Energy Flow in Ecosystems

I. Introduction to Ecosystems

• An ecosystem encompasses all species occurring together (the community) and the abiotic environment (non-living factors like water, substrate, nitrogen, carbon).

  • These components interact dynamically, forming a complex web where matter cycles and energy flows.

II. Energy Flow in Ecosystems

• Primary Energy Source: The ultimate source of energy for most ecosystems is the sun.

• Producers (Autotrophs): Organisms, primarily plants, algae, and some bacteria, that convert light energy (or chemical energy) into chemical energy through photosynthesis (or chemosynthesis).

  • Photosynthesis equation: $6CO<em>2 + 6H</em>2O + \text{Light Energy} \rightarrow C<em>6H</em>{12}O<em>6 + 6O</em>2$

  • They form the base of the food web, creating organic matter from inorganic compounds.

• Consumers (Heterotrophs): Organisms that obtain energy by consuming other organisms.

  • Primary Consumers (Herbivores): Feed directly on producers (e.g., deer, rabbits).

  • Secondary Consumers (Carnivores/Omnivores): Feed on primary consumers (e.g., wolves, humans).

  • Tertiary Consumers (Top Carnivores/Omnivores): Feed on secondary consumers (e.g., eagles, sharks).

• Decomposers (Detritivores): Organisms (e.g., bacteria, fungi) that break down dead organic matter (detritus) from all trophic levels, recycling nutrients back into the ecosystem.

III. Trophic Levels and Energy Transfer

• Trophic Level: Each step in a food chain or food web, representing the feeding position of an organism.

  • Energy flows unidirectionally through trophic levels, from producers to various levels of consumers.

• Energy Transfer Efficiency: Only a small fraction of energy is transferred from one trophic level to the next.

  • The 10% Rule: Approximately $10\%$ of the energy from one trophic level is incorporated into the biomass of the next trophic level. The remaining $90\%$ is lost as metabolic heat, or used for life processes, or not consumed.

  • This inefficiency limits the number of trophic levels in an ecosystem.

IV. Ecosystem Productivity

• Primary Productivity: The rate at which producers convert energy into organic compounds.

  • Gross Primary Production (GPP): The total amount of energy captured by producers via photosynthesis (or chemosynthesis) over a given period.

  • Net Primary Production (NPP): The energy remaining after producers account for their own respiration ($R_p$). This is the energy available to consumers.

    • Formula: $NPP = GPP - R_p$

  • NPP is a critical measure for understanding the amount of energy available to support the rest of the food web in an ecosystem.