Chapter 19 - Community Succession

Community Succession

  • Definition: Community succession refers to the process by which the species composition of a community changes over time, often following a disturbance

Clements vs. Gleason: Two Views of Community Dynamics

  • Frederick Clements

    • Advocated for the view that biological communities are stable and predictable.

    • Proposed that communities progress through a set of predictable stages toward a stable endpoint known as a climax community (closed community).

  • Henry Gleason

    • Suggested that community composition is not stable or predictable and can change based on chance after disturbances (open community).

Experimental Tests

  • A study on planktonic communities in identical ponds showed that communities do not develop identically despite similar habitats, suggesting that Gleason's view may be closer to reality:

    • Clementsian Hypothesis: Identical communities will always develop in identical environments.

    • Gleasonian Hypothesis: Identical communities will not develop identically.

  • Experiment with 12 identical ponds:

    • Setup: Ponds filled and sterilized simultaneously to eliminate preexisting organisms.

    • Observations after 1 year: All ponds contained different plankton species despite having a similar initial environment, leading to a conclusion that both hypotheses have merit.

Disturbance and Its Impact

  • Disturbance Definition: An event that removes individuals or biomass from a community and alters resource availability.

  • Types of Disturbances Include:

    • Fire

    • Deforestation

    • Floods

    • Windstorms

    • Disease epidemics

    • Herbivore outbreaks

Intermediate Disturbance Hypothesis

  • Proposed by J.P. Grime, Henry Horn, and Joseph Connell, suggesting that biodiversity peaks at intermediate levels of disturbance; too little disturbance leads to competitive exclusion by dominant species, while too much disturbance limits species diversity.

Types of Ecological Succession

  • Primary Succession: Occurs in lifeless areas where soil has not yet formed (e.g., after a volcanic eruption or glacial retreat).

    • Early colonizers such as mosses help create soil for later species.

  • Secondary Succession: Takes place in environments previously occupied by living organisms but where disturbances have removed them (e.g., after a forest fire or agricultural abandonment).

Observing Succession

  • Direct observation is the clearest method, exemplified by the volcanic eruption of Krakatau and the subsequent colonization.

  • Indirect observation can utilize chronosequences – sequences of communities over time in a specific location to study ecological changes.

Mechanisms of Succession

  • Facilitation: Early species make environmental conditions more favorable for later species.

    • Example: Alder shrubs enriching nitrogen levels in the soil.

  • Tolerance: Some species can establish themselves based on their ability to thrive under stress, independent of other species.

  • Inhibition: Early species can inhibit the establishment of later species through competition.

Changes in Climax Communities

  • Climax communities can change over time and with environmental conditions, not always reaching a static state.

  • Example: In northern deciduous forests, the species composition can shift from oak and hickory to sugar maple and beech as conditions evolve.

Transient Climax Communities

  • These communities are not persistent due to frequent disturbances preventing a stable climax community from forming.

Species Diversity Changes During Succession

  • Species richness typically follows a pattern of rapid increase after a disturbance, then plateaus, and can see a small decline as a climax community stabilizes.

Quantifying Community Similarity

  • Jaccard’s Index (J): Measures community similarity, ranging from 0 (no shared species) to 1 (identical species composition). It accounts for species only in one community (A, B) and those common to both (X).

By understanding the succession processes and community dynamics, ecologists can better manage ecosystems, predict future changes, and conserve biodiversity.