Transcript 20Molles5e

Succession and Stability
Chapter 20
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Outline
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Introduction
Community Changes During Succession
 Primary Succession
 Secondary Succession
Ecosystem Changes During Succession
 Disturbance
 Ecosystem Recovery
Mechanisms of Succession
Community and Ecosystem Stability
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Introduction
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Succession: Gradual change in plant and
animal communities in an area following
disturbance.
 Primary succession on newly exposed
geological substrates.
 Secondary succession following
disturbance that does not destroy soil.
Climax Community: Late successional
community that remains stable until
disrupted by disturbance.
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Community Changes During Succession
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Reiners et al. studied changes in plant
diversity during succession.
 Total number of plant species increased
with plot age.
 Species richness increased rapidly in early
years of succession and more slowly
during later stages.
 Not all groups increased in density
throughout succession.
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Primary Succession at Glacier Bay
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Secondary Succession in Temperate Forests
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Oosting found number of woody plant
species increased during secondary
succession at Piedmont Plateau.
 Johnston and Odum found increase in bird
diversity across successional sequence
closely paralleled increase in woody plant
diversity observed by Oosting.
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Succession in Stream Communities
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Fisher studied rapid succession in Sycamore
Creek, AZ.
 Evaporation nearly equals precipitation flows generally low and intermittent.
 Subject to flash floods.
 Observed rapid changes in diversity and
composition of algae and invertebrates.
 Invertebrates found refuge because
many adults in aerial stage.
 Re-colonized after flooding.
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Ecosystem Changes During Succession
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Chapin documented substantial changes in
ecosystem structure during succession at
Glacier Bay.
 Total soil depth and depth of all major soil
horizons show significant increase from
pioneer community.
 In addition, organic content, moisture,
and N concentrations all increased.
 Physical and biological systems are
inseparable.
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Ecosystem Changes During Succession
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Four Million Years of Ecosystem Change
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Chronosequences such as that found at
Glacier Bay are limited.
 Hawaiian Islands have formed over hot
spots on the Pacific tectonic plate, forming
an island chain varying greatly in age.
 Hedin et al. found differing patterns of
nutrient distribution across the
chronosequence.
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Recovery of Nutrient Retention
Following Disturbance
Bormann and Likens found felling trees in
Hubbard Brook substantially increased
nutrient losses.
 Herbicide used to suppress regrowth.
 When application stopped, succession
proceeded, nutrient losses decreased,
and primary production increased.
 Vegetation can’t account for all nutrient
loss.
 Reduced nutrient amounts in ecosystem.
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Recovery of Nutrient Retention
Following Disturbance
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Model of Ecosystem Recovery
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Biomass Accumulation Model
 Reorganization (10-20 yrs)
 Forest loses biomass and nutrients.
 Aggradation (100+ yrs)
 Ecosystem reaches peak biomass.
 Transition
 Biomass declines from peak.
 Steady-State
 Biomass fluctuates around mean.
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Mechanisms of Succession
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Clements
 Facilitation
Connell and Slayter
 Facilitation
 Tolerance
 Inhibition
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Mechanisms of Succession
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Facilitation
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Proposes many species may attempt to
colonize newly available space.
 Only certain species will establish.
 Colonizers “Pioneer Species” modify
environment so it becomes less suitable
for themselves and more suitable for
species of later successional stages.
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Tolerance
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Initial stages of colonization are not limited to
pioneer species.
 Early successional species do not
facilitate later successional species.
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Inhibition
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Early occupants of an area modify the
environment in a way that makes it less
suitable for both early and late successional
species.
 Early arrivals inhibit colonization by later
arrivals.
 Assures late successional species
dominate an area because they live a long
time and resist damage by physical and
biological factors.
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Successional Mechanisms in
Rocky Intertidal Zone
Sousa investigated mechanisms behind
succession of algae and barnacles in
intertidal boulder fields.
 If the inhibition model is in effect, early
successional species should be more
vulnerable to mortality.
 Results showed early successional
species had lowest survivorship and
were more vulnerable to herbivores.
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Community and Ecosystem Stability
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Stability: Absence of change.
Resistance: Ability to maintain structure and
function in face of potential disturbance.
Resilience: Ability to recover from
disturbance.
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Park Grass Experiment
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Hertfordshire, England
 Studied effects of fertilizer treatments.
 Continued for 150 years.
Silverton investigated ecosystem stability.
 Used community composition variability as
measure of stability.
 Represented composition as proportion of
community consisting of each plant form.
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Park Grass Experiment
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Dodd showed that although community
stability is present, populations of individual
species can change substantially.
 Stability depends on resolution an area is
investigated at.
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Desert Stream Stability
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Valett studied interaction between surface
and subsurface waters.
 Flash floods devastated biotic community.
 Spatial relationships of zones stable.
 Geomorphology of landscape.
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Review
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Introduction
Community Changes During Succession
 Primary Succession
 Secondary Succession
Ecosystem Changes During Succession
 Disturbance
 Ecosystem Recovery
Mechanisms of Succession
Community and Ecosystem Stability
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