IMAT Biology · Evolution and Ecology

Ecosystems, Energy Flow, Nutrient Cycles and Interactions

Energy flows one way through an ecosystem and is mostly lost as heat at each level, while carbon and nitrogen are recycled; species interact through competition, predation and partnerships.

All Evolution and Ecology notes

Why this matters

Only one past question, in 2018, sits here: two competing bacteria and which changes in conditions could favour one of them. Food chains, energy flow and the carbon and nitrogen cycles are standard syllabus topics that the ministry papers can ask at any time.

Concept 1 of 4: Levels of ecological organisation and trophic levels

Ecology zooms out in steps: one organism, then all of its species in a place, then all species in that place, then those species plus the non-living world round them. Within an ecosystem, organisms are sorted by what they eat, which is how energy and matter move.

Definition

Key terms:

Biotic factors are living (predators, competitors, disease); abiotic factors are non-living (light, temperature, water, pH, soil minerals).
Trophic levels: producers (autotrophs, mostly photosynthetic) → primary consumers (herbivores) → secondary and tertiary consumers (carnivores).
Decomposers (bacteria and fungi) break down dead matter and waste and return minerals to the soil; detritivores such as earthworms eat the dead matter.
A food chain is one feeding line; a food web is all the linked chains in an ecosystem. Arrows point in the direction energy moves, from the eaten to the eater.

Term	Meaning	Example
Population	All individuals of one species in one area	The perch in a lake
Community	All populations of all species in one area	Every plant, animal and microbe in the lake
Ecosystem	A community plus its abiotic environment	The lake with its water, mud, light and temperature
Habitat	The place where an organism lives	Shallow water among reeds
Niche	The role of a species: what it eats, when, where, and how it uses resources	A perch hunting small fish by day near the surface

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An ecosystem includes the non-living part

A community is only the living organisms. An ecosystem is the community together with its abiotic environment. Options that use the two words as if they meant the same are wrong.

Concept 2 of 4: Energy flow between trophic levels and the 10% rule

Most of the energy an animal eats is burned in its own respiration and leaves as heat, or is never eaten at all (roots, bones) or passes out in faeces. Only a small part becomes new body tissue for the next level to eat. So energy shrinks fast up a food chain, which is why chains rarely have more than four or five levels.

Definition

Energy enters as light, is fixed by producers, and flows one way: it is not recycled.

On average only about 10% of the energy at one level becomes biomass in the next (the real range is roughly 5 to 20%).
Gross primary productivity (GPP) is the energy plants fix; net primary productivity is what is left after their own respiration: NPP = GPP minus respiration.
A pyramid of energy is never inverted. Pyramids of numbers (one oak tree, many caterpillars) and of biomass (in the sea) can be.
Persistent poisons such as DDT and mercury are not broken down, so they become more concentrated at each level (biomagnification).

Energy passed up a food chain

E_{k+1} \approx 0.1 \times E_{k} \qquad \text{efficiency} = \frac{E_{k+1}}{E_k} \times 100\%

$E_k$ energy at trophic level k (producers are level 1)
$E_{k+1}$ energy passed to the next level

Worked example

Grass in a meadow stores 50,000 kJ per m² per year. Using the 10% rule, how much energy reaches each level of the chain grass → grasshopper → frog → snake?

Practice this conceptself-check · 4 quick reps

Energy flows, matter cycles

Carbon and nitrogen atoms go round and round between organisms and the environment. Energy does not: it enters as light and leaves as heat. An option saying decomposers return energy to the producers is wrong; they return minerals.

Most energy is lost as heat from respiration

The roughly 90% that does not reach the next level is mainly used in respiration and lost as heat, with the rest in uneaten parts and faeces. It is not stored in the predator's body.

Concept 3 of 4: The carbon cycle and the nitrogen cycle

Living things are built from a fixed stock of atoms, so the atoms must be reused. Carbon goes into living matter by photosynthesis and back out as carbon dioxide. Nitrogen is plentiful in the air, but almost nothing can use it as a gas: bacteria do nearly all the work of moving it in and out of living things.

Definition

Carbon cycle: photosynthesis removes $\mathrm{CO_2}$ from the air; respiration (by all organisms), decomposition and combustion return it. Oceans dissolve $\mathrm{CO_2}$ ; fossil fuels are carbon locked away for millions of years. Burning them and cutting forests raise atmospheric $\mathrm{CO_2}$ and add to the greenhouse effect. Nitrogen cycle: $\mathrm{N_2}$ makes up about 78% of air, but plants take in nitrogen mainly as nitrate $\mathrm{(NO_3^-)}$ and ammonium $\mathrm{(NH_4^+)}$ to make amino acids and nucleic acids.

Nitrogen fixation needs the enzyme nitrogenase, which oxygen damages.
Nitrification needs oxygen; denitrification happens where oxygen is short, as in waterlogged soil.

Process	Change	Done by
Nitrogen fixation	$\mathrm{N_2} \rightarrow \mathrm{NH_3}$ or $\mathrm{NH_4^+}$	Rhizobium in legume root nodules, free-living Azotobacter, cyanobacteria; also lightning and the industrial Haber process
Ammonification	Proteins and urea in dead matter and waste $\rightarrow \mathrm{NH_4^+}$	Decomposers (bacteria and fungi)
Nitrification	$\mathrm{NH_4^+} \rightarrow \mathrm{NO_2^-} \rightarrow \mathrm{NO_3^-}$	Nitrosomonas (first step), Nitrobacter (second step)
Assimilation	$\mathrm{NO_3^-}$ and $\mathrm{NH_4^+}$ taken up into amino acids	Plant roots, then animals by eating plants
Denitrification	$\mathrm{NO_3^-} \rightarrow \mathrm{N_2}$ back to the air	Anaerobic bacteria such as Pseudomonas

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Nitrification is not nitrogen fixation

Fixation turns nitrogen gas into ammonia. Nitrification turns ammonium into nitrite and then nitrate. They are done by different bacteria, and only fixation brings new nitrogen in from the air.

Concept 4 of 4: Interactions between species and population growth

Every species is affected by the others around it: some it eats, some eat it, some want the same food, some help it. When conditions change, the balance shifts: a change that hurts one competitor frees resources for the other.

Definition

Species interactions are named by the effect on each partner (gain, harm or none).

Competition can be within a species (intraspecific) or between species (interspecific). Two species with exactly the same niche cannot coexist for long: one excludes the other (competitive exclusion).
Species differ in their tolerance of temperature, pH and chemicals, so a change in conditions or an added toxin can tip a competition either way.
Population growth: with unlimited resources, numbers grow exponentially (J-shaped curve). In reality growth slows and levels off at the carrying capacity (K), the largest population the environment can support (S-shaped, logistic curve).
Density-dependent factors (competition, predation, disease) bite harder in crowded populations; density-independent factors (floods, frost) do not depend on numbers.

Interaction	Effect on each species	Example
Competition	Both harmed	Two plant species shading each other for light
Predation	Predator gains, prey harmed	Lynx and snowshoe hare
Parasitism	Parasite gains, host harmed	Tapeworm in a human gut
Mutualism	Both gain	Rhizobium in legume roots; the fungus and alga of a lichen
Commensalism	One gains, the other unaffected	Barnacles riding on a whale

Practice this conceptself-check · 4 quick reps

A change that harms one competitor helps the other

If two species share a resource, anything that slows one of them (a temperature or pH it tolerates badly, or a chemical it is sensitive to) leaves more of the resource for the other, which then grows faster. The change does not need to act on the winner directly.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Energy flow between trophic levels and the 10% rule
Energy passed up a food chain
$E_{k+1} \approx 0.1 \times E_{k} \qquad \text{efficiency} = \frac{E_{k+1}}{E_k} \times 100\%$

Reference tables (3)

Levels of ecological organisation and trophic levels5 rows

Term	Meaning	Example
Population	All individuals of one species in one area	The perch in a lake
Community	All populations of all species in one area	Every plant, animal and microbe in the lake
Ecosystem	A community plus its abiotic environment	The lake with its water, mud, light and temperature
Habitat	The place where an organism lives	Shallow water among reeds
Niche	The role of a species: what it eats, when, where, and how it uses resources	A perch hunting small fish by day near the surface

The carbon cycle and the nitrogen cycle5 rows

Process	Change	Done by
Nitrogen fixation	$\mathrm{N_2} \rightarrow \mathrm{NH_3}$ or $\mathrm{NH_4^+}$	Rhizobium in legume root nodules, free-living Azotobacter, cyanobacteria; also lightning and the industrial Haber process
Ammonification	Proteins and urea in dead matter and waste $\rightarrow \mathrm{NH_4^+}$	Decomposers (bacteria and fungi)
Nitrification	$\mathrm{NH_4^+} \rightarrow \mathrm{NO_2^-} \rightarrow \mathrm{NO_3^-}$	Nitrosomonas (first step), Nitrobacter (second step)
Assimilation	$\mathrm{NO_3^-}$ and $\mathrm{NH_4^+}$ taken up into amino acids	Plant roots, then animals by eating plants
Denitrification	$\mathrm{NO_3^-} \rightarrow \mathrm{N_2}$ back to the air	Anaerobic bacteria such as Pseudomonas

Interactions between species and population growth5 rows

Interaction	Effect on each species	Example
Competition	Both harmed	Two plant species shading each other for light
Predation	Predator gains, prey harmed	Lynx and snowshoe hare
Parasitism	Parasite gains, host harmed	Tapeworm in a human gut
Mutualism	Both gain	Rhizobium in legume roots; the fungus and alga of a lichen
Commensalism	One gains, the other unaffected	Barnacles riding on a whale

Watch out for (5)

An ecosystem includes the non-living part
→ Levels of ecological organisation and trophic levels
Energy flows, matter cycles
→ Energy flow between trophic levels and the 10% rule
Most energy is lost as heat from respiration
→ Energy flow between trophic levels and the 10% rule
Nitrification is not nitrogen fixation
→ The carbon cycle and the nitrogen cycle
A change that harms one competitor helps the other
→ Interactions between species and population growth

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