Environmental Studies Grade 12 Life Sciences

Environmental Studies:
Population and community ecology:
1 Population Ecology:
 Ecology is 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 are the fluctuations in the size of a population and the
factors, both physical and social that regulates the fluctuations.
Individual  populations  community  with non-living org. in ecosystem 
biosphere.
 Biosphere is the part of the earth where living organisms are found.
 An ecosystem is made up of groups of different species of organisms that
interact with each other and with the environment.
 An organism is an individual form of life composed of a single cell or a complex
of cells that are capable of growing and reproducing.
 A community is a group of different species that inhabit and interact in a
particular area.
 A 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.
 An individual is a single organism capable of independent existence.
 A population is a group of organisms of the same species that occupy the
same area and can breed freely with each other.

Demographics of populations:
I.e. the statistics such as the size, age distribution, growth rate, density, etc. of
populations.

What affects the size of a population?
Population size is the total number of individuals in a population. It can increase or
decrease over time with a change in 1/+ of the following:
 Natality – birth rate in animals or the production of seeds in plants
 Mortality – death rate
 Immigration – individuals move into a population and stay
 Emigration – individuals leave a population and do not return.
Natality rate – number of births per 1000 individuals of a population per year.
Population density – increase population size and population density
Mortality rate – number of deaths per 1000 individuals of a population per year.
Population density – decrease population size and population density.
For humans, the:

,  Birth rate is the number of births per 1000 people in a year.
 Death rate is the number of deaths per 1000 people per year.

Populations will therefore:
 Grow 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.
Closed population is a population with no immigration or emigration, the only
parameters affecting any change in the population numbers will be births or deaths,
e.g. fish in a small pod.

How is the growth of a population regulated?
 Individuals within 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.
Exponential - increasing more and more rapidly.
 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 - 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 – the population density that the environment can support over a
relatively long period of time. It is determined by limiting factors that exist in that
environment.
 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 is an ecosystem is self-regulating. (Negative feedbacks)




Limiting Factors:
Factors that help to regulate the growth of a population.

,  May be:

1. Density independent factors:
 limit the growth of a population as a result of natural factors and not
because of the density or number of the organisms. E.g.
 physical factors, e.g. rainfall, temperature, humidity, acidity,
salinity
 catastrophic events, e.g. floods, fire, drought, volcanic
eruptions, tsunami, earthquakes.
2. Density dependent factors:
 Have a greater effect when the population density is high. This is
because, when organisms are more crowded, they:
 Compete more for resources such as food, light, oxygen, space,
and shelter
 Are more easily found by predators
 Spread disease and parasites more readily.
These limiting factors collectively build up environmental resistance.

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, i.e. 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.

How is population size estimated?
1. Direct Methods:
 Involve counting every single individual in a
population. This is called 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.

 Can be used for individuals that are:
o Slow moving, e.g. snails, tortoises or
o Stationary, e.g. plants or
o Usually stay in a fixed position, e.g. 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 species of a large tree.