Energy flows, matter cycles
Two things move through every ecosystem: energy and matter. Energy arrives as sunlight, is passed along food chains in chemical form, and eventually leaves as heat. It cannot be recycled, so ecosystems need a constant supply. Matter, the atoms of carbon, nitrogen and other elements, is not lost: it is used again and again as it moves between the air, soil, water and living things. This is nutrient cycling.
Producers, consumers and decomposers
Producers, mainly plants and algae, capture light energy by photosynthesis and build it into sugars. Consumers obtain energy by eating: herbivores (primary consumers) eat producers, and carnivores (secondary and tertiary consumers) eat other animals.Decomposers, mostly fungi and bacteria, feed on dead organisms and waste. They are essential, because they release the nutrients locked in dead matter back into the soil and air.
Food chains and food webs
A food chain shows one path of feeding, such as plants → rabbits → foxes. The arrows point in the direction energy moves, from the eaten to the eater. Real ecosystems contain many linked chains, forming afood web. Each feeding step is a trophic level.
Energy transfer between levels
Only a small part of the energy at one level becomes new biomass at the next. Some of the plant is never eaten, some of what is eaten is not digested and leaves as waste, and most of the energy that is absorbed is used for respiration and lost as heat. Typically about 5–20% passes on; the often-quoted “10% rule” is a rough average, not a law. In the simulator the efficiencies are not fixed: they come out of the model and change as populations change. This is why food chains rarely have more than four or five levels.
Pyramids of energy
A pyramid of energy shows how much energy each trophic level stores per unit of time. It is always wider at the bottom, because energy is lost at every step. Pyramids of numbers or biomass can sometimes be inverted (one tree can feed thousands of insects), but a pyramid of energy cannot.
The carbon cycle
Photosynthesis removes carbon dioxide from the air and turns it into organic compounds.Feeding passes them along food chains. Respiration by plants, animals and decomposers returns carbon dioxide to the air, and decomposition releases the carbon in dead matter. Some carbon is stored for a long time in wood, soils, oceans and fossil fuels; burning fossil fuels releases it much faster than it was stored.
The nitrogen cycle
Organisms need nitrogen to make proteins and DNA, but most cannot use the nitrogen gas that makes up 78% of the air.Nitrogen-fixing bacteria, some living in the root nodules of plants such as peas and clover, turn it into ammonium. Decomposers release ammonium from dead matter, nitrifying bacteriaconvert it to nitrates, and plants absorb nitrates through their roots. Denitrifying bacteria return some nitrogen to the air, and nitrates can also be washed out of soil by rain.
Adding and removing organisms
Because organisms are linked, a change in one population spreads through the web. Remove a predator and its prey may increase until their food runs short; remove the prey and the predator declines. Remove the decomposers and nutrients stop cycling, so producers and everything that depends on them decline. Reducing the energy input, for example by shading, shrinks every level.
Real-world applications
Farmers rotate crops with legumes to add nitrogen to soil, and composting relies on decomposers. Fisheries scientists use energy transfer to estimate how many fish an ecosystem can support. Conservationists study food webs to predict the effects of losing or reintroducing species, such as wolves in Yellowstone.
Common misconceptions
- “Energy is recycled by decomposers.” Decomposers recycle nutrients; the energy ends up as heat.
- “Arrows in a food chain show who eats whom.” They show the direction energy moves: from food to feeder.
- “Exactly 10% is passed on.” The share varies widely between ecosystems.
Key takeaways
- Energy flows one way through ecosystems and is lost as heat; matter is recycled.
- Producers capture energy, consumers obtain it by eating, and decomposers return nutrients.
- Only a small, variable share of energy passes from one trophic level to the next.
- Carbon and nitrogen cycle between the air, soil and living things, driven largely by microbes.
Model note: this is a simplified meadow with six groups of organisms and nitrogen as the limiting nutrient. All amounts are relative units chosen for teaching, not measurements, and no number in it should be taken as a constant for real ecosystems.