Ecology & Ecosystems
High SchoolZoom out from the cell to the whole living planet. Ecology studies the interactions between organisms and their environment — how energy flows, matter cycles, and populations rise and fall. It is where all of biology comes together, and where its lessons matter most for our future.
Levels and energy flow
Ecology is organised in levels: organism → population → community → ecosystem → biosphere. An ecosystem is a community of living organisms plus their non-living (abiotic) environment, functioning together.
Energy enters most ecosystems as sunlight, captured by producers (plants, algae) through photosynthesis. It then flows through trophic levels along a food chain:
(with decomposers recycling matter at every level)
Crucially, energy flow is one-way and lossy. Only about 10% of the energy at one trophic level is passed to the next; the rest is lost as heat (through respiration), in movement, and in undigested or excreted material. This 10% rule explains why food chains rarely exceed 4–5 links and why there is far more plant biomass than top predators — the basis of the ecological pyramid.
Biogeochemical cycles
Unlike energy, matter is recycled. The carbon cycle: CO₂ is fixed by photosynthesis, passed along food chains, and returned to the air by respiration, decomposition and combustion of fuels. The nitrogen cycle: nitrogen-fixing bacteria convert inert N₂ into usable nitrates; plants absorb them; decomposers and denitrifying bacteria return nitrogen to the soil and air. Decomposers (bacteria and fungi) are the recyclers that keep matter available.
Populations and interactions
Populations grow when resources allow, but are checked by limiting factors (food, space, disease, predators) toward the environment's carrying capacity. Species interact through competition, predation, mutualism, and parasitism — relationships that shape communities.
Why energy is lost but matter is not
The two master rules of ecosystems seem to conflict but do not. Energy flows through; matter cycles round. Energy obeys thermodynamics: every transfer loses usable energy as heat (respiration keeps every organism alive but "wastes" energy from the chain's point of view), so the ecosystem needs a constant fresh input from the Sun. Matter (carbon, nitrogen, water) cannot be created or destroyed, only rearranged, so the same atoms are used again and again — the carbon in your body has been in countless organisms before. Take away decomposers and the cycles would seize up: nutrients would stay locked in dead bodies and life would grind to a halt.
Because ~90% of energy is lost at each step, there is only enough left to support a handful of trophic levels. It also means eating lower on the food chain feeds far more people from the same land: a field of grain feeds many more humans directly than if that grain is first fed to cattle. Energetics is not just theory — it constrains agriculture and ecology alike.
Worked example: energy transfer and estimating a population
Part A — the 10% rule. A grassland's producers fix 100 000 kJ of energy. Estimate the energy reaching a tertiary consumer (a hawk).
- Producers. 100 000 kJ available.
- Primary consumers (grasshoppers): ~10% passes on → 10 000 kJ.
- Secondary consumers (shrews): ~10% → 1 000 kJ.
- Tertiary consumer (hawk): ~10% → 100 kJ. Only one-thousandth of the original energy reaches the top — vividly showing why apex predators are few.
Part B — mark–release–recapture. Estimate a population of woodlice you cannot count directly.
- Capture & mark. Catch 60 woodlice, mark them harmlessly, and release them. (Marked, first sample = 60.)
- Recapture later. After they remix with the population, catch a second sample of 50. Of these, 15 bear marks.
- Apply the Lincoln index. N = (first sample × second sample) ÷ (marked recaptured).
- Compute. N = (60 × 50) ÷ 15 = 3000 ÷ 15 = 200 woodlice (estimated total population).
- Note the assumptions. Valid only if marks don't harm or wash off, marked animals mix randomly, no significant births/deaths or migration between samples, and marked animals are equally likely to be recaught. State assumptions — an estimate is only as good as its model.
Test yourself
Q1 Why are food chains rarely longer than four or five links?
Because only about 10% of energy is transferred from one trophic level to the next (the rest is lost as heat via respiration, in movement and in waste). After four or five transfers, so little energy remains that it cannot support another viable population of predators — there is not enough food energy to sustain them. The steady loss of usable energy at each step sets a hard ceiling on food-chain length.
Q2 Explain the roles of bacteria in the nitrogen cycle.
Several bacteria drive the cycle. Nitrogen-fixing bacteria (e.g. Rhizobium in legume root nodules) convert inert atmospheric N₂ into ammonia/nitrogen compounds plants can use. Decomposers/ammonifying bacteria break down dead organisms and waste, releasing ammonium. Nitrifying bacteria oxidise ammonium → nitrites → nitrates (the form plants absorb). Denitrifying bacteria convert nitrates back to N₂ gas, returning it to the atmosphere. Without these microbes, nitrogen — essential for proteins and DNA — would remain locked in unusable forms.
Q3 Distinguish the flow of energy from the cycling of matter in an ecosystem.
Energy flows one way: it enters as sunlight, passes through trophic levels, and is continuously lost as heat at every step, so it must be constantly replenished from the Sun — it is not recycled. Matter cycles: atoms of carbon, nitrogen, oxygen and so on are reused indefinitely, passing between organisms and the environment through biogeochemical cycles, driven largely by decomposers. In short: energy is a river (flows through and is gone); matter is a wheel (turns round and round).
Q4 What is carrying capacity, and name factors that determine it.
Carrying capacity is the maximum population size an environment can sustain indefinitely given its resources. A population growing toward it slows and levels off as resources run short. It is set by limiting factors: availability of food and water, space/territory, light and nutrients (for producers), plus biotic checks like predation, disease and competition, and the accumulation of waste. When a population exceeds carrying capacity, deaths rise and/or births fall until it returns toward the sustainable level.
Q5 A pesticide (like DDT) is present at low, harmless concentrations in water. Explain how it can still kill top predators.
Through bioaccumulation and biomagnification. The pesticide is persistent and fat-soluble, so it is not excreted but stored in organisms' tissues (bioaccumulation). At each step up the food chain, a predator eats many contaminated prey, concentrating the toxin — so its level magnifies at higher trophic levels (biomagnification). By the time it reaches top predators (e.g. birds of prey), the concentration can be thousands of times the water level — high enough to cause harm, such as the eggshell thinning DDT caused in raptors. A dose harmless at the bottom becomes lethal at the top.
How the ideas connect
Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.
The key facts, visualised
Worked problems, step by step
Follow each solution line by line, then try to reproduce it on paper before moving on.
Example 1Producers hold 10000 kJ. How much reaches the secondary consumer?
- 10% to primary consumer: 1000 kJ
- 10% of that to secondary: 100 kJ
Example 2In capture-recapture, 60 marked, then 50 caught of which 20 marked. Estimate population.
- N = (first x second) / recaptured marked
- (60 x 50) / 20 = 150
Now you try
Work each one out first, then tap to reveal the worked answer.