Senior Science
Preliminary Course
Local Environment
Section 1
Populations
SenSc\Prelim\LE\LE Notes01
Local Environment – Section 1
:::
Populations
8.5.1 The distribution, diversity and numbers of plants and animals found in
ecosystems are determined by the biotic and abiotic factors
8.5.1.a
Describe the differences between abiotic and biotic features of the
environment
8.5.1.b
Compare the abiotic characteristics of aquatic and terrestrial environments
8.5.1.c
Identify the factors determining the distribution and abundance of a species
in each environment
8.5.1.d
Describe and explain the short- and long-term consequences on the
ecosystem of members of the same species competing for resources
8.5.1.e
Explain the need to use sampling techniques to make population estimates
when total counts cannot be made
8.5.1.i
Perform a first-hand investigation using transect, random quadrat, capturerecapture and tagging/marking methods to make estimates of real or
simulated populations of organisms and use the available evidence to
discuss the advantages and disadvantages of these methods
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8.5.1.a
Describe the differences between abiotic and biotic features of the
environment
The environment is everything that is external to the organism. Both nonliving
and living factors interact to make up the total environment of organisms.
Abiotic Factors
Abiotic factors are the non-living features of the environment. These factors rarely
threaten the survival of a species but will greatly affect their populations. They effect the
type of ecosystem and therefore the distribution and abundance of organisms within that
ecosystem.
These factors can be physical or chemical.
Physical factors
Temperature and humidity
Wind
Light
Soil structure
Topography (landform)
Fire
Currents (tides, wave action)
Seasonal change
Amount of living space and shelter
Chemical factors
Water (rainfall)
Oxygen, carbon dioxide
Salinity
pH
Soil nutrients
Biotic factors
Biotic factors are the living organisms that occur in the environment. The distribution
and abundance of organisms in an ecosystem is greatly affected by the organisms that
live in the ecosystem. An organism may be affected by:


Activities of members of the same species
– Competing for resources
– A mate for reproduction
Activities of members of different species.
– A source of food
– A predator or a parasite
– A disease causing organism
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8.5.1.b
Compare the abiotic characteristics of aquatic and terrestrial environments
Abiotic features of Aquatic and Terrestrial Environments
Abiotic features of the environment vary greatly from place to place. The greatest
diversity exists between the abiotic features of an aquatic (water) environment and a
terrestrial (land) environment.
The table below shows differences in the abiotic feature of these two environments.
Abiotic feature
Aquatic environment
Terrestrial environment
Buoyancy
Water exerts an upward force Air provides little floatation or
on objects in it allowing support for organisms.
floatation.
Viscosity
Water has a greater resistance Air
has
a
smaller
Refers to the resistance to to movement then air.
resistance to movement
movement – “stickiness”
than water.
Light
Available light decreases with Light is readily available
depth.
Water
Abundant supply of water
Availability
greatly
Oxygen availability
The availability of oxygen is low
Oxygen availability is high
Nutrients
Dissolved minerals are readily Availability
of
dissolved
available
minerals
varies
greatly
depending on soil type and
quality, and water.
Temperature
variation
In large bodies of water,
temperature
is
relatively
constant.
Temperature also
decreases with depth.
of
water
varies
On land large and rapid
temperature
changes
can
occur.
Temperature also
decreases with altitude.
Notes Questions
1. Name two abiotic features of the environment.
2. Name two biotic features of the environment.
3. What is the difference between availability of nutrients in an aquatic environment
compared to a terrestrial environment?
4. Which environment has the smallest viscosity?
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8.5.1.c
Identify the factors determining the distribution and abundance of a
species in each environment
Numbers of Organisms
Population refers to members of the same species living in the same habitat.
 The distribution of an organism refers to where an organism is found.
 The abundance of an organism refers to the numbers of an organism in the area.
All the organisms living in the same area makes up a community [eg the marine animals
and plants found on a coastal rock platform make up a rock platform community]. A
community of organisms and their environment interacting as a unit is termed an
ecosystem.
The particular set of abiotic and biotic factors determines the distribution and abundance
of a species. The factors include:
 European rabbits are rarely found in Northern Australia. The main reason affecting
their distribution is the climate (particularly heat) (Climate is a set of abiotic factors)
 An adequate supply of water and a balance of inorganic nutrients are needed in the
soil. Some plants will not grow in sandy soil because it cannot hold enough water or
nutrients.
 Members of the same species can influence their own numbers in a community.
Under population can be a problem
– Large herds allow members to protect their young from predators.
– Low numbers may mean finding a mate is very difficult (eg whales)
Notes Questions
5. What is a community?
6. Why aren’t European rabbits usually found in Northern Australia?
7. Coral is a limestone formation formed in the ocean by millions of tiny organisms
called coral polyps.
An orangutan is a tree dwelling, small ape that lives in the
tropical rainforests of Asia.
a. Identify the type of environment in which each organism lives.
b. Outline at least three abiotic features of each environment.
c. Explain why these two organisms occupy different environments. Identify the
factors determining the distribution and abundance in your answer.
2. Research
Select one other aquatic and one other terrestrial organism
a. Name an organism that lives in each environment.
b. Identify the factors determining the distribution and abundance of a species in
each environment.
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8.5.1.d
Describe and explain the short- and long-term consequences on the ecosystem
of members of the same species competing for resources
Competition and Ecosystems
An ecosystem is any space containing living organisms interacting with each other and
with the non-living parts of that environment. In such a system there is an exchange of
materials and energy between the organisms and their environment.
In any community, all the organisms are part of one, large food web. Therefore, the
numbers of one species will affect the numbers of other species. Changes to any part of
a community – living or non-living – can set off a whole series of other changes.
Organisms belonging to the same species use the same resources. If these resources
are limited the individuals must compete with one another. Competition for resources
limits population size. Population increases create more intense competition for
resources that has a greater effect on limiting the population.
For example, flowers planted close together compete for resources. As a result they will
grow poorly and will produce few blooms. Only if they are thinned, by the death of the
weakest plants, will they grow well. The same sort of competition for space, light, water,
and nutrients operates in a forest and keeps down the numbers of the trees.
The graph opposite shows the changes in
populations in hare and lynx over a period
of ninety years. The population of the hare
is limited by available food (plants), and the
number of predators (lynx).
The
environment determines the amount of
plant food available. If there is plenty of
plant food available the population of the
hare increases. A bigger hare population
means more food is available for the lynx
population. As a consequence the lynx
population may increase.
More lynx
means more hares will be killed and the
population of hare will decrease.
Source Keeton W W, Biological Science, Norton & Co., New York, 1972
To summarise,
More plant food (grass)
 Increased hare population
 Increased lynx population and less plant food available.
 Decreased hare population
 Fewer lynx and more grass
 More hares
 More lynx population and less grass.

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Generally, plant populations tend to be limited by competition for resources, particularly
light, water and minerals (the abiotic environment). The population of plant eating
organisms tends to be limited by the availability of food and the number of predators.
Predator populations tend to be limited by competition for food (plant eating organisms).
In the short-term there is usually a constant change in the population of individual
species. The short-term consequence on the ecosystem is a constant change in
available resources for the organism. This occurs because, as the population changes,
resource-use changes. More resources mean a bigger population, which then results in
fewer resources per individual resulting in a population decrease.
In the long term the population of an individual organism remains fairly constant. The
long-term consequence for the ecosystem is stability. Stability occurs because
competition produces environmental changes to return populations to an average level.
It is important to remember that stable ecosystems are not static – they do slowly
change. Further, they can change rapidly if there are major changes in the environment.
Notes Questions
8. What is an ecosystem?
9. What is the effect of competition on the population size of a species?
10. In the example given (hare and lynx)
a. What happened to the population of lynx when the number of hare increased?
b. Name two factors that could cause the hare population to decrease?
11. Name the two factors that generally limit the population of plant eating organisms?
12. What is a predator?
13. What happens to population size in the short term?
14. What is the long-term consequence of competition for resources by organisms?
15. Read the following article about coyotes; then answer the questions that follow
The San Diablo Valley in California was a rich farming area. The main ‘crop’ was
cattle. There were coyotes in the valley that lived mainly on small rodents (rat-like
plant eating animals). The coyotes also killed and ate the occasional café. The
ranchers decided to lay baits to reduce the coyote ‘pest’ population. In the end they
destroyed the whole coyote population. This left the rodents with no predator.
Two years later, there was almost no life left in the San Diablo Valley. “It’s just like a
waste land,” said one rancher. “I guess we’ve learnt our lesson the hard way,” said
another.
a. What resource were the coyotes competing for?
b. What resource were the rodents competing for?
c. What was the short-term consequence of the rodents having no predator?
d. What was the long-term consequence of the coyotes being killed?
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16. Read the following article about a bird species; then answer the questions that follow
Source: ‘Biological Science: the web of life’ (pp247 – 249)
In 1912 a Dutch scientist studied the numbers of bird species living in 129 hectares
of woodland in the Netherlands. This study is still continuing. These birds are about
the size of a sparrow. They feed on insects and have an abundant supply for both
themselves and their young. They roost at night in holes in the trees and in crevices
amongst rocks. They use these places as nesting sites during the breeding season.
During spring each pair rears one or two broods, with two or three birds in each. We
might expect that the bird population would grow from year to year. The study found
that each year the number of birds in the area was almost the same.
In autumn, when the young birds had grown up and developed their adult plumage,
‘fighting’ occurred. The birds were competing for nesting places for the next spring.
Pairs of birds tried to defend a territory around a tree hole. Eventually one pair was
left in possession of the tree hole. Most of the birds that were driven away died
during the winter because they were unable to find suitable shelter.
a. Name the resource the birds competing for.
b. Identify the short-term consequence of the birds competing for the same resource
from one year to the next.
c. Identify the long-term consequence of the birds competing for this resource.
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17. Read the following article about water snails; then answer the questions that follow
1. Another way of examining the short and long-term consequences of competition for
resources is examining a predator/prey graph.
A fresh water pond was studied over two years by a group of biologists. They found
that water snails ate a water plant Nitella. A freshwater fish, the minnow, in turn ate
the water snails. The table below shows the numbers of each organism over the
two-year period.
Time in months
0 (Jan)
2
4
6 (June)
8
10
12 (Jan)
14
16
18 (June)
20
22
24 (Jan)
No of Nitella plants
340
320
200
150
200
270
320
350
280
220
200
230
320
a.
b.
c.
d.
No of snails
100
130
180
170
120
100
100
130
180
230
210
150
130
No of minnow
120
80
30
20
100
150
40
10
10
50
100
160
100
Plot the graph (line graph, line of best fit) for each population on the same grid.
Describe the change in population of the Nitella population.
Describe the change in population of the snail population.
Explain why the population of the snails varies.
i.
In terms of the population of the Nitella.
ii.
In terms of the population of the minnow.
e. Explain why the population of the minnow varies.
f. Use this graph to explain the long-term consequences for the snail population.
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Activity Dingos, Wallabies and Grasses
The dingos and wallabies game can show the inter-relationships between plants and
animals. The game shows how the population of predators is indirectly related to the
plant population through its prey. A simple food chain shows the connections. The
main objective of the activity is to introduce the concept of food chains.
THE GAME
Some animals eat only plants. Others in turn eat these animals.
Instructions and rules
1.
Go to an area where you will have plenty of room to move.
2.
Divide into three groups: dingos, wallabies and grasses.
3.
Wallabies are identified by pieces of cloth in their back pocket, their tails.
4.
Dingos form a line separating the wallabies from the grasses.
5.
Grasses are immobile.
6.
When a signal is given the wallabies must try to get to the grasses.
7.
Dingos try to catch the wallabies by pulling tails out.
8.
Wallabies are safe when frozen in a crouching position.
9.
Wallabies may not move or get grasses without standing up and wallabies must
get food within each round or they die and become grasses.
10. When a dingo catches a wallaby, the wallaby becomes a dingo, but if the dingo
fails to catch a wallaby within the round it “dies” and becomes grass too.
11. When the wallaby gets food, the food becomes a wallaby.
12. Dingos may only get 1 wallaby in each round and wallabies may only get 1 grass.
What to do
 Players stand in lines in the ratio of 5 grasses, 4 wallabies and 3 dingos (see
diagram)
grasses
dingos
 Each round pasts approximately 15 to 20 seconds.
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wallabies
 At the end of each round the score is recorded as follows:
Round
0
1
2
Numbers
W
D
G
8
6
10
Total
24
24
24
Graph the results after several rounds.
QUESTIONS AND DISCUSSION
1. Outline what happens to the wallaby population if there is a shortage of grass.
2. What would happen to the wallaby population if there were no grass?
3. For one round, state the number of dingoes, wallabies and grasses. Explain how
these numbers affect the numbers in the next round.
4. Discuss the way the three populations change and the reasons why.
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8.5.1.e
Explain the need to use sampling techniques to make population estimates
when total counts cannot be made
8.5.1.i
Perform a first-hand investigation using transect, random quadrat, capturerecapture and tagging/marking methods to make estimates of real or
simulated populations of organisms and use the available evidence to
discuss the advantages and disadvantages of these methods
Sampling techniques
To count the total population of a plant or animal species in an area could be an
enormous job and take a long time. When the organisms are in huge numbers,
scattered over a large area, mobile or difficult to see or capture it is often impossible to
complete a total count of all the organisms in an area.
To make the job easier, only certain areas or positions are studied. This involves a
method called sampling. Example of the sampling techniques used to estimate
population numbers are:
 Selection of random quadrats
 Line transect
 Capture and recapture
 Tagging / marking methods.
The Random Quadrat Method
The random quadrat method is usually used for estimating large numbers of small
organisms that cannot move, living in a relatively defined area. A sample area is
selected by placing a square frame, called a quadrat, on the ground. A one square
metre frame is generally used. The number of organisms within the quadrat is counted
several times and averaged.
An estimate of the total population is determined by moving the quadrat around the area
being studied. Each new position of the quadrat is selected at random. Counts in each
sample are made and the total area calculated. The number of sample plots used
depends on time available, accuracy required and the total area being investigated and
a number of other factors.
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Activity
Estimating Populations using random quadrats
What to do
Each dot in the rectangle represents an individual organism. Your task is to estimate
the population of this organism. Follow the instructions (over page) to estimate the
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‘population’ of dots.
Aim
To estimate the number of organisms shown on the map, using the quadrat method.
Method
1.
2.
3.
4.
5.
6.
Mark out a 2cm x 2cm square on the map.
Count the number of organisms (dots) in this square (square A)
Record this number.
Repeat steps 1,2& 3 for TWO other areas (square B & C).
Measure the size of the rectangle map area in cm2and then
Calculate the number of 2cm x 2cm squares in this rectangle map area by dividing
total area by 4.
Results
Number of organisms in each 2cm x 2cm square
Square A
+
_______
Square B
+
Square C
_______
=
________
Average (Total – 3)
______
Size of Rectangle Map Area
Length
x
______
breadth
=
total area
_______ cm2
______
Number of 2cm x 2cm squares
=
total area divided by 4
=
______
Interpreting data
To estimate the population, multiply the average obtained by the number of squares.
Average
______
x
Num. of squares
=
______
______
Conclusion
Write a conclusion stating the estimated population
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population estimate
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Activity
Estimating Population II using random quadrats
In this section the quadrat method will be used to estimate the populations of two weeds
(bindii and dandelion) that occur on one grassed area within the school grounds.
Information
What to do
Equipment
- 1 metre ruler; string quadrat (1m2)
1. Write a heading
2. Write an aim into your book
Method [VGMANS]
a. For validity the area of each quadrat
should be kept the same throughout the
investigation.
3. Write a draft method to perform this first
hand investigation. The questions below
will help.
a. What
variables
need
to
be
controlled?
b. How many times should the
organisms be counted for each
quadrat?
c. How many quadrats should be used?
d. How should total area be measured?
e. Will any diagrams be used?
b. The quantity being measured is the
number of organisms.
4. Edit your draft method after it has been
proof read by at least one other person.
c. The activity is a practical investigation.
5. Complete the activity and collect data on
the number of organisms in each
quadrat.
In this investigation there is no independent
variable or dependent variable. This is
because the effect of one variable on
another is not being investigated. The
activity is concerned with collecting data for
one factor.
d. Field data is recorded on a working
sheet (rough). It is then transferred to a
data table. In this investigation both sets
of data should be handed in. The design
of the data table needs to be considered.
e. To get a reliable estimate the organisms
in a quadrat should be counted at least
twice. Also the more quadrats counted
the more reliable the estimate.
6. Record data
a. How will rough data be recorded?
b. How will the table be set out to
record data?
7. Write a conclusion.
8. Answer the discussion questions
f. A safety issue in this first hand
investigation could be to check if the
grassed area has been recently sprayed
with herbicide.
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Discussion questions
1. Explain why there is no dependent or dependent variable in this investigation. [2 mk]
2. Identify two variables is kept constant in this investigation?
[1 mark]
3. In any investigation problems are encountered. The method used can be improved.
In this investigation you needed to make a number of decisions to solve particular
problems. These decisions related to factors such as:
 One weed was easy to count and one weed was relatively difficult to count.
 How many sample plots (quadrats) should be used.
 How many times the populations of weeds were counted for each quadrat.
 Where to put the quadrat.
 Other issues.
a. Outline some of the problems encountered as you planned and performed your
investigation.
[4 marks]
b. Outline any improvements you made while performing the investigation. [2 mks]
The Line Transect
Sometimes it is useful to determine the change in population along a line between two
points. For example, on a rock platform the numbers of particular organisms change as
you walk along a line moving away from the wave area. This change is influenced by a
number of abiotic factors. The change in the population along a transect can be
measured by using a quadrat at intervals or by making appropriate observations.
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Activity
Capture / Recapture
Counting the population of organisms that can move can be very difficult. It is usual to
calculate an estimate after counting a number of samples of the population. A technique
known as: capture – recapture can be used.
In this method
 A number of organisms are captured and counted
 These captured organisms are then “tagged”.
 The tagged organisms are then released.
Once released the organisms move around in their environment.
 Another group of these organisms are (re)captured – this will include some tagged
and some untagged organisms.
 The number of tagged and the number of untagged organisms is counted.
 The data is recorded in a table like the one below.
No. of captured,
tagged organisms
=
Recaptured Sample
When “recaptured”,
no of tagged organisms
1
2
3
4
=
When “recaptured”,
No of untagged organisms =

An estimate of the population can be calculated using the following formula:
Pop. estimate

=
No recaptured & untagged x No captured & tagged
No recaptured & tagged
The “recapturing” steps can be repeated several times to get several estimates.
These estimates can be graphed or averaged to get a more reliable population
estimate.
What to do
Your teacher will guide you through this exercise.
1. Estimate the size of one population to demonstrate the capture / recapture method.
2. Write a laboratory report using the appropriate scaffold.
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The investigation is to use the quadrat method to estimate three populations
Marking criteria
Marks
Aspect of Report
Comment
Presentation
 Neatness
 Follow laboratory report scaffold
Method
 Point form
 Steps can be followed
 Number – quadrats, counts
Results
 Field data attached in appendix
 Data table (heading, ruled, ????)

Discussion
 Responses to questions
Other
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