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Chapter 50
Community Ecology
Chapter 50
Climate and the Distribution of
Ecological Communities

Communities are assemblages of large numbers of species
that 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 temperature
and precipitation, as well as variations occurring in these two
variables.
• Examples: tropical wet forests, subtropical deserts, temperate
grasslands, temperate forests, boreal forests, and tundra.
(Fig. 50.1)
Figure 50.1
Climate and the Distribution of
Ecological Communities

Productivity is positively correlated with temperature and
humidity.

Communities have a characteristic pattern or type of
disturbance.
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 Gleason’s view.
(Fig. 50.9)
Figure 50.2a
Climate characteristics
40
Belém, Brazil
Temperature
(ºC)
Average:
HIGH
30
Very low
variation
20
50
Variation:
VERY LOW
40
30
Variation
20
10
0
J
F M A
M
J
J
Month
A
S
O
N D
Precipitation
(cm)
Annual total:
HIGH
Variation:
HIGH
Figure 50.2b
Appearance
Figure 50.3
Dense, dry air
descends, warms,
and absorbs moisture
30ºN
Dry
Hadley
cell
Cooled air
is pushed
poleward
Wet
Equator
Warm air rises and cools,
dropping rain
Dry
30ºS
Atmosphere
(not to scale)
Figure 50.4a
Climate characteristics
40
Temperature
(ºC)
Average:
HIGH
Yuma, Arizona
30
20
Variation
Variation:
MODERATE
10
(Freezing)
0
20
10
0
J
F
M
A
M
J
J
Month
A
S
O
N
D
Precipitation
(cm)
Annual total:
VERY LOW
Variation:
LOW
Figure 50.4b
Appearance
Figure 50.5a
Climate characteristics
40
Temperature
(ºC)
Average:
MODERATE
Variation:
MODERATE
(Freezing)
Denver, Colorado
30
20
10
0
20
J
F
M A
M
J
J
Month
A
S
O
N
D
Precipitation
(cm)
10
Annual total:
LOW
0
Variation:
MODERATE
Figure 50.5b
Appearance
Figure 50.6a
Climate characteristics
40
Chicago, Illinois
Temperature
(ºC)
Average:
MODERATE
Variation:
MODERATE
(Freezing)
30
20
10
0
20
J
F
M
A M
J
Month
J
A
S
O
N
D
Precipitation
(cm)
10
Annual total:
MODERATE
0
Variation:
LOW
Figure 50.6b
Appearance
Figure 50.7a
Climate characteristics
40
Dawson, Yukon, Canada
Temperature
(ºC)
Average:
LOW
Variation:
VERY HIGH
(Freezing)
30
20
10
0
Precipitation
(cm)
–10
20
–20
10
Annual total:
LOW
–30
0
Variation:
LOW
J
F
M
A M
J
Month
J
A
S
O
N
D
Figure 50.7b
Appearance
Figure 50.8a
Climate characteristics
40
Barrow, Alaska
Temperature
(ºC)
Average:
VERY LOW
Variation:
HIGH
(Freezing)
30
20
10
0
Precipitation
(cm)
–10
20
–20
10
Annual total:
VERY LOW
–30
0
Variation:
LOW
J
F
M
A M
J
Month
J
A
S
O
N
D
Figure 50.8b
Appearance
Figure 50.9
EXPERIMENT TEST ON COMMUNITY STRUCTURE
Clement hypothesis:
Biological communities
have a predictable
composition.
REJECTED
Plankton species
(numbered rather than named, for simplicity)
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.
10 11
1234 Ponds
12
1 2 3 456789
1
3. Plot results.
10
20
30
40
50
60
Total
species
in each
pond
35 31 38 35 39 31 35 30 31 37 33 34
Gleason hypothesis:
The composition of biological
communities is largely a
matter of chance.
SUPPORTED
Figure 50.9
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.
Clement hypothesis:
Biological communities
have a predictable
composition.
REJECTED
Plankton species
(numbered rather than
named, for simplicity)
10 11
1234 Ponds
12
1 2 3 45 6 7 8 9
1
10
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.
20
30
40
50
60
Total
species
in each
pond
35 31 38 35 39 31 35 30 31 37 33 34
Gleason hypothesis:
The composition of biological
communities is largely a
matter of chance.
SUPPORTED
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. (Fig. 50.10a–c)
Figure 50.10a
Giant sequoias after a fire
Figure 50.10b
Fire scars in the growth rings
Figure 50.10c
Reconstructing history from fire scars
Number of fires
per century
50
40
30
20
10
0
0
400
800
Years A.D.
1200
1600
2000
How Predictable Are Community Assemblages?

Succession
• Succession is the recovery and development of communities
after 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. (Fig. 50.11)
• 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. (Fig. 50.12a,b)
Figure 50.11
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; longlived trees begin to invade.
Late-successional
community
Long-lived tree species mature.
Climax community
Figure 50.11
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; longlived trees begin to invade.
Late-successional
community
Long-lived tree species mature.
Climax community
Figure 50.12a
Hypothesis 1: Only one successional pathway occurs in Glacier Bay.
Proposed successional
pathway:
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
Alaska
Glacier
Bay
Direction of
glacial retreat
Figure 50.12b
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
PATHWAY 3
Climax
No hemlock?
?
Early successional
Mid-successional
Late-successional
Climax
Figure 50.12 b.1
Hypothesis 2: Three distinct successional pathways occur in Glacier Bay.
Spruce
PATHWAY 1
Alder
Early-mid successional
Late-mid successional
Climax
Hemlock
Cottonwood
Figure 50.12 b.2
Hypothesis 2: Three distinct successional pathways occur in Glacier Bay.
PATHWAY 2
No hemlock?
?
Early successional
Mid-successional
Late-successional
Climax
Figure 50.12 b.3
Hypothesis 2: Three distinct successional pathways occur in Glacier Bay.
PATHWAY 3
No hemlock?
?
Early successional
Mid-successional
Late-successional
Climax
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.
(Fig. 50.14)
Figure 50.14
Tropical forest
Canopy
Boreal forest
Canopy
Subcanopy
Epiphytes
Vines
Understory
trees and shrubs
Understory
shrubs
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. (Fig. 50.13)
• Several hypotheses have been proposed to explain this
gradient, but no simple answer exists.
Figure 50.13
Number of vascular plant
species per 10,000 km2
10000
60º
8000
30º
0º
6000
Equator
30º
4000
60º
2000
0
0
10
20
30
40
Latitude (degrees North or South)
50
60
Species Diversity in Ecological Communities

Communities with high diversity are more productive, more
resistant, and more resilient than those with low diversity.
(Fig. 50.16a,b)
Figure 50.15
1 species per plot
24 species per plot
Total plant cover (%)
65
55
45
35
25
1
2
4
6
8
12
Number of plant species per plot
24
Figure 50.16 a,b
Change in biomass:
One year before drought to
peak of drought
(a) Resistance to disturbance
Completely
resistant
0.0
– 0.5
–1.0
–1.5
0
5
10
15
20
25
30
Number of plant species before drought
(b) Resilience after disturbance
Change in biomass:
Before drought to four
years after
0.35
0.00
Completely
resilient
–0.35
–0.70
0
5
10
15
20
Number of plant species 2 years after drought
Essay 50.1, Figure 1, left
Shading indicates
burned areas
Lake
Yellowstone
Park boundary
Essay 50.1, Figure 1, right
Box 50.1, Figure 1
Community 1
Community 2
Community 3
Species richness:
6
6
5
Species diversity:
0.59
0.78
0.69
A
B
C
Species
D
E
F
Applying Ideas, Question 1
40
30
Temperature
(ºC)
20
10
40
30
20
10
0
J
F
M
A
M
J
J
Months
A
S
O
N
D
Precipitation
(cm)