Population Ecology
Ecology
The study of the interactions of organisms with their physical and biological environments and how these
determine the distribution and make up of populations within an ecosystem
Population ecology
Concerned w fluctuations in population size and the factors that regulate populations
Population: a group of organisms of the same species that occupy the same area and can breed freely with each
other
Species: a group of organisms that share characteristics and are able to breed among themselves but not with
other groups
Population Size
Total number of individuals in a population
Can increase or decrease with a change in:
- Natality: birth rate in animals or the production of seeds in plants
- Mortality: death rate
- Immigration: individuals move into a population & stay
- Emigration: Individuals leave a population & don’t return
For humans: birth rate is number of births and death rate is number of deaths per 1000 people
Populations will fluctuate as they:
- Grow, when birth and immigration exceed death and emigration
- Decline, when death and emigration exceed birth and immigration
- Remain stable when birth and immigration = death and emigration
In a closed population (no emigration or immigration) parameters that affect population are births or deaths
eg: fish in a pond
,Estimating population size
Direct methods
Include counting every single individual in a population (census)
Can only be used:
- When organisms are large enough to be seen
- The area in which the animals are being counted is not too large
Can be used for individuals that are:
- Slow moving
- Stationary
- Usually stay in a fixed position
If area is too large:
- Aerial photographs can be taken to show the whole area in which the population occurs
- Helicopters can be used to count larger animals
- Census forms can be filled by humans accounting for everyone in a household
Indirect methods
Counting a sample number of the population and then using simple calculations to estimate the total size
1. Quadrat method
Counting the number of individuals in a quadrat and then using it to calculate the population size of
the total area with the aid of this formula:
𝑛𝑢𝑚𝑏𝑒𝑟𝑠 𝑖𝑛 𝑠𝑎𝑚𝑝𝑙𝑒 × 𝑠𝑖𝑧𝑒 𝑜𝑓 𝑤ℎ𝑜𝑙𝑒 ℎ𝑎𝑏𝑖𝑡𝑎𝑡
Total population (N): 𝑁 = 𝑠𝑖𝑧𝑒 𝑜𝑓 𝑞𝑢𝑎𝑑𝑟𝑎𝑡
As distribution is not uniform it is important to measure different parts (random sampling)
Method:
- Measure total area
- Use same size quadrat for each sample
- Distribute quadrat at random
- Count individuals in quadrat and repeat sample several times: ‘number in sample’
- Calculate population size
2. Mark-recapture method
A known number of individuals is caught and marked and then released
After a time period this is repeated
, Formula is used based on the fact that the ratio of unmarked to marked individuals is the same as the
ratio of the population as a whole
Suitable for:
- Mobile animals eg: butterflies, birds
- Not easily visible eg: fish in a dam
Method:
- Mark out a well-defined area
- Capture as many individuals as possible and mark them
- Release them
- Allow time for them to mix with unmarked
- Recapture
- Count the total number recaptured and those marked
- Calculate using Petersen index:
𝑀×𝐶
𝑃= 𝑅
P - estimated population
M - Marked animals in first capture
C - total animals caught in second sample
R - total number of marked animals in second same
Precautions for a reliable result
- Short time should pass between first & second sampling to avoid births/deaths
- Sampling should be repeated several times for an average population to be calculated
- Marking must not damage individual
- Marked animal must mix freely with the rest before a new sample is taken
- No immigration or emigration is allowed (population must be closed)
Limiting factors
Factors or resources that help regulate growth of population by causing birth rate to decline or death rate to
increase or both
May be:
- Density independent factors that limit growth as a result of natural factors and not because pf the
density or the number of organisms eg:
- Physical factors: rainfall, temp, humidity
- Catastrophic events: floods, fire, drought
Ecology
The study of the interactions of organisms with their physical and biological environments and how these
determine the distribution and make up of populations within an ecosystem
Population ecology
Concerned w fluctuations in population size and the factors that regulate populations
Population: a group of organisms of the same species that occupy the same area and can breed freely with each
other
Species: a group of organisms that share characteristics and are able to breed among themselves but not with
other groups
Population Size
Total number of individuals in a population
Can increase or decrease with a change in:
- Natality: birth rate in animals or the production of seeds in plants
- Mortality: death rate
- Immigration: individuals move into a population & stay
- Emigration: Individuals leave a population & don’t return
For humans: birth rate is number of births and death rate is number of deaths per 1000 people
Populations will fluctuate as they:
- Grow, when birth and immigration exceed death and emigration
- Decline, when death and emigration exceed birth and immigration
- Remain stable when birth and immigration = death and emigration
In a closed population (no emigration or immigration) parameters that affect population are births or deaths
eg: fish in a pond
,Estimating population size
Direct methods
Include counting every single individual in a population (census)
Can only be used:
- When organisms are large enough to be seen
- The area in which the animals are being counted is not too large
Can be used for individuals that are:
- Slow moving
- Stationary
- Usually stay in a fixed position
If area is too large:
- Aerial photographs can be taken to show the whole area in which the population occurs
- Helicopters can be used to count larger animals
- Census forms can be filled by humans accounting for everyone in a household
Indirect methods
Counting a sample number of the population and then using simple calculations to estimate the total size
1. Quadrat method
Counting the number of individuals in a quadrat and then using it to calculate the population size of
the total area with the aid of this formula:
𝑛𝑢𝑚𝑏𝑒𝑟𝑠 𝑖𝑛 𝑠𝑎𝑚𝑝𝑙𝑒 × 𝑠𝑖𝑧𝑒 𝑜𝑓 𝑤ℎ𝑜𝑙𝑒 ℎ𝑎𝑏𝑖𝑡𝑎𝑡
Total population (N): 𝑁 = 𝑠𝑖𝑧𝑒 𝑜𝑓 𝑞𝑢𝑎𝑑𝑟𝑎𝑡
As distribution is not uniform it is important to measure different parts (random sampling)
Method:
- Measure total area
- Use same size quadrat for each sample
- Distribute quadrat at random
- Count individuals in quadrat and repeat sample several times: ‘number in sample’
- Calculate population size
2. Mark-recapture method
A known number of individuals is caught and marked and then released
After a time period this is repeated
, Formula is used based on the fact that the ratio of unmarked to marked individuals is the same as the
ratio of the population as a whole
Suitable for:
- Mobile animals eg: butterflies, birds
- Not easily visible eg: fish in a dam
Method:
- Mark out a well-defined area
- Capture as many individuals as possible and mark them
- Release them
- Allow time for them to mix with unmarked
- Recapture
- Count the total number recaptured and those marked
- Calculate using Petersen index:
𝑀×𝐶
𝑃= 𝑅
P - estimated population
M - Marked animals in first capture
C - total animals caught in second sample
R - total number of marked animals in second same
Precautions for a reliable result
- Short time should pass between first & second sampling to avoid births/deaths
- Sampling should be repeated several times for an average population to be calculated
- Marking must not damage individual
- Marked animal must mix freely with the rest before a new sample is taken
- No immigration or emigration is allowed (population must be closed)
Limiting factors
Factors or resources that help regulate growth of population by causing birth rate to decline or death rate to
increase or both
May be:
- Density independent factors that limit growth as a result of natural factors and not because pf the
density or the number of organisms eg:
- Physical factors: rainfall, temp, humidity
- Catastrophic events: floods, fire, drought
