Food Webs and Energy Transfer

Food Web Structure * Primary Producers: Autotrophs (e.g., photosynthetic organisms) form the base (Trophic Level 1). * Consumers (Heterotrophs): * Primary Consumers: Herbivores (Trophic Level 2). * Secondary Consumers: Carnivores eating herbivores (Trophic Level 3). * Tertiary Consumers: Carnivores eating other carnivores (Trophic Level 4). * Food webs show complex interconnections, defining an organism's trophic level by the highest level it feeds at. * Detritivores: Organisms that directly consume dead organic matter or waste. * Decomposers: (e.g., bacteria, fungi) Release enzymes to break down organic matter externally, returning nutrients to the environment. # Primary Productivity * Gross Primary Productivity (GPP): Total energy fixed by producers. * Net Primary Productivity (NPP): The biomass available to higher trophic levels; calculated as $NPP = GPP - R$ (where $R$ is energy used by producers for respiration/metabolism). * Factors influencing NPP: Temperature, precipitation, sunlight, nutrients, soil, wind. * The open ocean, despite lower productivity per unit area, contributes significantly to global NPP due to its vast expanse. # Energy Transfer in Ecosystems * First Law of Thermodynamics: Energy cannot be created or destroyed; total energy in an ecosystem remains constant, though its form changes. * Second Law of Thermodynamics: Disorder (entropy) increases naturally; energy transfers are not 100% efficient. * 10% Rule: On average, only about 10% of energy from one trophic level is incorporated into the next. The remaining energy is lost primarily as heat (respiration) or waste. * At each trophic level, energy taken in is allocated to: * Respiration (for organism's own metabolic processes). * Incorporation into new biomass (available to the next trophic level). * Waste (available to detritivores/decomposers). * The sum of these three components equals the energy initially taken in at that level. # Implications of Energy Transfer * Limited Trophic Levels: The rapid loss of energy restricts food chains to typically 3-5 trophic levels. * Population Size: The number of individuals decreases significantly at higher trophic levels due to insufficient energy to support large populations. * Biomass Pyramid: Explains why there's a large base of producers supporting smaller populations of consumers. * Dietary Efficiency: Eating lower on the food chain (e.g., eating plants) can support more individuals.