Download Lesson 2 Investigating populations

Today we are covering from the specification:
Investigating Populations - Key Terms
Habitat
Abundance
Random sampling
Systematic sampling
Quadrat
Transect
Frequency
Percentage cover
Mark-release-recapture
Studying Habitats
• When studying a habitat, ecologists will first estimate the
populations of the species living there.
• The abundance of each species can never be known exactly, but
sampling can give reasonably accurate estimates.
Why would it be wrong to try to count every individual of a
population?
• Small samples are studied at random locations, and then scaled
up to fit the entire habitat.
• There are of course, a range of ecological techniques at an
ecologist’s disposal.
Definitions
Write a definition for the terms:
- Habitat
- Abundance
- Quantitative data
- Qualitative data
Introducing Sampling Techniques
The approach to sampling can be in one of two ways:
1. Random
2. Systematic
Random Sampling:
This is usually employed when trying to eliminate bias. Two
numbered axis can be laid out over the sample area. Generation of
random numbers provides co-ordinates for areas to study.
Systematic Sampling:
A similar grid is laid over the entire area, but samples are taken at
regular intervals. Time-consuming... but more reliable?
How do we go about sampling in the field?
Random walking
One random sampling technique is random walking.
This involves choosing a random number between 0 and
360 to act as a compass bearing, then choosing another
random number to determine how many paces to walk
before reaching the sample point.
start
sample point
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Quadrats
• A quadrat is just a fancy square used by biologists/ecologists.
• There are two types of quadrat:
Frame Quadrat
Point Quadrat
How would you use each of these in the field? What data
could you collect?
Random sampling and quadrats
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More About Quadratting
Three things to consider when using quadrats:
1. The size of the quadrat:
Depends on the size of species being investigated, or what kind of
groups or colonies the species live in.
2. The number of samples being taken in the area:
The more samples you take in the habitat, the more reliable the
results will be - depends if time is an issue.
3. The position of each quadrat:
Producing unbiased results within a small time-frame is the best
idea. Random sampling would work well.
A problem that arises during quadrat sampling is the clumping of
plants. To get around this... We measure the ‘mean density’ or
‘percentage cover’ instead.
How could you use a
quadrat to estimate the
population of daisies in
this field?
Random Sampling
• Use 2 tape measures at right angles along
adjacent sides of the field.
• Use a random number generator (e.g. on your
calculator) to generate pairs of numbers.
• Use these pairs as coordinates to locate a random
point on the field.
• Place the quadrat and count the number of
daisies within.
• Repeat so you have at least 10 results.
• Scale up to find the mean population density –
how would you do this?
Mean Density
• You count all the individuals of a single species in a quadrat. Do this
for several quadrats (as painstaking as it may be).
• The quadrat must be of a known size.
• Plug the numbers into the following formula:
Estimated mean
density
=
Total number of individuals counted
Number of quadrats x Area of quadrat
Let’s have a go at some fieldwork!
What to do with your results
We use statistical tests to see if the results are
significant.
• Χ2 (Chi-squared) – used to test whether there is a
significant difference between your observed
results and some theoretical expected data.
• Spearman’s rank – used to test for correlation
between sets of data from the same sample.
• Standard error and 95% confidence limits – used
to test for differences between mean values.
Standard Error and 95% Confidence
Limits
• Is there a significant difference between the
mean population densities in the two areas
we tested?
You’ll get one
of these in the
EMPA.
Statistics and EMPAs
- State null hypothesis
- Which test will you use?
- Why?
- Calculate test statistic
- Interpret the test statistic in relation to your
null hypothesis. Use the words probability
and chance in your answer.
Null hypothesis
Results of an experiment could be due to
random chance.
Only way to support your hypothesis is to
reject a null hypothesis.
Null hypothesis states there is no
link/correlation/difference between results.
→ Depends on statistical test used.
Probabilities
• We normally work at the 5% probability
level (P=0.05).
• To reject the null hypothesis (and accept
your own hypothesis), you must be sure
that there is ≤5% probability that the
results are due to chance.
Why use SE?
Standard Error with 95% Confidence
Limits
What can this test tell you?
If there is a statistically significant
difference between two (or more) means.
What is the null hypothesis?
There is no difference between the mean
………… (for ………… and ………...).
Worked example
A student investigated the variation in the
length of mussel shells on two different
locations on a rocky shore.
The student measured the shell length of
10 mussels at each location.
What is the null hypothesis?
There is no statistically significant difference
between the mean lengths of the two samples
of mussels.
If the 95% confidence limit around the means
do not overlap, then you can reject the null
hypothesis.
e
Calculating SE and 95% CL
1. State null hypothesis
2. Calculate mean and standard deviation
3. Calculate SE and 95% confidence limits:
Step 2: Calculate mean and SD for
both groups
Group 1:
SD = 
(x – x)2
n–1
=

1296
= 12
10 – 1
Group 2:
SD = 
(x – x)2
n–1
=
Group 1 mean = 58mm

368
10 – 1
= 6.4
Group 2 mean = 31mm
Step 3: Calculate the SE for both groups
Group 1:
SE =
SD
=
n
12
 10
= 3.8
Group 2:
SE =
SD
n
=
6.4
 10
= 2.0
Step 3: Calculate the 95% confidence limits
Mean ± 2 x SE
Group 1:
Upper limit = 58 + (2 x 3.8) = 66
Lower limit = 58 – (2 x 3.8) = 50
Group 2:
Upper limit = 31 + (2 x 2.0) = 35
Lower limit = 31 – (2 x 2.0) = 27
Calculating SE and 95% CL
4. Compare 95% confidence limits (you may
wish to use a graph).
If the confidence limits
overlap:
If the confidence limits do
not overlap:
- the means are not
significantly different
- the means are
significantly different
- accept null hypothesis
- reject null hypothesis
- >5% probability the
differences in means are
due to chance.
- ≤5% probability the
differences in means are
due to chance.
Step 4: Plot means and confidence limits
Mean ± 2 x SE
Mean
shell
length/
mm
Data set
The 95% confidence limits do not overlap, therefore there is a
significant difference between the two means. We reject the
null hypothesis as there is equal to or less than a 5%
probability that the differences in means are due to chance.
Now try the examples on the sheet.
Remember:
1. State your null hypothesis.
2. Calculate the mean and standard deviation.
3. Calculate the standard error.
4. Calculate the 95% confidence limits.
5. Check to see whether the confidence limits overlap or
not.
6. Write a conclusion, stating:
1. Whether or not the confidence limits overlap
2. Whether you accept or reject the null hypothesis
3. What the probability is that the differences between
means occurred by chance.
Now:
• Use the data you collected (you can also
collect data from other groups to provide a
more reliable mean) to work out whether or
not there is a significant difference in the
population density for the two areas.
Transects
• Transect sampling is more of a systematic technique, but can be
adapted to a random technique if required.
Line transects
A line transect is useful for
examining the effect of a
change in habitat on
biodiversity; for example,
the effect of a stream
running through a field or
wood.
A line is drawn through the
area to be examined. Any
species touching the line at
fixed intervals (e.g. 1 m) is
recorded.
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Line transect data
This graph shows the presence of each species along the
line of the sample.
0
1
2
3
4
5
6
distance along transect (m)
7
A line transect does not reflect the density of each species
along the line, so is therefore only useful as a very basic
analytical tool.
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Belt transects
A belt transect is similar to a
line transect, but provides
more detailed data.
Rather than simply recording
the type of species touching
the line, quadrats are taken at
regular intervals along the line
to identify the number/density
of the species along the belt.
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Belt transect data
The graph for a belt transect shows the density of the
species present in the area, rather than just
presence/absence.
no. individuals
120
100
grass
moss
reed
dandelion
clover
80
60
40
20
0
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1
2
3
4
5
quadrat no.
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7
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Transects
• Use a tape measure or transect line.
• Allow us to see changes as you move across
an area,
e.g. does the population density of bluebells change
as you move further into the woods?
• Line transect/belt transect?
• With/without quadrats?
• Placed randomly
Depends upon
what you’re
investigating!
What about animals?
1.
2.
3.
4.
What we’ve seen up until now is fine for sampling plant
populations, but studying animals and insects is trickier.
A set of animals are caught and then marked in some way.
They’re then released back into the community.
After a specified length of time, the community is revisited and a
number of individuals are caught again.
The number of marked individuals is counted.
The population size is calculated:
Studying animal populations
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Field study
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Common sampling techniques
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Learning check – use the book to help you answer the
following questions
1. What is the difference between random and systematic sampling?
2. What types of animal can be sampled using a frame quadrat?
20 min
3. In the context of quadrat sampling, what is meant by species frequency?
4. If there are 2100 daisies, evenly distributed in a 10 m by 10 m area, what would you expect
the species density of the daisies to be?
5. What is meant by percentage cover?
6. If you obtained three ‘hits’ with a needle on a particular species of plant, while using a tenneedle point quadrat, what percentage cover would you record for that plant species?
7. In what kind of environment would you collect samples along a transect line?
8. What precaution must be taken when marking mobile animals as part of the mark–release–
recapture technique?
9. When using the mark–release–recapture technique, why must a reasonable length of time be
left between release and recapture?
10. If 20 rabbits are marked and released and then, of 15 recaptured a week later, 5 are found to
be marked, what would you estimate the size of the rabbit population to be in the area
sampled?
Learning check
1. What is the difference between random and systematic sampling?
In random sampling, sampling points are chosen randomly. In systematic
sampling, sample points are taken at regular intervals or in some other
fixed pattern.
2. What types of animal can be sampled using a frame quadrat?
Sessile, or non-moving, animals and sedentary, or slow-moving, animals.
3. In the context of quadrat sampling, what is meant by species frequency?
The number of quadrats in which a particular species is found.
4. If there are 2100 daisies, evenly distributed in a 10 m by 10 m area, what
would you expect the species density of the daisies to be?
21 per m2.
5. What is meant by percentage cover?
The proportion of an area covered by a plant or sessile or sedentary
animal.
Learning check
6. If you obtained three ‘hits’ with a needle on a particular species of plant, while
using a ten-needle point quadrat, what percentage cover would you record for that
plant species?
30%
7. In what kind of environment would you collect samples along a transect line?
One in which there is a transition between communities along an environmental
gradient, e.g. a beach.
8. What precaution must be taken when marking mobile animals as part of the mark–
release–recapture technique?
The marking must not affect the animals’ mobility or survival chance.
9. When using the mark–release–recapture technique, why must a reasonable length
of time be left between release and recapture?
To allow marked animals to mix in with the unmarked population.
10. If 20 rabbits are marked and released and then, of 15 recaptured a week later, 5 are
found to be marked, what would you estimate the size of the rabbit population to
be in the area sampled?
60
Key words & definitions
Key word
Definition
Habitat
The place where an organism normally lives, which is characterised by physical
conditions and the species of other organisms present.
Random sampling
Random sampling is used to avoid any bias in collecting data. Avoiding bias
ensures that the data obtained is valid.
Systematic
sampling
Samples are taken at regular intervals within a set sampling area. Timeconsuming... But more reliable?
Abundance
Counting the number of individuals of a species in a given space.
Frequency
Frequency is the likelihood of a particular species occurring in a quadrat. It gives
a quick idea of the species present and their general distribution within an area.
However it does not provide information on the density and detailed distribution
of species.
Percentage cover
Is an estimate of the area within a quadrat that a particular plant species covers.
Advantage – data be collected rapidly & individual plants do not need be counted
Disadvantage – less useful organisms occur in several overlapping layers.
Mark-releaserecapture
Known number animals caught, marked and released. Some time later given
number of individuals are collected randomly and mark recorded.