Community Dynamics and Ecological Succession

Historical Perspectives and Models of Succession

  • Historical context:
    • The debate regarding why plant succession occurs has spanned approximately 100 years, leading to the proposal of numerous ecological models.
    • The most widely accepted model attributes succession to species adaptations and life history patterns that dictate species interactions, including competitive dynamics and symbiotic relationships.

Autogenic vs. Allogenic Drivers of Succession

  • Mechanisms driving community structure and change:
    • Initial community composition at a given location is determined by two main factors:
    • Allogenic factors: Physical environmental variables external to the organisms, such as water availability, soil pH, climate, and external forces.
    • Surrounding community composition: The pool of surrounding organisms available for immigration into the newly available or disturbed site.
    • Autogenic change: Changes in the physical environment caused directly by the living organisms within the community itself.
    • Interplay leading to ecological succession:
    1. Organisms establish and modify their immediate physical environment through autogenic processes (such as light attenuation and soil organic matter accumulation).
    2. Environmental modifications alter selective pressures operating on the site.
    3. Early pioneer communities become poorly adapted to the modified conditions (e.g., light-demanding pioneer species cannot survive under the shade created by their own canopy).
    4. Succession proceeds as an orderly replacement of one community by another, driven by continuous immigration from surrounding areas and changing physical conditions.
    5. Communities eventually reach a relatively stable equilibrium termed the climax stage, which persists until a major ecological disturbance resets the site.

Changes in Species Diversity During Succession

  • Patterns of species richness over successional time:
    • Climax communities are not the most diverse; they typically display only modest species diversity.
    • Species diversity reaches its peak during intermediate successional stages when members of two adjacent successional communities overlap and coexist (a mix of growth types).
    • Empirical timeline example (Oak-pine forest succession in New York and Michigan models):
    • Year 0: Major ecological disturbance resets the ecosystem.
    • Years 1–6 (Herbaceous stage): Species richness rises rapidly, reaching a peak of approximately 30 species around years 4–6 during the transition phase where shrubs and tree saplings establish alongside herbaceous species.
    • Years 10–20 (Shrub stage): Species richness declines as dense shrub canopy shades out light-demanding herbaceous species.
    • Years 50–200+ (Tree stages to Climax community): Species richness levels off at a modest equilibrium (around 10–15 species).

Succession species diversity graph

Successional Dynamics of Heterotrophic Organisms

  • Succession in decomposer communities (Heterotrophic Succession):
    • Decomposition of decaying structural biomass, such as a fallen tree log, follows an orderly sequence of heterotrophic species over time.
    • Sequential degradation of wood tissue:
    • Phloem invasion: Phloem—the part of the bark between the woody vessel and outer corky periderm rich in sugars—is invaded first by specialized wood borers and bark insects.
    • Cellulose breakdown: Brown-rot fungi target and break down cellulose polymers.
    • Lignin breakdown: White-rot fungi synthesize a specialized battery of enzymes to decompose lignin. Lignin is the rigid polymer holding plant cell walls together, found in much higher concentrations in woody plants than in herbaceous plants.
    • Detailed organismal sequence on a decomposing log:
    • Sawflies lay eggs in rotting wood tissue.
    • Long-horned beetles bore entry holes under the bark.
    • Downy woodpeckers excavate holes while searching for wood-boring insects.
    • Bark beetles engrave galleries into the wood.
    • Carpenter moth caterpillars burrow deeply into sapwood.
    • Termites infest moist, rotting wood.
    • Black carpenter ants create structural galleries.
    • Dry-rot fungi continue internal decay.
    • Plant seedling roots penetrate softening log remains.
    • Soil fungi and bacteria complete wood decomposition, returning essential minerals to the soil.

Heterotrophic succession in rotting log

  • Successional changes in animal fauna:
    • Faunal shifts across successional stages occur due to two main drivers:
    • Changes in plant species composition, which fundamentally alters food availability and nutritional options.
    • Structural changes in vertical canopy vegetation, modifying nesting options, shelter, and microclimates.
    • Avian and mammalian sequence along plant structural stages:
    • Grass stage: Inhabited by grasshopper sparrow, meadowlark, and meadow mouse.
    • Low shrub stage: Inhabited by field sparrow and cottontail rabbit.
    • High shrub stage: Colonized by song sparrow, towhee, purple finch, and white-tailed deer.
    • Shrubs and trees transition stage: Occupied by ruffed grouse, robin, junco, short-tailed shrew, and Nashville warbler.
    • Low trees stage: Inhabited by red squirrel, black-throated green warbler, and red fox.
    • High trees stage: Dominated by white-footed mouse, veery, and high-canopy bird species.

Changes in fauna during succession

Fire Ecology and Plant Adaptations

  • Ecological role of wildfire:
    • Wildfire acts as a critical natural disturbance mechanism (ignited naturally by lightning strikes or through human activity).
    • Fire restarts ecological succession, removes dense canopy cover, reduces competition, and opens habitat for grazing animals (such as deer and buffalo/bison).
  • Specific plant adaptations to fire regimes:
    • Fire-resistant bark: Insulation that protects vascular cambium from lethal heat damage.
    • Root resprouting: Ability to regenerate aerial shoots from underground root reserves post-fire.
    • Serotinous cones: Cones sealed with heat-sensitive resin that remain tightly closed until high temperatures melt the seal, releasing seeds onto fresh, nutrient-rich ash beds (e.g., Jack pine - Pinus banksiana).
    • Soil seed banks: Long-lived seeds stored dormant in the soil for years or decades until fire removes competitors and opens the canopy, permitting germination in full sunlight (e.g., Fireweed - Chamaenerion angustifolium).
    • Growth habit changes: Ontogenetic modifications in growth form designed to withstand ground fires:
    • Pinus palustris (Longleaf pine) remains in a dense, fire-resistant ground "grass-stage" for several years to build a thick root system while protecting its main bud. Following a fire event that clears competing vegetation, it executes rapid stem elongation ("fire-induced growth") to lift its growing tip above fire height.

Longleaf pine growth habit

Case Studies in Fire-Dependent Ecosystems

  • Kirtland's Warbler (Setophaga kirtlandii):
    • An endemic bird species (found in only one small geographic region) native to Michigan and listed on the Endangered Species List.
    • Strict habitat specificity: Kirtland's warbler nests exclusively on the ground under dense, young stands of Jack pine (Pinus banksiana).
    • Because Jack pine requires heat from fire to open its serotinous cones and regenerate, Kirtland's warbler is obligately dependent on periodic wildfires.

Jack pine serotinous cones and Kirtland's warbler

  • Kitty Todd Nature Preserve (Ohio):
    • Located in Ohio; characterized by nutrient-poor, highly sandy soils with sufficient rainfall to support oak trees (Quercus spp.), but insufficient water retention to establish a closed canopy forest.
    • Prescribed burns (intentionally set fires, typically conducted in the spring) are a vital ecological management tool used to maintain open savanna conditions and prevent succession to closed canopy forest.

Review and Practice Questions

  • Practice Question 1: In Michigan, species diversity is highest:
    • A. immediately after an ecological disturbance
    • B. during the earliest stages of pioneer community formation
    • C. during the latter stages of the herbaceous plant community formation when shrubs and tree saplings start to become established.
    • D. during the middle of the formation of the tree sere
    • E. when the community reaches climax stage.
    • Correct Choice: C — Species richness peaks when late herbaceous species overlap with newly establishing shrub and tree saplings prior to canopy closure.
  • Practice Question 2: Wildfires — Which of the following statements can be true?
    • a) Wildfire are always bad. As a result, Smokey the Bear says, "Only you can prevent forest fires".
    • b) Some communities are dependent on fire.
    • c) Fire is used as a management tool by the Michigan Department of Natural Resources (DNR).
    • d) Wildfire can be beneficial to some animal communities.
    • Correct Choice: Statements b, c, and d are true.