Transcript 9/28 Communities

Ecology = Scientific study of natural
communities
Process
Experimental and
observational tests
provisionally accept,
begets
revise or reject
are tested by
Predictions
lead to
Pattern
Hypotheses
suggest
delimit
made of
Observations
Principles
Types of Terrestrial Ecosystems
• Biomes are major groupings of plant and animal communities
defined by a dominant vegetation type.
• Each biome is associated with a distinctive set of abiotic conditions.
• The type of biome present in a terrestrial region depends on
climate—the prevailing, long-term weather conditions found in an
area.
• Weather consists of the specific short-term atmospheric conditions
of temperature, moisture, sunlight, and wind.
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Distinct Biomes Are Found throughout the World
Barrow
Dawson
Chicago
Konza Prairie
Yuma
Belém
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Solar Radiation per Unit Area Declines with Increasing Latitude
North pole
Small amount of
sunlight per unit area
Low angle of
incoming sunlight
Moderate angle of
incoming sunlight
Sunlight directly
overhead
Large amount of
sunlight per unit area
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Global Air Circulation Patterns Affect Rainfall
Circulation cells exist at the equator …
Atmosphere (not to scale)
Hadley
cell
Hadley
cell
Warm air rises
and cools,
dropping rain
Cooled air is
pushed poleward
Dense, dry air
descends, warms,
and absorbs moisture
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Global Air Circulation Patterns Affect Rainfall
… and at higher latitudes.
There are three
circulation cells in the
Northern Hemisphere
There are also three
circulation cells in the
Southern Hemisphere
(draw them in)
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Figure 50-11
Figure 50-12
Tropical wet forests are
extremely rich in species
Figure 50-13
Figure 50-14
Saguaro cacti are a prominent
feature of the Sonoran Desert in
the southwestern part of
North America
Figure 50-15
Figure 50-16
Grasses are the
dominant lifeform
in prairies and
steppes
Figure 50-17
Figure 50-18
Temperate forests are
dominated by broadleaved deciduous trees
Figure 50-19
Figure 50-20
Boreal forests are
dominated by
needled-leaved
evergreens, such
as spruce and fir
Figure 50-21
Figure 50-22
Arctic tundra is dominated
by cold-tolerant shrubs,
lichens, and herbaceous
plants
Increases in Rainfall Variability Affects Grassland Biomes
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Increases in Rainfall Variability Affects Grassland Biomes
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Increases in Rainfall Variability Affects Grassland Biomes
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What factors influence the type
and extent of prairies found in
central North America?
Pattern occurs at different scales
How Predictable Are Communities?
• Frederick Clements promoted the view that biological communities
are stable, integrated, and orderly entities with a highly predictable
composition.
• Clements argued that communities develop by passing through a
series of predictable stages dictated by extensive interactions
among species, and that this development culminates in a stable
final stage called a climax community.
• “All the stages which precede the climax are stages of growth.”
• ”As an organism the formation arises, grows, matures and dies.”
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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
How Predictable Are Communities?
• Henry Gleason, in contrast, contended that the community found in
a particular area is neither stable nor predictable.
• According to Gleason, it is largely a matter of chance whether a
similar community develops in the same area after a disturbance
occurs.
• Which viewpoint is more accurate?
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Whittaker’s studies of
tree populations
Identical Communities Do Not Develop in Identical Habitats
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Identical Communities Do Not Develop in Identical Habitats
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Identical Communities Do Not Develop in Identical Habitats
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Disturbance and Change in Ecological Communities
• Community composition and structure may change radically in
response to changes in abiotic and biotic conditions.
• A disturbance is any event that removes some individuals or
biomass from a community.
• The important feature of a disturbance is that it alters some aspect
of resource availability.
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Disturbance and Change in Ecological Communities
•
The impact of disturbance is a function of three factors:
(1) type of disturbance
(2) frequency of disturbance
(3) Severity of disturbance
•
Most communities experience a characteristic type of
disturbance, and in most cases, disturbances occur with a
predictable frequency and severity.
•
This is called a community's disturbance regime.
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History of Disturbance in a Fire-Prone Community
Fire scars in the growth rings
Fire scars
Reconstructing history from fire scars
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Box 50.1, Figure 1
How do we measure diversity?
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
Simpson’s Index of Diversity
Proportion of sample represented by species (pi)
D = 1 /  (pi)2
Species
A
Species
B
Species
C
Species
D
Species
E
Community
1
0.20
0.20
0.20
0.20
0.20
Community
2
0.50
0.30
0.10
0.07
0.03
If you plug these numbers into the formula for Simpson’s Index, D = 5.00 for the community where all 5 species are at equal
abundances (Row 3). In contrast, D = 2.81 for the community with the same 5 species at very unequal abundances (Row 4). As this
example illustrates, Simpson’s index is more sensitive to changes in the abundant (rather than rare) species in a community.
Joseph Connell’s
Intermediate Disturbance Hypothesis
Observational Evidence for IDH