What is the ten percent rule?

Only about 10% of the energy at one trophic level is passed to the next; roughly 90% is lost, mostly as heat from cellular respiration and life processes, plus undigested waste. This is why food chains are short (usually three to five levels) and why an energy pyramid is widest at the producers and narrows upward, so top predators are few. It also explains why energy flows one way and is lost, in contrast to matter, which cycles.

What is the difference between how energy and matter move through an ecosystem?

Energy flows one way: it enters as sunlight, is captured by producers, passes up the trophic levels, and is lost as heat at every step, so it must be constantly resupplied. Matter cycles: atoms of carbon, nitrogen, oxygen, and hydrogen are reused over and over as they move between living things and the non-living environment through the carbon, nitrogen, and water cycles. The summary to remember is energy flows, matter cycles.

What is carrying capacity?

The carrying capacity is the maximum population size an environment can support over time, given its resources. On a logistic (S-shaped) growth curve it is the level at which the population levels off and then fluctuates around. The population settles there because, as it grows, density-dependent limiting factors (competition, disease, predation) reduce births and increase deaths until they balance. An exponential (J-shaped) curve, by contrast, shows growth with no limits.

How do humans reduce biodiversity?

Through climate change (extra greenhouse gases from burning fossil fuels and deforestation), habitat loss (clearing land for farming and cities), pollution (of air, water, and soil), overexploitation (overfishing and overhunting), and invasive species (non-native species that spread with no natural predators and outcompete or prey on natives). These pressures raise extinction rates and lower both genetic and species diversity, which also reduces ecosystem stability. This is the focus of standard B.DI.3.

OhioBiology

Ohio Biology EOC B.DI (Ecology and Interdependence): ecosystems and levels of organization, energy flow and food webs, the cycling of matter, population dynamics and carrying capacity, species interactions, and ecosystem stability and human impact

Q: What does the ecology side of the Diversity strand (B.DI) on Ohio's Biology EOC cover?

It covers the levels of ecological organization (organism to biosphere) and the biotic and abiotic factors of an ecosystem; the one-way flow of energy through trophic levels in food chains and webs, with the ten percent rule and energy pyramids; the cycling of matter through the carbon, nitrogen, and water cycles; population dynamics, carrying capacity, and density-dependent versus density-independent limiting factors (B.DI.2); the interactions between species (predation, competition, and the three forms of symbiosis); and ecosystem equilibrium, succession, and how human activities reduce biodiversity (B.DI.3). The classification side is in a separate guide.

A deep-dive guide to the ecology side of the Diversity and Interdependence strand (B.DI) on Ohio's Biology EOC: ecosystems and levels of organization, energy flow and food webs with the ten percent rule, the cycling of carbon, nitrogen, and water, population dynamics and carrying capacity, species interactions, and ecosystem stability and human impact.

Generated by Claude Opus 4.817 min readB.DIUpdated 2026-06-13

Reviewed by: AI editorial process; not yet individually human-reviewed

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What the ecology topics demand
Ecosystems and levels of organization
Energy flow and food webs
The cycling of matter
Population dynamics and carrying capacity
Species interactions
Ecosystem stability and human impact
Check your knowledge

What the ecology topics demand

The Diversity and Interdependence strand (B.DI) has a classification side and an ecology side. This guide covers the ecology side: how ecosystems are structured, how energy and matter move through them, what controls population size, how species interact, and how ecosystems stay stable or are disrupted. (The classification side, with taxonomy, phylogeny, the domains and kingdoms, and biodiversity, is in the diversity and classification guide.) The main content statements are B.DI.2 (Ecosystems: equilibrium and disequilibrium, carrying capacity) and B.DI.3 (Loss of Diversity: climate change, Anthropocene effects, extinction, invasive species). The recurring crosscutting concepts are energy and matter and stability and change, and the science practices most in play are analyzing and interpreting data (reading graphs and food webs) and constructing explanations.

This guide ties together the matching topic pages, each with its own practice questions: ecosystems and levels of organization, energy flow and food webs, the cycling of matter, population dynamics and carrying capacity, species interactions, and ecosystem stability and human impact.

Ecosystems and levels of organization

Ecology arranges life into nested levels: organism, population (one species), community (all species), ecosystem (community plus the non-living environment), biome (a large climate region), and biosphere (all life). The two boundaries the EOC tests are population (one species) versus community (all species) and community (living only) versus ecosystem (living plus non-living). An ecosystem is made of biotic factors (living: plants, animals, fungi, bacteria) and abiotic factors (non-living: sunlight, temperature, water, soil, oxygen). The system view matters because a change in one part affects many others.

Energy flow and food webs

Energy enters as sunlight and flows one way. Producers (autotrophs) capture it by photosynthesis; consumers (herbivores, then carnivores) pass it along; decomposers recycle dead matter. Each feeding position is a trophic level, shown in a food chain (one path) or a food web (many linked paths). The ten percent rule says only about 10% of energy moves to the next level (about 90% lost, mostly as heat from respiration), so food chains are short and an energy pyramid narrows upward. The slogan is energy flows, matter cycles.

The cycling of matter

Unlike energy, matter cycles. In the carbon cycle, photosynthesis removes carbon dioxide and respiration, decomposition, and combustion return it; burning fossil fuels adds extra carbon dioxide. In the nitrogen cycle, plants cannot use nitrogen gas directly, so nitrogen-fixing bacteria convert it to a usable form and denitrifying bacteria return it to the air; bacteria are essential. In the water cycle, water moves by evaporation, transpiration, condensation, precipitation, and runoff. These are biogeochemical cycles, and human activity can disrupt them.

Population dynamics and carrying capacity

A population grows when births and immigration exceed deaths and emigration. With unlimited resources it grows exponentially (a J-shaped curve); in reality, limited resources give logistic growth (an S-shaped curve) that levels off at the carrying capacity, the maximum the environment can support. Limiting factors are density-dependent (stronger when crowded: competition, disease, predation) or density-independent (act regardless of density: drought, fire, floods). As a population nears carrying capacity, density-dependent factors balance births and deaths, and it fluctuates around the carrying capacity at equilibrium.

Species interactions

Species interact through predation (predator kills and eats prey; includes herbivory), competition (two organisms need the same limited resource, harming at least one), and symbiosis (a close, long-term relationship). The three symbiosis types are defined by who benefits and who is harmed: mutualism (both benefit), commensalism (one benefits, the other unaffected), and parasitism (one benefits, the other harmed). Classify by writing the outcome for each organism as +, 0, or -. These interactions act as limiting factors and shape the community.

Ecosystem stability and human impact

An ecosystem at equilibrium is dynamically stable; a disturbance causes disequilibrium. Ecological succession (primary from bare rock with no soil, slow; secondary where soil remains, faster) rebuilds a stable community. Greater biodiversity makes ecosystems more stable. Humans reduce biodiversity (B.DI.3) through climate change (greenhouse gases from fossil fuels), habitat loss, pollution, overexploitation, and invasive species, raising extinction rates and lowering genetic and species diversity.

Revision order

Start by nailing the levels of organization and the biotic/abiotic split, since the rest builds on them. Then learn energy flow: producers, consumers, decomposers, trophic levels, and the ten percent rule (practice the calculation), always saying "energy flows, matter cycles." Next master the cycling of matter, keeping photosynthesis (carbon out of the air) and respiration (carbon back) paired, and remembering that bacteria run the nitrogen cycle. Move to population dynamics: read S-shaped curves, find the carrying capacity, and sort density-dependent from density-independent factors. Learn the species interactions by classifying outcomes as +, 0, or -. Finish with stability and human impact, contrasting primary and secondary succession and tying each human impact to a loss of biodiversity.

The ecology side of the Diversity strand (B.DI) on Ohio's Biology EOC covers the levels of organization (organism, population as one species, community as all species, ecosystem as community plus non-living environment, biome, biosphere) and biotic versus abiotic factors; the one-way flow of energy through producers, consumers, and decomposers across trophic levels in food chains and webs, with the ten percent rule (about 90% lost as heat) and energy pyramids; the cycling of matter through the carbon cycle (photosynthesis removes carbon dioxide, respiration, decomposition, and combustion return it, fossil fuels add more), the nitrogen cycle (bacteria fix and recycle nitrogen because plants cannot use nitrogen gas directly), and the water cycle, summed up as energy flows and matter cycles; population dynamics, where exponential (J-shaped) growth is unlimited and logistic (S-shaped) growth levels off at the carrying capacity, held by density-dependent and density-independent limiting factors (B.DI.2); species interactions (predation, competition, and the symbioses mutualism, commensalism, and parasitism, classified by who benefits and who is harmed); and ecosystem stability, where equilibrium is disturbed into disequilibrium and rebuilt by primary or secondary succession, and human activities (climate change, habitat loss, pollution, overexploitation, invasive species) reduce biodiversity and cause extinction (B.DI.3). The recurring themes are energy and matter and stability and change.

Check your knowledge

A mix of recall and reasoning questions covering the ecology topics. Attempt them under timed conditions, then check against the solutions.

State the difference between a population and a community. (2 marks)
Classify sunlight, a fish, water temperature, and an algae as biotic or abiotic factors. (2 marks)
Producers store 20,000 units of energy. Using the ten percent rule, calculate the energy available to a secondary consumer. (1 mark)
State the key difference between how energy and matter move through an ecosystem. (1 mark)
Name the process by which nitrogen-fixing bacteria make atmospheric nitrogen usable to plants. (1 mark)
A growth curve rises, then levels off at about 800 organisms. Name the curve shape and state what the level represents. (2 marks)
Classify "competition for food" and "a wildfire" as density-dependent or density-independent. (2 marks)
A bee gets nectar from a flower while pollinating it. Name this relationship and state who benefits. (2 marks)
State the difference between primary and secondary succession. (2 marks)

Solutions

Q1: A population is all the individuals of one species in an area; a community is all the different populations (all species) living and interacting in that area.
Q2: Biotic: the fish and the algae (living). Abiotic: sunlight and water temperature (non-living).
Q3: Primary consumer $\approx 2{,}000$ ; secondary consumer $\approx 200$ units (10% at each step).
Q4: Energy flows one way and is lost as heat; matter cycles and is reused ("energy flows, matter cycles").
Q5: Nitrogen fixation.
Q6: A logistic (S-shaped) curve; the level at about 800 is the carrying capacity, the maximum population the environment can support.
Q7: Competition for food is density-dependent; a wildfire is density-independent.
Q8: Mutualism; both benefit (the bee gets nectar/food, the flower is pollinated).
Q9: Primary succession begins on bare ground with no soil (and is slow); secondary succession begins where soil remains after a disturbance (and is faster).

Sources & how we know this

Ohio's Learning Standards and Model Curriculum for Science — Ohio Department of Education and Workforce (2022)
Biology State-Tested Course Resources — Ohio Department of Education and Workforce (2024)