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Energy flows unidirectionally through ecosystems: solar energy is captured by producers through photosynthesis (gross primary productivity, GPP), with net primary productivity (NPP = GPP − respiration) available to consumers. Ecological efficiency averages roughly 10% between trophic levels — about 90% of energy is lost as heat through respiration, excretion, and inefficient consumption. This explains why food chains are short (typically 3–5 levels) and why biomass pyramids are narrow at the top. Unlike nutrients, energy cannot be recycled; it must continuously enter ecosystems from solar input.
Calculate how much primary productivity is needed to support a top predator through multiple trophic transfers. Compare ecological efficiency across different ecosystem types. Construct biomass and energy pyramids and understand why inverted biomass pyramids (e.g., phytoplankton supporting zooplankton) are possible but inverted energy pyramids are not.
From your study of trophic levels and food webs, you know that organisms are organized into feeding levels — producers, primary consumers, secondary consumers, and so on. From cellular respiration, you know that organisms extract energy from organic molecules and lose much of it as heat. Energy flow connects these ideas: it traces how energy enters an ecosystem, passes through trophic levels, and is progressively lost, explaining fundamental patterns like why there are more plants than herbivores and more herbivores than top predators.
Energy enters most ecosystems as sunlight captured by primary producers through photosynthesis. The total energy fixed is called gross primary productivity (GPP), but producers use a substantial fraction of this energy for their own respiration — building and maintaining cells, growing roots, reproducing. What remains after the producers' own metabolic costs is net primary productivity (NPP), and this is the energy actually available to the rest of the food web. NPP varies enormously across ecosystems: tropical rainforests and coral reefs are highly productive, while deserts and open oceans produce far less per unit area. Understanding NPP tells you the energy budget that herbivores, predators, and decomposers must share.
The critical concept is ecological efficiency — the fraction of energy at one trophic level that gets transferred to the next. On average, this is roughly 10%, though it varies from about 5% to 20% depending on the organisms and ecosystem. The other 90% is lost through three main pathways: metabolic heat from respiration (organisms burn energy to live), unconsumed biomass (not all plant material gets eaten; not all prey gets caught), and undigested material (feces and other waste). This compounding loss explains the pyramid of energy: if producers fix 10,000 kcal, herbivores capture about 1,000, secondary consumers about 100, and tertiary consumers about 10. After just four or five transfers, there is simply not enough energy left to support another trophic level — this is why food chains rarely exceed five links.
Unlike nutrients, which cycle through ecosystems and can be reused indefinitely, energy follows the second law of thermodynamics: with each transfer, usable energy is irreversibly degraded into heat. An ecosystem is therefore an open system that requires continuous energy input from the sun. This has practical consequences: producing a kilogram of beef requires roughly ten times the plant biomass as producing a kilogram of grain for direct human consumption, because each trophic transfer loses ~90% of the energy. It also explains why biomass pyramids can occasionally appear inverted — in open ocean ecosystems, phytoplankton have low standing biomass but reproduce so rapidly (high turnover rate) that they support a larger biomass of zooplankton at any given moment — but energy pyramids never invert, because thermodynamics does not permit more energy to flow out of a level than flows in.