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Succession and Stability

Chapter 20

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Outline

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• Clements Facilitation

• Connell and Slayter Facilitation Tolerance Inhibition

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Mechanisms of Succession

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Facilitation

• 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

• Initial stages of colonization are not limited to pioneer species. Early successional species do not

facilitate later successional species.

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Inhibition

• 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

• 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

• 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

• 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

• 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

• Introduction• Community Changes During Succession

Primary Succession Secondary Succession

• Ecosystem Changes During Succession Disturbance Ecosystem Recovery

• Mechanisms of Succession• Community and Ecosystem Stability