1. Topic 2.3
Measuring Biotic Factors

2. Measuring Biotic factors
• Measuring biotic factors requires estimating the
abundance of species or the biodiversity of an
ecosystem
• Species richness is simply the number of different
species that are counted in a sample area
• Biodiversity is a measurment which combines:
– The number of different species present
– The relative numbers of individuals of each species
• Biodiversity can be expressed as Simpson’s Biodiversity
Index

3. Calculating Population Size
Population size
=
total number of organisms in all quadrats
number of quadrats thrown
x
Area of field
Area of one quadrat
• Throw 10 quadrats and collect the data. Use it to
calculate the population size of daisies.
• How close were you to the actual population size?

4. Simpson’s Diversity Index
Biologists use a more complicated formula called
Simpson's Diversity Index which calculates
biodiversity rather than simply population size:
Where:
D = N (N – 1)
Σn (n – 1)
D = diversity index (simply a number with no units)
N = total number of organisms of all species found
n = total number of individuals found of the species you are interested in
Use the calculation to calculate the diversity index for daisies and
clover, by throwing 10 quadrats in total

5. Biodiversity
• We don’t just have to count the number of individuals. We could also
measure:
Abundance - estimates plentifulness of a species or target group
according to a predefined scale, such as rare, infrequent, abundant, etc.
Density - abundance, measured by actual count, per unit area. Counts are
averaged when more than one sample is taken.
Frequency - the percentage of sample plots in which a species or target
group appears. Frequency data are often grouped into classes, such as A = 120%, B = 21-40%, C = 41-60%, D = 61-80%, E = 81-100%.
Cover - commonly measured as "Total Cover", which is the percentage of
all vegetation covering the ground surface inside the quadrat, or as "Species
Cover", which is the percentage of the target species covering the ground
surface inside the quadrat.
Biomass - the dry weight of the total vegetation or target species inside a
quadrat. Sampling consists of clipping the vegetation to ground level, sorting
by species if required, oven drying the sorted clippings, and weighing. Figure
3 shows new growth inside a quadrat that had previously been clipped.

6. Other Methods to Determine Population Size
and/or Biodiversity
• Mark-recapture
• Animals cannot be counted in the same way as
plants (they move around)
• The Lincoln Index is used to estimate population
size (note: not biodiversity)

7. The Lincoln Index
N = n1 x n2
m
Where:
N = total population size of animal of interest in the study site
n1 = number of animals captured on the first day
n2 = number of animals recaptured on the second day
m = number of marked animals in the sample recaptured on the second day
• Can you see a potential problem that might lead to inaccurate
results?
• What kind of ethical considerations must you take into account
during mark-recapture sampling?

8. The Lincoln Index
• Cut out 100 squares of graph paper individually
• Put them all into a tray
• Capture 20 pieces at random, mark them all with a cross and
replace them in the population (this is the end of day 1
sampling)
• Recapture 20 pieces of paper at random and record how
many of them have a cross (this is the end of day 2 sampling)
• Calculate the population size of pieces of paper using the
Lincoln Index. How close was this to the actual population
size of 100?

9. Other Methods to Determine Population Size
and/or Biodiversity
• Chemical Fogging
– http://edroness.blogspot.mx/2013/10/measuring-biodiversity.html
– This is a method created by Dr. Terry Erwin to
measure insect biodiversity in the rainforest
canopy
– It involves using an insecticide to kill all the insects
in a section of canopy and collecting the bodies as
they fall
– Can this be justified ethically?

10. Constructing a Dichotomous Key
• Create a dichotomous key from a set of rightfooted shoes
• http://oregonstate.edu/trees/dichotomous_key.html