Chapter 20 - Succession Stability

Succession and Pioneer Community

Succession - Gradual change in plant/animal communities post-disturbance

-Primary Succession - Newly exposed geological substrates (not very common)

-Secondary Succession - Following disturbance that doesn’t destroy soil (ex; wildfires)

Pioneer Community - Plants/lichen colonize in the first few years after disturbance

Climax and Disclimax Community

Climax Community - late successional community that remains stable until disrupted by disturbance

Disclimax Community - maintained only under continual disturbance

Community Changes by Succession

Succession changes all aspects of community structure;

1. Richness

2. Dominance/evenness

3. Composition of species

4. Species diversity

-Changes are both predictable/unpredictable

Primary Succession at Glacier Bay

As glaciers receded, number of plant species in the bay increased

Secondary Succession at Boreal Forest

Frequently disturbed (by humans and fires)

Killed plants → Increase in light penetration → woody recruitment/early colonizers → increase in nutrient availability → stimulate microbial activity

Recently Burned Differences

Pre-Fire - Variation in forest species composition

Post-Fire - regular pattern changes over time

-Moderate fire, no primary succession just hardwood regeneration

• Replacement by confiers/spruce budworm

Succession in Rocky Intertidal Communities

Boulders cleaned bare of organisms

-Wanted to examine what occurs with successions

Results: Number of species increased until 1-1.5 years after disturbance; begins to level off at 5 species

Succession in Stream Communities

98% of algal and invertebrates biomass lost, ecological succession occurred in < 2 months

-Flash floods remove the diversity, but diversity returned in 20 days

Shallow Lake Succession

Experience succession on 2 time scales;

1. Seasons or years; Disturbance

2. Geological time scales; Sedimentation

-Most lake eventually disappear

-Sedimentation - Drives successions

Paludification

Most common way of peatland formation

Ecosystem Changes During Succession

Succession Increases;

1. Biomass

2. Primary production

3. Respiration

4. Nutrient retention

-Time is also important: weathering of bedrock releases lots of nutrients (like phosphorus)

-Physical and biological systems therefore inseperable

Succession and Soil Change in Glacier Bay

Succession change the soil’s

1. Organics

2. Water

3. Phosphorus

4. Bulk Density

Hubbard Experimental Forest and Nutrient Retention

Clear logged forest

-Then used herbicides to slow down succession

Result: Significant lowering of nutrient retention in the forest

Nutrient Loss on Logged Peatlands

After logging, exposed peat does not recover over time, peat depth continued to be reduced

-Only a temporary spike in nutrients, then they returned back to normal

Successional Change Models

1. Clements

-Serial replacement of species that facilitate each other

-Climax community seen as a “super-organism”

2. Gleason

-Species distributed independently of each other

-Individualistic approach — outcome can be altered

Why Gleason More Accepted

1. Not always the climax community

2. Climax concept = general concept of succession

Communities are groups of individuals, not a “super-organism”

Egler

Clarified distinction between Clement and Gleason, provided validity to both

Two alternative ways of succession:

1. Relay Floristics - one wave of species replaced by the next until climax — little overlap

2. Initial Floristics - different species dominate at different time periods — great deal of overlap

Western Boreal Forest and Succession

Follows the initial floristics

-Explains the high amount of productivity (which attracts lots of bird species) —> Greater diversity

Connell and Slayter Mechanism of Succession

1. Facilitation

2. Tolerance

3. Inhibition

Facilitation

Many species attempt colonization, those with particular characteristics can do it

-Colonization of new sites done by Pioneer Species

1. Species modifies environment to be less suitable for them, but more suitable for later species

Climax Community = Replacement of early stages until no longer facilitate colonization

Tolerance

Initial Stage - not limited to a few pioneer species

-Juvenile of species dominant at climax can establish in earliest successional stages

-Early colonizers do not facilitate colonization

Climax Community = When list of tolerance species is exhausted

Inhibition

Any species can colonize area during early stages of succession

-Early occupants modify environment to make area less suitable for any other species

Climax Community - long lived species resistant to damage (physical or biological)

Successional Mechanism in Rocky Intertidal Zone

Evidence for inhibition of later successional species

-Ulva algae species inhibits the colonization by red algae

-Evidence for facilitation; colonization by intertidal plants allow for other species to grow

Primary Succession Following Deglaciation

In Glacier Bay Alaska, no single mechanism determines the pattern of succession

Community and Ecosystem Stability

Stability results from:

1. Resistance - ability to maintain structure/function

2. Resilience - ability to recover from disturbance

Time - long time required to understand the stability of an ecosystem

Scale - depends on how we view stability

a) Community life form - little change, very stable

b) Species level - lots of change/differences, unstable

Other Factors in Community Stability

Stable spatial structure in landscape;

E.g., Desert (bedrock) and streams (hydrology)