Notes on Ecological Succession

Ecological Succession Overview

  • Definition: Ecological succession refers to the process where community composition changes over time due to disturbances or environmental changes.

Types of Succession

  • Primary Succession:

    • Occurs in environments where there is no life or organic soil.

    • Example: After a volcanic eruption or glacier retreat (e.g., bare rock starts to develop life).

    • Layers of progression:

    • Pioneer Stage:

      • Often includes lichens and mosses, which can colonize bare rock.

    • Intermediate Stage

      • Grasses, shrubs, shade-intolerant trees

    • Climax community

      • Shade-tolerant trees

    • Example timeline:

      • Year 0: Bare rock

      • Year 5: Lichens and small annual plants

      • Year 100: Shrubs and pioneer species

      • Century-long development towards a climax community.

  • Secondary Succession:

    • Happens after a disturbance that does not destroy the soil.

    • Example: Abandoned agricultural fields recover after disturbances like fire, pollution, or human activity.

    • Typical progression:

    • Year 1: Remnants of crops

    • Year 2: Wild grasses establish

    • Year 5: Mature grasses, sedges appear

    • Year 20: Woody shrubs and young trees (juniper, birch, maple).

Factors Influencing Succession

  • Biodiversity and Succession:

    • Biodiversity tends to increase as communities progress through successional stages.

    • Early succession has low diversity, while later stages may see a stabilization or slight decrease in diversity.

  • Traits of Species:

    Early Successional Species Traits:

    • Good dispersal ability, can thrive in high-light and low-water environments, short-lived.

    • r-strategists

    • Light seeds, good at dispersing to newly disturbed habitat

    • Examples: Crabgrass, wildflowers

    Late Successional Species Traits:

    • Slower growth, longer lifespan, shade tolerance, requiring more complex biological interactions.

    • K-strategists

    • Heavy seeds: energy reserve, grow in shade

    • Examples: Oak and hickory trees.

Successional Mechanisms

  • Abiotic Factors:

    • Distance: early successional species need to disperse far enough

    • Tolerance: colonising species must tolerate the abiotic conditions

  • Biotic Interactions:

    • Facilitation: Species A enhances the probability that Species B will establish.

    • Inhibition: Species A reduces the chances of Species B establishing.

Case Studies

  • Glacier Bay Chronosequence:

    • Shows the progression from pioneer stage to a climax community.

    • Community composition changes with time indicated by different stages (e.g., Alder, Spruce).

  • Role of N-fixing Plants in soil formation:

    • N-fixing plants help create nutrient-rich soil, essential for later successional species to thrive.

    • Experiment showed significantly higher plant biomass in control plots with N-fixing species compared to those without.

Climax Communities

  • Definition and Characteristics:

    • Traditional climax communities refer to stable end points of succession which may not be truly stable.

    • Examples of dynamic climax communities: Fire-maintained ecosystems that require disturbance (like fire) to renew succession pathways.

Summary of Key Points

  • Ecological succession is a non-deterministic process that results in community changes over time influenced by environmental factors and species interactions.

  • The concept of a climax community is evolving, reflecting the understanding that ecological communities are dynamic and continuously changing rather than static end points.

  • Communities can adapt and respond to changes, and some ecosystems may not reach a 'final' stage but instead continue to evolve.