Download Community Ecology

• Chapter 41 Community
Interactions
Interactions
• Interspecific (interactions
between populations of
different species within a
community):
•Predation (including
parasitism) may involve a
keystone species/predator
•Competition
•Commensalisms
•Mutualism
Predation defense
•
Cryptic (camouflage) coloration
•
Aposematic (warning) coloration
•
Mimicry- superficial resemblance to
another species
–
–
Batesian- palatable/ harmless species mimics an
unpalatable/ harmful model
Mullerian- 2 or more unpalatable,
aposematically colored species resemble each
other
Competition: a closer look
• Interference - actual fighting over
resources
• Exploitative - consumption or use of
similar resources
• Competitive Exclusion Principle 2
species with similar needs for the
same limiting resources cannot
coexist in the same place
• Gause experiment (interspecific)
– Paramecium aurelia
• Has competitive edge
– Paramecium caudatum
• Driven to extinction
– Lotka-Volterra competition model
describes the outcome of competition
between two species over ecological time
Competition evidence
•
Resource partitioning- sympatric
species consume slightly different
foods or use other resources in slightly
different ways
•
Character displacement- sympatric
species tend to diverge in those
characteristics that overlap
Sympatric vs. Allopatric Speciation
Ex: Anolis lizard sp. perching sites in the
Dominican Republic
Ex: Darwin’s finch beak size on the
Galapagos Islands
Community structure
• Community - an assemblage of
populations living close enough
together for potential interaction
• Richness (number of species) &
abundance…….
• Species diversity
• Hypothesis:
•Individualistic- chance assemblage
with similar abiotic requirement
•Interactive- assemblage locked into
association by mandatory biotic
interactions
Species diversity
• Ecological communities differ in species
number and composition
– tropics > temperate
– remote islands < large islands
– continents > islands
Species Diversity Indices

Species Richness (S) - the total number of different organisms present. It does not take into account the
proportion and distribution of each species within the local aquatic community.

Simpson Index (D) - a measurement that accounts for the richness and the percent of each species from a
biodiversity sample within a local aquatic community. The index assumes that the proportion of
individuals in an area indicate their importance to diversity.

Shannon-Wiener index (H’) - Similar to the Simpson's index, this measurement takes into account
species richness and proportion of each species within the local aquatic community. The index comes
from information science. It has also been called the Shannon index and the Shannon-Weaver index in
the ecological literature.
7
Species diversity
• Comprised of
– species richness: number of species present
– heterogeneity of species
• equitability or evenness
• relative abundance of each species present in the
community
8
Measurement of species diversity
• Species richness
–
–
–
–
–
9
number of species present in community
first and oldest concept of diversity
simplest estimate of diversity
only residents are counted
treats common and rare species with the same
weight
Measurement of species diversity
• Heterogeneity of species
– uses relative abundance to give more weight to
common species
– possibilities in a 2-species community:
Species A
Species B
10
Comm 1
99
1
100
Comm 2
50
50
100
Measurement of species diversity
• Shannon-Wiener diversity function
s
H' = - (pi) [ln(pi)]

H’ = Shannon-Wiener index of species diversity
s = number of species in community
pi = proportion of total abundance represented
by ith species
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Shannon-Wiener diversity index
Community 1
Species
N
A
99
B
1
pi
ln(pi)
pi[(ln(pi)]
pi
ln(pi)
pi[(ln(pi)]
Community 2
Species
N
A
50
B
50
12
Shannon-Wiener diversity index
Community 1
Species
N
pi
ln(pi)
pi[(ln(pi)]
A
99
0.99
-0.010
-0.010
B
1
0.01
-4.605
-0.046
100
1.00
-0.056
H’
0.056
Community 2
Species
N
pi
ln(pi)
pi[(ln(pi)]
A
50
0.50
-0.693
-0.347
B
50
0.50
-0.693
-0.347
100
1.00
H’
-0.694
0.694
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Measurement of species diversity
• Shannon-Wiener diversity function
– values range from near zero to ???
– increased values indicate increased diversity
– index has no units; value only as comparison
between at least two communities
14
Species diversity
• What increases species diversity (H’)?
– increasing the number of species in the
community (s)
– increasing the equitability of the abundances of
each species in the community
15
Evenness
• Measurement of equitability among species in the
community
• Pielou evenness
E = H’ / Hmax
E = Pielou evenness
H’ = calculated Shannon-Wiener diversity
Hmax = ln(s) [species diversity under maximum
equitability conditions]
– values range from near zero to 1
16
Diversity and evenness
Community 1 Community 2
s
2
2
H’
0.056
0.694
Hmax
0.693
0.693
E
0.081
1.000
17
Practice problem
Community 1
Species
N
A
62
B
97
C
110
D
84
E
16
pi
ln(pi)
pi[(ln(pi)]
18
Practice problem
Community 2
Species
N
A
88
B
10
C
0
D
211
E
27
pi
ln(pi)
pi[(ln(pi)]
19
Practice problem
Community 1 Community 2
s
H’
Hmax
E
20
Simpson’s Index
•
Many diversity indices have been
developed that combine different
measures of biodiversity. One is called
the Simpson’s Index.
•
The Simpson’s Index includes BOTH
species richness and species evenness
in a single number.
Ocean sunfish (Mola mola).
© iStockphoto.com/Todd Winner
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How to Calculate
Simpson’s Index
•
D is the Simpson’s Index
•
n is the total number of organisms of a
particular species
•
N is the total number of organisms of
all species
•
∑ means “add up”!
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∑ n(n - 1)
D =
N(N - 1)
Let’s Try an Example
n
•
•
You have studied a specific site, and
have counted the individuals of five
different species.
n is the total number of organisms of a
particular species.
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Species A
12
Species B
3
Species C
7
Species D
4
Species E
9
Simpson’s Index
n
D=
∑ n(n - 1)
N(N - 1)
∑ n(n - 1) = 264
n-1
n(n - 1)
Species A
12
11
132
Species B
3
2
6
Species C
7
6
42
Species D
4
3
12
Species E
9
8
72
∑ n(n - 1)
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264
Simpson’s Index
∑ n(n - 1)
D =
=
N(N - 1)
264
N(N - 1)
N = total number of all individuals = 35
N - 1 = 34
N(N - 1) = 1190
D
=
264
1190
=
0.22184
This area would score 0.22184 on the Simpson’s Index. The scale ranges from
0–1, with 1 representing the lowest biodiversity. Therefore, the score for this
area indicates a high level of biodiversity.
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Biodiversity at Sites in
Honduras
The following calculations are based on sampling conducted in Honduras by
Canadian Museum of Nature research scientist Bob Anderson.
N
N(N - 1)
∑ n(n - 1)
2996
7 120 892
El Pital 2050 m
233
El Pital 2650 m
Site
D
Species
Richness
1 600 002
0.2247
61
54 056
6856
0.1268
22
5411
29 273 510
12 873 694
0.4398
46
Cerro Puca
311
96 410
19 126
0.1984
27
Santa Barbara
839
703 082
55 514
0.0789
44
Cerro Montecristo
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Biodiversity at Sites in
Honduras
•
Which site has the highest species richness?
•
Which site is the most diverse according to the Simpson’s Index? (HINT: Has the lowest
D).
•
Do any sites have both a low Simpson’s Index and high species richness? Which one(s)?
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The Niche
• Ecological niche - the sum total of an
organism’s use of biotic and abiotic
resources in its environment; its
“ecological role”
• Fundamental - the set of resources a
population is theoretically capable of
using under ideal conditions
• Realized- the resources a population
actually uses
• Thus, 2 species cannot coexist in a
community if their niches are identical
Ex: Barnacle sp. on the coast of Scotland
Succession
• Ecological succession - transition in
species composition over ecological
time
• Primary - begun in lifeless area; no
soil, perhaps volcanic activity or
retreating glacier
• Secondary - an existing community
has been cleared by some disturbance
that leaves the soil intact
• Climax Communities
A climax community is one that has reached the stable stage. When extensive
and well defined, the climax community is called a biome. (Ex. tundra,
grassland, desert, and the deciduous, coniferous, and tropical rain forests.)
Stability is attained through a process known as succession, whereby relatively
simple communities are replaced by those more complex.