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Summary - Life Sciences (Biology)
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1. Environmental studiesPopulation and community ecology
Definitions
• Individual – a single, separate organism, animal or plant, capable of
independent existence.
• Population – a group of individuals belonging to the same species (so they
can interbreed) that live in the same area at the same time.
• Community – a group of different populations that live together and interact in
a defined area.
• Ecosystem – all the organisms that live in an area and interact with each other
and with the physical environment.
Population size
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 one or more of the following:
• Natality – birth rate in animals or the productions of seeds in plants.
• Mortality – death rate
• Immigration – individuals move into a population and stay.
• Emigration – individuals leave a population and do not return.
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.
In a closed population, with no immigration or emigration, the only parameters
affecting any change in population numbers with be birth or deaths, e.g. fish in a
small pond.
Population size fluctuates seasonally and annually depending on the resources
available.
Limiting factors
The factors that help to regulate the growth of a population are known as the limiting
factors.
Limiting factors may be:
• Density independent factors that limit the growth of a populations 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, tsunamis,
earthquakes.
• Density dependant factors that have a greater effect when the population
density is high. This is because, when organisms are more crowded, they:
- compete for more resources such as food, light, oxygen, water, space and
shelter.
- are more easily found by predators.
- spread diseases and parasites more readily.
These limiting factors collectively build up environmental resistance.
environmental resistance = the total number of factors that stop a population from
reproducing at its maximum rate.
Carrying capacity
Eventually a balance is reached, and the population stabilizes at a particular size or
number. This is known as carrying capacity.
carrying capacity = the maximum population size that the environment can support.
Implications of carrying capacity for a species
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 and
therefore the number of species that can be supported.
, • eventually not being able to support the population, which will decrease
rapidly and cause species to become extinct.
Sampling methods
It is important to be able to measure the size of a population to see if it is changing
over time. This can be done using the following methods
A. Mark-recapture method
This method is suitable for animals that are:
- mobile, e.g. butterflies, birds, antelope
- not easily visible, e.g. fish in a dam.
Method
- Mark out a well-defined area.
- Capture as many individuals as possible and mark them, e.g. metal tag on
the ear or a spot of oil-based paint.
- Release the marked individuals back into the environment.
- Allow them time to mix thoroughly with the unmarked individuals of the
same species.
- Recapture as many individuals as possible.
Calculations
- Count the total number and count the number of those which have been
marked.
- Calculate the total population by using the Peterson index as below:
𝑀𝑥𝐶
𝑃=
𝑅
P = estimated population
M = total number of marked animals
C = total number of animals caught in second sample.
R = total number of marked animals in second sample, i.e. recaptured.
Precautions
- Only a short time should pass between the first and second sampling, so
that no births and deaths can occur.
- Sampling should be repeated several times and an average population
calculated.
, - The marking must not damage the individual or affect its movement or
behaviour.
- The marked animal must mix freely with the rest of the population before a
new sample is taken.
- No immigration or emigration is allowed, i.e. the population must be
closed.
B. Quadrat method
The purpose of using a quadrat is to enable comparable samples to be
obtained from areas of consistent size and shape.
Method
- Measure the size of the total area.
- Use a wooden frame of known diameters as a quadrat. Count the
individuals in each quadrat.
- Several samples should be taken and the number of individuals per
quadrat calculated. This is the “number sample” In the formula.
- Using the formula below, calculate the size of the population of the total
area.
Calculation
Total population = (N)
𝑛𝑢𝑚𝑏𝑒𝑟𝑠 𝑖𝑠 𝑠𝑎𝑚𝑝𝑙𝑒 𝑥 𝑠𝑖𝑧𝑒 𝑜𝑓 𝑤ℎ𝑜𝑙𝑒 ℎ𝑎𝑏𝑖𝑡𝑎𝑡
𝑁=
𝑠𝑖𝑧𝑒 𝑜𝑓 𝑞𝑢𝑎𝑑𝑟𝑎𝑡
Precautions
- The same quadrat size must be used for each sample.
- Quadrats should be distributed at random.
C. Census
Method
A census involves counting every single individual in a population.
If the area is too large to count every individual at one time either:
- ariel photography 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.
Limitations
This method can only be used for individuals that are:
- slow moving, e.g. snails, tortoises or
- stationary, e.g. plants or
- usually stay in fixed position, e.g. barnacles, muscles.
It is limited to:
- populations where organisms are large enough to be seen.
- areas in which the animals are being counted is not too large.
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