NDA Biology · Ecology and Environment

Ecosystems, Biomes and Ecological Interactions

An ecosystem is a community of living organisms (biotic) interacting with their non-living surroundings (abiotic); energy flows one way through trophic levels while organisms feed on one another and live in named biomes around the world.

All Ecology and Environment notes NDA Biology strategy

Why this matters

This is the machinery half of the chapter — 6 PYQs. The bank tests it four ways: pick the valid food chain, name an organism's trophic level (producer / primary consumer), recognise a symbiotic relationship (the bee-and-flower mutualism is a favourite), and classify a land or aquatic biome from its description. All EASY or MODERATE — get the trophic-level vocabulary and the three symbiosis types cold and these marks are free.

Concept 1 of 5

What an ecosystem is — biotic and abiotic components

Intuition

An ecosystem is everything in one place that interacts — the living things plus the non-living surroundings they depend on. A pond is an ecosystem: the fish, plants and microbes (living) plus the water, sunlight, temperature and dissolved minerals (non-living). Knowing this split tells you what each exam term is.

Definition

An ecosystem is a community of living organisms interacting with each other and with their physical environment. Its two component groups are:

Biotic components — all the living organisms, sorted by how they get food: producers (green plants, algae — make their own food), consumers (animals that eat others), and decomposers (bacteria, fungi — break down dead matter).
Abiotic components — the non-living physical and chemical factors: sunlight, temperature, water, air, soil, and minerals.

Two things flow through every ecosystem: energy (one-way, from the Sun through producers to consumers) and nutrients (recycled in cycles by decomposers).

Worked example

In a forest ecosystem, sort these into biotic vs abiotic: a deer, sunlight, soil minerals, a mushroom, rainfall, an oak tree.

Biotic = living. The deer, the mushroom and the oak tree are all organisms.
Abiotic = non-living. Sunlight, soil minerals and rainfall are physical/chemical factors.
Within the biotic group note the roles: oak tree = producer, deer = consumer, mushroom = decomposer.

Answer:Biotic: deer, mushroom, oak tree. Abiotic: sunlight, soil minerals, rainfall.

Practice this concept4 quick reps

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
Name the two component groups of an ecosystem.
2.
Which biotic group makes its own food?
3.
Which organisms break down dead matter?
4.
Give three abiotic factors of a pond.

Concept 2 of 5

Trophic levels, food chains and the 10% law

Intuition

A food chain is the path energy takes as one organism eats another: grass to grasshopper to frog to snake. Each step is a trophic (feeding) level, and the arrow always points TO the organism that gets the energy. Only about a tenth of the energy passes up each step, which is why chains are short and top predators are few.

Definition

A food chain is a linear sequence showing who eats whom, with energy passing from one feeding level to the next. The trophic levels are:

Producers (1st level) — green plants and algae; trap solar energy to make food.
Primary consumers (2nd level) — herbivores that eat producers (caterpillar, goat, grasshopper, deer).
Secondary consumers (3rd level) — small carnivores/omnivores that eat herbivores (frog, small fish).
Tertiary / top consumers (4th level) — top carnivores that eat secondary consumers (snake, hawk).

The arrow in a food chain points from the eaten to the eater (toward the energy receiver). By the 10% law (Lindeman), only about 10% of the energy at one level is passed to the next — the rest is lost as heat — so a typical chain has only 4–5 links.

The 10% law of energy transfer

E_{n+1} \approx 0.10 \times E_{n}

E_nenergy available at trophic level n
E_{n+1}energy passed to the next (higher) level

Worked example

In the chain Grass → Goat → Tiger, name the trophic level of each organism, and state which is the primary consumer.

Grass makes its own food by photosynthesis → producer (1st level).
The goat is a herbivore that eats the grass → primary consumer (2nd level).
The tiger eats the goat → secondary consumer (3rd level).
The primary consumer is the herbivore — the goat.

Answer:Grass = producer, goat = primary consumer, tiger = secondary consumer. The goat is the primary consumer.

Practice this conceptself-check · 4 quick reps

Try it yourself

Which of these is a valid food chain, and why are the others invalid: (a) Grass, goat and human; (b) Goat, cow and human?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
What trophic level is a caterpillar that eats leaves?
2.
Which trophic level traps solar energy?
3.
By the 10% law, if producers hold 2000 units of energy, how much reaches the primary consumers?
4.
Which way does the arrow point in a food chain?

From the bank · past-year question

Example 2Ecology and EnvironmentEASY

Which one of the following group of organisms forms a food chain?

[Q91 · Apr · 2018]

A food chain must START with a producer

'Goat, cow and human' is not a food chain — goat and cow are both herbivores and neither eats the other. A valid chain begins with a producer (grass) and each later organism eats the one before it.

Primary consumer = herbivore, not 'the first animal you see'

The primary consumer is the organism that eats the PRODUCER — a herbivore (caterpillar, goat, grasshopper). A frog or sparrowhawk eats other animals, so they are secondary or higher consumers, not primary.

Drill 1 more on trophic levels, food chains and the 10% law

Concept 3 of 5

Nutrition modes — autotrophs, heterotrophs, decomposers

Intuition

Organisms are grouped by how they obtain carbon and energy. Autotrophs make their own food from simple inorganic raw materials (mainly carbon dioxide); heterotrophs must eat other organisms; decomposers feed on dead matter. The NDA tests the definition directly — 'organisms that use CO₂ as their main carbon source' are autotrophs.

Definition

Three nutrition modes, classified by carbon and energy source:

Autotrophs — 'self-feeders'; build their own food from carbon dioxide using light (photosynthesis) or chemical energy (chemosynthesis). They are the producers.
Heterotrophs — 'other-feeders'; cannot fix CO₂, so they obtain carbon by eating other organisms. All animals are heterotrophs.
Decomposers (saprotrophs) — heterotrophs that feed on dead organic matter, recycling nutrients back to the soil (bacteria, fungi).

Mode	Carbon source	Examples
Autotroph	Carbon dioxide (CO₂), fixed via photosynthesis or chemosynthesis	Green plants, algae, cyanobacteria NDA 2023 — organisms using CO₂ as their principal carbon source are AUTOTROPHS.
Heterotroph	Organic matter from other organisms	All animals, fungi, most bacteria
Decomposer	Dead organic matter (detritus)	Bacteria, fungi, earthworms
Parasite	Living host's body	Tapeworm, Plasmodium, Cuscuta

Autotroph = producer (makes food from CO₂); heterotroph = consumer (eats others); decomposer = recycler of dead matter.

Practice this conceptself-check · 4 quick reps

Try it yourself

An organism captures atmospheric carbon dioxide and builds its own sugars using sunlight. What nutrition mode is this, and is the organism a producer or a consumer?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
What do we call organisms that use CO₂ as their principal carbon source?
2.
Are all animals autotrophs or heterotrophs?
3.
Which organisms feed on dead organic matter?
4.
Autotrophs occupy which trophic level?

From the bank · past-year question

Example 3Ecology and EnvironmentEASY

Organisms capable of using

CO_2

as principal carbon source are called :

[Q102 · Apr · 2023]

Decomposers are heterotrophs, not autotrophs

A common distractor calls decomposers a kind of autotroph. They are not — they cannot fix CO₂. They are heterotrophs that feed on DEAD matter (saprotrophs), while autotrophs are the green producers that make food from CO₂.

Concept 4 of 5

Ecological interactions — mutualism, commensalism, parasitism

Intuition

When two species live closely together, the relationship is named by who benefits and who is harmed. The NDA favourite is the bee-and-flower partnership: the bee gets nectar and the flower gets pollinated — both win, so it is mutualism. Learn the win/lose pattern for each type.

Definition

Interactions between two species, named by their effect on each partner (+ benefit, − harm, 0 no effect):

Mutualism (+ / +) — both partners benefit (bee and flower; lichen = alga + fungus; nitrogen-fixing bacteria in legume roots).
Commensalism (+ / 0) — one benefits, the other is unaffected (orchid growing on a tree; remora fish on a shark).
Parasitism (+ / −) — one (parasite) benefits, the host is harmed (tapeworm, Plasmodium, Cuscuta).
Predation (+ / −) — the predator kills and eats the prey (lion and deer).
Competition (− / −) — both compete for the same limited resource.

Interaction	Effect (species 1 / 2)	Example
Mutualism	Benefit / Benefit (+ / +)	Bee and flower (nectar for pollination); lichen NDA 2023 — the flower-and-honeybee relationship helps the flower with POLLINATION (a mutualism).
Commensalism	Benefit / No effect (+ / 0)	Orchid on a tree; remora on a shark
Parasitism	Benefit / Harm (+ / −)	Tapeworm, Plasmodium, Cuscuta on host
Predation	Benefit / Harm (+ / −)	Lion eats deer
Competition	Harm / Harm (− / −)	Two plants competing for light/water

Read the sign pair: both + is mutualism; + and 0 is commensalism; + and − (host kept alive) is parasitism.

Practice this conceptself-check · 4 quick reps

Try it yourself

A honey-bee visits a flower, drinks its nectar, and in doing so carries pollen from flower to flower. Name the interaction and state how the flower benefits.

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
In the bee-flower relationship, how does the flower benefit?
2.
What interaction has both partners benefiting (+ / +)?
3.
A tapeworm living in a host gut is which interaction?
4.
An orchid growing on a tree, neither helping nor harming it, is which interaction?

From the bank · past-year question

Example 4Ecology and EnvironmentEASY

Relationship between a flower and honey-bee will help the flower for/in

[Q71 · Sep · 2023]

Bee + flower benefits the flower with POLLINATION, not 'germination' or 'size'

The distractors offer 'quick germination of pollen' or 'increase in size'. The flower's benefit from the bee is pollination — the bee carries pollen between flowers, enabling cross-pollination. This is a mutualism (the bee gets nectar).

Don't confuse commensalism with mutualism

In mutualism BOTH species benefit (+ / +). In commensalism one benefits and the other is unaffected (+ / 0). The bee-flower pair is mutualism because the bee also gains food.

Concept 5 of 5

Biomes — land and aquatic ecosystems

Intuition

A biome is a large region defined by its climate and the community of plants and animals adapted to it — desert, grassland, tropical forest, temperate forest, taiga, tundra. The NDA describes a biome by its trees, climate and animals and asks you to name it; it also tests the ocean as a biome where phytoplankton are the main producers.

Definition

Major land (terrestrial) biomes, identified by climate and vegetation:

Tropical rainforest — hot, very high rainfall year-round; dense evergreen trees; the richest biodiversity.
Temperate (deciduous) forest — high rainfall, cold-to-mild seasons; deciduous trees (maple, oak, hickory, beech) that shed leaves; raccoons, squirrels, deer.
Taiga (boreal forest) — cold; coniferous evergreens (pine, spruce).
Grassland / Savanna — moderate rainfall; grasses, grazing herbivores.
Desert — very low rainfall; cacti, xerophytes.
Tundra — coldest; no trees, mosses and lichens.

In aquatic biomes (oceans, lakes), the main producers are tiny floating phytoplankton, which carry out most of the ocean's photosynthesis and produce most of its organic carbon.

Biome	Climate	Signature life
Tropical rainforest	Hot, high rainfall all year	Dense evergreen trees; greatest biodiversity
Temperate forest	High rainfall, cold-to-mild seasons	Deciduous trees (maple, oak, hickory); raccoons, squirrels NDA 2024 — deciduous maple/oak/hickory + raccoons + cold-to-mild + high rainfall = TEMPERATE forest.
Taiga / Boreal	Cold	Coniferous evergreens (pine, spruce)
Desert	Very low rainfall	Cacti, xerophytes, reptiles
Tundra	Coldest, frozen	No trees; mosses, lichens
Ocean (aquatic)	Saltwater	Phytoplankton = main producers NDA 2018 — phytoplankton produce most of the ocean's organic carbon (true); algae are NOT limited to the cold-water biome (false).

Identify a land biome from its trees + climate; remember phytoplankton are the ocean's primary producers.

Practice this conceptself-check · 4 quick reps

Try it yourself

A forest has high rainfall, temperatures from cold to mild, deciduous trees such as maple and oak, and animals like raccoons and squirrels. Which biome is it?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
A forest of deciduous maple and oak with cold-to-mild seasons is which biome?
2.
Which biome has the greatest biodiversity?
3.
What are the main producers in the ocean?
4.
Coniferous pine and spruce forests in a cold climate make which biome?

From the bank · past-year question

Example 5Ecology and EnvironmentMODERATE

Identify the land biome on the basis of the given characteristics : 1. Their climates are characterised by high rainfall and temperatures that vary from cold to mild. 2. These forests contain primarily deciduous trees – including maple, oak, hickory and beechwood. 3. Raccoons, opossums, bats and squirrels are found in the trees. Select the correct land biome from the options given below :

[Q119 · Apr · 2024]

Deciduous + cold-to-mild = temperate, not tropical or boreal

Tropical forests are hot and evergreen; boreal/taiga forests are coniferous evergreens. A forest of deciduous maple/oak/hickory with cold-to-mild seasons and high rainfall is a temperate forest.

Phytoplankton are everywhere in the ocean, not just cold water

Phytoplankton (and algae) produce most of the ocean's organic carbon — true — but they are NOT restricted to cold-water biomes. A statement claiming algae are produced only in cold water is false.

Drill 1 more on biomes — land and aquatic ecosystems

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)

Trophic levels, food chains and the 10% law
The 10% law of energy transfer
$E_{n+1} \approx 0.10 \times E_{n}$

Reference tables (3)

Nutrition modes — autotrophs, heterotrophs, decomposers4 rows

Mode	Carbon source	Examples
Autotroph	Carbon dioxide (CO₂), fixed via photosynthesis or chemosynthesis	Green plants, algae, cyanobacteria NDA 2023 — organisms using CO₂ as their principal carbon source are AUTOTROPHS.
Heterotroph	Organic matter from other organisms	All animals, fungi, most bacteria
Decomposer	Dead organic matter (detritus)	Bacteria, fungi, earthworms
Parasite	Living host's body	Tapeworm, Plasmodium, Cuscuta

Autotroph = producer (makes food from CO₂); heterotroph = consumer (eats others); decomposer = recycler of dead matter.

Ecological interactions — mutualism, commensalism, parasitism5 rows

Interaction	Effect (species 1 / 2)	Example
Mutualism	Benefit / Benefit (+ / +)	Bee and flower (nectar for pollination); lichen NDA 2023 — the flower-and-honeybee relationship helps the flower with POLLINATION (a mutualism).
Commensalism	Benefit / No effect (+ / 0)	Orchid on a tree; remora on a shark
Parasitism	Benefit / Harm (+ / −)	Tapeworm, Plasmodium, Cuscuta on host
Predation	Benefit / Harm (+ / −)	Lion eats deer
Competition	Harm / Harm (− / −)	Two plants competing for light/water

Read the sign pair: both + is mutualism; + and 0 is commensalism; + and − (host kept alive) is parasitism.

Biomes — land and aquatic ecosystems6 rows

Biome	Climate	Signature life
Tropical rainforest	Hot, high rainfall all year	Dense evergreen trees; greatest biodiversity
Temperate forest	High rainfall, cold-to-mild seasons	Deciduous trees (maple, oak, hickory); raccoons, squirrels NDA 2024 — deciduous maple/oak/hickory + raccoons + cold-to-mild + high rainfall = TEMPERATE forest.
Taiga / Boreal	Cold	Coniferous evergreens (pine, spruce)
Desert	Very low rainfall	Cacti, xerophytes, reptiles
Tundra	Coldest, frozen	No trees; mosses, lichens
Ocean (aquatic)	Saltwater	Phytoplankton = main producers NDA 2018 — phytoplankton produce most of the ocean's organic carbon (true); algae are NOT limited to the cold-water biome (false).

Identify a land biome from its trees + climate; remember phytoplankton are the ocean's primary producers.

Watch out for (7)

A food chain must START with a producer
→ Trophic levels, food chains and the 10% law
Primary consumer = herbivore, not 'the first animal you see'
→ Trophic levels, food chains and the 10% law
Decomposers are heterotrophs, not autotrophs
→ Nutrition modes — autotrophs, heterotrophs, decomposers
Bee + flower benefits the flower with POLLINATION, not 'germination' or 'size'
→ Ecological interactions — mutualism, commensalism, parasitism
Don't confuse commensalism with mutualism
→ Ecological interactions — mutualism, commensalism, parasitism
Deciduous + cold-to-mild = temperate, not tropical or boreal
→ Biomes — land and aquatic ecosystems
Phytoplankton are everywhere in the ocean, not just cold water
→ Biomes — land and aquatic ecosystems

Mastery check — 2 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Ecology and EnvironmentEASY

Which one of the following organisms represents the primary consumer category in an ecosystem?

[Q55 · Apr · 2019]

Example 2Ecology and EnvironmentMODERATE

Directions : The following six (6) items consist of two statements, Statement I and Statement II. Examine these two statements carefully and select the correct answer using the code given below. Code: (a) Both the statements are individually true and Statement II is the correct explanation of Statement I (b) Both the statements are individually true but Statement II is not the correct explanation of Statement I (c) Statement I is true but Statement II is false (d) Statement I is false but Statement II is true

Statement I : Phytoplankton produce most of the organic carbon in the ocean. Statement II : Algae are produced in the cold water biome.

[Q55 · Sep · 2018]

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