Energy Flow and Biogeochemical Cycles

Every ecosystem runs on two different things moving through it at the same time. Energy flows in one direction, starting with the sun and ending as heat, and it is never reused. Matter (carbon, nitrogen, water, and other nutrients) cycles around again and again, reused by one organism after another. This sheet covers both halves: how energy flows through feeding relationships, and how key nutrients cycle through the ecosystem. Every organism in an ecosystem fits one of three feeding roles. Producers (autotrophs) make their own food, almost always through photosynthesis, and form the base of every food chain. Green plants, algae, and cyanobacteria are producers. Consumers (heterotrophs) eat other organisms to get energy. Primary consumers (herbivores) eat producers. Secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers. Many consumers are omnivores that eat at more than one level. Decomposers (fungi and bacteria) break down dead organisms and waste, releasing the nutrients locked inside back into the soil, water, and air, where producers can absorb them again.

A food chain is a single pathway showing who eats whom, drawn with arrows pointing from the organism being eaten to the organism that eats it. The arrow shows the direction energy flows, not the direction of eating. Each step in the chain is a trophic level, a feeding level counted from the producer up. Example: grass → grasshopper → frog → hawk. Read this as grass is eaten by the grasshopper, the grasshopper is eaten by the frog, and the frog is eaten by the hawk. Energy from the sun, captured by the grass, flows up the chain in that same direction.

Real ecosystems are never just one tidy chain. Most consumers eat more than one type of food and are eaten by more than one type of predator. A food web is a diagram of many overlapping food chains, showing all the feeding connections in an ecosystem at once. A hawk might eat frogs, snakes, and mice, and a frog might be eaten by both hawks and snakes, all in the same web. Food webs matter because they show how ecosystems stay connected. If one species disappears, every food chain that passed through it is disrupted, which is why food webs are a far more realistic model of a real ecosystem than any single food chain.

An energy pyramid stacks trophic levels from producers at the bottom to top predators at the top, with the width of each level showing how much energy is available there. The pyramid gets narrower going up because of the ten percent rule: on average, only about 10% of the energy at one trophic level gets passed on to the next trophic level. The other roughly 90% is used up or lost as heat through the consumer's own cellular respiration, movement, and everyday life, or it leaves as waste that was never digested. Example: if producers in a field store 10,000 kcal of energy, primary consumers that eat them gain only about 1,000 kcal (10% of 10,000). Secondary consumers gain about 100 kcal (10% of 1,000), and tertiary consumers gain about 10 kcal (10% of 100).

Carbon moves between the atmosphere, living things, and long-term storage through a few key processes. Photosynthesis removes CO₂ from the atmosphere and builds it into glucose: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Cellular respiration does the reverse, breaking glucose down for energy and releasing CO₂ back into the atmosphere: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Decomposition releases the carbon stored in dead organisms back into the air and soil as decomposers break them down. Combustion, burning wood or fossil fuels, releases stored carbon quickly. Fossil fuels formed from the remains of ancient organisms buried and compressed over millions of years, so burning them releases carbon that had been locked away for a very long time.

About 78% of the atmosphere is nitrogen gas, N₂, but most organisms cannot use N₂ directly because the bond holding the two nitrogen atoms together is very strong. Getting nitrogen into a usable form takes several steps carried out by different bacteria. Nitrogen fixation converts N₂ into ammonia (NH₃) or ammonium (NH₄⁺), done mainly by nitrogen-fixing bacteria that live in the root nodules of legume plants like beans and peas, and to a smaller extent by lightning. Nitrification converts ammonium into nitrites (NO₂⁻) and then nitrates (NO₃⁻), done by nitrifying bacteria in the soil. Assimilation is producers absorbing nitrates through their roots to build proteins and DNA, which then pass to consumers that eat those producers. Ammonification happens when decomposers break down waste and dead organisms, releasing nitrogen back into the soil as ammonium. Denitrification converts nitrates back into N₂ gas, done by denitrifying bacteria, returning nitrogen to the atmosphere and completing the cycle.

Water moves between the atmosphere, land, and oceans through a repeating cycle. Evaporation turns liquid water from oceans, lakes, and rivers into water vapor, driven by heat from the sun. Transpiration is water vapor released from the leaves of plants. Evaporation and transpiration together are often called evapotranspiration. Water vapor rises, cools, and undergoes condensation, forming the tiny water droplets that make up clouds. Once droplets grow large and heavy enough, precipitation (rain, snow, or sleet) falls back to Earth. From there, water either runs off into rivers and lakes as runoff or soaks into the ground as infiltration, refilling groundwater, and the cycle repeats.

1. In the food chain oak leaves → caterpillar → songbird → hawk, which organism is the secondary consumer? 2. If producers in a pond store 5,000 kcal of energy, about how much energy is available to primary consumers, and about how much to secondary consumers, using the ten percent rule? 3. Which nitrogen cycle process converts N₂ gas into a form producers can eventually use, and which process returns nitrogen to the atmosphere as N₂ gas? 4. A food web is generally considered more realistic than a single food chain. Why? 5. Using energy-flow-trophic-levels.csv, apply the ten percent rule stated on this sheet to the producers' (phytoplankton) energy to predict how much energy should be available to the primary consumers (zooplankton). Then compare your prediction to the actual value for primary consumers in the file. 6. Using the file, calculate the actual transfer efficiency, as a percent, from primary consumers (zooplankton) to secondary consumers (minnows). Use efficiency = (energy at the higher trophic level ÷ energy at the trophic level below it) × 100. 7. Calculate the transfer efficiency from secondary consumers (minnows) to tertiary consumers (bass) the same way. Is it closer to your answer for Question 6, or to the ten percent rule? 8. If Lake Kestrel had a fifth trophic level, quaternary consumers that feed on bass, use the ten percent rule and the bass energy in the file to predict about how much energy would reach them. Based on this sheet's explanation, why do food chains rarely reach a fifth trophic level?