Ecosystem

Detritus food chain (DFC) begins with detritus or dead organic matter. Detritivores (e.g. earthworm, millipedes, slugs) perform fragmentation process on dead decaying material in any ecosystem. Humus, a dark nutrient rich substance, is degraded by microorganisms releasing inorganic nutrients into the soil. This process is known as mineralisation. By the process of leaching, water soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.

Decomposition is carried out by the enzymes of bacteria and fungi, and its rate is controlled by the chemical composition of the detritus and by climatic factors, mainly temperature and soil moisture.
Decomposer microbes need water for their growth and for their extracellular enzymes to act. A warm and moist environment favours decomposition, whereas dryness inhibits it.
Fallen logs are made of dry, dead wood with a low moisture content, so microbial activity in them is slow and they decompose slowly. This is the answer given in the answer key.
The surroundings of a fallen log are aerobic, not anaerobic, and wood is rich in cellulose, not low in it.
The chemical nature of wood also contributes: the decomposition rate is slower if the detritus is rich in lignin and chitin, and quicker if it is rich in nitrogen and water-soluble substances like sugars. Wood is rich in lignin and poor in nitrogen, so some books give poor nitrogen content as the answer to this question.

The pyramid has one primary producer (PP = 1), which means a single large tree.
A single tree supports a large number of insects as primary consumers (PC = 500), which are eaten by a smaller number of small insectivorous birds as secondary consumers (SC = 50).
Rats do not feed in hundreds on one tree, phytoplankton would be far more numerous than 1, and sheep are herbivores, not secondary consumers.

A pyramid of numbers shows the number of individuals at each successive trophic level, with the producers at the base.
In most ecosystems the producers are more numerous than the herbivores, and the herbivores more numerous than the carnivores, so the pyramid is upright: broadest at the base and narrowing towards the top.
The answer accepted in the key is the upright pyramid, labelled B, taking the forest as a whole with its large number of producers.
The pyramid of numbers becomes inverted only when a single large tree is considered as the ecosystem, since one tree supports many insects and birds.

By the 10 percent law, energy transferred to each trophic level is only 10% of the level below it. Working backwards from $T_4=1000$ J: $T_3=10{,}000$ J; $T_2=1{,}00{,}000$ J; $T_1=10{,}00{,}000$ J.

Ecological pyramids assume a simple, straight-line food chain and do NOT accommodate a food web involving several interconnected food chains — this is one of their limitations. So the statement 'It accommodates a food web' is not a real limitation because it does not correctly describe the pyramid (pyramids fail to accommodate food webs); the other three options are genuine, correctly stated limitations.

Energy content actually decreases (not increases) progressively from lower to higher trophic levels, following the 10 percent law of energy transfer — so the statement that energy content increases from first to fourth trophic level is incorrect.

Unlike the carbon or nitrogen cycles, the phosphorus cycle is a sedimentary cycle with no significant gaseous/atmospheric phase — there is no appreciable respiratory release of phosphorus into the atmosphere, so this statement is incorrect.

Detritus rich in lignin and chitin decomposes more slowly (these are resistant structural compounds), so this factor is an exception — it actually slows down decomposition rather than speeding it up, unlike detritus rich in sugars, a warm/moist environment, or the presence of aerobic microbes.

Hydrarch succession proceeds from phytoplankton to mesic conditions (A-IV); Xerarch succession proceeds from lichens to mesic conditions (B-III); Ecological succession in general involves a gradual change in species composition (C-I); Secondary succession is faster and reaches climax quickly (D-II).

Excess nitrates and phosphates (from agricultural runoff/fertilizers) entering water bodies cause eutrophication — rapid, excessive growth of algae and aquatic plants, leading to oxygen depletion and disruption of the aquatic ecosystem.

The amount of nutrients present in the soil (or any reservoir) at any given time is referred to as the standing state (or standing state of nutrients).

The stages of a hydrosere in order are: phytoplankton stage → rooted submerged plant stage → rooted floating and free-floating plant stage → reed-swamp stage → marsh-meadow stage → scrub (woodland) stage → forest (climax).
The second stage is therefore the submerged plant stage, occupied by plants such as $Hydrilla$ and $Vallisneria$.
$Azolla$ is a free-floating plant of the third stage.
$Typha$ belongs to the reed-swamp stage and $Salix$ to the woodland stage.
Hence the second stage is occupied by plants like $Vallisneria$.

Secondary succession begins in areas where the natural biotic communities have been destroyed, such as abandoned farm lands, burned or cut (deforested) forests and lands that have been flooded.
Because some soil or sediment is already present, it is faster than primary succession.
Succession that begins on a bare rock, where no living organisms ever existed, is primary succession.
It is not simply the same as primary succession at a faster pace: the starting conditions and the pioneer species are different, and it does not have to follow a primary succession.
Hence the correct statement is that it occurs on a deforested site.

A hydrosere is succession starting in water and ending in a mesic forest. Its stages are:
Phytoplankton stage (pioneers): e.g., $Volvox$
Submerged plant stage: e.g., $Hydrilla$
Submerged and free-floating plant stage: e.g., $Pistia$
Reed-swamp stage: e.g., $Scirpus$
Marsh-meadow and scrub stage: shrubs such as $Lantana$
Forest (climax) stage: trees such as oak
Correct sequence: $Volvox$ → $Hydrilla$ → $Pistia$ → $Scirpus$ → $Lantana$ → Oak

About 71% of carbon is found dissolved in oceans; annual fixation of carbon through photosynthesis is about $4\times10^{13}$ kg; plants capture only 2-10% of PAR (photosynthetically active radiation); productivity of oceans is only 55 billion tons.

Earthworm is a detritivore (ii).
Succession begins with pioneer species (i).
Pollination is an ecosystem service (iv).
Natality contributes to population growth (iii).
Correct match: a-ii, b-i, c-iv, d-iii.

Phosphorus is released from rock minerals (B) by weathering into the soil solution and is taken up by producers (C).
It passes to consumers, returns through litter fall (D) to detritus (A), and decomposition sends it back to the soil solution.

In the food web, vegetation/seeds form the producer level and the arrows show who eats whom.
(a) feeds on vegetation and is eaten by the lion, so it must be a large herbivore: deer.
(b) feeds on vegetation and is eaten by foxes, so it is a small herbivore: rabbit.
(c) feeds on grasshoppers and is eaten by snakes, so it is an insect-eating carnivore: frog.
(d) feeds on vegetation/seeds and is eaten by hawks, snakes and owls, so it is a small seed-eating herbivore: rat.
A cat, dog, bat or tortoise does not fit these feeding positions.
So (a) is deer, (b) is rabbit, (c) is frog and (d) is rat.

A species may occupy more than one trophic level in the same ecosystem, depending on what it eats.
Phytoplankton are producers (first trophic level only).
Zooplankton feed on phytoplankton and are primary consumers (second trophic level only).
Fishes in a pond feed on phytoplankton as well as on zooplankton, insects and smaller fishes, so they act as primary, secondary and even tertiary consumers.
The frog feeds mainly on insects and is a carnivore.
Hence fish occupy more than one trophic level in a pond ecosystem.

(ii) is correct: carnivores keep the population of herbivores in check. When most of the carnivores are removed, predation pressure falls and the population of deer increases.
(iii) is correct: according to the 10 percent law, only about 10 percent of the energy is transferred from one trophic level to the next; the rest is lost as heat in respiration. So very little energy is left after 3 to 4 transfers, and the number of trophic levels in a food chain is restricted to 3-4.
(i) is not correct: removal of tigers increases the number of herbivores, which graze more, so the vegetation decreases instead of increasing.
(iv) is not correct: food chains do not extend to 8 trophic levels, because the energy available becomes too small to support them.
So the two correct statements are (ii) and (iii).

Stability (resistance) is the ability of an ecosystem to resist change, i.e. to remain unchanged when a disturbance acts on it.
Resilience is the ability of an ecosystem to recover and return to its original state after the disturbance is removed.
An ecosystem that is easily damaged has low resistance, i.e. low stability.
An ecosystem that recovers after some time once the damaging effect stops has high resilience.
So such an ecosystem has low stability and high resilience (e.g. a grassland that is quickly damaged by fire or grazing but grows back soon).