spread of disease.
Density- The population size is limited Floods, droughts, fires,
independent due to a natural event. extreme temperatures.
Population density: The number of individuals per unit of area or volume.
Example: Disease spreads more easily in a crowded population.
Environmental resistance: The combined effect of limiting factors that prevent a
population from growing at its maximum rate.
As population size increases, resources may become scarce and
environmental resistance increases.
Carrying Capacity
Carrying capacity (CC) is the maximum population an environment can
support sustainably with its resources.
It can change when environmental conditions or resources change.
When the population exceeds carrying capacity:
Resources become scarce
Competition increases
Natality and immigration may decrease
Mortality and emigration may increase.
Population size falls.
When the population falls below carrying capacity:
More resources are available per individual
Environmental resistance decreases
Population size may increase again.
Stable Populations
A stable population fluctuates around carrying capacity.
1. Population size rises above carrying capacity.
2. Resources per individual decrease.
3. Environmental resistance increases.
, 4. Natality and immigration may decrease, while mortality and emigration may
increase.
5. Population size falls below carrying capacity.
6. Resources per individual increase and environmental resistance decreases.
7. Population size can increase again.
This cycle may occur seasonally or annually.
Unstable populations
Population size rapidly exceeds carrying capacity.
Resources are used up.
Many individuals die or leave.
A population crash occurs.
Very unstable populations
Population size greatly exceeds carrying capacity.
Overuse of resources damages the habitat.
The population crashes.
Habitat damage lowers carrying capacity, so fewer organisms can survive in
future.
Example: Overgrazing removes vegetation and damages soil, reducing future food
production.
Population Growth Graphs
population growth curves can tell as the size of the population AS WELL As the
type of environment the species is in
Graphs show how population size changes over time.
X-axis: Time.
Y-axis: Population size.
Carrying capacity: The maximum population the environment can
support.
Exponential growth — J-shaped curve
Occurs when conditions are favourable and resources are plentiful.
At first: Growth is slow while organisms establish themselves.
Then: Growth becomes increasingly rapid as more organisms reproduce.
There are few limiting factors and little environmental resistance.
Remember: Plenty of resources → rapid growth → J-shaped curve.
