Ecosystem Dynamics: Energy Flows One Way, Matter Cycles
🎒 with Atlas
Atlas, a calm field-biologist guide in a sun-warmed meadow, kneels beside a pond sketching an energy-pyramid and a looping carbon arrow in a notebook while dragonflies hover overhead.
Trace energy as it flows one way from the Sun through producers, consumers, and decomposers.
Explain why roughly 90 percent of energy is lost between trophic levels, and describe this pattern as a pyramid.
Contrast one-way energy flow with the cycling of matter such as carbon and nitrogen.
Predict how carrying capacity and feedback keep population sizes relatively stable.
Analyze how removing one population can ripple through the rest of an ecosystem.
Key terms
Trophic level
A feeding position in a food chain, such as producer, herbivore, or top predator
Carrying capacity
The maximum population size an environment can sustain given available resources
Negative feedback
A self-correcting response that pushes a fluctuating value back toward a stable level
Decomposer
An organism that breaks down dead matter and returns nutrients to the environment
Trophic cascade
A chain of ecological effects triggered when one population is added or removed
The Ten Percent Pattern
Energy enters an ecosystem from the Sun and is captured by producers, then passes up trophic levels as organisms eat one another. At each transfer most energy is lost to respiration, heat, uneaten biomass, excretion, and incomplete assimilation, so only roughly five to twenty percent reaches the next level, averaged conveniently as ten percent. This rapid loss draws ecosystems as pyramids and explains why food chains rarely exceed four or five links before too little energy remains.
Matter Cycles, Energy Does Not
Unlike energy, matter is conserved and continually reused. Atoms of carbon, nitrogen, phosphorus, and water change form but are never consumed, looping from the abiotic environment into organisms and back through decomposition. Decomposers like fungi and bacteria are essential to this cycle because they release locked-up nutrients from dead tissue, restoring the supply that producers depend on. Remove them and nutrients would stall in corpses, starving the whole system.
Feedback Keeps Populations Stable
Each population has a carrying capacity set by food, space, water, and shelter. When numbers exceed it, scarce resources lower survival and reproduction, pushing the population back down; when numbers fall, resources recover and growth resumes. This negative feedback maintains relative stability. Because populations are interconnected, removing a key species such as a top predator weakens that feedback and can trigger a trophic cascade through the community.
Worked examples
Producers store 10,000 kcal. Estimate the energy reaching a secondary consumer.
Apply the ten percent rule to the first transfer: about ten percent of 10,000 kcal reaches the primary consumer, roughly 1,000 kcal.
Apply it again for the next transfer to the secondary consumer: about ten percent of 1,000 kcal reaches them.
That leaves approximately 100 kcal available at the secondary consumer level.
Answer: About 100 kcal reaches the secondary consumer.
Hi, I'm Atlas. Let's stand at this pond and watch an ecosystem run on two different rules at once.
First rule: ENERGY FLOWS ONE WAY. Sunlight reaches producers like grass and algae, which use photosynthesis to lock it into sugars. Primary consumers (herbivores) eat producers; secondary and tertiary consumers eat those. At every trophic level, most captured energy is lost and cannot be passed on — through respiration, through uneaten biomass that never gets consumed, through excretion, and through structural material that is not fully assimilated. On average, only about 5 to 20 percent transfers to the next level; ecologists use 10 percent as a convenient rough average, not a fixed law, because real ecosystems vary. That shrinking transfer is why we draw trophic levels as a pyramid and why food chains rarely exceed four or five links. Energy never loops back to the Sun, so it must be constantly resupplied.
Second rule: MATTER CYCLES. Atoms of carbon, nitrogen, phosphorus, and water are not used up — they change form and keep moving. Decomposers like fungi and bacteria break dead organisms down and return nutrients to soil, water, and air, where producers absorb them again. The same carbon atom can travel from air to leaf to deer to soil and back to air many times over.
Now for population dynamics. Each population has a CARRYING CAPACITY — the maximum size its environment can support given available food, space, water, and shelter. When a population overshoots that limit, dwindling resources push it back down; when numbers drop low, resources recover and the population can grow again. That push-and-pull is negative feedback, and it keeps a healthy ecosystem relatively stable. But populations are linked: remove a top predator and prey numbers may surge past carrying capacity, strip their food base, and then crash.
If you ever feel stuck, return to the two rules — energy one-way, matter in loops — and the activity below will let you sort the picture yourself.
Activity
Sort each card into the correct track: 'Energy Flow' (one-way, Sun to consumers) or 'Matter Cycling' (loops through decomposers and back to producers)
Practice
Explain why energy must be constantly resupplied while matter does not need resupplying.
Predict the short-term effect on prey populations when a top predator is removed from a stable ecosystem.
Common mistakes to avoid
Energy is recycled like matterEnergy flows one way and is lost as heat at each level, so it must be resupplied by the Sun; only matter cycles.
Decomposers are unimportant cleanup crewsDecomposers return essential nutrients from dead matter to soil and water, restarting the matter cycle that producers depend on.
Check your understanding
Approximately how much of the energy at one trophic level is passed on to the next level up?
Which statement correctly contrasts energy and matter in an ecosystem?
A student says decomposers are just unimportant 'cleanup crews.' What is wrong with that idea?
If a top predator is removed from a stable ecosystem, what is the most likely short-term result?
Recap
Ecosystems run on two rules: energy flows one way from the Sun and dissipates as heat, losing about ninety percent per trophic level, while matter cycles endlessly through decomposers. Carrying capacity and negative feedback keep populations stable, and removing a key species can cascade through the whole community.
Reflect
How does the ten percent rule explain why large predators are comparatively rare?
Biology