12.2 Transfer of Energy and Matter

Radiation from the Sun supplies most ecosystems. Photosynthesis converts a fraction of incoming light energy into chemical energy in organic compounds. Producers do not create energy. They use some captured chemical energy in respiration and retain the rest in new biomass, storage compounds and reproductive material. Gross primary productivity is the rate at which producers convert light energy into chemical energy; net primary productivity is the rate at which producer biomass stores energy after respiratory loss.

Net primary production is the energy available for producer growth and for transfer to consumers and decomposers. Consumers similarly partition ingested energy: some material is not absorbed and leaves as feces, assimilated energy supports respiration and activity, and the remainder becomes new biomass or offspring. Energy in uneaten parts, excreta and dead bodies enters detrital pathways rather than vanishing, but it is unavailable to the particular grazing or predatory link being measured.

Respiration converts chemical energy into ATP and ultimately releases heat. Heat spreads through the surroundings and cannot be recaptured by organisms to rebuild the same organized store of chemical energy. Ecosystems therefore need a continuing external energy supply. Matter can be rearranged and reused; usable energy flows one way and becomes increasingly dispersed.

Ecological efficiency is the percentage of energy in one trophic level incorporated into biomass at the next. It is often near 10% but is not a law or a fixed constant. Digestibility, body temperature regulation, activity, tissue composition and the fraction consumed all affect transfer. Endotherms may spend a large fraction on maintaining body temperature, whereas some ectotherms convert a greater fraction of assimilated energy into growth.

A pyramid of energy displays energy flow per unit area per unit time, so its bars cannot be inverted: each higher trophic level receives only part of the energy from below. Pyramids of numbers or standing biomass can sometimes be inverted because one tree can support many insects or phytoplankton can reproduce so rapidly that a small standing mass supports a larger consumer mass at one instant. Energy pyramids avoid that snapshot problem by measuring transfer through time.

Repeated losses limit food-chain length. If each of five transfers retained 10%, only one hundred-thousandth of the initial producer energy would reach the sixth trophic level. Top predators consequently require a broad productive base and often occupy large ranges. Harvesting producers generally supplies more food energy per area than feeding producers to animals and harvesting the consumers, although nutrition, land suitability, livelihoods and non-food animal products complicate real agricultural decisions.

Elements such as carbon, nitrogen and phosphorus are finite within an ecosystem boundary and move repeatedly through biotic and abiotic pools. Plants absorb inorganic ions such as nitrate, ammonium and phosphate and incorporate them into amino acids, nucleotides, phospholipids and other compounds. Feeding moves those atoms through a food web. Excretion, egestion and death supply organic matter to detritivores and saprotrophs, whose metabolism helps release inorganic forms for reuse.

Nutrient cycles require chemical transformations, many of them microbial. Decomposition and ammonification return nitrogen in organic compounds to ammonium. Nitrifying bacteria oxidize ammonium to nitrite and nitrate under aerobic conditions. Nitrogen-fixing organisms convert atmospheric nitrogen to ammonia, while denitrifying bacteria return nitrogen gas to the atmosphere under low-oxygen conditions. A diagram that merely draws arrows between plants, animals and soil misses these changes in chemical form.

Photosynthesis fixes atmospheric or dissolved carbon dioxide into organic compounds. Respiration by producers, consumers and decomposers returns carbon dioxide. Feeding transfers organic carbon among organisms, while death and waste transfer it to detritus. Combustion rapidly oxidizes organic carbon in biomass or fuel. Exchange between atmosphere and ocean can move carbon in either direction, and dissolved carbon participates in carbonate equilibria and formation of shells and sediments.

Waterlogged soils slow aerobic decomposition because oxygen diffusion is restricted. Plant production can then exceed decay, allowing partly decomposed organic matter to accumulate as peat. Methanogenic archaea in anaerobic microsites produce methane, while methanotrophs can oxidize part of it before it reaches the atmosphere. Peat forms over centuries to millennia and stores carbon, habitat structure and records of past vegetation. Extraction or drainage accelerates oxidation and releases stored carbon, so peat is not renewable on a human extraction timescale.

Over geological time, a small fraction of buried organic carbon became coal, oil and natural gas. Burning these fuels moves carbon from a slow geological store into the active atmosphere-ocean-biosphere system far faster than new fossil fuel forms. Deforestation adds a second effect: combustion and decomposition release carbon while reduced biomass can remove less carbon through photosynthesis. The carbon cycle still operates, but human activity changes the sizes of stores and rates of flux.

Reliable carbon evidence combines direct atmospheric measurements, ice-core records, remote sensing, flux towers and inventories. Seasonal oscillations in atmospheric carbon dioxide are superimposed on the long-term rise because photosynthesis and respiration vary through the year, especially across the land-rich Northern Hemisphere. A short downward segment in the seasonal curve does not contradict the multi-decadal upward trend; time scale determines the pattern being interpreted.