Chloë van Beukering Life Sciences Notes 2020
Population Ecology
• Population: a group of individuals of the same species who live together in the same habitat/
area, at the same time, and are likely to breed
• Population size governed by a number of factors, both physical and social
• Populations of animals exhibit different social organisations that enhance their survival within
the community
• The main goal of a population is to ensure survival of their species and therefore to maximise
their reproductive
• Community: all the different populations of organisms occupying the same area at the same
time
• Species: a group of individuals with similar characteristics which are capable of breeding and
producing fertile offspring
• Habitat: an environment where an organism lives and reproduces
• Ecological niche: the functional role the organism plays in its community, defines the resources
and conditions necessary for survival of each species
• Ecosystem: a unit of plants and animal communities in interaction with each other as well as
with non-living factors in a particular area
Changes in Population Size
Change in population density = ( births + immigration ) — ( deaths + emigration )
1. Natality (birth rate)
- Natural ability of a population to increase
- Percentage increase of the population per unit time
- In humans, it is expressed as the number of births per 1000 persons per year
2. Mortality (death rate)
- Number of deaths per 1000 people
- In a closed population (no immigration or emigration) the only factors affecting population size
are the birth rate and death rate
- Most natural populations are not closed, but are open, therefore individuals can emigrate or
immigrate as they wish
3. Immigration (entering a population)
- Individuals entering an area from another one
- Occurs especially when a population colonises a favourable new habitat
4. Emigration (exiting a population)
- Individuals leaving an area
- Occurs especially when an area become overpopulated, resulting in a lack of food or space
• Population size can also fluctuate because of seasonal or annual changes
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, Chloë van Beukering Life Sciences Notes 2020
Population Growth Patterns
• When food is abundant and growing conditions are favourable, a population has the potential
to increase in number from generation to generation
- Growth is geometric when each
generation’s increase is a constant
percentage of the total population size
- Also known as exponential growth
because the larger the population gets, the
faster it grows
- The “J-shaped” curve represents the typical
form of an exponential growth curve
- Eventually the population hits the
environmental carrying capacity
- As a population density approaches the
carrying capacity, competition becomes
more intense, mortality increase, the birth
rate drops and any of the following is
possible:
- The population may level out and stabilise
below the capacity
- This pattern is known as a logistic or “S-
shaped” growth curve
- The population may fluctuate around (or
below) the carrying capacity
- The population may briefly overshoot the
carrying capacity and then crash, resulting in
repeated cycles of “boom” and “bust”
Population Regulating Factors
• Populations would continue to grow if there weren’t any factors that limit or regulate their sized
• These factors include: competition, drought, disease, predation and climate-related changes
• These factors prevent a population from reaching its maximum reproductive growth rate and are
collectively called environmental resistance
• Carrying capacity is the maximum number of individuals of a certain species that an
environment can support indefinitely without depleting the resources
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Population Ecology
• Population: a group of individuals of the same species who live together in the same habitat/
area, at the same time, and are likely to breed
• Population size governed by a number of factors, both physical and social
• Populations of animals exhibit different social organisations that enhance their survival within
the community
• The main goal of a population is to ensure survival of their species and therefore to maximise
their reproductive
• Community: all the different populations of organisms occupying the same area at the same
time
• Species: a group of individuals with similar characteristics which are capable of breeding and
producing fertile offspring
• Habitat: an environment where an organism lives and reproduces
• Ecological niche: the functional role the organism plays in its community, defines the resources
and conditions necessary for survival of each species
• Ecosystem: a unit of plants and animal communities in interaction with each other as well as
with non-living factors in a particular area
Changes in Population Size
Change in population density = ( births + immigration ) — ( deaths + emigration )
1. Natality (birth rate)
- Natural ability of a population to increase
- Percentage increase of the population per unit time
- In humans, it is expressed as the number of births per 1000 persons per year
2. Mortality (death rate)
- Number of deaths per 1000 people
- In a closed population (no immigration or emigration) the only factors affecting population size
are the birth rate and death rate
- Most natural populations are not closed, but are open, therefore individuals can emigrate or
immigrate as they wish
3. Immigration (entering a population)
- Individuals entering an area from another one
- Occurs especially when a population colonises a favourable new habitat
4. Emigration (exiting a population)
- Individuals leaving an area
- Occurs especially when an area become overpopulated, resulting in a lack of food or space
• Population size can also fluctuate because of seasonal or annual changes
1 of 13
, Chloë van Beukering Life Sciences Notes 2020
Population Growth Patterns
• When food is abundant and growing conditions are favourable, a population has the potential
to increase in number from generation to generation
- Growth is geometric when each
generation’s increase is a constant
percentage of the total population size
- Also known as exponential growth
because the larger the population gets, the
faster it grows
- The “J-shaped” curve represents the typical
form of an exponential growth curve
- Eventually the population hits the
environmental carrying capacity
- As a population density approaches the
carrying capacity, competition becomes
more intense, mortality increase, the birth
rate drops and any of the following is
possible:
- The population may level out and stabilise
below the capacity
- This pattern is known as a logistic or “S-
shaped” growth curve
- The population may fluctuate around (or
below) the carrying capacity
- The population may briefly overshoot the
carrying capacity and then crash, resulting in
repeated cycles of “boom” and “bust”
Population Regulating Factors
• Populations would continue to grow if there weren’t any factors that limit or regulate their sized
• These factors include: competition, drought, disease, predation and climate-related changes
• These factors prevent a population from reaching its maximum reproductive growth rate and are
collectively called environmental resistance
• Carrying capacity is the maximum number of individuals of a certain species that an
environment can support indefinitely without depleting the resources
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