Ecological Hierarchy and Levels of Organization

The ecological hierarchy begins with the individual organism and builds up to interacting communities.

• Organism: The basic unit of study. Adapts to its environment behaviourally, morphologically, and physiologically.
• Population: A group of individuals of the same species living in a defined area. Key characteristics include birth rate, death rate, density, sex ratio, and age distribution.
• Population Growth Models:
  • Exponential Growth: $dN/dt = rN$ (where $r$ is the intrinsic rate of natural increase). Occurs when resources are unlimited, forming a J-shaped curve.
  • Logistic Growth: $dN/dt = rN(1 - N/K)$ (where $K$ is the carrying capacity). Occurs when resources are limited, forming a sigmoidal (S-shaped) curve.
• Biotic Interactions:
  • Mutualism ($+/+$): Both species benefit (e.g., Lichens: algae and fungi; Mycorrhizae: fungi and roots of higher plants).
  • Commensalism ($+/0$): One benefits, other unaffected (e.g., Orchid growing as an epiphyte on a mango branch; Barnacles on a whale).
  • Amensalism ($-/0$): One is harmed, other unaffected (e.g., Penicillium mold producing penicillin inhibiting bacteria; black walnut tree secreting juglone).
  • Parasitism/Predation ($+/-$): One benefits, other harmed.
  • Competition ($-/-$): Both species suffer due to shared limiting resources.

Ecosystems integrate communities with abiotic components, which scale up into regional biomes.

• Ecosystem: A functional unit consisting of a biotic community and its abiotic environment interacting to exchange energy and materials. Core features include energy flow (uni-directional), nutrient cycling (cyclic), and productivity.
• Biome: A large, regional ecological unit characterized by a dominant type of vegetation and associated climate (temperature and precipitation).
• Major Terrestrial Biomes:
  • Tundra: Cold, treeless, permafrost layer, low precipitation.
  • Taiga (Boreal Forest): Coniferous forests, cold winters, moderate rainfall.
  • Temperate Deciduous Forest: Broad-leaved deciduous trees, distinct four seasons, moderate precipitation.
  • Tropical Rainforest: High temperature, year-round rainfall, highest biodiversity, layered canopy.
  • Savanna: Tropical grassland with scattered trees, distinct wet and dry seasons.
  • Desert: Arid, extreme temperatures, specialized xerophytic adaptations (sunken stomata, thick waxy cuticle).

Biome Distribution Chart:

The biosphere represents the global ecological system where limiting factors govern the distribution of life.

• Biosphere: The thin global zone of life integrating all living beings and their physical environment (lithosphere, hydrosphere, and atmosphere).
• Limiting Factor Laws:
  • Liebig's Law of the Minimum (1840): Formulated by Justus von Liebig. It states that yield/growth is limited not by the total nutrients available, but by the essential nutrient in the shortest supply (limiting factor).
  • Shelford's Law of Tolerance (1913): Formulated by Victor Shelford. It states that an organism's distribution is controlled by the factor for which the organism has the narrowest range of tolerance (both minimum deficiencies and maximum excesses can be limiting).
• Eury- vs Steno- organisms:
  • Eurythermal / Euryhaline: Organisms tolerant of a wide range of temperatures/salinities.
  • Stenothermal / Stenohaline: Organisms restricted to a narrow range of temperatures/salinities.