Ecological Tolerence

1. Definition of Ecological Tolerance

According to Mr. Smedes' AP Environmental Science curriculum, ecological tolerance refers to the range of abiotic conditions—such as temperature, salinity, dissolved oxygen, sunlight, and pH—in which an organism can endure, survive, and reproduce.

  • Individual vs. Species Tolerance:

    • Individual organisms within a population possess slight variations in tolerance due to genetic diversity.

    • A species has an overall range of tolerance encompassing the combined tolerances of all its individuals.

    • High genetic biodiversity within a population provides resilience, increasing the probability that some individuals will survive environmental shifts.

2. The Ecological Tolerance Curve

When plotted along an abiotic gradient, population density follows a characteristic bell-shaped curve categorized into three distinct physiological zones:

  1. Optimal Range

    • The ideal set of environmental conditions where organisms exhibit peak health, growth, and reproductive success.

    • Population density and species fitness reach their highest levels.

    • Example: Salmon thriving in cool, well-oxygenated freshwater streams.

  2. Zone of Physiological Stress

    • Conditions deviate above or below the optimal range, forcing organisms to expend excess energy maintaining homeostasis.

    • Organisms survive, but fitness is impaired; symptoms include stunted growth, reduced reproduction, hormonal stress, and lethargy.

    • Example: Warmer stream temperatures decreasing dissolved oxygen, causing fish to struggle for oxygen and suppress spawning.

  3. Zone of Intolerance

    • Extreme conditions that exceed the organism's physiological limits.

    • Direct mortality occurs due to thermal shock, suffocation, desiccation, or enzyme denaturation.

    • Population density falls to zero.

    • Example: Extreme thermal pollution or severe hypoxia resulting in complete fish kills.

3. Key Environmental Factors and Stressors

  • Temperature: Directly influences metabolic rates and enzyme function. In aquatic biomes, elevated temperatures also reduce the solubility of dissolved oxygen.

  • Salinity: Governs cellular osmosis. Fluctuations from storm runoff, sea-level rise, or drought force estuarine and freshwater organisms to tolerate shifts in osmotic pressure.

  • pH: Disrupts ionic balances across gill membranes and alters soil nutrient availability. In marine settings, lower pH (ocean acidification) hinders calcifying organisms from building calcium carbonate structures.