Ecological Succession

Core Definition

  • Ecological succession: gradual, directional change in species composition in an ecosystem over time.

  • Driven by interactions between organisms (biotic factors) and environment (abiotic factors).

  • Leads toward a relatively stable climax community.

Pioneer Species

  • First organisms to colonize a new or disturbed area.

  • Characteristics:

    • High tolerance to harsh conditions

    • Rapid reproduction

  • Examples: lichens, mosses, grasses

  • Function:

    • Break down rock

    • Build soil

    • Enable other species to establish

Types of Succession

1. Primary Succession

  • Occurs in areas with no soil.

  • Examples: volcanic rock, glacial retreat.

  • Sequence:

    1. Bare rock

    2. Pioneer species (lichens, mosses)

    3. Soil formation

    4. Grasses → shrubs → trees

  • Very slow process (hundreds to thousands of years).

2. Secondary Succession

  • Occurs in areas where soil already exists.

  • Examples: after fire, flood, deforestation.

  • Sequence:

    1. Surviving seeds/roots

    2. Grasses and small plants

    3. Shrubs

    4. Trees

  • Faster than primary succession (years to decades).

Primary vs Secondary

Feature

Primary Succession

Secondary Succession

Soil

Absent

Present

Starting point

Bare surface

Disturbed ecosystem

Pioneer species

Lichens/mosses

Grasses

Speed

Slow

Faster

Models of Succession

  • Facilitation: early species improve conditions for later ones.

  • Inhibition: early species prevent others until removed.

  • Tolerance: later species outcompete early ones without help.

Community Changes Over Time

  • Early stages:

    • Small, fast-growing species (r-selected)

    • Low biodiversity

  • Late stages:

    • Larger, competitive species (K-selected)

    • High biodiversity and stability

Factors Affecting Succession

  • Climate (temperature, rainfall)

  • Soil type and nutrients

  • Light availability

  • Disturbances (fires, floods)

  • Human activity

  • Invasive species

Invasive Species

  • Can alter or stop normal succession.

  • May dominate and reduce biodiversity.

  • Example: invasive shrubs replacing native plants.

Biomes and Climax Communities

Biome Definition

  • Large region defined by climate + dominant vegetation.

  • Climax community reflects long-term environmental conditions.

Major Biomes (Key Traits)

Temperate Deciduous Forest

  • Trees lose leaves seasonally

  • Oak, maple, hickory dominate

  • Animals: deer, bears, birds

Grasslands (Prairies)

  • Dominated by grasses

  • Few trees (low rainfall, strong winds)

  • Rich soil → agriculture

Desert

  • Very low water availability

  • Plants store water (cacti)

  • Animals adapted to heat (nocturnal, burrowing)

Chaparral (Scrubland)

  • Shrubs, fire-adapted plants

  • Fires common and necessary for regeneration

Taiga (Boreal Forest)

  • Cold climate

  • Evergreen conifers

  • Short growing season

Microclimates

  • Small areas with different climate conditions than surroundings.

  • Example: forest canopy vs forest floor.

Aquatic Succession

  • Occurs in lakes, rivers, oceans.

  • Example:

    • Open water → sediment buildup → marsh → land

  • Influenced by:

    • Pollution

    • Overfishing

    • Algal blooms

Key AP Biology Points

  • Succession is predictable but variable.

  • Primary = no soil, Secondary = soil present.

  • Pioneer species start ecosystem formation.

  • Climax communities are stable but not permanent.

  • Disturbance is a normal and necessary ecological force.

  • Human impact can permanently alter succession pathways.