Q.The second trophic level in a lake is

1. The concept. In population ecology, the death rate (mortality rate) is expressed on a per capita basis: it is the number of individuals that die per individual of the population, per unit time.

2. Substitute the data. The laboratory population has $N = 80$ fruit flies, and $8$ individuals died in the specified time interval:

3. Interpret the units. This is read as 0.1 individual per fruit fly per time interval — i.e. on average, one-tenth of an individual dies per fly per interval (equivalently, 10% mortality over that interval). This mirrors the standard NCERT example in which 4 out of 40 flies dying gives a death rate of $4/40 = 0.1$ individuals per fruit fly per week.

4. Rule out the others. $0.001$ and $0.01$ would correspond to far smaller mortality fractions, and $1$ would mean the entire population died.

ANSWER: B

Step 1 — What Cuscuta is. Cuscuta (dodder, 'amarbel') is a total stem parasite: it has lost its chlorophyll and its leaves are reduced to scales, so it cannot photosynthesise. It commonly grows on hedge plants such as Duranta.

Step 2 — How it feeds. Its twining stems wrap around the host's shoot and send haustoria — modified absorptive roots — through the host's tissue into the phloem, drawing off water and ready-made food. Note the body of the parasite stays on the outside of the host.

Step 3 — Classify the parasitism.

• Ectoparasite — lives on the external surface of the host (e.g. lice, ticks, Cuscuta).
• Endoparasite — lives inside the host's body (e.g. Ascaris, liver fluke, Plasmodium). Since Cuscuta lives on the host's surface, it is an ectoparasite → ectoparasitism.

Step 4 — Eliminate the rest.

• (B) 'Broad parasitism' — not a recognised ecological category (a distractor).
• (C) Predation — the predator kills and eats its prey; Cuscuta keeps its host alive and drains it slowly, which is parasitism, not predation.
• (D) Endoparasitism — would require Cuscuta to live wholly inside the host body, which it does not.

ANSWER: A

Step 1 — The disease

Pneumonia is a bacterial disease of humans that affects the alveoli (air sacs) of the lungs.

Causative agents: Streptococcus pneumoniae and Haemophilus influenzae.

Step 2 — Pathology (why it is dangerous)

In an infected person, the alveoli become filled with fluid, which severely impairs gaseous exchange:

• Symptoms: fever, chills, cough and headache; in severe cases the lips and finger-nails turn grey to bluish — cyanosis, the visible sign of poor oxygenation of the blood.
• Transmission: by inhaling the droplets/aerosols released by an infected person, or even by sharing glasses and utensils with them.

Step 3 — Evaluate the options

• (A) Streptococcus pneumoniae — a genuine causative agent. ✓
• (B) Haemophilus influenzae — also a genuine causative agent. ✓ (Despite its misleading name, this is a bacterium, not the influenza virus.)
• Since both A and B are correct, neither alone is the complete answer.
• (D) None is clearly false.

Step 4 — Conclude

When a question lists two agents that are both named in the syllabus for one disease, the inclusive option is the only complete one.

ANSWER: C

The key here is to know the role of ozone in different atmospheric layers. Many students confuse the "bad ozone" at ground level (a pollutant) with the "good ozone" high up that protects life. Let’s check each statement carefully.

1. Statement (A): "Most of the forests have been lost in tropical areas."

   This is true. Tropical rainforests (like the Amazon, Congo Basin, and Southeast Asia) have experienced massive deforestation due to logging, agriculture, and urbanization. Over half of the world’s original tropical forests are already gone.

2. Statement (B): "Greenhouse effect is a natural phenomenon."

   This is also true. Without the natural greenhouse effect (from water vapour, CO₂, methane, etc.), Earth’s average temperature would be about -18°C instead of the current 15°C. The problem today is the enhanced greenhouse effect from human emissions — but the phenomenon itself is natural.

3. Statement (C): "Eutrophication is a natural phenomenon in freshwater lakes."

   This is true, but with a nuance. Eutrophication — the enrichment of water with nutrients (especially phosphorus and nitrogen) leading to algal blooms and oxygen depletion — can occur naturally over centuries as a lake ages. However, human activities (fertilizer runoff, sewage) greatly accelerate it. The statement says it is a natural phenomenon, which is correct; it does not claim it is only natural.

4. Statement (D): "Ozone in the upper part of the atmosphere is harmful to animals."

   This is false. Ozone in the stratosphere (the "ozone layer") absorbs 97–99% of the Sun’s harmful ultraviolet (UV-B and UV-C) radiation. Without it, life on land would be severely damaged — skin cancer, cataracts, and DNA damage would skyrocket. The ozone layer is protective, not harmful.

   Watch out

   Don’t confuse this with tropospheric ozone (ground-level ozone), which is a harmful air pollutant formed from vehicle exhaust and industrial emissions. The question specifically says "upper part of the atmosphere" — that’s the stratosphere, where ozone is good.

The key here is to distinguish between two very different numbers that often get confused in ecology: the fraction of total solar radiation that is PAR, and the fraction of PAR that plants actually use.

Photosynthetically Active Radiation (PAR) is the portion of sunlight with wavelengths between 400 and 700 nm — the part that drives photosynthesis. About 50% of the total solar radiation reaching Earth's surface falls in this range. But that's not what the question is asking. The question asks: of the PAR that reaches the plant, how much is actually captured and used?

Plants do not absorb every photon that hits them. Leaves reflect some light (especially green), transmit some through the leaf, and only a fraction of the absorbed photons actually drives carbon fixation. The rest is lost as heat or fluorescence. On top of that, canopy structure, leaf angle, and non-photosynthetic tissues all reduce the effective capture.

The well-established ecological figure is that plants capture only about 2–10% of the incident PAR. This is the net primary productivity efficiency relative to the PAR input. The other options — 30–40% or 10–20% — are far too high; those numbers might correspond to the absorption efficiency of a single leaf in a lab, not a whole plant or canopy in the field.

1. Identify what the question is actually measuring.

   The phrase "captured by the plants" means the energy that ends up stored as biomass (gross primary productivity minus respiration losses) relative to the PAR that falls on the plant. This is the ecological efficiency of photosynthesis in natural conditions.

2. Recall the known range from ecology.

   In most terrestrial ecosystems, the efficiency of converting PAR into plant biomass is low — typically between 2% and 10%. Even the most productive crops (like sugarcane or maize) rarely exceed 6–8% over a growing season. Natural forests and grasslands are often at the lower end, around 2–4%.

3. Eliminate the wrong options.

   • Option (B) 30–40%: This is roughly the fraction of total solar radiation that is PAR, not the fraction captured. A common confusion.
   • Option (C) 10–20%: This is the upper limit for very efficient crops under ideal conditions, but the question asks about "the plants" in general, not just the best-case scenario. The standard textbook range is 2–10%.
   • Option (D) 0–10%: This is close, but the lower bound of 0% is misleading — plants do capture some PAR, and the accepted minimum is about 2%, not zero. The range 2–10% is the precise one.

4. Confirm with the standard ecological data. In NCERT and most ecology textbooks, the figure given is: "Of the incident PAR, about 2–10% is captured by plants and converted into biomass." This is a direct fact from the chapter on ecosystem productivity.

The concept: Biological magnification and eggshell thinning

DDT (dichlorodiphenyltrichloroethane) is a persistent organic pollutant. It does not break down easily in the environment and is fat-soluble, meaning it accumulates in the fatty tissues of organisms. When a predator eats prey contaminated with DDT, the chemical gets concentrated further up the food chain — this is called biological magnification (or biomagnification).

In birds of prey (like falcons, eagles, and pelicans), DDT at the top of the food chain reaches very high concentrations. The key effect is not direct poisoning of adult birds, but a subtle disruption of their calcium metabolism. DDT interferes with the enzyme carbonic anhydrase, which is needed to deposit calcium carbonate into eggshells. The result: eggs with abnormally thin, fragile shells.

Step-by-step reasoning

1. DDT enters the food chain

   DDT sprayed on crops is washed into soil and water. It is absorbed by small organisms like plankton and insects. These are eaten by fish and earthworms, which are in turn eaten by birds. Because DDT is stored in fat and not excreted, its concentration increases at each trophic level.

2. Top predators accumulate the highest doses

   Birds that feed on fish or earthworms in DDT-treated areas accumulate the chemical in their bodies. The concentration can be millions of times higher than in the surrounding environment.

3. DDT disrupts eggshell formation

   In female birds, DDT (and its metabolite DDE) inhibits the action of carbonic anhydrase in the shell gland. This enzyme normally helps deposit calcium carbonate to form a strong shell. Without it, the shell is deposited too thinly.

4. Eggs break before hatching

   When the parent bird sits on the nest to incubate the eggs, the thin shells crack under the pressure. The eggs break prematurely, killing the embryos. Very few chicks survive, so the bird population declines over time.

5. Why the other options are wrong

   • (A) Birds stopped laying eggs — DDT does not prevent egg-laying; birds continue to lay eggs, but the eggs are defective.
   • (B) Earthworms in the area got eradicated — DDT does reduce earthworm populations, but the primary cause of bird decline is eggshell thinning, not food shortage. Even where earthworms survive, birds still fail to reproduce.
   • (C) Birds became vulnerable to predators — DDT does not make adult birds more vulnerable to predation. The decline is due to reproductive failure, not increased predation.

The key to this question lies in understanding what "floc" actually is and the environment in which it forms. Sewage treatment has two main stages: primary (physical settling) and secondary (biological treatment). Floc formation is the hallmark of the secondary stage.

In secondary treatment, the sewage is constantly aerated — air is pumped in. This creates an aerobic (oxygen-rich) environment. The microorganisms that thrive here are primarily aerobic bacteria and fungi. They feed on the dissolved organic matter in the sewage, breaking it down into simpler substances. As they grow and metabolize, they secrete sticky, gelatinous substances (polysaccharides and proteins) that cause them to clump together. These clumps, called flocs, also trap suspended particles and other microbes.

The floc itself is a complex community, but the dominant, active decomposers are aerobic bacteria and fungi. The flocs eventually settle out in a sedimentation tank, leaving clarified water.

Let's eliminate the other options:

1. Option (A) — Anaerobic bacteria and fungus: Incorrect. The aeration tank is aerobic, not anaerobic. Anaerobic bacteria would not be the primary floc-formers here.
2. Option (C) — Autotrophic bacteria and yeast: Incorrect. While some autotrophic bacteria (like nitrifying bacteria) are present in the aeration tank, they are not the main agents of floc formation. The bulk of the organic matter breakdown is done by heterotrophic bacteria and fungi. Yeasts are not the primary floc-formers in this system.
3. Option (D) — Fungus and algae: Incorrect. Algae require sunlight for photosynthesis. Sewage treatment tanks are typically deep and opaque, so algae are not a significant component of the floc. The floc is formed in the dark, oxygen-rich water.