Ecological Succession and Tolerance & Adaptation

Ecological Succession and Tolerance & Adaptation

Ecological Succession

• Definition: Ecological succession describes the series of expected changes that occur within a community over time.

• Drivers of Change: Communities are dynamic and change is inevitable due to:

  • Older members dying.

  • New organisms immigrating.

  • Sudden disturbances (e.g., forest fires).

• Example: A forest one year after a forest fire looks significantly different from the same forest two years after the fire, demonstrating rapid change.

Primary Succession

• Definition: Primary succession refers to the establishment of new communities in environments that initially had no living organisms.

• Initial Environments: These environments typically include:

  • Bare rock.

  • New surfaces formed by volcanic eruptions.

  • New surfaces created by the movements of glaciers (e.g., bare rock left after a glacier's retreat).

Pioneer Species

• Definition: In primary ecological succession, the first organisms to colonize an uninhabited environment are called pioneer species.

• Role of Lichens: Lichens are classic pioneer species. They are the first organisms to grow on bare rock (e.g., after a glacial retreat).

  • As lichens, followed by moss, grow and die, their decaying matter is added to the rock.

  • This decaying organic matter begins the process of soil formation.

Stages of Primary Succession

1. Soil Formation: Lichens and moss initiate soil formation by adding decaying organic matter to the bare rock.

2. Grasses and Small Plants: Nutrients from the decaying organic matter support the growth of grasses and small plants.

   • As these grasses and small plants die and decay, they add more nutrients, leading to deeper, more fertile soil.

3. Small Shrubs: The increasingly deeper and water-retaining soils allow small shrubs to colonize and grow.

4. Climax Community: Finally, with mature soil, larger trees can grow, leading to the formation of climax communities.

Climax Communities

• Definition: Climax communities are the organisms that remain stable in an ecosystem over time.

• Development: They develop on mature soils, representing the stable, mature stage of ecological succession.

Secondary Succession

• Definition: Secondary succession is the reestablishment of climax communities in an ecosystem that has been dramatically altered by disturbances.

• Disturbances: Causes of ecosystem alteration include:

  • Fires

  • Floods

  • Disease

  • Human activity (e.g., clearing ground to grow a single crop).

• Process: Unlike primary succession which starts from bare ground, secondary succession begins on existing soil where a community was previously present.

  • The ground is cleared of the existing community.

  • New species begin to grow.

  • Small shrubs colonize.

  • Eventually, a new climax community forms, often restoring the ecosystem to its original condition.

Tolerance & Adaptation

Tolerance

• Definition: Tolerance is the ability of an organism to withstand stress from abiotic factors (non-living components of an environment).

• Abiotic Factors Examples: Temperature, salinity, oxygen levels, pH, light intensity, etc.

• Tolerance Range: For each abiotic factor, an organism has a specific tolerance range, which encompasses the minimum and maximum levels of that factor it can withstand.

• Environmental Dependence: An organism's tolerance range is influenced by its environment and the degree of variability it experiences for each factor.

• Graphical Representation: A typical population curve along an abiotic factor's gradient shows a high population within an optimal range, decreasing towards zero at the Lower Limit of Tolerance and the Upper Limit of Tolerance.

Adaptation

• Evolutionary Basis: Organisms, both plants and animals, exhibit evolutionary adaptation, where their tolerance for an abiotic factor can change over generations through variability and natural selection. This concept is explored further in the Evolution unit.

Examples of Adaptation and Tolerance

• Temperate vs. Tropical Plants:

  • Temperate Zone Plants: Have a wider tolerance range for temperature because their environment experiences greater temperature variability. However, they are generally less efficient at any single given temperature.

  • Tropical Zone Plants: Have a narrower tolerance range for temperature as their environment is more stable in terms of temperature. They are better adapted and more efficient within this smaller range but cannot withstand significant temperature changes.

• Migration (Response to Varying Abiotic Factors):

  • Animals: If abiotic factors in a habitat vary over time beyond an organism's tolerance, animals may migrate to a more favorable habitat.

  • Plants: While individual plants cannot physically migrate, they