Skills: 1.Classifying species as autotrophs, consumers, detritivores or saprotrophs from a knowledge of their mode of nutrition. 2.Setting up sealed mesocosms to try to establish sustainability. 3. Testing for association between two species using the chi-squared test with data obtained by quadrat sampling. 4. Recognizing and interpreting statistical significance.

1. Topic 4.1 Ecology
IB DP – CORE

2. Understandings:
 Species are groups of organisms that can potentially interbreed
to produce fertile offspring.
 Members of a species may be reproductively isolated
in separate populations.
 A community is formed by populations of different species living
together and interacting with each other
 A community forms an ecosystem by its interactions with the
abiotic environment
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3.  Species have either an autotrophic or heterotrophic
method of nutrition (a few species have both methods)
 Autotrophs obtain inorganic nutrients from the abiotic
environment
 Consumers are heterotrophs that feed on living organisms
by ingestion
 Detritivores are heterotrophs that obtain organic nutrients
from detritus by internal digestion
 Saprotrophs are heterotrophs that obtain organic nutrients
from dead organisms by external digestion
 The supply of inorganic nutrients is maintained by nutrient
cycling
 Ecosystems have the potential to be sustainable over long
periods of time
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4. Species and Population
 A species is a group of organisms that can potentially
interbreed to produce fertile, viable offspring.
 Members of a single species are unable to produce fertile,
viable offspring with members from a different species.
 When two different species do produce offspring by cross-
breeding, these hybrids are reproductively sterile (e.g. liger,
mule)
 A population is a group of organisms of the same species
that are living in the same area at the same time
 Organisms that live in different regions (i.e. different
populations) are reproductively isolated and unlikely to
interbreed, however are classified as the same species if
interbreeding is functionally possible
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5. Community, Habitat,
Ecosystem, Ecology
Community:
A group of populations living together and interacting with each other
within a given area
Habitat:
The environment in which a species normally lives, or the location of a
living organism
Ecosystem:
A community and its abiotic environment (i.e. habitat)
Ecology:
The study of the relationship between living organisms, or between living
organisms and their environment
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6. Environment
 It has 4 main components
 Hydrosphere (water)
 Atmosphere (gases)
 Lithosphere (rocks)
 Biosphere (all living beings)
 The first 3 are abiotic components while the 4th
is the Biotic
component
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7. Assessment Statement
Distinguish between autotroph and heterotroph.
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8. Autotrophs & heterotrophs
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Living organisms obtain chemical energy in one of two ways:
Autotrophs
•Synthesises its own organic molecules from simple inorganic substances (e.g. CO2,
nitrates)
•Energy for this process is derived from sunlight (photosynthesis) or via the oxidation of
inorganic molecules (chemosynthesis)
•Because autotrophs synthesise their own organic molecules they are commonly referred to
as producers
Heterotrophs
•Obtains organic molecules from other organisms (either living / recently killed or their non-
living remains and detritus)
•Because heterotrophs cannot produce their own organic molecules and obtain it from other
sources, they are called consumers
Mixotrophs
•Certain unicellular organisms may on occasion use both forms of nutrition, depending on
resource availability
•Euglena gracilis possess chlorophyll for photosynthesis (autotrophic) but may also feed on
detritus (heterotrophic)
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9. Heterotrophs
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 Consumers ingest organic molecules from living or recently killed
organisms
 Detritivores ingest organic molecules found in the non-living
remnants of organisms (e.g. detritus, humus)
 Saprotrophs release digestive enzymes and then absorb the
external products of digestion (decomposers)

10. Detrivores & saprotrophs
• Detrivores are the organism that consumes dead organic
matter.
• Ex.: earthworm, woodlice.
• Saprotrophs are the organisms that live on, or in, dead organic
matter. (digesting the food by secreting enzymes)
• Ex. Bacteria, Fungus.
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11. Autotrophs
 Most autotrophs derive the energy for this process from sunlight (via
photosynthesis)
 Some may derive the needed energy from the oxidation of inorganic
chemicals (chemosynthesis)
 Autotrophs obtain the simple inorganic substances required for this
process from the abiotic environment
 These nutrients – including carbon, nitrogen, hydrogen, oxygen and
phosphorus – are obtained from the air, water and soil.
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13. Vocabulary Practice
 Autotroph:
 Heterotroph:
 Consumer:
 Detritivore:
 Saprotroph:
 Species.
 Habitat.
 Population.
 Community
 Ecosystem:
 Ecology:
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14. Assessment Statement
 State that saprotrophic bacteria and fungi (decomposers)
recycle nutrients.
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Nutrient Cycling

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•Autotrophs obtain inorganic nutrients from the air, water
and soil and convert them into organic compounds
•Heterotrophs ingest these organic compounds and use
them for growth and respiration, releasing inorganic
byproducts
•When organisms die, saprotrophs decompose the
remains and free inorganic materials into the soil
•The return of inorganic nutrients to the soil ensures the
continual supply of raw materials for the autotrophs

17. Assessment Statement
Explain that energy can enter and
leave an ecosystem, but that
nutrients must be recycled.
Energy enters as light and usually leaves
as heat.
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Energy cycling

19. Nutrient Cycle
Nutrient Cycle vs. Food ChainNutrient Cycle vs. Food Chain
Food Chain
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20. The Carbon Nutrient Cycle
CO2 in
Atmosphere
Photosynthesis
feeding
feeding
Respiration
Deposition
Carbonate
Rocks
Deposition
Decomposition
Fossil fuel
Volcanic
activity
Uplift
Erosion
Respiration
Human
activity
CO2 in Ocean
Photosynthesis
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21. Positive and negative
association
 If two species are typically found within the same habitat, they
show a positive association
 Species that show a positive association include those that exhibit
predator-prey or symbiotic relationships

If two species tend not to occur within the same habitat, they
show a negative association
 Species will typically show a negative association if there is
competition for the same resources
 One species may utilise the resources more efficiently,
precluding survival of the other species (competitive exclusion)
 Both species may alter their use of the environment to avoid
direct competition (resource partitioning)
 If two species do not interact, there will be no association
between them and their distribution will be independent of one
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22. IB Assessment Statement
Distinguish between organic and inorganic nutrients.
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23. Organic and inorganic
compounds
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24. Essential Elements of Life
• About 25 of the 92 elements are essential to life
• Carbon, hydrogen, oxygen, and nitrogen make up 99% of living matter
• Most of the remaining 1% consists of calcium, phosphorus, potassium,
iron and sulfur
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25. Sustainability of Ecosystem
 Something is Sustainable - if something that can
continue indefinitely
 There are three requirements for sustainability
1. Nutrient availability
2. Detoxification of waste products
3. energy availability
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26. Mesocosms – Lab/ practical 5
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 Mesocosms are enclosed environments that allow a small
part of a natural environment to be observed under
controlled conditions
 A terrarium is a small transparent container (e.g. glass or
plastic) in which selected plants (or animals) are kept and
observed

27. Making a Self-Sustaining
Terrarium
 A terrarium can be created using a glass or plastic bottle with a
lid, according to the following steps:
Building a verdant foundation
 Add a bottom layer of pebbles, gravel or sand – this layer exists
for drainage (smaller vessels require thinner rock layers)
 Add a second thin layer of activated charcoal – this will prevent
mold and help to aerate the soil
 Spread a thin cover of sphagnum moss (or use an organic coffee
filter) to create a barrier between the lower layers and soil
 The final layer is the pre-moistened growing medium (i.e. potting
mix)
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28. Selecting the right plants
 Ideally, choose plants that are both slow growing and
thrive in a bit of humidity (e.g. most ferns, club moss, etc.)
 Inspect the plant thoroughly for any signs of disease or
insects before introducing to the terrarium
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29. Terrariums
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30. Maintaining appropriate
conditions
 Ensure the terrarium is placed in a location that provides a
continuous source of light
 Locate the terrarium in a place that does not experience
fluctuating temperature conditions (i.e. avoid direct sunlight)
 Do not initially over-water the plants – once the right
humidity is established, a terrarium can go months without
watering
 Occasional pruning may be required – however, as level of
soil nutrients decrease, plant growth should slow down
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31. Quadrat Sampling
 A quadrat is a rectangular frame of known dimensions
that can be used to establish population densities
Quadrats are placed inside a defined area in either a
random arrangement or according to a design (e.g.
belted transect)
 The number of individuals of a given species is either
counted or estimated via percentage coverage
 The sampling process is repeated many times in order to
gain a representative data set
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32. Chi-Square Test
A chi-squared test can be applied to data generated
from quadrat sampling to determine if there is a statistically
significant association between the distribution of two species
A chi-squared test can be completed by following five simple
steps:
Identify hypotheses (null versus alternative)
Construct a table of frequencies (observed versus expected)
Apply the chi-squared formula
Determine the degree of freedom (df)
Identify the p value (should be <0.05)
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Example of Chi-Squared Test Application
The presence or absence of two species of scallop was recorded in fifty
quadrats (1m2
) on a rocky sea shore.
The following distribution pattern was observed:
•6 quadrats = both species ;
•15 quadrats = king scallop only ;
•20 quadrats = queen scallop only ;
•9 quadrats = neither species

34.  Step 1: Identify hypotheses
 A chi-squared test seeks to
distinguish between two distinct
possibilities and hence requires
two contrasting hypotheses:
 Null hypothesis (H0): There
is no significant difference
between the distribution of two
species (i.e. distribution is random)
 Alternative hypothesis
(H1): There is a significant
difference between the
distribution of species (i.e. species
are associated)
 Step 2: Construct a table of
frequencies
 A table must be constructed that
identifies expected distribution
frequencies for each species (for
comparison against observed)
 Expected frequencies are
calculated according to the
following formula:
 Expected frequency = (Row
total × Column total) ÷ Grand
total
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35. 01/22/18By Mariam Ohanyan Date: 1.22.2018
Expected frequencies are calculated according to the following
formula:
Expected frequency = (Row total × Column total) ÷ Grand total

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The formula used to calculate a statistical value for the chi-squared test is
as follows
These calculations can be broken down for each part of the
distribution pattern to make the final summation easier
Where: ∑ = Sum ; O = Observed frequency ; E = Expected frequency
These calculations can be broken down for each part of the distribution
pattern to make the final summation easier

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38.  Based on these results the statistical
value calculated by the chi-squared test
is as follows:
 �2
= (2.20 + 2.38 + 1.59 + 1.73) = 7.90
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39.  Step 4: Determine the degree of
freedom (df)
 In order to determine if the chi-
squared value is statistically
significant a degree of freedom
must first be identified
 The degree of freedom is a
mathematical restriction that
designates what range of values
fall within each significance level
 Step 5: Identify the p value
 The final step is to apply the value
generated to a chi-squared
distribution table to determine if
results are statistically significant
 A value is considered significant if
there is less than a 5% probability
(p < 0.05) the results are
attributable to chance
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40. 01/22/18By Mariam Ohanyan Date: 1.22.2018
The degree of freedom is calculated from the table of
frequencies according to the following formula:
df = (m – 1) (n – 1)
Where: m = number of rows ; n = number of columns
When the distribution patterns for two species are being
compared, the degree of freedom should always be 1
 When df = 1, a value of greater than 3.841 is required for
results to be considered statistically significant (p < 0.05)
 A value of 7.90 lies above a p value of 0.01, meaning there is
less than a 1% probability results are caused by chance
 Hence, the difference between observed and expected
frequencies are statistically significant.

41.  As the results are statistically significant, the null hypothesis is
rejected and the alternate hypothesis accepted:
 Alternate hypothesis (H1): There is a significant difference
between observed and expected frequencies
 Because the two species do not tend to be present in the
same area, we can infer there is a negative association
between them
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42. Practice Question
 Two species of fir tree are found along the coast of Southern
California.
These two tree species are the Grand Fir (Abies grandis) and the
Noble Fir (Abies procera).
Their distribution patterns were establsihed via 150 quadrat
samples, yielding the following results:
25 = both present ; 30 = Noble Fir only ; 45 = Grand Fir only ; 50
neither present
Activity: Use the chi-squared test to determine if these two plant
species show association.
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