Environmental Systems and Societies for the IB Diploma Study and Revision Guide
All the revision notes you need to know for Ess topic 2.5. Well organised and has practice exam questions. Combined class notes with notes from a revision guide. Used for IB ESS exams.
The sampling system used depends on the areas being sampled:
QUICK CHECK QUESTION
Random sampling is used if the same habitat is found throughout the area. -One habitat has a Simpson’s index of 1.83 and another has an index of 3.65. What do these
Stratified random sampling is used in two areas different in habitat quality. values tell you about each habitat?
Systematic sampling is using along a transect where there is an environmental gradient.
KEY DEFINITION
Biomass- The mass of organic material in organisms or ecosystems, usually stated per unit
area.
METHODS FOR ESTIMATING THE BIOMASS OF TROPIC LEVELS
Biomass is calculated to indicate the total energy within a tropic level.
Biomass is a measure of the organic content of organism.
Water is not an organic molecule, and its amount varies from organism to organism
Water is not an organic molecule, and its amount varies from organism to organism, so
water is removed before biomass is measured. This is called dry weight biomass.
One criticism of the method is that it involves the killing of living organisms (although not
all the organisms in an area need to be sampled)
Problems exist with measuring biomass of very large plants such as trees,a nd with roots
and underground biomass.
Calculating dry weight biomass:
To obtain quantative samples, biological material is dreid to constant weight:
The sample is weighted is a container of known weight.
The sample is put in a hot oven (80oC)
After a specific length of time the sample is reweighted.
The sample is put back in the oven
This is repeated until the same mass is recorded from two successive readings.
No further loss in mass indicated that water is no longer present.
Biomass is recorded per unit area (e.g. per meter squared) so that tropic levels can be
compared. Not all organisms in an area need to be sampled:
The mass of one organism, or the average mass of several organisms, is taken.
This mass is multiplied by the total number of organisms to estimate total biomass.
This is called an extrapolation technique.
QUICK CHECK QUESTIONS
-Describe and evaluate methods for measuring three abiotic factors in a forest ecosystem.
-Explain the difference between percentage frequency and percentage cover.
-Which data are needed to estimate the size of an animal population? Write the equation
needed to calculate population size.
-Explain how biomass is calculated
DIVERSITY AND THE SIMPSON’S DIVERSITY INDEX
Species diversity refers to the number of species and their relative abundance. It can be
calculated using diversity indices. MEASURING CHNAGES ALONG AN ENVIRONMENTAL GRADIENT
Species diversity can be calculated using the Simpson’s diversity index, using the equation:
Ecological gradients are found where two ecosystems meet or where an ecosystem ends.
Abiotic and biotic factors change along an ecological gradient.
Transects are sued to measure changes along a gradient; this ensures that all parts of the
gradient are measured.
Where: D= Simpson’s index; N=total number of organisms of all species found; n= number of The whole transect can be sampled (a continuous transect) or samples can be taken at
individuals of a particular species. points of equal distance along the transect (an interrupted transect).
A line transect is the simplest transect, where a tape measure is laid out in the direction
Index values are relative to each other and not absolute, unlike measures of, say, temperature, of the gradient. All organisms touching the tape are recorded.
which are on fixed scale. A belt transect allows more samples to be taken- a band usually between 0.5m and 1m
is sampled along the gradient.
A diversity index is a numerical measure of species diversity calculated by using both the
number of species (species richness) and their relative abundance. Quadrats can be used to sample at regular intervals along a transect.
Comparisons can be made betwee areas containing the same type of organism in the Frame quadrats are empty frames of known area (e.g. 1m2)
same ecosystem. Grid quadrats are frames divided into 100 small squares.
A high value of D suggests a stable and ancient site, where all species have similar Point quadrats are made from a frame with 10 holes, inserted into the ground by a leg.
abundance (or ‘evenness’). They are used fro sampling vegetation that grows in layers. A pin is dropped through
A low value of D could suggest disturbance through, say, logging, pollution, recent each hole in turn and the species touched are recorded. The total number of pins
colonization or agricultural management, where one species may dominate. touching each species is converted to percentage frequency data (i.e. is a species
touches 7 out of 10 pins it has 70% frequency)
Similar habitats can be compared using D; a lower value in one habitat may indicate human
impact. Low values of D in the Arctic tundra, however, may represent stable and ancient sites. Zonation can be measured by recording biotic and abiotic factors at fixed heights along a
transect.
KEY DEFINITION
Diversity- A generic term for heterogeneity (i.e. variation or variety). The scientific meaning A cross staff is used to move a set distance (e.g. 0.6m) vertically up the transect.
of diversity becomes clear from the context in which it is used; it can refer to heterogeneity The staff is set vertically and a point measured horizontally from an eye-sight 0.6m from
of species or habitat, or to genetic heterogeneity. the base of the staff.
Biotic and abiotic factors are measured at each height interval.
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