EXAM REVIEW Topic 2: The ecosystem
2.1 Structure
Assessment Statement
2.1.1 Distinguish
between biotic and
abiotic (physical)
components of an
ecosystem.
2.1.2 Define the term
trophic level.
Notes
Abiotic (physical; non-organic) soil, oxygen, light, temperature, pH
Biotic (living; organic) biomass, organisms
2.1.3
Identify and explain
trophic levels in food
chains and food webs
selected from the local
environment.
Relevant terms (for example, producers, consumers, decomposers, herbivores, carnivores, top
carnivores) should be applied to local, named examples and other food chains and food webs.
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2.1.4
Explain the principles
of pyramids of
numbers, pyramids of
biomass, and pyramids
of productivity, and
construct such
pyramids from given
data.
Pyramids are graphical models of the quantitative differences that exist between the trophic levels
of a single ecosystem.
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A feeding level in the food chain.
A pyramid of biomass represents the biomass of each trophic level measured in units such as
grams of biomass per square metre (g m–2).
 Biomass may also be measured in units of energy, such as J m–2.
 Numbers and quantities of biomass and energy decrease along food chains; therefore the
pyramids become narrower as one ascends.
 HOWEVER, Pyramids of numbers can sometimes display different patterns, for
example, when individuals at lower trophic levels are relatively large. Similarly,
pyramids of biomass it is possible for greater quantities at higher trophic levels because
they represent the biomass present at a given time (such as seasonal variations).
 Both pyramids of numbers and pyramids of biomass represent storages.
Pyramids of productivity refer to the flow of energy through a trophic level and always show a
decrease along the food chain. (2nd Law)
Biomass, measured in units of mass or energy (for example, g m–2 or J m–2), is different from
productivity measured as a rate (amount per unit time) in units of flow (for example, g m–2 yr–1
or J m–2 yr–1).
2.1.5
Discuss how the
pyramid structure
affects the functioning
of an ecosystem.
2.1.6 Define the terms
species, population,
habitat, niche,
community and
ecosystem with
reference to local
Include
 concentration of non-biodegradable toxins in food chains
 limited length of food chains
 vulnerability of top carnivores.
Species – a group of organisms that interbreed and produce fertile offspring
Population – a group of organisms of the same species living in the same area at the same time,
which are capable of interbreeding
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examples.
Habitat – the environment in which a species normally lives
Niche – where an organism lives and everything it does. No two species have the exact same
niche.
Community – all the populations interacting in the same habitat. (Living parts of an ecosystem)
Ecosystem – the interaction of a community and the physical environment they inhabit
2.1.7 Describe and
explain population
interactions using
examples of named
species.
Include competition, parasitism, mutualism, predation and herbivory.
- Competition – occurs when species niches overlap.
- Mutualism ( +, +) is an interaction in which both species benefit. Example: Oak tree and
squirrel, who “plants” acorns)
- Parasite (+, – ) Example: trematodes embedded in frogs during development cause
deformities but do not kill the frog)
Notable interactions:
 the influences each species has on other species’ population size
 and on the carrying capacity of the others’ environment.
PRACTICE
The following graphs represent the changes in population density of two species:
Population
Density
Species Y
Species
X
Time
Which of the following most likely represents the relationship between these two
species?
X
Y
A.
predator
prey
B.
prey
predator
C.
parasite
host
D.
host
parasite
Continued Below
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PRACTICE
1.
Net Productivity of carnivores
12
In the diagram all values are in kJ m
–2
yr
−1
Carnivores
Respiration and other losses
Z
The values of W, X, Y and Z are
W
X
Y
Z
A.
72 000
8400
720
92
B.
8000
8400
720
92
C.
8000
7600
80
92
D.
8000
7600
80
68
Consumed by carnivores
Y
Decomposers and other losses
320
Net Productivity of herbivores
400
Herbivores
Respiration and other losses
X
Consumed by herbivores
W
Decomposers and other losses
32 000
Net Productivity of producers
40 000
Producers
2.2 Measuring abiotic components of the system
Assessment
Notes
2.2.1 List the significant abiotic (physical) factors of an ecosystem.
2.2.2. Describe and evaluate methods for measuring at least three abiotic (physical) factors within an ecosystem.
Know methods for measuring any three significant abiotic factors and how these may vary in a given ecosystem with depth,
time, distance.
For example:
• marine—salinity, pH, temperature, dissolved oxygen, wave action
• freshwater—turbidity, flow velocity, pH, temperature, dissolved oxygen
• terrestrial—temperature, light intensity, wind speed, particle size, slope, soil moisture, drainage, mineral content.
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2.3 Measuring biotic components of the system
Assessment
Notes
2.3.1
PRACTICE: Make a dichotomous key for a bee, an ant, a spider, a sparrow, a robin, an earthworm,
Construct simple keys oat grass, and a radish plant.
and use published
keys for the
identification of
organisms.
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2.3.2
Describe and evaluate
methods for
estimating abundance
of organisms.
(size of populations)
Methods should include capture–mark–release–recapture (Lincoln index) and quadrats for
measuring population density, percentage frequency and percentage cover.
The Lincoln Index (capture – mark – release – recapture) is a mathematical model used to
estimate the size of a population. Scientists capture a sample of the population they want to
measure. They mark these individuals and release them. Later, the scientists return and capture
another sample. Some of the individuals in the second sample will carry the mark from the first
sample. The scientists then use the following formula to estimate the size of the population:
 Where N is the total size of the population
n1 is the size of the first sample captured
n2 is the size of the second sample captured
m is the number of marked individuals recaptured in the second sample
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N = n1 x n2
m
PRACTICE: In the first sample, 23 pheasants were captured and marked. A week later, 21
pheasants were captured in the same area, and of these 8 were marked. What is the estimated size
of the population? (Ans. 60 pheasants)
Quadrat sampling is a method to determine the percentage frequency or cover of each species.
Used for plants. Often used along environmental gradients, such as the edge of a forest, or a
beach or streamside.
2.3.3 Describe and
evaluate methods for
estimating the
biomass of trophic
levels in a community.
2.3.4 Define the term
diversity.
2.3.5 Apply
Simpson’s diversity
index and outline its
significance.
Biomass is the total mass of living matter within a given unit of environmental area. Examples:
plant material, vegetation, animal waste used as fuel)
- Used to calculate the total energy in an organism or at atrophic level.
Dry weight measurements of quantitative samples can be used to estimate total biomass. (Water
must be removed for valid comparisons.)
Diversity is considered BOTH the number of different species (richness) and the relative numbers
of individuals of each species (abundance)
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Students are not required to memorize this formula but must know the meaning of the symbols:
D = diversity index
N = total number of organisms of all species found
n = number of individuals of a particular species
(Greek Epsilon =”sum of”)


A high value of D suggests a stable and ancient site.
A low value of D could suggest pollution, recent colonization or agricultural management.
PRACTICE: In a field experiment 60 edible dormice (Glis glis) were
captured using Longworth mammal traps laid out in a grid within a 500 m ×
500 m quadrat. Each individual was marked and released. Two days later a
second trapping exercise caught 50 edible dormice, 15 of whom were
previously marked. Calculate the Lincoln index for this population.
PRACTICE Figure 3 below shows the species composition of two areas of forest. There are 100 trees in each area of
forest. The Simpson’s diversity index for Ecosystem A is 1.38. Calculate Simpson’s diversity index for Ecosystem B.
2.4 Biomes
Assessment Statement
Notes
2.4.1 Define the term biome.
A biome is – An area distinguished by the environment of a particular climate and the community or organisms adapted to it.
2.4.2 Explain the distribution, structure and relative productivity of tropical rainforests, deserts, tundra and any other biome.
Refer to prevailing climate and limiting factors.
Example 1: Tropical rainforests are found close to the equator where there is high insolation and rainfall and where light and
temperature are not limiting.
Example 2: Temperate grassland or a local example.
Climate: temperature, precipitation and insolation.
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2.5 Function
Assessment Statement
Notes
2.5.1 Explain the role of producers, consumers and decomposers in an ecosystem.
2.5.2
Describe
photosynthesis and
respiration in terms of
inputs, outputs and
energy transformations.
2.5.3
Describe and explain
the transfer and
transformation of
energy as it flows
through an ecosystem.

Photosynthesis requires carbon dioxide, water, chlorophyll and certain visible
wavelengths of light to produce organic matter and oxygen.
 Transformation of light energy into the chemical energy of organic matter.
 Respiration requires organic matter and oxygen to produce carbon dioxide and water.
Without oxygen, carbon dioxide and additional waste products are formed.
 Energy is released in a form available for use by living organisms, but is ultimately lost as
heat. (2nd Law)
Explain pathways of incoming solar radiation incident on the ecosystem including:
• loss of radiation through reflection and absorption (loss of ice)
• conversion of light to chemical energy
• loss of chemical energy from one trophic level to another
• overall conversion of light to heat energy by an ecosystem
• re-radiation of heat energy to the atmosphere.
Storages of energy are illustrated by boxes in energy-flow diagrams. The flows of energy or
productivity are shown as arrows.
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Processes involving the transfer and transformation of carbon, nitrogen and water as they cycle
within an ecosystem should be described, and the conversion of organic and inorganic storage
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Construct and analyze simple energy-flow diagrams illustrating the movement of energy through
ecosystems, including the productivity of the various trophic levels.
2.5.4 Describe and
explain the transfer and
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transformation of
materials as they cycle
within an ecosystem.
noted where appropriate.
2.5.5
Define the terms gross
productivity, net
productivity, primary
productivity and
secondary productivity.
Productivity is production per unit time.
Construct and analyze flow diagrams of these cycles.
Gross productivity = (GP) -the total gain in biomass or energy per unit area per unit time. (kj m-2
yr-1)
 Gross primary productivity (GPP) is gained through photosynthesis
 Gross secondary productivity (GSP) is gained through absorption in consumers
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Net productivity (NP) - the total gain in biomass or energy in producers remaining after
respiratory losses (R)
Net secondary productivity - (NSP) - the gain by consumers in biomass or energy remaining
after respiratory loss (R).
2.5.6 Define the terms
and calculate the values
of both gross primary
productivity (GPP) and
net primary
productivity (NPP)
from given data.
Use the equation:
NPP = GPP – R
where R = respiratory loss
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The diagram below shows the main energy flows through an ecosystem.
Respiratory loss
13 000
Energy input
21 000
Producers
4200
2200
3300
Herbivores
720
270
380
X
90
Decomposers
1960
Respiratory loss
–2 –1
What is the net productivity of the decomposers in kJ m yr ?
A.
1 960
B.
3 050
C.
5 010
D.
6 970
2.5.7 Define the terms and calculate the values of both gross secondary productivity (GSP) and net secondary productivity
(NSP) from given data.
Use the equations:
NSP = GSP – R
 GSP = food eaten – fecal loss
 R = respiratory loss
NOTE: The term “assimilation” is sometimes used instead of “secondary productivity”.
k
j
2. Net Primary Productivity (NPP) is
A. b – c – d.
B. d + e + f.
C. e.
D. e – d.
carnivores
h
g
i
herbivores
e
R E
1. Gross Primary Productivity (GPP) is
A. b – c
B. b – a.
C. b.
D. b – c – d.
D E C O M P O S E R S
S P I R A T I O N
The diagram below shows the flow of energy through a food web.
d
sunlight not
used in
photosynthesis
f
producers
b
c
sun
a
3.The net productivity for the consumer community is
A. e + h.
B. e + h g j k i.
C. e g j.
D. e g j i k.
(Ans. 1. A, 2. A, 3.C )
The diagram below shows energy transfer in a cow.
energy respired,
lost as heat R
secondary
productivity
SP
energy
assimilated
Secondary Productivity (SP) is
A.
C + R.
B.
C – (R + U + F).
C.
C – (U + F).
D.
C+R+U+F
energy
consumed
C
energy lost
in urine U
energy lost
in faeces F
energy
consumed
C
2.6 Changes
Assessment
Notes
2.6.1 Explain the concepts of limiting factors and carrying capacity in the context of population growth.
Limiting factor – single factor that limits the growth, abundance, or distribution of a species in an ecosystem.
Carrying capacity – maximum population that a given area can support over time.
2.6.2
Describe and
explain S and J
population
curves.
Explain changes in both numbers and rates of growth in standard S and J population growth curves.
Population curves should be sketched, described, interpreted and constructed from given data.
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2.6.3 Describe
the role of
density
dependent and
density
independent
factors, and
internal and
external factors,
in the regulation
of populations.
2.6.4
Describe the
principles
associated with
survivorship
curves including,
K and r
strategists.
Density-dependent factors – factors that limit population growth as the population density increases.
- Ex. parasitism, disease,
- operate as negative feedback mechanisms leading to stability of the population.
Density-independent factors – limit population growth no matter what the population density.
- Many species, particularly r-strategists, are probably regulated by density independent factors
- ex. weather, natural disasters
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*Both types of factors may operate on a population.
K and r strategists represent idealized categories and many organisms occupy a place on the continuum.
Be able to interpret features of survivorship curves.
K-strategists (Type I survivorship curve )
 Slow growing/maturing species
 Few offspring
 Long lifespans
 Most reach full maturation
 Ex. humans, elephants
R –strategists (Type III survivorship curve)
 Inhabit unstable/changing
environments
 Reproduce early/often, mature
quickly
 Few reach full maturation (most
die young)
 Ex. most insects
PRACTICE: The figure below shows contrasting age-sex pyramids for two animal species.
A
(a)
B
State, giving a reason, which of the population pyramids in Figure 1 is more likely
to be an r-strategist. (2)
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2.6.5 Describe
the concept and
processes of
succession in a
named habitat.
2.6.6 Explain the
changes in
energy flow,
gross and net
productivity,
diversity and
mineral cycling
in different
stages of
succession.
SUCCESSION
Provide named examples of organisms from a pioneer community, seral stages and climax
community.
Ex. Oak savannah community:
mosses → grasses → shrubs → saplings → mature trees (such as oak)
The concept of succession, occurring over time, should be carefully distinguished from the concept of
zonation, which refers to a spatial pattern.
The diagram below shows succession in a sand dune ecosystem.
A
B
Y
X
Zonation – changes in biota and physical environment across an ecosystem due to distance rather than
time. (An example: different organisms prevalent in different conditions on the side of a mountain due to
changing altitude.)
EXAMPLE of zonation: The figure below shows how vegetation changes with altitude in the
Andean mountain chain in South America.
a. State the pattern of vegetation shown in the figure. (Answer: zonation)
b. Identify two limiting factors in the alpine meadows. [2]
c. Suggest one way the pattern of vegetation might change as a result of global warming. [1]
SUCCESSION
In early stages,
 gross productivity is low due to the initial conditions and low density of producers.
 the proportion of energy lost through community respiration is relatively low too,
 so net productivity is high (the system is growing and biomass is accumulating).
In later stages, with an increased consumer community,
 gross productivity may be high in a climax community.
 balanced by high respiration, so net productivity approaches zero
 the production: respiration (P:R) ratio approaches one.
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2.6.7 Describe
factors affecting
the nature of
climax
communities.
Climate and soil determine the nature of a climax community. (Soil factors include water content,
acidity, aeration, and the availability of nutrients.)
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Human factors frequently affect this process through, for example, fire, agriculture, grazing and/or
habitat destruction.
2.7 Measuring changes in the system
Assessment Statement
Notes
2.7.1 Describe and
Examples of environmental gradients
evaluate methods for
 Edge of forest
measuring changes in
 Shore to deep water
abiotic and biotic
 Changing altitude up the side of a mountain
components of an
Methods possible: succession of pitfall traps, biotic index, quadrat sampling, etc. along the
ecosystem along an
gradient
environmental gradient.
2.7.2 Describe and
evaluate the use of
environmental impact
assessments (EIAs).
*An EIA involves:
1. production of a baseline study before any environmental development
2. assessment of possible impacts
3. monitoring of change during and after the development.
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