AP Biology: Energy Flow and Trophic Levels – Food Chains, Food Webs, 10% Rule

Energy Flow and Trophic Levels – Food Chains, Food Webs, 10% Rule

Concept Summary

• Trophic level: Position an organism occupies in a food chain, defined by its energy source (producer, primary consumer, etc.); establishes energy flow direction.
• Food chain: Linear sequence of feeding relationships showing energy transfer from producers to top predators; oversimplifies real ecosystems.
• Food web: Interconnected food chains depicting multiple feeding relationships; highlights biodiversity and ecosystem stability.
• 10% Rule: Only ~10% of energy at one trophic level is transferred to the next (90% lost as heat, waste, or metabolic work); explains pyramid shape and limits trophic levels.
• Ecological efficiency: Ratio of energy passed to the next trophic level vs. energy received; typically 5–20%, with 10% as a general benchmark.

Core Questions

WHAT (definitional)

Q: What is a producer? A: Autotroph (e.g., plants, algae) that converts sunlight or inorganic chemicals into organic molecules via photosynthesis/chemosynthesis. Trap/Clarification: Producers are not always photosynthetic (e.g., deep-sea chemosynthetic bacteria).

Q: What is gross primary productivity (GPP) vs. net primary productivity (NPP)? A: GPP = total energy fixed by producers; NPP = GPP minus energy used for respiration (available to consumers). Trap/Clarification: NPP is what matters for food chains—GPP includes "self-maintenance" energy.

WHY (causal/explanatory)

Q: Why does the 10% Rule limit the number of trophic levels? A: Energy loss at each transfer (90%) leaves insufficient energy to support higher-level consumers (e.g., apex predators). Trap/Clarification: The rule is an average—actual efficiency varies (e.g., ectotherms transfer more energy than endotherms).

Q: Why are food webs more stable than food chains? A: Redundant feeding pathways buffer against species loss; loss of one prey species has less impact. Trap/Clarification: Stability-complexity—overly complex webs can collapse if keystone species are removed.

HOW (process/application)

Q: How do you calculate energy available at the next trophic level? A: Multiply current level’s energy by 0.10 (10% Rule) or use actual ecological efficiency (e.g., 0.15 for ectotherms). Trap/Clarification: Energy biomass-energy availability—digestibility and assimilation matter (e.g., cellulose is hard to extract).

Q: How do you identify a keystone species in a food web? A: Look for species whose removal disproportionately alters community structure (e.g., sea otters controlling urchin populations). Trap/Clarification: Keystone-dominant (e.g., wolves are keystone but not the most abundant).

CAN (conditions/possibilities)

Q: Can energy flow "backward" in a food chain? A: No—energy flows unidirectionally (producers-consumers-decomposers); nutrients cycle, but energy dissipates as heat. Trap/Clarification: Detritivores/decomposers recycle nutrients, not energy.

Q: Under what conditions can a trophic level have more biomass than the level below it? A: In aquatic ecosystems (e.g., phytoplankton-zooplankton), where producers have high turnover rates (short lifespans, rapid reproduction). Trap/Clarification: This inverts the biomass pyramid but not the energy pyramid (energy still flows upward).

Quick Facts & Traps

• Fact: Primary consumers (herbivores) directly depend on NPP; secondary/tertiary consumers rely on energy passed through multiple transfers.
• Trap: "All energy is lost as heat."-Reality: Some is lost as waste (feces/urine) or used for growth/reproduction (not heat).
• Fact: Detritivores/decomposers (e.g., fungi, bacteria) process ~80–90% of NPP in some ecosystems, closing nutrient cycles.
• Trap: "Food chains are realistic."-Reality: They’re oversimplified; most organisms eat multiple prey (food webs dominate).
• Fact: Endotherms (birds/mammals) have lower ecological efficiency (~1–5%) than ectotherms (~10–20%) due to high metabolic costs.
• Trap: "More trophic levels = more energy."-Reality: More levels = less energy at the top (pyramid shape).

Rapid-Fire True/False

• Statement: "The 10% Rule means 10% of an organism’s energy is stored in its biomass." Answer: FALSE Why the common mistake happens: Confuses energy transfer (between trophic levels) with energy storage (within an organism).

• Statement: "A food web with 10 species is always more stable than one with 5." Answer: FALSE Why the common mistake happens: Assumes complexity alone ensures stability; ignores species interactions (e.g., competition, predation intensity).

• Statement: "Producers always have the highest biomass in an ecosystem." Answer: FALSE Why the common mistake happens: Overlooks inverted biomass pyramids in aquatic systems (e.g., phytoplankton < zooplankton).