Environmental Studies
Population Ecology
- Ecology is the study of how organisms interact with their physical and biological
environments, which influences the distribution and composition of populations within
an ecosystem.
- Individuals form populations, and various populations together create a community.
Communities, along with non-living elements in their environment, form an ecosystem.
All ecosystems on Earth collectively make up the biosphere.
Biosphere: is the part of the earth where living organisms are found
Ecosystem: groups of different species of organisms that interact with each other and
with the environment
Organism: is an individual form of life, such as a bacterium, protest, fungus, plant or animal,
composed of a single cell or a complex of cells that are capable of growing and reproducing
Community: a group of various species that live and interact within a specific area.Species:
is a group of closely related organisms that are very similar to each other and are usually
capable of interbreeding and producing fertile offspring
Individual: is a single organism capable of independent existence
Population: is a group of organisms of the same species that occupy the same area and can
breed freely with each other
What affects the size of a population?
Population size can increase/ decreases with a change in one or more of the following:
Natality: birth rate of a population (+)
Mortality: death rate of a population (-)
Immigration: individuals move into a population and stay (+)
Emigration: individuals leave a population and do not return (-)
Populations will therefore:
- Increase when birth and immigration exceed death and emigration
- Decline when death and emigration exceed birth and immigration
- Remain stable when birth and immigration approximately equal death and emigration
In a closed population, with no immigration or emigration, the only parameters affecting
any change in population numbers will be births or deaths, e.g. Fish in a small pond
How is the growth of a population regulated?
- If a few individuals enter an unoccupied area where there is no shortage of food or
other resources and no predators, they will reproduce and the number of individuals will
increase exponentially as the numbers increase, more demands are made on the available
resources and this builds up environmental resistance which causes the birth
rate/immigration rate to decrease and the death rate/emigration rate to increase
Environmental resistance is the total number of factors that stop a population from
reproducing at its maximum rate
- Eventually a balance is reached and the population stabilizes at a particular size or
number. This number is the carrying capacity of the ecosystem
- Carrying capacity is the population density that the environment can support
- The population fluctuates around the carrying capacity until the environment changes
again
- Population size fluctuates seasonally and annually depending on the resources available
- The population size in an ecosystem is self- regulating
1
, Limiting factors
- The factors that help to regulate the growth of a population are known as limiting
factors/environmental resistance. Limiting factors may be:
Density independent factors that limit the growth of a population because of natural
factors ( rainfall, temperature, humidity, acidity, salinity, floods, fire, drought, volcanic
eruptions, tsunami, earthquakes)
Density dependent factors have an effect when the population density is high. When
organisms are more crowded, they:
- Compete more for resources such as food, light, oxygen, water, space and shelter
- Are more easily found by predators as there is more of them
- Spread disease and parasites more readily.
Stable and unstable populations
- A stable population is one in which numbers decrease when its size exceeds the carrying
capacity but increase again when numbers fall below the carrying capacity, ie. One that
fluctuates around the carrying capacity
- An unstable population develops if the population far exceeds the carrying capacity
This results in the habitat:
- Deteriorating rapidly, which leads to a lowering of the carrying capacity
- Eventually not being able to support the population, which will decrease rapidly and
possibly become extinct
Other ways to regulate population size:
- Birth control and sterilization
- Relocation
How is population size estimated?
- It is important to be able to measure the size of a population to see if it is changing
over time
- The choice regarding what method is determined by the size of the organisms and the
mobility of the organisms
Direct methods
The total count of all individuals in the population, this is known as a census. This method
can only be used:
- For populations where organisms are large enough to be seen
- Where the area in which the animals are being counted is not too large
Direct methods can be used for individuals that are:
- Slow moving, e.g. Snails, tortoises or stationery, e.g. Plants or usually stay in fixed
position, eg barnacles, mussels
- If the area is too large to count every individual at one time either: Aerial photographs
can be taken to show the whole area in which the population occurs, e.g. Penguins, seals
or a species of a large tree
- Helicopters can be used to count larger animals, e.g. Elephant and buffalo in game
reserves
Indirect methods
The counting of only part of the population and then estimate the total size of the
population
Quadrat method
- This method involves counting the number of individuals in small, measured areas
(quadrats) and then using these numbers to calculate the population size of the total
area with the aid of the following formula:
2
