Community Change & Stability

Succession: Change in Species Composition

• Succession refers to the change in the species composition of communities over ecological time, typically ranging from 1 to 500 years.
• Examples of species involved in succession include lichens, mosses, grasses, herbs, shrubs, tree seedlings, aspen, black spruce, jack pine, paper birch, balsam fir, and white spruce.
• The process starts with a pioneer community and progresses toward a climax community over time.

Two Types of Succession

• Primary Succession: Succession that occurs on land not previously occupied by vegetation. Example: recent volcanic deposits.
• Secondary Succession: Succession that occurs following a disturbance of vegetated land. Example: regrowth of a forest after logging.

Stages of Succession

• Sere: A recognized successional sequence.
• Seral Stage: A particular phase (community) within a sere.
• Pioneer Stage (Community): The earliest seral stage.
• Climax: The final seral stage, representing the endpoint of succession.
• Disclimax: A successional endpoint maintained by disturbance, such as fire-maintained pine forests.
• Ecesis: Establishment phase.

Clementsian View of Succession

• Nudation: The disturbance process that sets the stage for succession, including consideration of residuals (what remains after the disturbance).
• Migration: The dispersal process involving initial colonists with significant dispersal capabilities. Early succession often involves wind-dispersed seeds, while later stages involve animal-dispersed seeds.
• Ecesis: The establishment phase, where recruits must grow and reproduce under harsh and extreme initial conditions.
• Reaction: Facilitation by organisms that change the environment, favoring different suites of migrants and residuals.
• Competition: Includes all biotic interactions. Plant density increases, leading to greater resource competition and increased importance of other biotic interactions like herbivory and seed predation.
• Stabilization: The climax stage is reached.

Traditional View of Succession

• The traditional view of succession is linear, progressive, and predictable, with a single endpoint.
• Succession starts with new land (primary succession) and progresses through pioneer and seral stages to a climax stage.
• Disturbances at any stage can set back succession and initiate secondary succession.
• Micro-cyclic succession involves small, non-cumulative changes.

Disturbance: The Starting Point

• Disturbance is defined as an event that disrupts community structure and changes resource availability.
  • Disturbances can be natural or anthropogenic.
  • Disturbances vary in intensity, extent, and frequency, significantly influencing the speed and direction of succession.
• Examples of disturbances include:
  • Blast damage to conifer forests around Mt. St. Helens.
  • Fire damage in lodgepole pine forests in Yellowstone National Park.
  • Hurricane Katrina.
  • Modern agriculture as human-managed succession.

Studying Succession

• Plant ecologists study succession through:
  • Repeated sampling of the same site over time using permanent plots or repeat photographs, which is usually a very long process.
  • Analysis of chronosequences, studying communities of different ages and inferring successional changes.
• Example: Primary Succession at Glacier Bay, Alaska
  • Demonstrates the progression from a pioneer stage with fireweed to a Dryas stage, an Alder stage, and finally a Spruce stage over time (1760-1941).

Secondary Succession Example

• Forest Cleared in Southern Poland
  • Shows stages of regrowth after 7, 15, 30, 95, and 150 years.

Predictability of Succession

• Succession on Lake Michigan Sand Dunes:
  • Henry C. Cowles (1901) studied diagrammatic profiles across Indiana sand dunes.
  • Stages include:
    • Pioneer Stage: Dune grasses stabilize the nutrient-poor, sandy substrate.
    • Early Successional Tree Stage: Fast-growing, short-lived trees such as cottonwoods.
    • Mid-Successional Pine Forest.
    • Late-Successional Hardwood Forest: Oaks transition to Beech-Maple (climax).

Dependence on Initial Conditions

• The outcome of succession may depend on initial conditions.
• Initial conditions include:
  • Damp depressions, upper beach, eroding surfaces, depositing crests, and steep lee slopes.
• Physiographic processes include:
  • Foredune initiation and marram blowout initiation.
• Pioneer vegetation includes:
  • Rush meadow, cottonwood, sand reed grasses, and shrubs.
• Conifers (mostly temporary):
  • Jack and white pines, arbor vitae, and balsam fir.
• Climax dominants:
  • Tallgrass prairie, red maple swamp, black oak, white oak, red and white maples, sugar and red beech hemlock, and birches (paper and yellow).

Secondary Succession in North Carolina Piedmont

• Years after abandonment:
  • 0: Cropland.
  • 1: Crabgrass, horseweed.
  • 2: White aster, ragweed.
  • 5: Broomsedge.
  • 10: Broomsedge, pine seedlings.
  • 20: Young pines, Broomsedge.
  • 30: Shortleaf pine (drier sites) or Loblolly pine (moister sites).
  • 40: Shortleaf pine, hardwood understory or Loblolly pine, hardwood understory.
  • 60: White oak, post oak, hickory, dogwood, etc. or White oak, many hickories, dogwood, sourwood, etc.

Species Replacement

• Species and communities replace one another due to:
  • Changes in resource availability and abiotic environment during succession.

Resource and Abiotic Environment Changes

• Examples:
  • Percentage of soil moisture changes with dune age, $r^2 = 0.943$
  • Total soil C (Mg/ha) changes with dune age, $r^2 = 0.921$
  • Proportion of full sunlight decreases with dune age.
  • Total soil N (Mg/ha) changes with dune age, $r^2 = 0.906$

Adaptations

• Early and late successional plants are adapted to different light and soil environments.
• Differences in tolerances, resource requirements, and life-history strategies lead to species replacement.

Ecosystem Trends During Succession

Community Stability

• Stability refers to whether some communities are more stable than others when facing disturbance or stress.
• Stability can mean:
  • Resistance: The ability of a community to remain unchanged during stress.
  • Resilience: The ability of a community to return to normal following disturbance.

Vegetation Change in Southern Texas

• Pre-settlement (ca. 150 years ago): Open grassland/savanna.
• Today: Dense, thorn woodland dominated by Mesquite and shrubs due to overgrazing and lack of fire.

Development of Woody Plant Clusters

• Shows the development of woody plant clusters in southern Texas grasslands over time (10-15 years, 30-40 years, 50-60 years).
• Dominant species include Prosopis, Zanthoxylum, Celtis, Condalia, Opuntia, Diospyros, Ziziphus, and Schaefferia.

Succession Pattern

• Hypothesized pattern of succession from grassland to savanna to woodland involving cluster expansion and coalescence.

Conceptual Model

• Conceptual model of changes in community structure as a function of grazing pressure and fire.

Multiple Stable States

• Multiple stable states are possible, represented by gravitational potential.

Diversity and Stability

• The relationship of whether diverse communities are more stable and better able to resist disturbance.
• Plant biomass in prairie plots experiencing drought is related to plant species diversity (Dave Tilman, 1996).