Species Diversity Concepts
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Transcript of 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
• 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
• 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
• 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
Spp1
Spp 2
Spp 3
Spp 4
Spp 5
Comm
A
10 10 10 10 10
Comm
B
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 1N = 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
Maple
Red
Maple
Yellow
Birch
Red
Oak
White
Ash
Total
#
Trees
56 48 12 6 3 125
((56*55)/(125*124))+ ((48*47)/(125*124)) + …………. ….((3*2)/125*124)) = 0.35509
See Excel
Show how index changes
Simpson Diversity Index (D)
• 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 speciesln=natural logarithm
Shannon-Weiner Index (H')
Sugar
Maple
Red
Maple
Yellow
Birch
Red
Oak
White
Ash
Total
#
Trees
56 48 12 6 3 125
Pi 56/125
0.44
48/125
0.38
12/125
0.096
6/125
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
• 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?