Energy Flow, Productivity, and Ecological Pyramids

1. Energy Flow and the Laws of Thermodynamics

Energy flow is the movement of energy through an ecosystem, from sunlight (or chemical sources) through producers and consumers and ultimately out as heat. It follows two physical laws. The first law of thermodynamics (conservation of energy) says energy is neither created nor destroyed, only transformed — so the energy entering a system equals the energy stored plus the energy leaving. The second law of thermodynamics says that every energy transformation increases entropy, a measure of disorder or unusable energy, so some energy is always lost as low-quality heat. This is why energy must be continuously resupplied by the sun and cannot be recycled the way nutrients are.

2. Primary Production and Its Controls

Primary production is the amount of organic matter (chemical energy) made by autotrophs — mostly plants and algae — from inorganic materials. Gross primary production (GPP) is the total energy fixed by photosynthesis. Net primary production (NPP) is what remains after producers use some fixed energy in respiration (R), the metabolic "burning" of sugars to power life. NPP is the energy actually available to herbivores and decomposers. Terrestrial productivity (land plants) is typically limited by temperature, water, and nutrients — especially nutrient limitation, where the scarcity of nitrogen or phosphorus caps growth. Aquatic productivity (phytoplankton and algae) is often limited by light limitation (depth and turbidity) and by nutrients; when excess nutrients enter water, eutrophication — an algal bloom and subsequent oxygen depletion — can result.

3. Secondary Production and Ecological Efficiencies

Secondary production is the generation of biomass by heterotrophs (consumers and decomposers) from the food they eat. Only a fraction of what an animal eats becomes its own tissue. Assimilation efficiency is the proportion of ingested energy that is digested and absorbed rather than lost in feces. Production efficiency is the proportion of assimilated energy that becomes new biomass rather than being used in respiration. Trophic efficiency is the proportion of production at one trophic level that becomes production at the next — usually about 5–20%. The widely quoted ten-percent rule summarizes this as roughly 10% of energy passing from one level to the next.

How it works

1. Sunlight is captured by producers, converting inorganic carbon into GPP.
2. Producers respire part of that energy, leaving NPP.
3. Herbivores consume producers, absorbing only a fraction (assimilation efficiency).
4. Herbivores use most absorbed energy in respiration; the rest becomes secondary production.
5. Carnivores repeat the process, and decomposers process dead material at every step.
6. At each transfer, heat is lost to the environment, so available energy shrinks with each level.

Common confusions

Memory aids

Remember "Gross Reduces to Net" — GPP Reduces by R (respiration) to give NPP. The energy pyramid is a "shrinking paycheck": every level takes a cut, and the top gets the least.

Quick review

Topic Recap

• Energy flows one-way through ecosystems, degraded to heat at every step (laws of thermodynamics, entropy).
• GPP is total fixed energy; NPP = GPP − R is what remains for consumers.
• Primary production is limited by light and nutrients in water, and by climate and nutrients on land.
• Secondary production and the efficiencies (assimilation, production, trophic) determine how much energy reaches higher levels.
• Ecological pyramids (energy, biomass, numbers) visualize these losses; only the energy pyramid is always a true pyramid.

Knowledge Check

1. Write the equation relating NPP, GPP, and respiration, and define each term.
2. Why can energy not be recycled in an ecosystem the way nutrients can?
3. Name two factors that commonly limit aquatic productivity and two that limit terrestrial productivity.
4. A caterpillar eats 100 J of leaf, absorbs 40 J, and converts 10 J into its own biomass. What are its assimilation and production efficiencies?
5. Under what circumstance can a biomass pyramid be inverted?

Answers and Rationales

1. NPP = GPP − R, where GPP is total carbon fixed by photosynthesis, R is autotrophic respiration, and NPP is the energy remaining for consumers. This isolates what producers actually make available.
2. Because the second law of thermodynamics means each transformation loses energy as heat (increasing entropy), so energy must be continuously resupplied; only matter can be recycled through biogeochemical cycles.
3. Aquatic: light limitation and nutrient limitation. Terrestrial: temperature/water and nutrient limitation. Any two from each set are correct.
4. Assimilation efficiency = absorbed ÷ ingested = 40/100 = 40%. Production efficiency = biomass ÷ absorbed = 10/40 = 25%.
5. In aquatic systems where phytoplankton grow and turn over so rapidly that their standing biomass is smaller than the consumers above them, even though energy still decreases upward.