C4.1 Populations and communities
Understandings:
Interaction and interdependence
Molecules
C4
SL
Key words:
Population
Community
Speciation
Random sampling
Sampling error
Sessile
Quadrat
Standard deviation
Motile
Lincoln Index
Carrying capacity
Density dependent factors
Density independent factors
Logarithmic
Exponential
Sigmoid
Intraspecific
Interspecific
Herbivory
Predation
Competition
Mutualism
Parasitism
Pathogenicity
Endemic
Invasive
Competitive exclusion
Chi squared
Null hypothesis
Critical value
Statistically significant
P value
Top-down
Bottom-up
Allelopathy
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Guiding questions
How do interactions between organisms regulate sizes of populations in a community?
What interactions within a community make its populations interdependent?
C4.1.1—Populations as interacting groups of organisms of the same species living in an area
A population is:
Reproductive isolation - When two populations can no longer interbreed (for a number of different reasons).
Reproductive isolation can lead to speciation
C4.1.2—Estimation of population size by random sampling
What is the difference between an estimate and a guess?
What is the difference between an estimate and a guess?
An estimate is based upon data.
In ecology it is often impossible to count all of the individuals in a population.
An estimate can be made via random sampling
Nature of science
Random sampling, instead of measuring an entire population, inevitably results in sampling error. In this case the difference between the estimate of population size and the true size of the whole population is the sampling error.
C4.1.3—Random quadrat sampling to estimate population size for sessile organisms
Sessile = stationary
How many daisies are in this field?
How would you count them?
Quadrats
You can use a square quadrat to sample a particular area.
Quadrats come in different shapes and sizes.
Many quadrats are have a side length of 50cm.
What is the area that they cover?
Many quadrats are have a side length of 50cm.
What is the area that they cover?
0.25m2
What does using a random number generator avoid?
What does using a random number generator avoid?
Bias
A student wants to estimate the number of daisies in a field.
The field is 10,000m2
The student generates 5 random coordinates.
She places the bottom left of a square quadrat with side length 50cm on each of these coordinates.
She obtains the following results
| 1 | 2 | 3 | 4 | 5 |
Number of daisies | 12 | 21 | 7 | 19 | 32 |
Estimate the number of daisies in the entire field
Calculate the mean of the following sets of data:
1, 10, 114, 22, 432, 5, 654, 98, 32, 32
140, 141, 142, 139, 138, 140, 140, 141, 139, 140
Calculate the mean of the following sets of data:
1, 10, 114, 22, 432, 5, 654, 98, 32, 32
Mean = 140
140, 141, 142, 139, 138, 140, 140, 141, 139, 140
Mean = 140
How can we differentiate these data sets?
Standard deviation
The standard deviation is a measure of spread around the mean.
Calculate the standard deviation of the following sets of data:
1, 10, 114, 22, 432, 5, 654, 98, 32, 32
Mean = 140
140, 141, 142, 139, 138, 140, 140, 141, 139, 140
Mean = 140
Calculate the standard deviation (of a sample) of the following sets of data:
1, 10, 114, 22, 432, 5, 654, 98, 32, 32
Mean = 140, Standard deviation = 222
140, 141, 142, 139, 138, 140, 140, 141, 139, 140
Mean = 140, Standard deviation = 1.15
C4.1.4—Capture– mark– release– recapture and the Lincoln index to estimate population size for motile
organisms
Sessile = stationary
Motile = mobile
Can you use a quadrat to count the number of lions in a nature reserve?
Capture
Mark
Release
Recapture
Do you think this sampling process has any limitations?
C4.1.5—Carrying capacity and competition for limited resources
How many frogs do you think you will find in these two environments?
The maximum number of of a species an environment can support is its carrying capacity
What things do you think affect an environment's carrying capacity for the following?
Animals
Plants
Animals
Plants
C4.1.6—Negative feedback control of population size by density-dependent factors
Could you describe, interpret and form a conclusion about this graph?
Could you describe, interpret and form a conclusion about this graph?
Two types of factors which affect the size of populations:
Density dependent factors
Density independent factors
Can you give some examples of each type?
Could you describe, interpret and form a conclusion about this graph?
C4.1.7—Population growth curves
Please draw a graph of the following data.
Exponential growth:
Exponential growth is a process that increases quantity over time at an ever-increasing rate. It occurs when the instantaneous rate of change of a quantity with respect to time is proportional to the quantity itself. Wikipedia
Logarithms
100 can be expressed as 102
10 is the based
2 is the exponent
Log10(100) = 2
Now plot the same data on a semi logarithmic scale (x axis normal scale, y-axis logarithmic scale).
Use base 10
https://www.omnicalculator.com/math/log
Nature of science
The curve represents an idealized graphical model. Models are often
simplifications of complex systems.
C4.1.8—Modelling of the sigmoid population growth curve
If toxic waste products accumulate then there may also be a death phase.
GROWTH OF YEAST PRACTICAL
C4.1.9—Competition versus cooperation in intraspecific relationships
Intraspecific - within a species
Interspecific - between species
Competition and cooperation
How does this picture represent intraspecific relationships? (DIC)
Intraspecific competition
Limited resources.
May challenge directly (fighting)
Challenge indirectly by depleting resources
Intraspecific cooperation
More common in social animals
C4.1.10—A community as all of the interacting organisms in an ecosystem
C4.1.11—Herbivory, predation, interspecific competition, mutualism, parasitism and pathogenicity as
categories of interspecific relationship within communities
Interspecific relationships
Herbivory - An animals eating a plant (may or may not kill the plant)
Predation - Animals killing other animals for food
Interspecific competition - Two or more species competing for the same resources
Multalism - A cooperation between two species where both benefit
Parasitism - One organism living on or within another organism
Pathogenicity - Pathogen lives inside the host and causing disease
Please think of an example of each of these six different interspecific relationships
C4.1.12—Mutualism as an interspecific relationship that benefits both species
Root nodules in Fabaceae (legume family),
Mycorrhizae in Orchidaceae (orchid family)
Orchid seeds have practically no energy stores. They rely on a symbiotic relationship with mycorrhizal fungi to gain the energy to germinate.
Zooxanthellae in hard corals
In which kingdom do the zooxanthellae algae belong?
In which kingdom do the zooxanthellae algae belong?
Protoctists
What is the name of the mutually beneficial relationship where one organism lives inside another?
What is the name of the mutually beneficial relationship where one organism lives inside another?
Endosymbiosis
Which other examples of endosymbiosis do you know?
What is the name of the mutually beneficial relationship where one organism lives inside another?
Endosymbiosis
Which other examples of endosymbiosis do you know?
Mitochondria and chloroplasts were once independent unicellular organisms!
C4.1.13—Resource competition between endemic and invasive species
Endemic species - Native to a specific region
Alien species - Foreign (and artificially introduced) to a specific region
If an alien species disrupts the natural ecosystem it is said to be invasive
Can you name an introduced species which has become invasive, for example in Crevillente reservoir?
What makes a non native, invasive species potentially so problematic?
What makes a non native, invasive species potentially so problematic?
Competes for same niche (invasive species may have a wider niche)
May have no natural predator
May be better suited to environment
May reproduce rapidly
May bring disease
Competitive exclusion
No two species can coexist if they occupy the exact same niche.
Why do you think some invasive species become so problematic?
C4.1.14—Tests for interspecific competition
What might you expect the distribution of two species to be if they were in direct competition with one another?
What might you expect the distribution of two species to be if they were in direct competition with one another?
They would not be found in the same location.
Interspecific competition can be investigated using:
Laboratory experiments
Field observations by random sampling
Field manipulation by removal of one species.
Nature of science
hypotheses can be tested by both experiments and observations.
C4.1.15—Use of the chi-squared test for association between two species
Chi squared
Chi square is a statistical test to compare the observed with an expected value.
Goodness of fit
Test for association
Testing for association between two species using the chi-squared test with data obtained by quadrat sampling.
Contingency tables
If we look at a particular spot, there are four possible combinations.
Nettles are present and so are dock leaves; nettles are present but dock leaves are absent; Nettles are absent and dock leaves are present; or both are absent.
100 quadrats were sampled and the following results were obtained.
| Number of times Dock leaves present | Number of times Dock leaves absent | TOTAL |
Number of times Nettles present | 52 | 12 | 64 |
Number of times Nettles absent | 4 | 32 | 36 |
TOTAL | 56 | 44 | 100 |
I am interested to see whether there is any correlation between where you find nettles and dock leaves.
What is the null hypothesis?
What is the alternative hypothesis?
I am interested to see whether there is any correlation between where you find nettles and dock leaves.
What is the null hypothesis?
There is no correlation between where nettles and dock leaves are found
What is the alternative hypothesis?
There is a correlation between where nettles and dock leaves are found (could be a positive or negative correlation)
If you presume there is an equal chance of finding nettles in each of your quadrats - Given your results, what is the chance that you would find nettles to be present in any given quadrat?
If you presume there is an equal chance of finding nettles in each of your quadrats - Given your results, what is the chance that you would find nettles to be present in any given quadrat?
It’s the times nettles were found divided by the times you looked.
If we looked 100 times and found nettles on 64 of those times, the probability would be 64% (64/100=0.64)
Given your results, what is the chance that you would find a dock leaf to be present?
Given your results, what is the chance that you would find a dock leaf to be present?
It’s the times dock leaves were found divided by the times you looked.
If we looked 100 times and found dock leaves on 56 of those times, the probability would be 56% (56/100=0.56)
Now, what is the chance that you found both nettles and dock leaves.
It’s the probability of finding nettles multiplied by the probability of finding dock leaves.
36% (0.64 x 0.56 = 0.3584)
If we simplify this we get the equation
Expected frequency = row total x column total
Grand total
Add the expected values in the following table (use brackets)
| Number of times Dock leaves present | Number of times Dock leaves absent | TOTAL |
Number of times Nettles present | 52 (64x56/100=36) | 12 | 64 |
Number of times Nettles absent | 4 | 32 | 36 |
TOTAL | 56 | 44 | 100 |
If we simplify this we get the equation
Expected frequency = row total x column total
Grand total
Add the expected values in the following table (use brackets)
| Number of times Dock leaves present | Number of times Dock leaves absent | TOTAL |
Number of times Nettles present | 52 (36) | 12 (28) | 64 |
Number of times Nettles absent | 4 (20) | 32 (16) | 36 |
TOTAL | 56 | 44 | 100 |
Chi squared
Test the null hypothesis.
Null hypothesis = There is no significant difference between the observed and the expected values.
Calculate the chi squared value for the table
| Number of times Dock leaves present | Number of times Dock leaves absent | TOTAL |
Number of times Nettles present | 52 (36) | 12 (28) | 64 |
Number of times Nettles absent | 4 (20) | 32 (16) | 36 |
TOTAL | 56 | 44 | 100 |
Calculate the chi squared value for the table
X2 = (52-36)² / 36 + (12-28)² / 28 + (4-20)² / 20 + (32-16)² / 16
X2 = 45
What does this chi squared value mean?
Should we accept or reject the null hypothesis?
What does this chi squared value mean?
Should we accept or reject the null hypothesis?
We need to know the degrees of freedom
We need to use a critical value table
The usual level of significance is 0.05 (=5%)
This suggests that the result would be due to random chance only 5% of the time - this is widely considered as statistically significant
For contingency tables the degrees of freedom are the number of columns - 1 multiplied by the number of rows - 1
(m-1)(n-1)
How many degrees of freedom for our test?
What is our critical value?
For contingency tables the degrees of freedom are the number of columns - 1 multiplied by the number of rows - 1
(m-1)(n-1)
How many degrees of freedom for our test? 1
What is our critical value? 3.84
Is our chi squared value of 54 to the right or left of our critical value of 3.84?
Do we reject or accept our null hypothesis?
Right Reject, Left Accept
Practice Data-based questions on p209 of textbook
C4.1.16—Predator–prey relationships as an example of density-dependent control of animal populations
What are the two different types of factor that affects the population size?
What are the two different types of factor that affects the population size?
Density dependent factors
Density independent factors
Use this graph to explain a typical predator prey relationship.
What type of feedback is this?
Large numbers of prey
Increase in predators
Decrease in prey
Decrease in predators
Increase in prey
C4.1.17—Top-down and bottom-up control of populations in communities
Referring to the trophic level above and the trophic level below, state how the population of mice is controlled.
Referring to the trophic level above and the trophic level below, state how the population of mice is controlled.
More predators eg snakes, fewer mice = Top down control
Less food (eg. grain), fewer mice = Bottom up control
C4.1.18—Allelopathy and secretion of antibiotics
“Allelo” = Others
“Pathy” = Suffering
Allelopathy - Where plants produce chemicals that interfere with the growth of other organisms.
Salvia phenomenon
Secretion of antibiotics
Can you…
Define the term “population”?
Describe sampling methods and explain how sampling error is reduced?
Describe the uses of the Lincoln index?
Describe what things affect the carrying capacity of an environment?
Compare and contrast density dependent and independent factors on population size?
Explain the sigmoid curve of population growth?
Give examples of intraspecific cooperation and competition?
Give examples and describe some categories of interspecific relationships?
Describe competitive exclusion and how invasive species can affect endemic species?
Understand when and how to use chi squared calculations?
Differentiate top down and bottom up control of populations?
Describe the term “allelopathy”?