Population Ecology
Limiting Factors
Factors that help to regulate the growth of a population:
1. Density Independent Factors
These limit the growth of populations as a result of natural factors, e.g. physical factors
such as rainfall, temperature, humidity. Catastrophic factors such as floods, fires,
drought.
2. Density Dependent Factors
Have a greater effect when the population density is high, e.g. food shortage,
competition, predation, invasive species, spread of disease.
These factors build up environmental resistance.
What affects size of population
Population size = N
1. Natality (B) – birth rate
2. Mortality (D) – death rate
3. Immigration (I) – move into population
4. Emigration (E) – move out population
B+I>D+E (population increase)
B+I<D+E (population decrease)
B+I=D+E (stable population)
How is growth of a population regulated?
Population size fluctuates seasonally and annually, depending on the resources available.
Sampling Methods
1. Mark-recapture method
Method:
Mark out well-defined area.
Capture as many individuals as you can and mark them.
Release the marked ones back into environment.
Allow for them to mix.
Recapture again.
Count the total number and count the number of marked ones.
Calculate size:
total number of marked animals ∙total number animalscaught ∈2nd sample
total number of marked animals∈2nd sample
2. Quadrat Method
, Method:
Measure the size the total area.
Use a wooden frame of known diameters, same size for each sample.
Quadrats must be distributed randomly.
Count the individuals in each quadrat.
Calculate:
number∈ sample ∙ ¿ whole habitat
¿ quadrat
3. Census
Forms which are filled in, to account everyone in a household on any day.
Why is random sampling important?
The distribution of individuals may not be uniform throughout the area. It is important to sample
as many quadrats as possible to achieve a true reflection.
Precautions must be taken for a reliable result:
A short time should pass between the 1st and 2nd sampling. Must be a closed population.
Sampling should be repeated several times and an average population calculated.
Marking must not damage the organism or affect the movements or behavior.
The marked animal must mix freely with the rest of the population before a new sample
is taken.
Predator-prey relationship
Example – Lion – Zebra relationship (CO-EVOLUTION)
The adaption of speed. Fastest lions catch and eat prey
more easily than the slower lions, so they survive and
reproduce. Gradually, faster lions make up more of the
population. The fastest zebras can escape the lions, so
they survive and reproduce. Gradually, the faster zebras
make up more of the population.
Example – Shark-fish relationship
Sharks keep populations of other fish healthy and in
balance in the ecosystem. Sharks eat old, sick, and slower
fish. This prevents spread of disease and strengthens the
gene pools of the prey species. Stronger healthier fish
will reproduce. Sharks also prevent the population
increasing rapidly and becoming too dominant.
, Competition
Species compete for resources, e.g. light, space, water, shelter, food.
Requirements to survive and reproduce:
Tolerate the physical environment.
Obtain energy and nutrients.
Cope with competition.
Avoid predators.
How food webs impact populations:
Populations of species in food web must be kept in balance. The removal of one species from a
food web will have a large impact on the ecosystem. If top predators are removed, prey
populations would rapidly increase.
Competitive Co-existence
Occurs when 2 competing species co-exist in the same habitat, they have overlapping niches and
compete for the same resource.
Example – Strategy among plants
Resource partitioning in a forest:
The vegetation is divided into layers called stratification. This is the partitioning of light, where
the light diminishes as the rays pass through different layers. Different species have created
different niches by stratification.
Example – Lion and Leopard Competitive Co-existence
relationship (Co-existing
predators)
Avoid competition by hunting at
different times of the day –
leopards during day, lions during
night or early morning. Hunt
different prey. Hunt in different
Competitive Exclusion
areas of habitats.
Competitive
Exclusion
Occurs when 1 of the 2 competing
species is more successful than the
other. The successful species
survives and other disappears, which can lead to extinction.
Limiting Factors
Factors that help to regulate the growth of a population:
1. Density Independent Factors
These limit the growth of populations as a result of natural factors, e.g. physical factors
such as rainfall, temperature, humidity. Catastrophic factors such as floods, fires,
drought.
2. Density Dependent Factors
Have a greater effect when the population density is high, e.g. food shortage,
competition, predation, invasive species, spread of disease.
These factors build up environmental resistance.
What affects size of population
Population size = N
1. Natality (B) – birth rate
2. Mortality (D) – death rate
3. Immigration (I) – move into population
4. Emigration (E) – move out population
B+I>D+E (population increase)
B+I<D+E (population decrease)
B+I=D+E (stable population)
How is growth of a population regulated?
Population size fluctuates seasonally and annually, depending on the resources available.
Sampling Methods
1. Mark-recapture method
Method:
Mark out well-defined area.
Capture as many individuals as you can and mark them.
Release the marked ones back into environment.
Allow for them to mix.
Recapture again.
Count the total number and count the number of marked ones.
Calculate size:
total number of marked animals ∙total number animalscaught ∈2nd sample
total number of marked animals∈2nd sample
2. Quadrat Method
, Method:
Measure the size the total area.
Use a wooden frame of known diameters, same size for each sample.
Quadrats must be distributed randomly.
Count the individuals in each quadrat.
Calculate:
number∈ sample ∙ ¿ whole habitat
¿ quadrat
3. Census
Forms which are filled in, to account everyone in a household on any day.
Why is random sampling important?
The distribution of individuals may not be uniform throughout the area. It is important to sample
as many quadrats as possible to achieve a true reflection.
Precautions must be taken for a reliable result:
A short time should pass between the 1st and 2nd sampling. Must be a closed population.
Sampling should be repeated several times and an average population calculated.
Marking must not damage the organism or affect the movements or behavior.
The marked animal must mix freely with the rest of the population before a new sample
is taken.
Predator-prey relationship
Example – Lion – Zebra relationship (CO-EVOLUTION)
The adaption of speed. Fastest lions catch and eat prey
more easily than the slower lions, so they survive and
reproduce. Gradually, faster lions make up more of the
population. The fastest zebras can escape the lions, so
they survive and reproduce. Gradually, the faster zebras
make up more of the population.
Example – Shark-fish relationship
Sharks keep populations of other fish healthy and in
balance in the ecosystem. Sharks eat old, sick, and slower
fish. This prevents spread of disease and strengthens the
gene pools of the prey species. Stronger healthier fish
will reproduce. Sharks also prevent the population
increasing rapidly and becoming too dominant.
, Competition
Species compete for resources, e.g. light, space, water, shelter, food.
Requirements to survive and reproduce:
Tolerate the physical environment.
Obtain energy and nutrients.
Cope with competition.
Avoid predators.
How food webs impact populations:
Populations of species in food web must be kept in balance. The removal of one species from a
food web will have a large impact on the ecosystem. If top predators are removed, prey
populations would rapidly increase.
Competitive Co-existence
Occurs when 2 competing species co-exist in the same habitat, they have overlapping niches and
compete for the same resource.
Example – Strategy among plants
Resource partitioning in a forest:
The vegetation is divided into layers called stratification. This is the partitioning of light, where
the light diminishes as the rays pass through different layers. Different species have created
different niches by stratification.
Example – Lion and Leopard Competitive Co-existence
relationship (Co-existing
predators)
Avoid competition by hunting at
different times of the day –
leopards during day, lions during
night or early morning. Hunt
different prey. Hunt in different
Competitive Exclusion
areas of habitats.
Competitive
Exclusion
Occurs when 1 of the 2 competing
species is more successful than the
other. The successful species
survives and other disappears, which can lead to extinction.
