8.2 Energy Flow Through Ecosystems

Endotherms, such as birds and mammals, use heat produced by their own metabolism to keep a steady body temperature. That lets them stay active in the cold, but it costs a lot of energy, so they need much more food than ectotherms of the same size. Smaller endotherms lose heat faster because they have more surface area relative to their volume, so they have higher metabolic rates per gram: a shrew must eat almost constantly. Ectotherms, such as reptiles, amphibians and most fish, lack efficient internal ways to hold their temperature, so their body temperature tracks the environment. They regulate it with behavior, such as basking in the sun, moving into shade or huddling together.

Energy balance matters. A net gain of energy is stored (as fat or starch), used to grow, or put into more offspring. A net loss means losing mass, producing fewer offspring and eventually death. Reproductive strategies track energy, too: many organisms reproduce in seasons when food is plentiful, biennial plants store energy in their first year and flower in their second, and some species switch between asexual and sexual reproduction. Aphids reproduce asexually all summer when food is abundant, then reproduce sexually as conditions worsen in fall.

Ecologists study life at several levels: populations (one species in an area), communities (all species in an area), ecosystems (communities plus the nonliving environment) and biomes (large regions with similar climate and life, like deserts or tundra).

Autotrophs (producers) capture energy from the environment. Photosynthetic organisms use sunlight; their energy capture is primary productivity. Chemosynthetic organisms, such as bacteria and archaea at deep-sea vents, get energy from small inorganic molecules like hydrogen sulfide, sometimes without oxygen. Heterotrophs (consumers) get energy by eating organic matter: herbivores, carnivores, omnivores, scavengers and decomposers all break down carbohydrates, lipids and proteins that originally came from producers.

Trophic levels are feeding steps: producers, then primary consumers (herbivores), secondary consumers, tertiary and quaternary consumers. Decomposers break down dead material from every level. In a food chain or web, arrows point in the direction energy flows, from the eaten to the eater.

At each trophic level, most energy is used for the organism's own metabolism and lost as heat, or is never eaten or digested. On average, only about 10% of the energy at one level becomes biomass at the next (the actual figure varies, often between about 5% and 20%). That's why energy pyramids narrow quickly and food chains rarely have more than four or five levels. Changes at the bottom ripple up: less sunlight or fewer producers means less energy for every level above.

Gross primary productivity (GPP) is the total energy producers capture. Net primary productivity (NPP) is what's left after producers' own respiration (NPP = GPP − R), and it's the energy available to consumers.

Unlike energy, matter is recycled. Atoms move between abiotic reservoirs (air, water, rock, soil) and biotic reservoirs (living things), and the total amount is conserved. The cycles are linked: for example, water carries nitrogen and phosphorus, and photosynthesis moves both carbon and water.