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NDA Biology · Ecology and Environment

Environment and Biodiversity

The environmental-issues half of the chapter: greenhouse gases and global warming, biogas and biomass as renewable energy, the variety of life (biodiversity) and how it is conserved, and the depletion of groundwater.

All Ecology and Environment notesNDA Biology strategy

Why this matters

6 PYQs, EASY-to-MODERATE, and almost all are 'spot the odd one out' or 'which statement is NOT correct'. The bank reliably tests: which gas is NOT a greenhouse gas (oxygen), what biogas is mostly made of (methane), which activity does NOT reduce biodiversity (sacred groves help it), and which is NOT a cause of groundwater depletion (afforestation helps recharge). Learn the short fact lists and the recall is automatic.

Concept 1 of 4

Greenhouse gases and global warming

Intuition

Greenhouse gases trap the Sun's heat in the atmosphere — like glass in a greenhouse — keeping the Earth warm. Too much of them causes global warming. The NDA tests which gases ARE greenhouse gases; the trick is that oxygen and nitrogen, the main gases of the air, are NOT greenhouse gases.

Definition

Greenhouse gases (GHGs) absorb and re-emit the Earth's outgoing heat, warming the lower atmosphere. The major ones are:

  • Water vapour (H₂O) — the most abundant greenhouse gas.
  • Carbon dioxide (CO₂) — the main man-made driver (fossil-fuel burning).
  • Methane (CH₄) — far stronger per molecule than CO₂; from cattle, paddy fields, biogas.
  • Nitrous oxide (N₂O) and CFCs / ozone — minor but potent.

Oxygen (O₂) and nitrogen (N₂) are NOT greenhouse gases — even though they make up most of the air, they do not trap heat.

GasGreenhouse gas?Note
Water vapour (H₂O)YesMost abundant GHG
Carbon dioxide (CO₂)YesMain man-made driver
Methane (CH₄)YesStrong; from cattle, paddy, biogas
Oxygen (O₂)NoA main gas of air, but does NOT trap heat
NDA 2023 — Oxygen is NOT a main greenhouse gas (the odd one out).
Nitrogen (N₂)No78% of air, but not a GHG
The major GHGs are water vapour, CO₂ and methane. Oxygen and nitrogen are NOT greenhouse gases.
Practice this conceptself-check · 4 quick reps

Try it yourself

From water vapour, oxygen, carbon dioxide and methane, which one is NOT a main greenhouse gas?

Practice — Level 1 (4 reps)

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

  1. 1.
    Name the three major greenhouse gases.
  2. 2.
    Is oxygen a greenhouse gas?
  3. 3.
    Which greenhouse gas is the most abundant?
  4. 4.
    Which greenhouse gas is the main man-made driver of warming?

From the bank · past-year question

Example 1Ecology and EnvironmentEASY
Which one of the following is not a main greenhouse gas?

[Q90 · Apr · 2023]

Oxygen and nitrogen are NOT greenhouse gases

They are the two main gases of the air, which tempts students to mark them as greenhouse gases. They are not — only gases that absorb outgoing heat (water vapour, CO₂, methane, N₂O) qualify.

Concept 2 of 4

Biogas and biomass — renewable energy from waste

Intuition

Biomass is plant and animal waste used as an energy source. Let cow-dung, crop residue and sewage rot WITHOUT oxygen and microbes produce biogas (gobar gas), a clean fuel. The NDA tests two facts: biogas is mostly methane, and its heating value is actually HIGH — so a statement calling it low is wrong.

Definition

Biomass is organic matter (plant/animal waste, crop residue, dung) used as a renewable energy source. Biogas (gobar gas) is produced when biomass — cow-dung, crop residues, vegetable waste, sewage — is allowed to decompose by bacteria in the absence of oxygen (anaerobic digestion).

  • Composition — biogas is mostly methane (CH₄), about 50–70%, the rest mainly CO₂ with traces of H₂ and H₂S.
  • It is a renewable, clean fuel — burning it gives heat with little smoke, and the process reduces soil and water pollution.
  • High heating value — because methane is its main component, biogas has a HIGH calorific (heating) value. A claim that biogas has a very low heating capacity is INCORRECT.

Worked example

Cow-dung and crop waste are sealed in a tank with no air and allowed to rot. The gas collected burns with a clean flame. What is this gas mostly made of, and is its heating value high or low?
  1. Decomposing biomass without oxygen is anaerobic digestion — it produces biogas (gobar gas).
  2. Biogas is roughly 50–70% methane, the rest mostly carbon dioxide.
  3. Methane is a good fuel, so biogas has a HIGH heating (calorific) value, not a low one.
Answer:Mostly methane; its heating value is high.
Practice this conceptself-check · 4 quick reps

Try it yourself

Which of these statements about biogas is NOT correct: (i) biomass is a renewable energy source; (ii) gobar gas forms by decomposing dung and sewage without oxygen; (iii) biogas reduces soil and water pollution; (iv) the heating capacity of biogas is very low?

Practice — Level 1 (4 reps)

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

  1. 1.
    Biogas is mostly composed of which gas?
  2. 2.
    Is biogas produced in the presence or absence of oxygen?
  3. 3.
    Is the heating value of biogas high or low?
  4. 4.
    Name two raw materials used to make gobar gas.

From the bank · past-year question

Example 2Ecology and EnvironmentEASY
Which one of the following is the largest composition in biogas?

[Q140 · Apr · 2020]

Biogas heating value is HIGH, not low

Because biogas is mostly methane, it burns well and has a high calorific value. A statement claiming 'heating capacity of biogas is very low' is the INCORRECT one the bank wants you to catch (NDA 2020).

Biogas is the largest component methane — not carbon dioxide

CO₂ is present (the second-largest part), but the LARGEST component is methane (~50–70%). Marking carbon dioxide as the main component is the classic error.
Drill 1 more on biogas and biomass — renewable energy from waste

Concept 3 of 4

Biodiversity, hotspots and conservation

Intuition

Biodiversity is the variety of all living things. It is being lost to deforestation, hunting and encroachment — but traditional protected areas such as sacred groves actually preserve it. The NDA tests the named facts (who coined 'biodiversity', what a hotspot is, roughly how many hotspots exist) and asks which activity does NOT reduce biodiversity.

Definition

Biodiversity is the variety of life — the number and variety of species, genes and ecosystems. Key recall facts:

  • The term 'biodiversity' was coined by Walter G. Rosen (1986).
  • A biodiversity hotspot is a region with exceptionally rich, threatened biodiversity; the term was coined by Norman Myers (1988).
  • There are about 36 hotspots recognised worldwide — NOT more than 100.
  • Causes of biodiversity loss: large-scale deforestation, over-exploitation of forest produce, hunting, pollution, and encroachment into forests.
  • Conservation measures: national parks, wildlife sanctuaries, biosphere reserves, and traditional sacred groves (forest patches protected by local communities) — these INCREASE biodiversity, they do not reduce it.
FactDetail
Term 'biodiversity' coined byWalter G. Rosen (1986)
Term 'biodiversity hotspot' coined byNorman Myers (1988)
Number of hotspots worldwideAbout 36 (NOT more than 100)
NDA 2021 — 'More than 100 hotspots are identified' is the INCORRECT statement.
Causes of lossDeforestation, over-exploitation, encroachment, hunting
Sacred grovesConserve biodiversity (do NOT reduce it)
NDA 2020 — maintaining sacred groves is NOT a cause of biodiversity decrease.
Rosen coined 'biodiversity' (1986); Myers coined 'hotspot' (1988); ~36 hotspots; sacred groves protect, not harm.
Practice this conceptself-check · 4 quick reps

Try it yourself

Which of these is NOT a reason for the decrease in biodiversity: large-scale deforestation, exploitation of forest produce, maintaining sacred groves, or encroachment in forest areas?

Practice — Level 1 (4 reps)

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

  1. 1.
    Who coined the term 'biodiversity'?
  2. 2.
    Who coined the term 'biodiversity hotspot'?
  3. 3.
    Roughly how many biodiversity hotspots are recognised worldwide?
  4. 4.
    Do sacred groves increase or decrease biodiversity?

From the bank · past-year question

Example 3Ecology and EnvironmentMODERATE
Which one of the following statements about biodiversity is not correct?

[Q62 · Sep · 2021]

Sacred groves CONSERVE biodiversity

In a 'which is NOT a cause of biodiversity decrease?' question, the answer is the conservation activity — maintaining sacred groves protects forest, so it does not reduce biodiversity (NDA 2020).

There are about 36 hotspots, not 'more than 100'

Rosen (1986) coined 'biodiversity' and Myers (1988) coined 'hotspot' — both true. The false statement is that more than 100 hotspots exist; the recognised figure is about 36 (NDA 2021).
Drill 1 more on biodiversity, hotspots and conservation

Concept 4 of 4

Groundwater depletion and conservation

Intuition

Groundwater is the water stored underground in soil and rock. It is depleted by over-pumping, by losing the forests that help rain soak in, and by paving land with concrete so rain runs off instead of recharging. The NDA asks which activity does NOT deplete groundwater — and planting trees (afforestation) actually recharges it.

Definition

Groundwater is recharged when rainwater seeps into the soil. Causes that DEPLETE it versus actions that CONSERVE it:

  • Depleting causes: excessive pumping of groundwater (over-extraction); loss of forests (less seepage, more runoff); large-scale concrete construction (paving blocks rain from soaking in).
  • Conserving actions: afforestation (planting trees, whose roots and litter let water soak in and recharge the aquifer); rainwater harvesting; check dams.

Afforestation INCREASES recharge — it is the opposite of a depletion cause.

ActivityEffect on groundwater
Excessive pumpingDepletes — over-extraction lowers the water table
Loss of forestsDepletes — bare soil means more runoff, less seepage
Large-scale concrete buildingsDepletes — paving stops rain soaking in
Afforestation (planting trees)Conserves / recharges — roots and litter aid seepage
NDA 2020 — afforestation is NOT a cause of groundwater depletion (it recharges it).
Pumping, deforestation and concreting deplete groundwater; afforestation and rainwater harvesting recharge it.
Practice this conceptself-check · 4 quick reps

Try it yourself

Which of these is NOT a cause of groundwater depletion: afforestation, loss of forests, excessive pumping, or construction of large concrete buildings?

Practice — Level 1 (4 reps)

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

  1. 1.
    Which activity recharges groundwater rather than depleting it?
  2. 2.
    How does large-scale concrete construction affect groundwater?
  3. 3.
    Name two ways to conserve groundwater.
  4. 4.
    Does loss of forests increase or decrease groundwater recharge?

From the bank · past-year question

Example 4Ecology and EnvironmentEASY
Which one of the following is NOT a cause of depletion in groundwater?

[Q137 · Apr · 2020]

Afforestation RECHARGES groundwater — it is the odd one out

In a 'which is NOT a cause of groundwater depletion?' question, pick the conservation action. Afforestation helps rainwater seep in and recharge the water table, so it does not deplete groundwater (NDA 2020).

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.

Reference tables (3)

Greenhouse gases and global warming5 rows
GasGreenhouse gas?Note
Water vapour (H₂O)YesMost abundant GHG
Carbon dioxide (CO₂)YesMain man-made driver
Methane (CH₄)YesStrong; from cattle, paddy, biogas
Oxygen (O₂)NoA main gas of air, but does NOT trap heat
NDA 2023 — Oxygen is NOT a main greenhouse gas (the odd one out).
Nitrogen (N₂)No78% of air, but not a GHG
The major GHGs are water vapour, CO₂ and methane. Oxygen and nitrogen are NOT greenhouse gases.
Biodiversity, hotspots and conservation5 rows
FactDetail
Term 'biodiversity' coined byWalter G. Rosen (1986)
Term 'biodiversity hotspot' coined byNorman Myers (1988)
Number of hotspots worldwideAbout 36 (NOT more than 100)
NDA 2021 — 'More than 100 hotspots are identified' is the INCORRECT statement.
Causes of lossDeforestation, over-exploitation, encroachment, hunting
Sacred grovesConserve biodiversity (do NOT reduce it)
NDA 2020 — maintaining sacred groves is NOT a cause of biodiversity decrease.
Rosen coined 'biodiversity' (1986); Myers coined 'hotspot' (1988); ~36 hotspots; sacred groves protect, not harm.
Groundwater depletion and conservation4 rows
ActivityEffect on groundwater
Excessive pumpingDepletes — over-extraction lowers the water table
Loss of forestsDepletes — bare soil means more runoff, less seepage
Large-scale concrete buildingsDepletes — paving stops rain soaking in
Afforestation (planting trees)Conserves / recharges — roots and litter aid seepage
NDA 2020 — afforestation is NOT a cause of groundwater depletion (it recharges it).
Pumping, deforestation and concreting deplete groundwater; afforestation and rainwater harvesting recharge it.

Watch out for (6)

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 EnvironmentMODERATE
Which one of the following statements is NOT correct?

[Q124 · Apr · 2020]

Example 2Ecology and EnvironmentEASY
Which one of the following is NOT a reason of decrease in biodiversity?

[Q136 · Apr · 2020]

Drill every past-year question on this subtopic

6 questions from the bank — paginated, with cart and Word-export support.

Drill the 6 questions