Transcript Species diversity

Species diversity
• Ecological communities differ in species
number and composition
– tropics > temperate
– remote islands < large islands
– continents > islands
1
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
2
Measurement of species diversity
• Species richness
– 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
3
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
Comm 1
99
1
100
Comm 2
50
50
100
4
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
6
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
8
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
9
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
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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
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Practice problem
Community 1
Species
N
A
62
B
97
C
110
D
84
E
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pi
ln(pi)
pi[(ln(pi)]
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Practice problem
Community 2
Species
N
A
88
B
10
C
0
D
211
E
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pi
ln(pi)
pi[(ln(pi)]
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Practice problem
Community 1 Community 2
s
H’
Hmax
E
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Species
diversity
indices
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Commonness, rarity and dominance
• Preston’s log normal distribution
model
– a few common species with high
abundances
– many rare species with low abundances
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Commonness, rarity and dominance
• MacArthur’s broken stick model
– random breaks in a stick  log normal
distribution of pieces
– results in a few large pieces and many small
pieces
17
Commonness, rarity and dominance
• Community organization
– model 1
• a few very common species
• many rare species
– model 2
• a few very common and very rare species
• most species of intermediate abundance
18
Fig. 22.1, p. 435: Relative abundance of Lepidoptera
captured in a light trap in England (6814 individuals
representing 197 species).
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Biogeography
• Observations of relationships between
– area and number of species
– distance from source
• Island biogeography
– E.O. Wilson and Robert MacArthur
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Island biogeography
• Island communities: well-defined, captive
• Variables
– size
– degree of remoteness
– elevation
• Simple community structure
• Increase in area  increase in number of
species
21
Island biogeography
• Habitats considered as “insular”
because they are isolated from other
communities
– caves
– mountain tops
– some peninsulas
– wildlife or game preserves
22
Fig. 24.14, p. 502: Number of land-plant species on the
Galapagos Islands in relation to the area of the island.
23
Fig. 24.15, p. 503: Species-area curve for amphibians
and reptiles of the West Indies.
24
Island biogeography
• Relationship between remoteness and
number of species
– increase distance from mainland  decrease
number of species
– number of species present is dependent on
immigration from mainland
• rate is a function of the number of species already
present on the island
• number of species present = balance between
immigration and extinction
25
Fig. 24.17, p. 504: Equilibrium model for biota on a
single island.
26
Fig. 24.18, p. 504: Equilibrium model for biota on
several islands of different size and remoteness.
27
Island biogeography
• Small species are found on more islands
than are large species
• Number of herbivore species > carnivores
• Number of generalist herbivore species >
specialist herbivores
28
Island biogeography
• Species:area relationship
– log : log relationship
– 10-fold decrease in area  50% decrease in
number of species
29
Island biogeography
• Species:area relationship
30
Latitudinal diversity gradients
• Abundance and diversity patterns
– latitude
– elevation
– mountainsides
– peninsulas
31
Fig. 22.5, p. 438: Number of tree species in Canada
and U.S.
32
Fig. 22.6, p. 439: Number of species of land birds in
North and Central America.
33
Fig. 22.7, p. 440: Number of species of calanoid
copepods in top 50 m of transect from tropical Pacific
to Arctic Ocean.
34
Fig. 22.9, p. 440: Number of species of mammals in
continental North America.
35
Fig. 22.10, p. 440: Species richness of mammals in
North and South America in relation to latitude.
36
Latitudinal diversity gradients
• Tree species
– Malaysia (4
acres): 227
– Michigan (4
acres): <15
• Ant species
– Brazil: 222
– Trinidad: 134
– Cuba: 101
– Utah: 63
– Alaska: 7
37
Latitudinal diversity gradients
• Snake species
– Mexico: 293
– U.S.: 126
– Canada: 22
• Fish species
– Amazon R: >1000
– Central American
rivers: 450
– Great Lakes: 172
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Latitudinal gradient hypotheses
•
•
•
•
•
•
•
History (time)
Spatial heterogeneity
Competition
Predation
Productivity
Environmental stability (climate)
Disturbance
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Latitudinal gradient hypotheses
• History (time) hypothesis
– tropical habitats older, more stable
– support for
• geological past of temperate less constant than
tropics due to glaciation
• all communities diversify with time
– argument against
• as glaciers moved in, species moved south to
escape
• history hypothesis can not be tested
40
Latitudinal gradient hypotheses
• Spatial heterogeneity hypothesis
– higher diversity in tropics due to increase in
number of potential habitats
–  environmental complexity moving away from
equator
• macro level: e.g., topographic features
• micro level: e.g., particle size, vegetation
complexity
41
Latitudinal gradient hypotheses
• Spatial heterogeneity hypothesis
– Hutchinson’s n-dimensional niche  
specialization
– types of diversity defined by spatial
heterogeneity
• within-habitats ( diversity)
• between-habitats ( diversity)
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Diversity defined by spatial heterogeneity
Between habitat diversity ()
Temperate
Tropical
No. species per habitat
10
10
No. different habitats
10
50
Within-habitat diversity ()
Temperate
Tropical
No. species per habitat
10
50
No. different habitats
10
10
43
Latitudinal gradient hypotheses
• Competition hypothesis
– less competition in temperate and polar
environments compared to tropics because
these populations are more regulated by
extreme environmental conditions than by
biological factors
– populations maintained <K due to weather,
etc. and major sources of mortality are abiotic
– since population sizes small, decreased
competition for resources
44
Latitudinal gradient hypotheses
• Competition hypothesis
– no weather extremes in tropics, 
populations can increase to densities at
which competition for resources is necessary
– promotes species diversity through
specialization  resource partitioning
–  and  diversity higher in tropics due to
organisms being more specialized to habitats
45
Fig. 22.14a, p. 447. Niche breadth versus niche overlap
determined by competition within the community.
46
Latitudinal gradient hypotheses
• Predation hypothesis
– increased species diversity in tropics is
function of increased number of predators
that regulate the prey species at low densities
– decreases competition among prey species
– allows coexistence of prey species and
potential for new additions
47
Fig. 22.16, p. 449. Janzen-Connell model for increased
diversity of tropical rainforest trees: seed predation
versus distance of seed from tree versus seed survival.
48
Latitudinal gradient hypotheses
• Predation hypothesis
– there is more selective pressure on prey
evolving avoidance mechanisms than in
becoming better competitors
– cropping principle
• remove predators and prey start competing
• predation increases diversity by reducing
intraspecific competition among prey species
49
Community anchored by keystone starfish Heliaster
in northern Gulf of California.
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Latitudinal gradient hypotheses
• Predation hypothesis
– cropping principle in lakes
• top predators (fish) feed on zooplankton
• if fish are removed  community diversity
decreases, becomes dominated by a few species
of large, grazing zooplankton
• add fish  diversity of small zooplankton and their
invertebrate predators increases
51
Latitudinal gradient hypotheses
• Productivity hypothesis
– tropics support a greater number of species
because more resources are available,
allowing for more specialization
– in general:  production   diversity
– exceptions
• marshes: high production, relatively low diversity
• deserts: low production, high diversity
52
Latitudinal gradient hypotheses
• Environmental stability (climatic)
hypothesis
– annual climate in tropics more stable than
temperate or polar climates
– constant climate  finer specializations and
adaptations, shallower niches
– tropical species  number of broods / year 
 potential for evolutionary change   rate
of speciation
53
Latitudinal gradient hypotheses
• Environmental stability (climatic)
hypothesis
– high diversity habitats generally found in
stable climates; low diversity habitats
associated with severe and/or unpredictable
climates
54
Latitudinal gradient hypotheses
• Disturbance hypothesis
– if community disturbance frequency is very
high  local extinction of species  
species diversity
– if community disturbance frequency is very
low  competitive exclusion by dominant
species   species diversity
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Latitudinal gradient hypotheses
• Disturbance hypothesis
– intermediate disturbance hypothesis
• moderate disturbance maximizes diversity
• leads to patches at local level
– intermediate disturbance  high species
diversity in some communities (not all)
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Fig. 22.20, p. 453. Model for intermediate disturbance
hypothesis.
57
Fig. 22.21, p. 453. Effect of periwinkle grazing on algae
diversity.
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Fig. 22.21, p. 453. Effect of periwinkle grazing on algae
diversity.
Community dominated
by one algal species
Predator limits number
of possible algal species
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Basic concepts related to energy
flow and trophic structure
• Energy moves through community and
is lost as heat
• Nutrients move through the community
in cycles and are retained
60
Basic concepts related to energy
flow and trophic structure
• Niche
– sum of all parameters that enable an organism
to live in its biotic and abiotic environments
• competition, food gathering, predator escape, mate
location, reproduction, etc.
• temperature, moisture, nutrients, soil structure,
salinity, etc.
– Hutchinsonian niche: n-dimensional
hypervolume
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Basic concepts related to energy
flow and trophic structure
• Trophic level
– Lindeman (1942)
• classification of animals according to location in
lake
• lake trophic groups
–
–
–
–
benthic
demersal
plankton
nekton
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Basic concepts related to energy
flow and trophic structure
• Trophic level
– Lindeman (1942)
• described food chain with primary producers at
base and other trophic levels of animals based on
feeding relationships
• more accurately described as food web, since few
organisms other than plants occupy only one
feeding level
63
Food webs and energy flow
• Trophic levels
– ecosystem feeding levels
– biomass and usable energy  as  level
– most systems support only four trophic levels
– aquatic communities have slightly longer food
chains than terrestrial communities
– ultimate food chain length limited by inefficiency
of energy transfer from one trophic level to the
next
64
Food webs and energy flow
• Food chains
– sequence of organisms where each is the
food source for the next
• Food webs
– represent energy flow through ecosystem
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Trophic levels
Tertiary consumers (top carnivores)
Secondary consumers (carnivores)
Primary consumers (herbivores)
Primary producers (plants)
66
Food chain model
Heat
First Trophic
Level
Second Trophic
Level
Third Trophic
Level
Fourth Trophic
Level
Producers
(plants)
Primary
consumers
(herbivores)
Secondary
consumers
(carnivores)
Tertiary
consumers
(top carnivores)
Heat
Heat
Heat
Solar
energy
Heat Heat
Heat
Heat
Detritivores
(decomposers and detritus feeders)
Heat
67
Figure 23.6, p. 465. Hypothetical food web model.
68
Food web terminology
• Top predators: species eaten by nothing else in the food web
• Basal species: species that feed on nothing within the food web
• Intermediate species: species that have both predators and prey
within the food web
• Trophic species: groups of organisms that have identical sets of
predators and prey
• Cycles within food web: which species eat which other species
• Interaction: any feeding relationship within food web
• Connectance: number of actual interactions in food web divided by
number of possible interactions
• Linkage density: average number of interactions per species in the
food web
• Omnivores: species that feed on more than one trophic level
• Compartments: groups of species with strong linkages among group
members but weak linkages to other groups of species
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Figure 23.8, p. 467. Distribution of food chain lengths
in the Ythan Estuary, NE Scotland.
95 species
5518 food chain lengths counted
70
Food web of a rocky intertidal community, northern Gulf of
California (after Paine 1966).
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Producers
Producers
Producers
Producers
Rocky intertidal community food web
(Paine’s 1966 study)
• (Producer level omitted from original figure)
• Level 1
– herbivorous gastropods and chitons
– filter feeding bivalves
– suspension feeding barnacles and
brachiopods
• Levels 2-4: carnivorous gastropods
• Level 5: top carnivore
– Heliaster starfish
72
Keystone species
• Usually the top carnivore
• Presence or absence determines
community structure and composition
73
Food web of a rocky intertidal community, northern Gulf of
California (after Paine 1966).
Top carnivore
74
Producers
Producers
Producers
Producers
Food web of a rocky intertidal community, northern Gulf of
California (after Paine 1966).
Top carnivore
X
Species outcompeted in absence of
keystone species
X
X
Space competitor
X
X
X
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Producers
Producers
Producers
Producers
Keystone species
• Paine (1974): Pacific rocky intertidal
community
– dominated by Pisaster starfish
– remove starfish → mussel Mytilus californiensis
↑ → excludes all other invertebrate species
– Mytilus becomes numerically dominant
– Pisaster feeds on Mytilus → prevents Mytilus
domination of community → ↑ community
diversity
76
Figure 23.3, p. 462. Simplified Antarctic marine food web.
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Fig. 23.4, p. 464. Food web of boreal forest of northwest Canada.
78
Generalizations about food webs
• Size of animal increases with increase in
trophic level
• Abundance decreases with increase in
trophic level
• Large animals can not exist on small
animals as prey
• Small carnivores are limited to prey that can
fit into their mouths
79
Which trophic level is most important?
• Studies by Charles Elton in two square
miles of Wytham Woods
• Which species could be removed without
changing the community?
– top carnivore, except keystone species
– lower levels are food source for higher levels
– importance of top carnivores <<< herbivores
80
Which trophic level is most important?
• Dependent on complexity of community
– increased number of interconnections in
community → increased complexity of food
web → increased stability of community
structure → alternate food sources should one
be removed
– redundancy model versus rivet model
81
Which trophic level is most important?
• Determining species importance
– species with highest biomass
– where nutrients accumulate
– where energy accumulates
82