Ecosystem Ecology: Energy Flow and Nutrient Cycling

Fundamentals of Ecosystems

  • Definition: An ecosystem consists of a biotic community of organisms and the abiotic environment that affects it.
  • Organization Hierarchy:   - Biosphere: Global interaction of ecosystems.   - Community: Different species living in the same area.   - Population: Organisms of the same species in the same area; this is the level where evolution occurs.   - Organism: The individual level upon which natural selection acts.
  • Ecosystem Dynamics:   - Energy Flow: Unidirectional movement from solar energy to chemical energy; dissipated as heat and cannot be recycled.   - Chemical Cycling: Materials cycle continuously between biotic and abiotic components.

Trophic Levels and Food Webs

  • Primary Producers (Autotrophs): Synthesize food from inorganic sources (sunlight or chemicals); form the base of the ecosystem.
  • Consumers (Heterotrophs):   - Primary Consumers: Herbivores.   - Secondary Consumers: Carnivores that eat herbivores.   - Tertiary Consumers: Secondary carnivores.
  • Decomposer Food Chain: Involves primary decomposers (bacteria, archaea, fungi) feeding on dead organic matter.
  • Food Web Structure: Energy flow follows the direction from the consumed organism to the consumer. Chain lengths are typically short, usually less than 6 levels.

Energy Productivity and Transfer Efficiency

  • Primary Productivity Formulas:   - Gross Primary Productivity (GPP): Total carbon fixed during photosynthesis.   - Net Primary Productivity (NPP): Energy available to consumers after plant respiration (RR).   - Equation: NPP=GPP−RNPP = GPP - R
  • Efficiency Metrics:   - Solar Capture: Only approximately 0.8%0.8\% of total solar energy (1,254,000 kcal/m2/year1,254,000\,kcal/m^2/year) is captured as GPP.   - Production Efficiency: Percentage of assimilated energy incorporated into new biomass. High in invertebrates (10%−40%10\%-40\%), lower in ectotherms (fish ~10%10\%), and lowest in endotherms (birds/mammals 1%−2%1\%-2\%).   - Trophic-Level Transfer Efficiency: Generally estimated at 10%10\%. Approximately 90%90\% of energy is lost between levels as heat or waste.

Global Patterns in Productivity

  • Terrestrial Patterns: Highest NPP is found in tropical wet forests, wetlands, and tropical seasonal forests.
  • Aquatic Patterns: Highest NPP per unit area occurs in coral reefs, algal beds, and estuaries. The open ocean has low NPP per unit area but contributes significantly to global total NPP due to its vast size.
  • Biosphere Composition: Primary producers account for 99.9%99.9\% of the biosphere by mass.

Ecological Regulation and Biomagnification

  • Top-Down Control: Occurs when a consumer limits a prey population.
  • Trophic Cascade: A phenomenon where changes in top-down control cause ripple effects multiple links away in the food web (e.g., wolf reintroduction in Yellowstone National Park affecting elk and aspen).
  • Biomagnification: The concentration of certain materials increases at higher trophic levels. Key examples include:   - DDT (e.g., 0.04 ppm0.04\,ppm in producers vs. 99 ppm99\,ppm in gulls).   - Mercury (accumulates as Methylmercury in top predators like shark and tuna).   - PCBs.

Questions & Discussion

  • How much biomass gets eaten dead versus alive? In a typical forest, 60%−65%60\%-65\% of biomass becomes dead wood/leaves, whereas in marine systems, 93%−95%93\%-95\% of algae is consumed dead.
  • Primary producers make up what percent of the biosphere? They comprise 99.9%99.9\% by mass.