Population ecology
Individual - a single organism that forms part of a species.
Species - a group of organisms with similar characteristics, which can interbreed and produce
fertile offspring. (giraffes, rhinos, zebras)
Population - a group of organisms of the same species that are found in a particular area and can
crossbreed freely. (Population of giraffes in the Kruger National Park)
Community - a group of populations occurring in a particular area. (The lion, zebra, giraffe, rhino
populations in the Kruger National Park)
Ecosystem - a unit of plant and animal communities (biotic/living) in interaction with each other as
well as with the non-living (abiotic) factors in a particular area.
Environment - the external factors, living and non-living, surrounding an organism and influencing
its development and survival.
Habitat - the specific environment (home) in which a plant or animal species normally occurs.
Niche - specific role an organism will fill in the ecosystem. (Decomposers)
Population size
The population size refers to the total number of individuals in a population.
The population density refers to the number of individuals of a population per unit area.
[E.g. 10 sheep per hectare (100m x 100m)]
Population parameters (4 main factors)
Natality - birth rate of a population. (number of live births per thousand individuals per year)
Mortality - death rate of a population. (number of deaths per thousand individuals per year)
Immigration - one-way movement of organisms into an area where they become established.
Emigration - one-way movement of organisms out of an area to become established.
Migration - fluctuating population.
Natality and immigration cause an increase in population size.
Mortality and emigration cause a decrease in population size.
, Closed population - a population where immigration and emigration do not occur. (Fish in a dam or
animals on an island)
Only parameters that affect a closed population are natality and mortality.
Determining population size
The choice of method is determined by:
- the size of the organisms
- the mobility of the organisms
Direct techniques
Involves the total count of all individuals in the population.
Census - directing counting of individuals
Census methods include:
- direct contact counting of smaller-sized organisms or humans.
- direct counting from vehicle/helicopter/plane of larger organisms.
- counting from aerial photographs taken of larger moving organisms.
Indirect techniques
Involves the counting of only part of the population.
This tally/count is used as a sample to determine the total estimated population size by means of
statistical calculations.
Mark-recapture technique
- A specific area is demarcated.
- A number of individuals are caught, counted and marked and they are known as the 1st sample.
- The marked individuals are released into the environment.
- Sufficient time must be allowed for the marked individuals to mix with the rest of the population.
- Another group of individuals is caught and counted and they are known as the 2nd sample.
- The number of marked individuals in the 2nd sample are counted
This data is used to determine the estimated size of the population using the following formula:
𝑚𝑥𝑠
P= 𝑡
, P = estimated population size
m = total number of animals captured and marked in 1st sample
s = total number of animals captured in 2nd sample
t = number of marked individuals in 2nd sample
The mark-recapture method can be improved by repeating the sampling a few times and then
calculating an average population size.
This technique is only valid under the following conditions:
- Marking methods should not injure/damage the organism.
- The marks should be clearly visible for the duration of the investigation.
- The marking methods should not affect the movement or behaviour of the organism.
- Marked organisms should be allowed enough time to mix with the rest of the population before
the next sample is taken.
- The population must be closed so no immigration/emigration takes place.
- The period between the two samplings should be sufficiently brief so no births/deaths occur.
Quadrats
A quadrat is a square frame of metal/wood/plastic of known size. E.g. 1 m2
- An area where organisms must be counted is demarcated and the surface area determined.
- The quadrat is placed on the ground in the demarcated area and the organisms concerned are
counted inside the quadrat.
- This is the number of individuals per unit area. E.g. 1 m2
- This process is repeated in different areas of the demarcated area by placing the quadrat
randomly on the ground. (Random sampling: each individual has an equal/independent chance)
- The average number of individuals per quadrat is calculated.
The total number of individuals in the population is determined using the following formula:
Population size = average number of individuals per quadrat x surface area of the area
surface area of the quadrat
The reliability of this technique depends on the following:
- the exact number of organisms in each quadrat must be known.
- the surface area of the quadrat must be known.
- the surface area of the demarcated area must be known.
- the quadrat must be placed randomly in the demarcated area.
The quadrat method is suited to plant populations and sessile/slow-moving invertebrates.
Individual - a single organism that forms part of a species.
Species - a group of organisms with similar characteristics, which can interbreed and produce
fertile offspring. (giraffes, rhinos, zebras)
Population - a group of organisms of the same species that are found in a particular area and can
crossbreed freely. (Population of giraffes in the Kruger National Park)
Community - a group of populations occurring in a particular area. (The lion, zebra, giraffe, rhino
populations in the Kruger National Park)
Ecosystem - a unit of plant and animal communities (biotic/living) in interaction with each other as
well as with the non-living (abiotic) factors in a particular area.
Environment - the external factors, living and non-living, surrounding an organism and influencing
its development and survival.
Habitat - the specific environment (home) in which a plant or animal species normally occurs.
Niche - specific role an organism will fill in the ecosystem. (Decomposers)
Population size
The population size refers to the total number of individuals in a population.
The population density refers to the number of individuals of a population per unit area.
[E.g. 10 sheep per hectare (100m x 100m)]
Population parameters (4 main factors)
Natality - birth rate of a population. (number of live births per thousand individuals per year)
Mortality - death rate of a population. (number of deaths per thousand individuals per year)
Immigration - one-way movement of organisms into an area where they become established.
Emigration - one-way movement of organisms out of an area to become established.
Migration - fluctuating population.
Natality and immigration cause an increase in population size.
Mortality and emigration cause a decrease in population size.
, Closed population - a population where immigration and emigration do not occur. (Fish in a dam or
animals on an island)
Only parameters that affect a closed population are natality and mortality.
Determining population size
The choice of method is determined by:
- the size of the organisms
- the mobility of the organisms
Direct techniques
Involves the total count of all individuals in the population.
Census - directing counting of individuals
Census methods include:
- direct contact counting of smaller-sized organisms or humans.
- direct counting from vehicle/helicopter/plane of larger organisms.
- counting from aerial photographs taken of larger moving organisms.
Indirect techniques
Involves the counting of only part of the population.
This tally/count is used as a sample to determine the total estimated population size by means of
statistical calculations.
Mark-recapture technique
- A specific area is demarcated.
- A number of individuals are caught, counted and marked and they are known as the 1st sample.
- The marked individuals are released into the environment.
- Sufficient time must be allowed for the marked individuals to mix with the rest of the population.
- Another group of individuals is caught and counted and they are known as the 2nd sample.
- The number of marked individuals in the 2nd sample are counted
This data is used to determine the estimated size of the population using the following formula:
𝑚𝑥𝑠
P= 𝑡
, P = estimated population size
m = total number of animals captured and marked in 1st sample
s = total number of animals captured in 2nd sample
t = number of marked individuals in 2nd sample
The mark-recapture method can be improved by repeating the sampling a few times and then
calculating an average population size.
This technique is only valid under the following conditions:
- Marking methods should not injure/damage the organism.
- The marks should be clearly visible for the duration of the investigation.
- The marking methods should not affect the movement or behaviour of the organism.
- Marked organisms should be allowed enough time to mix with the rest of the population before
the next sample is taken.
- The population must be closed so no immigration/emigration takes place.
- The period between the two samplings should be sufficiently brief so no births/deaths occur.
Quadrats
A quadrat is a square frame of metal/wood/plastic of known size. E.g. 1 m2
- An area where organisms must be counted is demarcated and the surface area determined.
- The quadrat is placed on the ground in the demarcated area and the organisms concerned are
counted inside the quadrat.
- This is the number of individuals per unit area. E.g. 1 m2
- This process is repeated in different areas of the demarcated area by placing the quadrat
randomly on the ground. (Random sampling: each individual has an equal/independent chance)
- The average number of individuals per quadrat is calculated.
The total number of individuals in the population is determined using the following formula:
Population size = average number of individuals per quadrat x surface area of the area
surface area of the quadrat
The reliability of this technique depends on the following:
- the exact number of organisms in each quadrat must be known.
- the surface area of the quadrat must be known.
- the surface area of the demarcated area must be known.
- the quadrat must be placed randomly in the demarcated area.
The quadrat method is suited to plant populations and sessile/slow-moving invertebrates.
