Community Ecology

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• Community Ecology

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Climate and the Distribution ofEcological
Communities

• Communities are assemblages of large numbers of
  speciesthat all interact with each other.
• Areas with different climatic characteristics
  contain different ecological communities.
• Climate types are classified using the Koeppen
  Classification System.
• Categorizes climate types based on annual
  temperatureand precipitation, as well as
  variations occurring in these two variables.
• Examples tropical wet forests, subtropical
  deserts, temperate grasslands, temperate forests,
  boreal forests, and tundra.

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Climate and the Distribution ofEcological
Communities

• Productivity is positively correlated with
  temperature and humidity.
• Communities have a characteristic pattern or type
  of disturbance.

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How Predictable Are Community Assemblages?

• Two views of community dynamics exist.
• Clements believed that communities are stable,
  integrated, orderly, and predictable entities.
• Gleason believed communities are neither stable
  nor predictable, but a matter of history and
  chance.
• Historical and experimental data support
  Gleasons view.

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Climate characteristics
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Belém, Brazil
Temperature (ºC)
30
Very low variation
Average HIGH
20
50
Variation VERY LOW
40
30
Variation
Precipitation (cm)
20
Annual total HIGH
10
Variation HIGH
0
J
J
J
F
M
M
A
A
S
O
N
D
Month
7
Appearance
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Climate characteristics
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Yuma, Arizona
Temperature (ºC)
30
Average HIGH
20
Variation
Variation MODERATE
10
0
(Freezing)
Precipitation (cm)
20
Annual total VERY LOW
10
Variation LOW
0
J
J
J
F
M
M
A
A
S
O
N
D
Month
9
Appearance
10
Climate characteristics
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Denver, Colorado
Temperature (ºC)
30
Average MODERATE
20
Variation MODERATE
10
0
(Freezing)
Precipitation (cm)
20
Annual total LOW
10
Variation MODERATE
0
J
J
J
F
M
M
A
A
S
O
N
D
Month
11
Appearance
12
Climate characteristics
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Chicago, Illinois
Temperature (ºC)
30
Average MODERATE
20
Variation MODERATE
10
0
(Freezing)
Precipitation (cm)
20
Annual total MODERATE
10
Variation LOW
0
J
J
J
F
M
M
A
A
S
O
N
D
Month
13
Appearance
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Climate characteristics
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Dawson, Yukon, Canada
Temperature (ºC)
30
Average LOW
20
Variation VERY HIGH
10
0
(Freezing)
Precipitation (cm)
20
10
Annual total LOW
10
20
Variation LOW
0
30
J
J
J
F
M
M
A
A
S
O
N
D
Month
15
Appearance
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Climate characteristics
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Barrow, Alaska
Temperature (ºC)
30
Average VERY LOW
20
Variation HIGH
10
0
(Freezing)
Precipitation (cm)
20
10
Annual total VERY LOW
10
20
Variation LOW
0
30
J
J
J
F
M
M
A
A
S
O
N
D
Month
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Appearance
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EXPERIMENT TEST ON COMMUNITY STRUCTURE
1. Construct 12 identical ponds. Fill at the same
time with sterile water so that there are
no preexisting organisms.
2. After one year, examine water samples from
each pond under the microscope. Count the number
of plankton species in each sample.
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11
Ponds
12
1234
3. Plot results.
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2
3
4
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Plankton species (numbered rather than named, for
simplicity)
1
10
20
30
Total species in each pond
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Gleason hypothesis The composition of
biological communities is largely a matter of
chance.
Clement hypothesis Biological communities have a
predictable composition.
50
60
35 31 38 35 39 31 35 30 31 37 33 34
REJECTED
SUPPORTED
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EXPERIMENT TEST ON COMMUNITY STRUCTURE
1. Construct 12 identical ponds. Fill at the same
time with sterile water so that there are
no preexisting organisms.
2. After one year, examine water samples from
each pond under the microscope. Count the number
of plankton species in each sample.
3. Plot results.
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How Predictable Are Community Assemblages?

• Disturbance and change in ecological communities.
• Disturbance is any event that removes some
  individuals or biomass from a community.
• The characteristic type of disturbance found in a
  community is known as its disturbance regime.
• Important management decisions hinge on
  understanding the disturbance regimes of any
  community.

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Giant sequoias after a fire
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Fire scars in the growth rings
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Reconstructing history from fire scars
50
40
30
Number of fires per century
20
10
0
0
400
800
1200
1600
2000
Years A.D.
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How Predictable Are Community Assemblages?

• Succession
• Succession is the recovery and development of
  communitiesafter a disturbance occurs.
• Primary succession removes all organisms and
  soil, while secondary succession leaves soil
  intact.
• A distinct sequence of communities develops as
  succession proceeds.
• Succession is greatly impacted by the particular
  traits of the species involved, how species
  interact, the short-term weather conditions, and
  the overall environmental conditions.
• Glacier Bay, Alaska provides an excellent case
  study in succession.

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Old field
Disturbance ends, site is invaded by short-lived
weedy species.
Pioneering species
Weedy species replaced by longer-lived herbaceous
species and grasses.
Early successional community
Shrubs and short-lived trees begin to invade.
Mid-successional community
Short-lived tree species mature long- lived
trees begin to invade.
Late-successional community
Long-lived tree species mature.
Climax community
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Climax community
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Hypothesis 1 Only one successional pathway
occurs in Glacier Bay.
Proposed successionalpathway
Soils exposed less than 20 years willow and Dryas
20 km
Soils exposed 45-80 years sitka alder,
scattered cottonwood
N
Soils exposed 100 years sitka alder, scattered
spruce
Soils exposed 150-200 years dense sitka spruce
and western hemlock
Glacier Bay
Alaska
Glacier Bay
Direction of glacial retreat
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Hypothesis 2 Three distinct successional
pathways occur in Glacier Bay.
Spruce
PATHWAY 1
Alder
Early-mid successional
Late-mid successional
Climax
Hemlock
Cottonwood
PATHWAY 2
No hemlock?
?
Early successional
Mid-successional
Late-successional
Climax
PATHWAY 3
No hemlock?
?
Early successional
Mid-successional
Late-successional
Climax
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Hypothesis 2 Three distinct successional
pathways occur in Glacier Bay.
Spruce
PATHWAY 1
Alder
Early-mid successional
Late-mid successional
Climax
Hemlock
Cottonwood
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Hypothesis 2 Three distinct successional
pathways occur in Glacier Bay.
PATHWAY 2
No hemlock?
?
Early successional
Mid-successional
Late-successional
Climax
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Hypothesis 2 Three distinct successional
pathways occur in Glacier Bay.
PATHWAY 3
No hemlock?
?
Early successional
Mid-successional
Late-successional
Climax
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Recent data indicate that all three pathways
exist.

• In pathway 1, spruce arrive early and in high
  densities. Alder never forms continuous groves
  and western hemlock arrive after spruce.
• In pathway 2, alder arrives early and forms
  continuous thickets. Spruce arrives early as well
  but develops low-density stands under the alder
  thicket. Western hemlock is absent.
• In pathway 3, alder arrives early and forms
  continuous thickets. Cottonwood and spruce form a
  low-density forest int eh alder thickets. Western
  hemlock is absent.

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Species Diversity in Ecological Communities

• Quantifying diversity can be simple or complex.
• Research has focused on why some communities are
  more diverse than others and why diversity is
  important.

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Tropical forest
Boreal forest
Canopy
Canopy
Subcanopy
Epiphytes
Vines
Understory trees and shrubs
Understory shrubs
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Species Diversity in Ecological Communities

• On a global scale, a latitudinal gradient of
  species diversity exists for most taxa.
• Species diversity declines as latitude increases.
  
• Several hypotheses have been proposed to explain
  thisgradient, but no simple answer exists.

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10000
60º
8000
30º
Equator
0º
6000
Number of vascular plant species per 10,000 km2
30º
4000
60º
2000
0
0
10
20
30
40
50
60
Latitude (degrees North or South)
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Species Diversity in Ecological Communities

• Communities with high diversity are more
  productive, more resistant, and more resilient
  than those with low diversity.

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Shading indicates burned areas
Lake Yellowstone
Park boundary
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