Q.How much of the net primary productivity of a terrestrial ecosystem is eaten and digested by herbivores?

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: $\text{Death rate}=\frac{8}{80}=0.1$ …

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 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. ✓ …

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. …

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. …

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. …

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. …