BM

Succession and Disturbance

Change Over Time: Succession and Disturbance

Definition of Succession

  • Succession: Change in the species composition in communities over time due to both biotic (living) and abiotic (nonliving) factors.

    • Examples of abiotic factors: climate, soil, nutrients, water, etc.

  • Succession is often described as directional, implying a beginning and an end, but is more complex than a simple linear path.

Types of Succession

  • The major types of succession are Primary Succession and Secondary Succession.

Primary Succession

  • Occurs when bare land is being colonized for the first time by living organisms.
    • Slow development of biotic community.
    • Examples:
    • Bare rock (e.g., after a volcanic eruption, such as Mount Saint Helens in 1980)
    • Shifting sand dunes
    • Land exposed by receding glaciers

Secondary Succession

  • Happens in an area where life previously existed but has been disrupted by a catastrophic event, yet the soil remains intact.
    • Faster development of biotic community due to existing soil bioactivity.
    • Examples:
    • Abandoned farmland
    • Forests affected by fire or timber harvesting
    • Areas damaged by severe weather, such as hurricanes

Historical Perspectives on Succession

  • Two primary hypotheses from early 1900s:

1. Superorganism Hypothesis (Frederick Clements, 1916)

  • Plants function as a group striving for a predetermined end (climax community).
    • View: Succession is linear and directionally leads to a climax community, which is stable and resilient to disturbances.
    • Influences natural area management today.

2. Individualist Hypothesis (Henry Gleason, 1917)

  • Communities are products of unique individual species responding to fluctuating environmental conditions.
    • View: No predetermined endpoint; community composition changes gradually and unpredictably.

Key Concepts of Succession

  • Succession is not a simple path like a food chain; instead, it resembles a food web due to various interactions.
  • Early stages of succession often include pioneer species (e.g., lichens and mosses) that subsequently allow larger species to dominate.

Stages of Succession

  • Pioneer Stage: Early colonization by hardy species.
  • Intermediate Stages: Development of various vegetation typically occurs over years (5, 10, 20, 30-50 years).
  • Climax Community: The final stable community that may adjust slightly but is generally stable unless disturbed.

Progression Example (Old Field Succession)

  1. 1 Year After Abandonment: Grasses and weeds (pioneer stage).
  2. 5 Years: Intermediate stage with perennial weeds.
  3. 10 Years: Tree saplings become evident.
  4. 20 Years: Herbaceous plants dominated by trees and shrubs.
  5. 30-50 Years: Dense saplings dominate the canopy; increased shade affects forest floor species.
  6. 100 Years: Arrival of climax community; late-stage species join that are stable but dynamic.

Pioneer Species Characteristics

  • Adapted to harsh conditions.
  • Reproductive traits: High seed production, wind dispersal, rapid life cycle, often reproduce asexually.
  • Examples of Pioneer Species:
    • Lichens: Form symbiotic relationships, colonize bare rock.
    • Dryas: A nitrogen-fixing pioneer species.
    • Fireweed: Known for high adaptability and vegetative reproduction through rhizomes.

Mechanisms of Succession

  • Facilitation: Early species make habitat more suitable for later colonists (e.g., nitrogen fixation).
  • Inhibition: Early settlers create conditions unfavorable for later species, often through shading.
  • Tolerance: Early species tolerate conditions for late colonists without altering suitability significantly.

Classic Examples of Succession

Sand Dune Succession

  • Observations by Henry Cowells (late 1800s):
    • Change in dune ecology occurs rapidly, influencing plant colonization.
    • Successional trajectory from grasses to shrubs to forests observed through distance from shorelines.

Glacier Retreat Succession

  • Permanent Plots: Used to observe changes in species richness with distance from glacial edges.
  • Pioneer Stage: Dominated by fireweed, followed by Dryas, Alder, and ultimately Spruce forests over decades.

Agents of Change

  • Disturbances: Events that create opportunities for species to replace one another.
  • Types of disturbances can be abiotic (e.g., volcanic eruptions, hurricanes) or biotic (e.g., grazing, diseases).
  • Intermediate Disturbance Hypothesis: Suggests that species diversity peaks at intermediate levels of disturbance where natural renewal can occur.

Impacts of Human Activity

  • Humans alter disturbance frequency and intensity through actions such as farming and fire suppression, creating ecosystems that may be more vulnerable to catastrophic events.