ESS Topic 2.3 - Measuring Biotic Components of the Ecosystem
2.3.1 Construct simple keys and use published keys for the identification of organisms.
Read pp.309-312 in the IBO ESS Course Companion.
In class, develop a dichotomous key for all the organisms listed in Figure 16.4: "A picture key of
pond animals". Your key should start with the largest groups of organisms and split them into
successively smaller groups until each organism is alone in its own group. No characteristic
should be used twice to subdivide different groups.
Here’s another selection of critters to classify with your own dichotomous key:
Complete the "To-do" activity accompanying Figure 16.5: "A selection of arthropods" on p.312
2.3.2 Describe and evaluate methods for estimating abundance of organisms.
ESS Topic 2.3 - Measuring Biotic Components of the Ecosystem
Because it's usually impractical to sample every single organism in an ecosystem, we use
statistical sampling to estimate the abundance of species within the ecosystem.
A transect is a line across an ecosystem along which quadrats may be placed for sampling.
Quadrats are square frames of a specific size, in which we count the number of each species of
organism.
•
The size of the quadrat depends on the size of organism anticipated (i.e. small quadrats
for small organisms and large quadrats for large organisms).
•
The number of quadrats used for sampling depends on the number of species found. If
we plot the number of species found against the number of quadrats used, once the
number of species is stable, it's no longer necessary to add more quadrats - we can
assume we've found all the species in the sample area. Refer to Fig. 16.6 on p.315 of the
IBO ESS Course Companion.
•
Placing quadrats: Quadrats can be placed randomly, continuously, or systematically.
◦
Random quadrats - use random number tables (easily generated online) to
determine placement of quadrats. Click here for an example of how to do this.
◦
Continual quadrats - place quadrats adjacently along a transect line and every
species along the line. This is quite accurate but can be extremely timeconsuming.
◦
Systematic quadrats - place quadrats along a transect at regular, predetermined intervals.
Estimating abundance depends on the type of organism being sampled.
•
Plants are stationary, so we can use the percentage cover to estimate their abundance.
Simply put, estimate the % of each quadrat covered by each species' leaf area. The total
% in a quadrat do NOT have to total 100%!
•
We can also count the density of individuals - how many per square meter.
•
Frequency of individuals is the % of the number of quadrats where a particular species is
found (i.e. Acacia senegalensis was present in 47 of 92 quadrats, for a frequency of
51%).
Here's a Youtube video podcast showing how an AP Environmental Science class in North
Carolina uses some of these field sampling techniques.
Capture, mark, release, recapture: Animals within an ecosystem are caught and marked so that
they may be tracked in the future, then released back into their habitat. At a subsequent time,
another capture event is held, and previously-marked animals are noted in the field notes. We
must be careful to make sure that the way the animals are marked doesn't make them more
prone to be killed and eaten by predators or harm their chances of survival in any other way
(i.e. making it more difficult to hunt or find food for themselves).
ESS Topic 2.3 - Measuring Biotic Components of the Ecosystem
The Lincoln Index is an important tool for estimating population size via capture-release
programs. This method is frequently used but is not perfect. See some of the assumptions on
p.317 of the IBO ESS Course Companion for further explanations. In class, compelte the Lincoln
Index calculations on p.317. The formula for calculating the Lincoln index is explained below:
N = (n1 x n2)/m2 where:
•
•
•
•
N = total population
n1 = number of animals first marked and released
n2 = number of animals captured in the second sample
m2 = number of marked animals in the second population
2.3.3 Describe and evaluate methods for estimating the biomass of trophic levels in a
community.
Biomass, as the name implies, is a measurement of the mass of living material at a trophic level
or within an ecosystem. Because water is NOT living, we do not include it in our calculations.
Since all organisms are made of roughly the same organic molecules in similar proportions, a
measure of their dry weight is a rough measure of the energy they contain. Therefore, material
brought into the lab must be dried completely before measuring its mass. Normally, this is
accomplished by placing the material in a warm - not hot - drying oven and allowing it to dry
completely over a day or two before weighing it.
•
•
•
•
Use the Lincoln index (above) to estimate the total population of a secies of organisms.
Once the mass of an average organism within the population is known, that mass can be
multiplied by the estimated population to determine the total biomass of the
population of that species.
This process is repeated until all species at a trophic level have been accounted for.
(Source: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/F/FoodChains.html)
2.3.4 Define the term diversity.
Diversity (commonly 'biodiversity' when discussing ecological principles) is a measure of the
relative abundance of different living organisms within an ecosystem. Diversity is not simply the
number of different species found in an ecosystem - consideration must be given to the relative
abundance of all species within the system.
Two ecosystems may have the same number of species (called 'species richness'), but vary
considerably in their diversity. If one or two species make up most of the total number of living
ESS Topic 2.3 - Measuring Biotic Components of the Ecosystem
organisms within the system, that system is considered less diverse than one in which the
numbers of every species are relatively similar (called 'species evenness').
Look at the example on p.318 (recreated below). Both ecosystem 1 and ecosystem 2 have the
same species richness - they each contain 3 different species. They also have similar numbers of
organisms (70 and 72, respectively). However, the vast majority of organisms in ecosystem 2
are from a single species - species 'A'. Because the distribution of species in ecosystem 2 is
skewed heavily to species A, ecosystem 2 is considered less diverse than ecosystem 1. How
does Ecosystem # 3 compare to #1 and #2 in terms of species richness and species evenness?
Diversity of different ecosystems
Ecosystem #1
Ecosystem #2
Ecosystem #3
Species
# of Individuals
A
25
B
24
C
21
A
65
B
3
C
4
A
28
B
32
C
12
Because ESS examines the science of ecosystems, we must be able to quantify or measure the
diversity of ecosystems for the sake of comparison. How do we do that? Read below...
2.3.5 Apply Simpson’s diversity index and outline its significance.
Simpson's diversity index is a mathematical model used to compare the relative biodiversity of
different ecosystems. A full explanation can be found on p.318 of the IBO ESS Course
Companion. In short, the higher the numerical value of Simpson's diversity index, the greater
the biodiversity of a system.
D = [N*(N-1)]/[Σ(n*(n-1)]
Where...
• D = diversity index
ESS Topic 2.3 - Measuring Biotic Components of the Ecosystem
• N = total number of organisms of all species found
• n = number of individuals of a particular species
Complete the "To do" activities # 1-3 on p.318 in class. In addition, respond to the following
prompts in your notebook:
1.
2.
Calculate the diversity index 'D' for a sample from ecosystem #3, the North American
temperate deciduous forest. Sample data are listed in Table 1, below.
What would happen to 'D' if all the oaks were harvested, leading to a 40% decline in the
squirrel population, a decline of 25% in the woodpecker population, and a 30% decline
in the warbler population in ecosystem #3?
Table 1: Species distribution in the eastern deciduous forest of North America, October 2009.
Species
# of individuals per species
Red oak (Quercus rubrum)
55
Silver maple (Acer saccharinum)
60
Eastern white pine (Pinus strobus)
32
Eastern white-tail deer (Odocoileus virginianus)
88
Eastern grey squirrel (Sciurus carolinensis)
110
Red bellied woodpecker (Melanerpes carolinus)
50
Yellow-rumped warbler (Dendroica coronata)
42
This site has a good step-by-step explanation of Simpson's Diversity Index.