Ecology is the study of the mutual interaction among living organisms and between
living organisms and the environment in which they live.
Population ecology is the part of ecology that focuses on the factors influencing the
population size, growth rate, growth forms and distribution of individuals inside a
population.
TERMINOLOGY
― individual: a single organism that forms part of a species
― species: a group of organisms with similar characteristics, which produce fertile
offspring
― population: a group of organisms of the same species that are found in a
particular are that can crossbreed freely
― community: a group of populations occurring in a particular area // groups of
different species living in a particular area
― ecosystem: 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 (biotic) and non-living (abiotic),
surrounding an organism and influencing its development and survival
― habitat: the specific environment in which a plant or animal species normally
occurs
POPULATION SIZE
o Population size refers to the total number of individuals in a population
o population density refers to the number of individuals of a population per unit
area
POPULATION PARAMETERS
Four main factors, known as parameters, have an effect on population size:
― natality: the birth rate of a population, normally expressed as the number of live
births per thousand individuals per year
― mortality: the death rate of a population, normally expressed as the number of
deaths per thousand individuals per year
― immigration: the one-way movement of organisms into an area where they
become established
― emigration: the one-way movement of organisms out of an area to become
established
,- Natality and immigration cause an increase in population size
- Mortality and emigration cause a decrease in population size
Closed population: population where immigration and emigration do not occur, the
only parameters affecting population size are natality and mortality
DETERMINING POPULATION SIZE
population size can be determined by direct or indirect techniques/methods
(determined by size and mobility of organisms)
direct techniques
- involve the total count of all individuals in the population
- direct counting of individuals is known as a census
- 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
- tally/count is used as a sample to determine the total estimated population size by
means of statistical calculations
- includes the following;
mark-recapture technique
quadrat technique
, mark-recapture technique
- a specific area is demarcated
- individuals are caught, counted and marked (1st sample), then released into the environment
- sufficient time is allowed for marked individuals to mix with the rest of the population
- another group are caught and counted and are known as 2nd sample
p – estimated population size
- number of marked individuals in 2nd sample are counted
m- total numbers captured and marked in 1st sample
- this data is used to determine estimated size of population
s – total number of animals captured in 2nd sample
𝑚 ×𝑠
P= t – number of marked individuals in 2nd sample
𝑡
- can be made more accurate by repeating sampling and then calculating average population
size
- technique is only valid under following conditions:
marking methods should not damage or injure the organism
marks should be clearly visible
marking method shouldn’t affect movement of behaviour of organism
marked organisms should be allowed enough time to mix before the next sample is taken
population must be closed (no immigration or emigration)
period between samplings must be brief (no births or deaths)
quadrats
- usually a square frame of metal, wood or plastic, of known size
- area where organisms must be counted is demarcated and surface area is determined
- quadrat is placed in demarcated are and the organisms concerned are counted inside quadrat
- this is the number of individuals per unit area
- this process is repeated a few times in different demarcated areas by placing quadrat on ground
- average number of individuals per quadrat is calculated
- total number of individuals is calculated by:
𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑎𝑟𝑒𝑎
Population size = average number of individuals per quadrat x
𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑞𝑢𝑎𝑑𝑟𝑎𝑡
- reliability of this technique depends on:
exact number of organisms must be known
surface area of quadrat must be known
surface area of demarcated area must be known
quadrat must be placed randomly in demarcated area
- quadrat can be any shape, data must be collected from the same shaped quadrats to be
comparable
- quadrat size depends of type of organism (suited to plant populations/slow moving invertebrates)
living organisms and the environment in which they live.
Population ecology is the part of ecology that focuses on the factors influencing the
population size, growth rate, growth forms and distribution of individuals inside a
population.
TERMINOLOGY
― individual: a single organism that forms part of a species
― species: a group of organisms with similar characteristics, which produce fertile
offspring
― population: a group of organisms of the same species that are found in a
particular are that can crossbreed freely
― community: a group of populations occurring in a particular area // groups of
different species living in a particular area
― ecosystem: 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 (biotic) and non-living (abiotic),
surrounding an organism and influencing its development and survival
― habitat: the specific environment in which a plant or animal species normally
occurs
POPULATION SIZE
o Population size refers to the total number of individuals in a population
o population density refers to the number of individuals of a population per unit
area
POPULATION PARAMETERS
Four main factors, known as parameters, have an effect on population size:
― natality: the birth rate of a population, normally expressed as the number of live
births per thousand individuals per year
― mortality: the death rate of a population, normally expressed as the number of
deaths per thousand individuals per year
― immigration: the one-way movement of organisms into an area where they
become established
― emigration: the one-way movement of organisms out of an area to become
established
,- Natality and immigration cause an increase in population size
- Mortality and emigration cause a decrease in population size
Closed population: population where immigration and emigration do not occur, the
only parameters affecting population size are natality and mortality
DETERMINING POPULATION SIZE
population size can be determined by direct or indirect techniques/methods
(determined by size and mobility of organisms)
direct techniques
- involve the total count of all individuals in the population
- direct counting of individuals is known as a census
- 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
- tally/count is used as a sample to determine the total estimated population size by
means of statistical calculations
- includes the following;
mark-recapture technique
quadrat technique
, mark-recapture technique
- a specific area is demarcated
- individuals are caught, counted and marked (1st sample), then released into the environment
- sufficient time is allowed for marked individuals to mix with the rest of the population
- another group are caught and counted and are known as 2nd sample
p – estimated population size
- number of marked individuals in 2nd sample are counted
m- total numbers captured and marked in 1st sample
- this data is used to determine estimated size of population
s – total number of animals captured in 2nd sample
𝑚 ×𝑠
P= t – number of marked individuals in 2nd sample
𝑡
- can be made more accurate by repeating sampling and then calculating average population
size
- technique is only valid under following conditions:
marking methods should not damage or injure the organism
marks should be clearly visible
marking method shouldn’t affect movement of behaviour of organism
marked organisms should be allowed enough time to mix before the next sample is taken
population must be closed (no immigration or emigration)
period between samplings must be brief (no births or deaths)
quadrats
- usually a square frame of metal, wood or plastic, of known size
- area where organisms must be counted is demarcated and surface area is determined
- quadrat is placed in demarcated are and the organisms concerned are counted inside quadrat
- this is the number of individuals per unit area
- this process is repeated a few times in different demarcated areas by placing quadrat on ground
- average number of individuals per quadrat is calculated
- total number of individuals is calculated by:
𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑎𝑟𝑒𝑎
Population size = average number of individuals per quadrat x
𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑞𝑢𝑎𝑑𝑟𝑎𝑡
- reliability of this technique depends on:
exact number of organisms must be known
surface area of quadrat must be known
surface area of demarcated area must be known
quadrat must be placed randomly in demarcated area
- quadrat can be any shape, data must be collected from the same shaped quadrats to be
comparable
- quadrat size depends of type of organism (suited to plant populations/slow moving invertebrates)
