Transcript Species Diversity Concepts

Calculating Diversity
Class 3
Presentation 2
Outline
• Lecture
• Class room exercise to calculate diversity
indices
Why quantify biodiversity?
• Initially thought that more diversity = more
stable ecosystem*
• Now used to measure and track changes
*MacArthur, R. 1955. Fluctuations of animal
populations and a measure of community stability.
Ecology 35:533-536
How do we measure
biodiversity?
• Use functional categories
– Ecosystem, species, genetic
• Use theoretical categories*
– Alpha
– Beta
– Gamma
* Whittaker, R.H. 1960. Vegetation of the
Siskiyou Mountains, Oregon and California.
Ecol. Mono. 30:279-338.
Alpha diversity
• Diversity within a particular sample
• E.g. the number of species surveyed
Beta Diversity
• Changes in sample composition along an
environmental gradient
• E.g. composition of forest stands on the
slope of a mountain
Gamma Diversity
• Diversity due to differences in samples
when all samples combined
• E.g. diversity of a forest landscape
Describing Communities
• Two methods
– Describe physical attributes (e.g. age
class, size class)
– Describe number of species and their
abundance
Biodiversity
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Diversity of living things
Term often misused and overused
Current focus in conservation studies
Includes interest in genetic, species and
ecosystem diversity
• We will use species as our focus but
concepts can be used for genetic and
ecosystem diversity as well.
Species Richness
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Number of species in a community
The simplest measure
Can count all spp only is few simple ecosystems
Does not consider number of individuals
Difficulties
– When is it a specie?
• Aphids
• Clonal plants
– Cannot count all species with limited time
Species Richness
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How?
Identify organism groups of interest
Identify boundaries of community
Survey area for organisms of interest
Species diversity
• Species richness not very informative
• Each community has 5 spp & 50 individuals
Comm
A
Comm
B
Spp
1
Spp
2
Spp
3
Spp
4
Spp
5
10
10
10
10
10
46
1
1
1
1
Diversity indices
• To get a better description of the community
we need to get a measure of spp richness
and evenness of their distribution
• We usually use an index to represent several
different measures
– E.g. stock markets, air pollution, etc.
Diversity indices
• Over 60 indices used in ecology
• Indices used to measure proportional
abundance
• Two major forms:
– Dominance indices (e.g. Simpson index)
– Information indices (e.g. Shannon Weiner
index)
Simpson Diversity Index (D)
– Simpson’s index considered a dominance index
because it weights towards the abundance of the
most common species.
– measures the probability two individuals
randomly selected from a sample will belong
to the same category
– For example, the probability of two trees, picked at
random from a tropical rainforest being of the
same species would be relatively low , whereas in
the boreal forest would be relatively high.
Simpson Diversity Index (D)

Ds = (n1(n1 -1)/N(N-1))
Where:
Ds = Bias corrected form for Simpson Index
n1 = number of individuals of spp 1
N = Total number of spp in community
In this form as diversity increases index value
gets smaller
Simpson Diversity Index (D)
• To make it easier to read the index is often
read as:
• Reciprocal i.e. 1/ Ds
• Complimentary form: 1- Ds
• Here as diversity increases Index value
increases
Simpson Diversity Index (D)
Sugar Red Yellow
Maple Maple Birch
#
Trees
56
48
12
Red
Oak
White
Ash
Total
6
3
125
See Excel
((56*55)/(125*124))+
Show how
((48*47)/(125*124)) + …………. index
….((3*2)/125*124)) = 0.35509 changes
Simpson Diversity Index (D)
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Complimentary form = 1-D
= 1-0.35509 = 0.6449
Reciprocal 1/D
1/0.35509 = 2.816
Shannon-Weiner Index (H')
• The index measures the uncertainty of a
category in a particular set
• It is a measure of evenness
• For example, very low uncertainty the letter
y is the next letter in this string: yyyyyyy
(H' = 0)
Shannon-Weiner Index (H')
• Assumptions:
– All species represented
– Sample randomized (equal probability of being selected
in the sample)
H' = -  pilnpi
pi= proportion of the ith species
ln=natural logarithm
Shannon-Weiner Index (H')
Sugar
Red Yellow
Maple Maple Birch
#
Trees
p
56
48
12
Red
Oak
White
Ash
Total
6
3
125
56/125 48/125 12/125 6/125
0.44
0.38 0.096 0.048
3/125
0.024
-plnp 0.359 0.367 0.224 0.146 0.089 1.187
Shannon-Weiner Index (H')
• Index affected by both number of species
and evenness of their population
• Diversity increases as both increase
• Diversity maximum when all species
equally abundant
Evenness
• Can use Shannon Weiner index to get a
measure of evenness
• First calculate Hmax
• Evenness = H‘/ Hmax
• Evenness will vary between 1 and 0
Evenness
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In the last example
H‘= 1.1875
Hmax = 1.609
Therefore E = 1.1875/1.609 = 0.738
The closer to 1 the more even the
populations that form the community
Questions?