Change in Communities - PCB 3043

Formula:
   $RII = rac{(C - E)}{(C + E)}$
  - Where:
    - C = number or biomass of target individuals in presence of interactor
    - E = number or biomass of target individuals in absence of interactor
  - Scenario: Removing mussels (interactor species) to measure RII for gulls (target species).
  - Question: What is the expected RII?
    a) Positive
    b) Negative
    c) Zero
    d) Not enough information to answer
  - Data Provided:
    - With mussels:
      - Barnacles: 10
      - Gulls: 8
    - Without mussels:
      - Barnacles: 20
      - Gulls: 12
  - Calculated RII for gulls:
    - $RII_{gulls} = rac{(8 - 12)}{(8 + 12)} = rac{-4}{20} = -0.2$

Main Topics:
  - Agents of Change
  - Primary vs Secondary Succession
  - Models of Succession
  - Experimental Tests of Succession
  - Succession and Diversity

Key Concepts:
  - Communities are dynamic, experiencing both gradual and abrupt changes.
  - Agents of Change:
    - Abiotic Factors: e.g., waves
    - Biotic Factors: e.g., predation
  - Concepts of Disturbance and Stress:
    - Disturbance: An event (abiotic/biotic) that injures/kills individuals, creating opportunities for growth/reproduction.
    - Stress: A factor reducing growth/reproduction/survival, similarly creating opportunities for others.
  - Intensity and Frequency of Change:
    - Changes can be:
      - Intense but rare (e.g., volcanic eruptions) leading to significant change.
      - Mild but frequent (e.g., waves, predation) can promote diversity without major changes.
Succession Definition:
- The process through which species composition and community structure change over time due to abiotic and biotic agents.
- Succession often examines changes following disturbances.

Succession Defined:
  1. Primary Succession:
    - Involves colonization of lifeless habitats (no soil).
    - Extremely slow, as early species face harsh conditions and are crucial for transforming habitats.
  2. Secondary Succession:
    - Involves the reestablishment of communities where most organisms have been destroyed but some life remains.
  - Climax Stage: Stable endpoint community changes minimally until a disturbance resets the succession.

Primary Succession:
- Hawaiian Islands example showcasing stages from bare substrates to tropical forests over time.
Secondary Succession Illustrated with Crabgrass: Chronological progression from crabgrass to oak-hickory climax forests within specified years.
  - Timeline Overview:
    - Year 1: Crabgrass
    - Year 2: Crabgrass, horseweed
    - Year 3-25: Ragweed, heath aster
    - Year 25-100: Broomsedge, perennial flowers, shrubs, pines
    - Year 100-200: Remnant pines with young oak and hickory trees
    - Year 200+: Oak-hickory climax forest

Mt. St. Helens eruption provided a unique case for studying primary and secondary succession.
Notable features of the eruption included various zones including blowdown, scorch, and mudflow zones.
Plant Colonization Post-Eruption:
- Fireweed (Chamerion angustifolium) seeds dispersed post-eruption to colonize bare sites, crucial for nitrogen levels in nutrient-poor soils.
- The mutualistic relationship with nitrogen-fixing bacteria highly beneficial for new growth.

Clements vs. Gleason:
- Clements’ View: Succession is predictable and converges on a climax community, species being interdependent.
- Gleason’s View: Succession is less predictable and influenced by abiotic tolerances and space availability.

Three Conceptual Models of Succession:
  1. Facilitation Model: Early species support later colonization through environmental modifications.
  2. Tolerance Model: Establishment probability depends on dispersal ability and environmental persistence capabilities.
  3. Inhibition Model: Early species hinder the establishment of other species, requiring disturbance to promote change.

Example 1: Secondary succession in New England salt marshes, focusing on intertidal plant species.
Key Findings: Distichlis plays a dual role in facilitating or inhibiting colonization by Spartina or Juncus depending on salt stress.
Example 2: Primary succession in the rocky intertidal. Research by Wayne Sousa demonstrated effects of frequent disturbances on species colonization patterns, particularly with algal species.

Patterns of Species Richness: Typically rises quickly and may plateau or decline in later stages.
Diversity Complexity: Changes as evenness often declines in later stages, indicating shifts in species interactions and competition.

Predictability of Succession Paths: Suggests that while some patterns may be recognizable, alternate community development can occur under similar conditions.
Priority Effects: Early colonizers can influence future community structure significantly.
Alternative Stable States: Following disturbance, communities may resist change due to established structures.

Abiotic and biotic factors are significant agents of ecological change.
Succession represents the ongoing processes of change following disturbances.
Distinctions between primary and secondary succession are vital—primary is typically slower.
Key mechanisms during succession include facilitation, tolerance, and inhibition, with their importance potentially shifting over time.
Communities may follow varied successional paths and display alternative stable states.