molles: ecology 2 nd ed. succession and stability chapter 20

Molles: Ecology 2nd Ed.

SUCCESSION AND STABILITY

Chapter 20


Chapter Concepts

• What is succession?

• What are the causes of succession?

• How do community composition / diversity, and ecosystem energy flow / nutrient cycling change during succession?

• How do ecologists know this stuff?

• What are the implications in a human – dominated world?


What does it take to keep this lawn What does it take to keep this lawn looking like this?looking like this?

• What will happen to this lawn over time with What will happen to this lawn over time with no human intervention?no human intervention?


Primary Succession

• Establishment of a biological community on new substrates where no community had previously existed.

• Examples On land exposed by retreating glaciers On new substrates created by volcanic

lava flows


Primary Succession – Glacier Bay, Alaska

• Reiners et.al. (1971) Changes in plant diversity during

succession


Glacier Bay Succession

Soil Development

Barerock &soil

LichensMosses

GrassesForbs

ShrubsAspen

Blackspruce

Whitespruce

Incr

easi

ng

Ver

tica

lS

tru

ctu

re


Coral Reef Succession On Lava Flows

Lava Flow From Land Into Sea



Underwater Lava Flow


Secondary Succession

• Re-building of a biological community after a previously existing community is destroyed by a disturbance, but the soil remains.

• Examples: Recovery of forests after fire, wind storm, insect

pest outbreak, logging Re-growth of natural vegetation after farmland

abandonment


Old Field Succession



1 Year After Abandonment

Grasses and Weeds Dominate



5 Years After Abandonment

Goldenrod and Other Perennial Weeds Dominate




Tree saplings and shrubs begin to establish


Old Field

Succession

20 Years AfterAbandonment

Trees and shrubsbegin to replaceherbaceous plantspecies




Where Did the Grasses and Goldenrod Go ?


Definitions

• Climax Community – Late successional community Remains stable until disrupted by

disturbance

Primary vs. Secondary Succession

• Primary succession slower than secondary succession.

With no pre-existing seeds or root systems in the soil, establishment of organisms requires migration into the area from other locations.

Establishment of many plant species is delayed until soil development has occurred (sand, silt, clay, organic matter)

Causes of Succession

• Species differ in dispersal (migration) ability.

• Species differ in their environmental tolerances / requirements.

• Species differ in growth rate.

• Species differ in life span.

• Species differ in competitive ability.

Pioneer (r-selected) Species

• Produce large numbers of small, easily dispersed young.

• Are usually first species to arrive after disturbance.

• Grow fast in high resource environment.

• Tolerate harsh physical environments.

• Reach sexual maturity fast.

• Short life spans (require replacement via reproduction to remain on the site)

• Poor competitive ability.

Late-Succession (K-selected) Species

• Produce fewer, larger young that often have limited dispersal from parent.

• Usually arrive at disturbed site later (may require pioneers moderate environment first).

• Have slower growth rate.

• Longer time to sexual maturity.

• Long-lived (hold onto space / resources)

• Good competitive ability (able to displace pioneer species)

Community and Ecosystem Changes During Succession

• Species Diversity increases (but may decrease during late stages due to competitive exclusion).

• Net Primary Production increases during early stages, but declines during late stages due to increased respiration.

• Nutrient Cycling / Retention greatest during middle stages.

Species Richness During Primary Succession at Glacier Bay Study Plots

Fig 20.2

Change of Plant Growth Forms During Succession

Fig 20.3


Woody Plant Species Richness During Secondary Forest Succession (Eastern USA)

Piedmont Plateau

Woody Plant Species Richness During Secondary Forest Succession (Eastern USA)

Fig 20.4

Breeding Bird Species Richness During Secondary Forest Succession (Eastern USA)

Fig 20.5

Species Richness of Macroinvertebrate and Macroalgae Species During Secondary Succession on Intertidal Boulders

Fig 20.7

Algal species Diversity During Secondary Succession in Sycamore Creek After Flooding

Fig 20.8


Fig 20.9

Fig 20.8

How Do We Study Succession ?(Long-Term Research)

• 1. Establish permanently marked plots in an area recently affected by disturbance.

• 2. Record community / ecosystem variables

Species relative abundance / Species diversity

Biomass / Net Primary Production

Nutrient Pools and Input / Output Fluxes


How Do We Study Succession ?(Long-Term Research)

• 3. Re-measure variables in exactly the same plots at different times (often intervals of decades).

• 4. Change in community / ecosystem variables over time attributed to succession.

How Do We Study Succession ?(Space-for-Time Comparative Studies)

• 1. Record community / ecosystem variables in plots established in different areas that were disturbed at different times in the past. Species relative abundance / Species diversity Biomass / Net Primary Production Nutrient Pools and Input / Output Fluxes


•2. Compare community / ecosystem variables between these different areas. Differences are attributed to succession.

•3. Valid only if the following conditions are true:Different areas were all disturbed in exactly the same wayDifferent areas have similar environmental conditions

How Do We Study Succession ?(Space-for-Time Comparative Studies)

How Do Disturbance Characteristics Influence Succession?

• Short Return Time (High Frequency) Limits opportunity for immigration of species

and re-growth of populations. K-selected species eliminated. Limits opportunity for re-building biomass. Nutrient losses due to disturbance not fully

replenished before next disturbance.• Community dominated by r-selected species, with low

standing crop biomass and low nutrient availability.


• Large Magnitude + High Intensity Re-establishment of species populations requires long-

distance migration from undisturbed areas. Succession is retarded by slow immigration rate. NPP recovers slowly Biomass pool recover slowly Nutrient losses greater during slow recovery

• Community dominated by r-selected species, with rapid dispersal ability for a longer period.


• Small Magnitude or Low Intensity Re-establishment of species populations from surviving

individuals within / near disturbed area. Succession proceeds quickly. NPP recovers rapidly. Biomass pool recover rapidly. Nutrient losses quickly stop.

• Community dominated by species that are best able survive disturbance and compete for limited growing space (K-selected).

Implications for Human Activities

• Human populations typically increase the frequency, magnitude, and intensity of disturbance.

Entire landscapes dominated by r-selected species b/c succession never finishes before next disturbance.

Depletion of biomass, soil organic matter, and nutrient pools.


• Humans sometimes suppress disturbances (e.g. fire), increasing return time, but often with increase in magnitude and intensity.

Entire landscapes become dominated by K-selected species, and accumulate biomass.

Communities may become more susceptible to catastrophic disturbance (e.g., recent fires in western U.S.)

Effects of Fire SuppressionEastern Hardwood Forest

0

5

10

15

20

25

30

35

1900 1910 1920 1930 1940 1950 1960 1970

Year of Tree Establishment

Nu

mb

er

of

Tre

es

Oak Maple Sassafras Dogwood

Fire Suppression Begins

▲Logging

Num

ber

of t

rees

Effects of Fire SuppressionWestern Conifer Forests

0

200

400

600

800

1000

1200

1400

1600

1980 1960 1940 1920 1900 1880 1860 1840 1820 1800 1780

Year of Establishment

Nu

mb

er

of

Tre

es

Ponderosa Pine White Pine Other

Frequent Low-Intensity Fires

▼▼▼▼▼▼▼▼

Fire SuppressionBegins

Fuel build-up leads to catastrophic crown fire

Num

ber

of t

rees


• Humans depend on rapid growing, r-selected species for agriculture and forestry.

Humans use disturbance to prevent succession and maintain open environments required by these pioneer species.

Frequent human disturbance to maintain crop species can deplete soil organic matter and nutrient pools.

The End

molles: ecology 2 nd ed. succession and stability chapter 20

Documents

old field succession

disturbance slide

succession fig

succession species diversity

sea slide

causes of succession

coral reef succession

farmland abandonment