14.2 Applications and skills

14.2 Applications and skills
14.2.1 Using models: Limits of tolerance and the niche
14.2.2 Pyramids, energy and nutrient diagrams
14.2.3 Case studies: Human impacts on ecosystems
14.2.4 Field techniques: Using Transects
14.2.5 Lab: Modelling population growth
14.2.6 Lab: Nutrient content of soils (HL)
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14.2 Applications and skills
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In this unit
14.2.1 Using models: Limits of tolerance and the niche
14.2.2 Pyramids, energy and nutrient diagrams
14.2.3 Case studies: Human impacts on ecosystems
14.2.4 Field techniques: Using Transects
14.2.5 Lab: Modelling population growth
14.2.6 Lab: Nutrient content of soils (HL)
14.2.1 Using models: Limits of tolerance and the niche
Students often find it difficult to understand what is meant by ‘ecological niche’. A niche is not a physical place or
role. Rather, a niche considers the sum of the habitat requirements, both biotic and abiotic, for each species.
Limits of tolerance and zones of stress
A single limiting factor influences the distribution and success of species. An example is shown in Figure 14.2.1a.
Here, temperature affects the metabolic activity of three species of mussel of the genus Mytilus. There is more
energy available for growth (scope for growth) at different temperatures. The shape of the graph is characteristic of
the zone of stress model.
Figure 14.2.1a – Limit of tolerance examples
Source: www.asnailsodyssey.com
 Activity 1: Analysing data
Study Figure 14.2.1a and answer the following:
Which species has the highest scope for growth at 25°C?
Which species has the lowest overall scope for growth?
Does the data show evidence of niche overlap? Explain.
Compare the shapes on the graph to Figure 14.1.1a. How useful is the model in explaining the phenomenon?
A niche considers all limiting factors
Imagine that we now put together all of the tolerance data for all of the limiting factors in a single graph. It would
have too many dimensions to represent on a flat surface, but it would represent a species’ niche more realistically.
Figure 14.2.1b – A three-dimensional niche
Figure 14.2.1b shows the distribution of one plant species based on three limiting factors: latitude, elevation and
mean annual temperature. A niche tells us about species distribution based on n limiting factors.
Fundamental vs Realised niche
Removal experiments demonstrate that most species are limited in natural environments by interspecific
competition for resources. An example is shown in Figures 14.1.2c and d.
Figure 14.1.2c – Paramecium cultured separately
When three species of Paramecium are cultured separately, they show a normal pattern of sigmoid population
Figure 14.2.1d – Paramecium cultured together
When different species of Paramecium are cultured together, one species is limited.
Because competition excludes species from their full range of habitat, the realised niche is smaller than the
fundamental niche.
We use the terms ‘narrow’ and ‘broad’ to describe a species niche. Niches are narrowed as a result of
Figure 14.2.1e – Waterfowl populations
Distribution of waterfowl according to foraging habits. Foraging area has been partitioned as a natural
consequence of competition.
 Activity 2: Analysing data
Study Figures 14.2.1e and 14.2.1f and answer the following questions:
Which species of waterfowl show niche overlap?
Consider pairs of species and match each pair to the level of resource partitioning shown in Figure 14.2.1f.
Extension: How does resource partitioning relate to evolution?
Figure 14.2.1f – Levels of resource partitioning
Figure 14.2.1g – Woodland duck
The woodland duck's (Cairinini sp.) foraging range is limited by competitive exclusion.
 Key questions
Define niche
Distinguish between fundamental and realised niches
Explain how competitive exclusion leads to narrowing of realised niches
Figure 14.2.1h – Mussel
In the genus Mytilus, metabolism and growth are limited by temperature.
Figure 14.2.1i – P. bursaria
All three species of protists exploit the same type of resources and environment.
Figure 14.2.1j – P. caudatum limits the realised niche of the other paramecium species.
Figure 14.2.1k – P. aurelia has a wider realised niche when it is not in competition with other protists.
 Nature of Science
A good model explains phenomena and has predictive power. Discuss ways in which the limits of tolerance
model can predict interactions between species in a community.
14.2.2 Pyramids, energy and nutrient diagrams
The shape of a pyramid of net productivity is shown in Figure 14.2.2a. The amount of energy stored as biomass is
very high at the lowest trophic level, because a lot of producers are needed to support consumers.
Figure 14.2.2a – General shape for all pyramids of productivity
Since consumers depend on producers for energy, the distribution and abundance of life in any ecosystem is
determined by its net primary productivity (NPP). The average NPP of each of the world’s major biomes is shown in
Figure 14.2.2b.
Figure 14.2.2b – Net primary productivity of the Earth’s major ecosystems
 Activity 1: Interpreting energy diagrams
Explain how the shape of all energy pyramids illustrates the second law of thermodynamics.
Predict which of the ecosystems shown in Figure 14.2.2b can support the longest food chains. Explain your
Discuss the relationship between NPP and climate conditions in the major biomes. Refer to [link] Figure
4.1.2d [end link] and Figure 14.2.2b in your answer.
Application: Sustainable food
At each trophic level, energy is used in respiration, so the rates at which food is converted to biomass differ. Food
conversion rates are expressed as ratios of feed input to protein output, and are important considerations in
Figure 14.2.2c – Food conversion ratios
Amount of plant feed necessary to produce different types of food for human consumption. Cattle have the least
efficient food conversion rates: approximately 7kg of feed is needed to produce 1kg of cow protein.
Representing nutrient flows in Gersmehl diagrams
Nutrients such as carbon, nitrogen and phosphorus flow between three major stores in an ecosystem. The nutrient
stores are:
biomass (B)
litter (L)
soil (S)
Each ecosystem has a unique profile for the amount of nutrients in each of the stores, as well as the rate of flow, or
flux, between the stores. Some of the processes that contribute to rate of flux are shown in the following table.
Natural processes
Leaching, erosion, runoff, sedimentation, weathering, precipitation, decomposition, death, precipitation,
Human processes
Fertiliser use, sewage (eutrophication), harvesting, clear cutting (removal of nutrients)
Table 1: Processes that contribute to nutrient flow
Gersmehl diagrams are useful in visualising the inter-relationships between nutrient stores and flows. In a Gersmehl
Stores (B, L, S) are represented by circles. The size of the circle represents the amount of biomass held there.
Fluxes are represented by arrows. The thickness of the arrow represents the rate of flux, but the arrows
themselves may represent many processes.
The Gersmehl diagrams for three different ecosystems are shown in Figures 14.2.2d–f.
Figure 14.2.2d – Nutrient flow in a rainforest: the largest store is biomass (B)
There is high precipitation (P) all year round, so rapid transfer between stores and environment because moisture
and heat are ideal for decomposers.
Figure 14.2.2e – Nutrient flow in the taiga (boreal forest): the largest store is litter (L)
There is very little transfer between stores.
Figure 14.2.2f – Nutrient flow in a desert: the largest store is soil (S)
There is a high supply of litter but little transfer between stores.
 Key skills
Compare pyramids of energy from different ecosystems
Construct of Gersmehl diagrams to show relationships between nutrient stores and flows
 Concept help
Depending on the discipline, an energy pyramid may be called a pyramid of ‘biomass’ or ‘productivity’ or
simply ‘energy’. The general shape of all of these pyramids is the same when the following units are used:
Pyramid type
g m-2yr-1
J m-2yr-1
 Language tools
Litter refers to organic matter in and on the soil, including detritus and humus.
Soil refers to disintegrated rock particles and water. Soil is composed of inorganic nutrients only.
 Science and social responsibility (Aim 8)
Forests are often cleared to make room for grazing livestock, which then emit carbon dioxide, a greenhouse
gas, as a product of respiration. Eating less meat significantly reduces a person’s carbon footprint. Is a
vegetarian diet more socially responsible?
Figure 14.2.2g – Rainforest
In a rainforest, high precipitation and moisture lead to high productivity and biomass.
Figure 14.2.2h – Taiga
In a boreal forest, or taiga, the main store of nutrients is litter – mostly leaf needles.
Figure 14.2.2i – Desert
In a desert, biomass does not decompose due to dry conditions. The main nutrient store is soil.
 Activity 2
On the Gersmehl diagrams, identify which arrows represent the processes in the table.
 Course link
We have already learned about carbon flux in 4.1.3 Carbon cycling.
14.2.3 Case studies: Human impacts on ecosystems
We have already encountered numerous examples of how humans have an impact on ecosystems. Case studies are
often used in ecology to study general principles because controlled experiments are not always possible in the field.
As you go through this page, consider:
how the causes and consequences of these examples illustrate basic ecological concepts
ways that conservation/control programmes can be evaluated.
Invasive species: Cane toads in Australia
The poisonous cane toad, Bufo marinus, is native to central and south America and was introduced to Australia in
1935 in order to control a common sugar cane pest, the cane beetle, Dermolepida albohirtum (Figure 14.2.3d).
Cane toads have no natural predators in Australia, and are poisonous during all stages of their life. Any small
predator that attempts to eat a cane toad is almost always lethally poisoned, although some species are not
susceptible to the toxin.
The Australian government suggests that the toads are advancing at a rate of about 45 to 60km per year westward
from their current distribution.
Figure 14.2.3a – Distribution of cane toads in Australia
Source: Kearney et al., WP 2008, 'Modelling species distributions without using species distributions: the cane toad
in Australia under current and future climates’, Ecography, vol. 31, pp. 423-434
Scientists and policymakers are uncertain how to proceed in dealing with cane toads. When evaluating what type of
biological control is necessary, scientists must consider:
The life cycle of the invasive species. Cane toads disperse quickly because cannibalism is common. A tadpole has
a better chance of survival if it is far away from its more mature (and hungry) cousins.
The effect on local flora and fauna. Setting up traps has been unsuccessful since other amphibians often get
caught. Egg and tadpole collection is difficult because cane toads look very much like native amphibian species
during these stages.
Biomagnification: Mercury in fish
Mercury released by burning coal and incinerating wastes eventually settles in freshwater ecosystems.
Figure 14.2.3b – Location of mercury deposition in the United States (1mg = 10-6g)
Once in the water supply, elemental mercury is converted by specific bacteria into methylmercury, a persistent
organic neurotoxin that accumulates in fatty tissues and is biologically magnified through aquatic food chains.
Figure 14.2.3c – Methylmercury concentrations in organisms collected from Lake Washington.
 Activity
Using the information from Figures 14.2.3b and 14.2.3c and your own knowledge:
Explain the causes of mercury biomagnification.
Draw a food web for Lake Washington species. Name species that occupy more than one trophic level.
Categorise the species as having a high (>100mm/kg), medium (20–100mm/kg) or low (<20mm/kg)
concentration of methylmercury. Explain the categories.
Estimate the concentration of methylmercury in a smallmouth bass from Lake Norman. Justify your
 Course link
The cane toad on discussion here, Bufo marinus, is the same species that we encountered in Page 5.2.2. Redbellied black snakes in Australia have evolved mechanisms to avoid the poisonous toad. Often, the effect of a
disturbance on an ecosystem is impossible to predict!
Figure 14.2.3d – The cane beetle, Dermolepida albohirtum
Cane toads were originally brought to Australia to eat the beetles’ eggs during the breeding season. The
effectiveness of this programme remains in question today. (Photo credits: ©Alexander Dudley 2005)
Figure 14.2.3e – Cannibal cane toad
This cane toad was found with these immature cane toads in its stomach!
 Did you know?
If you don’t know the name of something, it helps to describe it as accurately as possible, using scientifically
appropriate language, on an internet search. It’s like a reverse glossary search!
Figure 14.2.3f – Lake Washington, Washington State, USA
Figure 14.2.3g – Lake Norman, North Carolina, USA
14.2.4 Field techniques: Using Transects
Transects are often used to sample the distribution of populations. The most common type is a line transect. A piece
of tape or string of fixed length is randomly placed in a straight line in a test site. The number of organisms that fall
on the line and their locations are noted. The process is repeated until a representative sample has been taken.
Figure 14.2.4a – Placement of a line transect
The number of organisms that fall on the line, or (as shown) within a fixed distance from the line can be sampled.
A belt transect involves laying two lines at a fixed distance from each other and counting the organisms within the
two lines. Sometimes this is done using a quadrat, so that an estimate not only of distribution but also of relative
abundance can be calculated.
Figure 14.2.4b – Belt transect
In this case, counting organisms within a 1 x 1m quadrat along a line transect is equivalent to using a belt transect
with two lines 1m apart.
A point transect is useful for sampling organisms in tall trees (e.g. birds and bats) or to determine canopy cover.
The researcher stands at a fixed point and measures the number of organisms detected within a certain radius of that
point. Sometimes a point is painted on the researcher’s boot for accuracy.
Figure 14.2.4c – Point count
In all cases, care must be taken to ensure that transects are done in random locations through the test sites.
Application: Primary succession in sand dunes
Sand dunes develop in coastal areas where there are high winds and little vegetation.
High tides deposit sand, which becomes an effective wind (and wave) breaker, leading to more sand deposition.
Grasses may eventually colonise the dunes if they provide enough wind cover.
Sand dune ecosystems demonstrate primary succession in action because the youngest dunes are closest to the
strand line (i.e. the beach) and the oldest dunes are furthest away.
Figure 14.2.4d – Primary succession in sand dune ecosystems
The diagram shows the successional gradient from the newest sand dunes (east) to the oldest sand dunes (west) with
respect to various abiotic factors.
Figure 14.2.4e – Data collected from a sand dune ecosystem near Sefton, UK
 Activity 1: Analysing data
Referring to the data in Figure 14.2.4e:
1. Name the pioneer species.
2. Describe the trends in abiotic factors that are typical of primary succession.
3. Identify which data was collected using transects and describe what type of transect was used.
4. Explain the trend in species number from early to late stages of primary succession.
5. Suggest a reason why marram grass does not appear in sites 3, 4 or 5.
Activity 2: Investigate the effect of disturbance on an ecosystem
Disturbances affect living and non-living parts of the ecosystem, including, but not limited to:
species diversity
nutrient cycling
water movement
leaf area index
percentage cover
relative abundance
edge effects (e.g. narrowing of realised niche, or defended territories)
For this task, work in groups of two or three. You can use any of the skills you have learned in this course,
including quadrat samples, transects, mark-recapture (HL), Simpson’s reciprocal index of diversity, and
statistical analysis such as the t-test and chi-square test. Have your plan approved by your teacher before you
begin your investigation.
 Key skills
Perform a transect.
Analyse data showing primary succession.
Investigate the effect of disturbance on an ecosystem.
 Course link
If you don’t know the name of something, it helps to describe it as accurately as possible, using scientifically
appropriate language, on an internet search. It’s like a reverse glossary search!
 Nature of Science
Avoiding bias: Be specific in your reporting about how you decided on the length and placement of your
transects. It’s best to follow an arbitrary rule or use a random number generator.
 Extended Essay ideas
Get to know your local ecosystem:
Combine your knowledge of statistics with your knowledge of population sampling methods to
investigate a limiting factor.
Perform a monitoring study of local indicator species.
Figure 14.2.4f – Sand dunes
Sand dunes on Schiermonnikoog, Netherlands (above) and Lake Huron, Canada. Sand dune ecosystems occur in
both saltwater and freshwater environments.
 Investigation hints
What constitutes a disturbance? A paved path through a park reduces connectivity for small insects
(island/edge effects).
Transects should be laid out in a way that you can collect a reasonable amount of data (minimum 40
It takes practice to determine how many samples is enough.
Keep track of the total length of your transect lines and the distance from the line in order to improve
precision of your method.
14.2.5 Lab: Modelling population growth
Do populations show logistic or exponential growth?
What are ideal conditions for growth?
Use these two techniques to investigate.
Technique 1: Counting yeast with a haemocytometer
Yeast, Saccharomyces cerevisiae, is a eukaryotic microorganism. You can monitor the growth of populations of
yeast over a period of days or weeks by taking population samples using serial dilutions and a haemocytometer,
which is a specialised tool for counting cells under a microscope.
You will need:
One packet dry brewers’ yeast
Erlenmeyer flask
Haemocytometer (and specialised cover slips)
0.1ml pipette
Distilled water
Test tubes
10ml graduated cylinder
1. Activate the yeast according to the package instructions (usually 15g in 100g water at 37°C and 1g sugar is
enough). This is your original population.
2. Once yeast is activated, after about 10 minutes, dilute a sample of the suspension with distilled water.
3. Add 0.1ml of the suspension into 9.9ml of distilled water in a test tube. Repeat the dilution two more times as
shown in Figure 14.2.5a. You have just performed a serial dilution with a dilution factor of 10-6.
Figure 14.2.5a – Serial dilution
The number of yeast cells in the original population is too large to count. Serial dilutions are a very useful
technique to learn for biological analysis.
4. Take a sample of the last dilution and mount it on the haemocytometer. First, place the cover slip over the
counting grid area. Then drop the sample at the edge of the cover slip, allowing it to move by capillary action
into the counting chamber.
Figure 14.2.5b – Side view of a haemocytometer
The sample is loaded at the end of the cover slip and pulled into the counting chamber by capillary action.
The cover slip is 0.1mm in height from the counting surface, so that the concentration of cells can be
calculated accurately.
5. Mount the slide under high power and count the number of yeast cells that are visible within 10 randomly
selected boxes. This will give you the population from a total volume of 0.25mm3.
Figure 14.2.5c – Counting grid
Counting is normally done in the smallest boxes (shown in blue), in the central area of the grid. Each of the
boxes is 0.5 x 0.5 x 0.1mm (height from coverslip).
6. Calculate the concentration of yeast in your original sample using the formula:
Concentration of sample =
Number of cells counted
Volume of squares counted x Dilution factor
where the volume of squares counted is 0.25 mm3, and the dilution factor is 10-6. The population should be
expressed as number of cells/mm3.
7. Keeping your original population at room temperature, repeat steps 3–6 every day for two weeks. Graph your
Technique 2: Modelling population growth in duckweed
Unlike most plant species, duckweed (Lemna sp.) grows continuously. It reproduces asexually by producing thalli
which break loose from the parent plant when its roots are large enough to support itself. Thalli are easy to count.
Figure 14.2.5d – Duckweeds are small freshwater autotrophs that consist of thalli
(leaves) and long roots
You will need:
Glass beaker
Healthy Lemna
Lamp or other continuous light source
Pond water (or dechlorinated tap water with added nutrient salts)
1. Place two healthy plants in a beaker with a set volume of pond water (e.g. 200ml). Count and record the
number of thalli on both plants.
2. Continue to count thalli every other day for 3–4 weeks. Make sure to replenish any volume of pondwater lost
to evaporation. Graph your results.
Variation: Compare the rates of population growth when the starting population in the same-volume beaker is much
 In the lab
Make sure your 0.1ml pipette is not contaminated when you do your serial dilutions.
More data is usually more reliable. Why not pool class data?
Yeast: How many times more concentrated is your original suspension compared with your diluted
Duckweed: Tap water can be dechlorinated by leaving it in a shallow pan overnight.
Duckweed: Make sure you specify a minimum size for counting thalli.
Figure 14.2.6e – Micrograph
Counting yeast is tricky! Take your time. Use a tally sheet or a handheld tally counter (below) so you don’t have to
look away from the microscope.
Figure 14.2.6f – Handheld tally counter
 Discussion
Do yeast and duckweed populations grow exponentially or logistically? Discuss the shape of the graphs.
Suggest variations to the main method that would (i) increase the rate of population growth, (ii) decrease
the rate of population growth and (iii) change the carrying capacity.
 IA: Exploration
Does competitive exclusion occur when two microorganisms are cultured together?
Figure 14.2.6g – A spectrophotometer as lab variation
You can also use a spectrophotometer to estimate population sizes in your samples of yeast (above). A sample
dilution is placed in a small cuvette and the optical density is read (below). Darker samples have more yeast.
Figure 14.2.5h – Optical density principle
Figure 14.2.5i – Duckweed pond
Duckweeds are food for frogs and ducks and respond quickly to eutrophication.
14.2.6 Lab: Nutrient content of soils (HL)
Soils are dynamic systems which shape and are shaped by their environments. Soil structure and nutrient content
have an impact on the type and distribution of plants growing on them and, in turn, the organisms affect the structure
of soils through death and decomposition.
Soil structure is generally determined by particle size. Clay particles have the smallest diameter, followed by silt and
sand. Most soil is composed of a combination of these three types of particle, as shown in Figure 14.2.6a.
Figure 14.2.6a – Soil classification system.
All soils are composed of a percentage of small (clay), medium (silt) and large (sand) particles. Different plants
grow best in different types of soil depending on their oxygen and nutrient requirements.
Application: Effects of waterlogging on the nitrogen cycle
Soils become waterlogged when water saturation prevents oxygen from reaching plant roots.
Waterlogging can occur because of excessive irrigation or poor drainage, often in flat clay soils.
Waterlogging leaves anaerobic conditions in the soil, so the plant roots are not able to respire. In addition, carbon
dioxide accumulates in the soil. The soil condition is said to be ‘anoxic’.
This affects the nitrogen cycle because anaerobic conditions favour denitrification over nitrification. More
nitrogen is released into the atmosphere.
Increased denitrification leaves less nitrogen available for plants.
This also affects the nitrogen cycle because waterlogging can cause runoff. Nitrogen in runoff water can also lead
to eutrophication in other ecosystems.
Figure 14.2.6b – Well aerated vs waterlogged soils
Waterlogged soils affect the nitrogen cycle by encouraging denitrification. Plant growth is also affected.
 Concept help
Bacterial species
NH3 → NO2- and NO3-
NO3- → N2
 Lab activity: Determining soil texture by feel
Here is a quick activity you can do to help you understand the different structures of soils and why certain soils
are more prone to waterlogging than others. All you need to do is obtain samples of different types of soil, then
follow the instructions given below. Download Figure 14.2.6c as PDF >
Figure 14.2.6c – Determining soil texture by feel
Skill: Assessing nutrient content of a soil sample
1. Nutrient content of a soil sample can be assessed indirectly by examining the plants growing there. Some
examples are given in the table below.
Plant appearance
Nutrient deficiency
Green glossy leaves
Yellow leaves, especially older leaves, new leaves lighter in colour
Leaf tips appear burnt, older leaves may be purple
Interveinal chlorosis from edges moving inwards
New leaves are distorted or irregularly shaped
Figure 14.2.6d – Interveinal chlorosis: yellowing between the veins and the edges of the leaf
2. Testing the pH of your soil sample using universal indicator is another indirect method of indicating which
nutrients are present.
Figure 14.2.6e – Soil pH and nutrient availability
3. Electrical conductivity can be measured to indicate the concentration of dissolved salts in the soil. In general,
the higher the salinity, the more nutrients are dissolved in the soil
Figure 14.6.2f – Electrical conductivity
Nitrates and dissolved salts increase conductivity of a soil sample.
4. The easiest way to test for nutrient content of soils is to use a commercial soil testing kit like the one shown
in Figure 14.6.2g. The chemicals in each of the tests indicate levels of nitrogen, potassium and phosphate.
Figure 14.2.6g – Commercial soil test kit
Chemical indicators change colour when nutrients are present.
 Key skills
Explain the impact of waterlogging on the nitrogen cycle.
Assess the nutrient content of a soil sample.
Figure 14.6.2h – Particle size
All soils are made of three sizes of particles
 Data analysis
From Figure 14.2.6a:
Identify the type of soil that is composed of 40% silt, 30% sand and 30% clay.
What is the minimum amount of clay necessary for a soil to be considered at least partially ‘loamy’?
Figure 14.6.2.i – Cassia obtusifolia
The sicklepod, Cassia obtusifolia, grows where soils are well aerated and is used as an indicator plant for soil
 IA: Exploration
Which soil conditions are best for plant growth?
What soil texture is most appropriate for different crops?
How does soil structure affect respiration rates?
Figure 14.6.2j – Raised beds
Gardeners use raised beds to prevent waterlogging
Figure 14.6.2.k – Fertiliser showing NPK ratings
Fertilisers show NPK ratings to indicate how much nitrogen, phosphorus and potassium they contain. Different
crops grow best with different ratios.
 Downloads
PDF of Figure 14.2.6c - Determining soil texture by feel
 Data analysis
At which pH is the availability of calcium highest?
At which pH range is the NPK rating highest?
Suggest the characteristics of plants grown in soils at pH4.5.