RowQ
The Vault
RowQ
The Vault
CBSE Class 12 Biology · 11 questions · 26 marks
An ecosystem is the working unit of ecology: a patch of the living world plus the physical surroundings it exchanges energy and matter with. This chapter follows the sunlight that enters a field or a pond as it is fixed by producers, passed along trophic levels with heavy losses at every step, and finally released by decomposers, and it shows how the same community changes in a predictable sequence over time. Most of the marks here come from three things done accurately — the energy budget, the three ecological pyramids and their exceptions, and the stages of succession.
Which ecological pyramid can never be inverted for any ecosystem?
Answer
The pyramid of energy is correct — energy is lost as heat at every transfer, so each higher trophic level must always contain less energy than the one below it, and the pyramid is invariably upright. The pyramid of number can be inverted for a single tree supporting thousands of insects, and the pyramid of biomass is inverted in the open sea where a small standing crop of phytoplankton supports a larger mass of zooplankton.
If the producers of a pond fix 50,000 kJ of energy in a year, the energy expected to be available to the secondary consumers, following the ten per cent law, is about:
Answer
500 kJ is correct — about 10 per cent passes from producers to the primary consumers, giving 5,000 kJ, and about 10 per cent of that reaches the secondary consumers, giving 500 kJ. The value 5,000 kJ belongs to the primary consumers and 50 kJ to the next level above. The losses at each step are respiratory heat and unassimilated matter.
The amount of living matter present in an ecosystem at a particular moment in time is called:
Answer
Standing crop is correct — it is the biomass present at a given moment and is a quantity, not a rate. Gross primary productivity is the total rate of organic matter production by producers, net primary productivity is what remains of it after respiratory loss, and secondary productivity is the rate at which consumers build new organic matter.
The pioneer community in a xerarch succession beginning on a bare rock surface is usually made up of:
Answer
Lichens is correct — they can colonise a bare rock face, and by secreting acids they dissolve the rock and begin the formation of a thin soil that later species can root in. Mosses follow once this soil has formed, phytoplankton are the pioneers of hydrarch succession in water, and shrubs appear only in the later stages once soil depth and moisture allow.
Assertion (A): The pyramid of biomass in an open ocean is inverted. Reason (R): The phytoplankton of the ocean have very short life spans and multiply rapidly, so a small standing crop supports a much larger mass of zooplankton feeding on them.
Answer
Both A and R are true and R is the correct explanation of A — biomass measures the material present at one instant, not the material produced over time. Because phytoplankton reproduce and are eaten within days, the mass standing at any moment is small even though the total quantity produced across a season is enormous. The longer-lived zooplankton accumulate, so their standing biomass exceeds that of the producers and the pyramid stands on its apex. Measured as energy rather than as standing biomass, the same ocean gives an upright pyramid.
Distinguish between gross primary productivity and net primary productivity, and state which of the two is available to herbivores.
Answer
Gross primary productivity is the total rate at which the producers of an ecosystem convert radiant energy into organic matter by photosynthesis, expressed as mass of organic matter per unit area per unit time. A substantial part of this is used up by the plants themselves in respiration. Net primary productivity is what remains after that respiratory loss is subtracted, so NPP = GPP − R. It is the net gain of the producer, and it is this net primary productivity, not the gross value, that is available as food to the herbivores and decomposers of the ecosystem.
A student asks why a food chain in a grassland does not continue for eight or nine links. Explain, and state one other consequence of the same principle.
Answer
Energy transfer between trophic levels is extremely inefficient. Following Lindeman's ten per cent law, only about a tenth of the energy present at one level is incorporated into the next; the rest is dissipated as heat in respiration, spent on movement and maintenance, or lost in the parts that are not eaten or not digested. Because of this, the energy remaining after three or four transfers is too small to support a viable population of another predator. If the grass holds 100,000 units, the grasshoppers hold about 10,000, the small birds about 1,000, and a hawk feeding on those birds about 100 — a further level would have only about 10 units to live on. A second consequence of the same principle is that the number and the biomass of organisms fall sharply at each higher level, which is why top carnivores are always few, need large territories, and are the first to disappear when a habitat shrinks.
Differentiate between primary and secondary succession, and state which of the two reaches its climax faster and why.
Answer
Primary succession begins on a surface where no living organisms have ever existed and no soil is present, such as a newly cooled lava flow, a bare rock face, or a newly formed pond. Secondary succession begins on a site that has lost its community but still retains soil, such as an abandoned farm field, a burnt forest, or a flood-scoured bank. Secondary succession is much faster. Soil formation is the slowest step of the whole process, and in secondary succession the soil, its nutrients, and often a buried store of seeds are already present, so colonisers establish quickly. Primary succession must build soil from bare rock through pioneers such as lichens before higher plants can root at all, and so may take thousands of years to reach a climax.
Trace the flow of energy through a terrestrial ecosystem from sunlight to the top carnivore, and explain why ecologists say that energy flow is unidirectional.
Answer
Source and capture: the sun is the only significant energy source for almost every ecosystem. Of the solar radiation falling on the earth, only about half is photosynthetically active radiation, the wavelengths that plants can actually use, and green plants capture roughly 2 to 10 per cent of even that fraction. Small as it is, this captured energy sustains the entire living world. Producers: the energy fixed by photosynthesis is the gross primary productivity. The plants themselves respire and lose part of it as heat, and what remains is the net primary productivity — the organic matter that is genuinely available to the rest of the ecosystem. Plants therefore form the first trophic level. Transfer along trophic levels: herbivores, the primary consumers, occupy the second trophic level and feed directly on plants. Primary carnivores that eat herbivores form the third level, and secondary carnivores that prey on those form the fourth. At every one of these transfers only about 10 per cent of the energy is incorporated into the body of the consumer; this is Lindeman's ten per cent law. A very large part of what an animal eats is never assimilated at all and leaves as faeces, and much of what is assimilated is burnt in respiration to power movement, growth, and maintenance, leaving the body as heat. Because of this steep loss, food chains rarely exceed three or four links. Two kinds of chain: the grazing food chain starts with living producers — grass to grasshopper to frog to snake — while the detritus food chain starts with dead organic matter and runs through detritivores and decomposers. In most terrestrial ecosystems the detritus chain carries the larger share of the energy, since a great deal of plant material dies without ever being eaten. Real ecosystems are not simple chains but food webs, since most organisms feed at more than one point. Why the flow is unidirectional: energy enters the system only as sunlight, and it can move only from producers upwards to consumers. It can never travel back — a plant cannot recover energy from a herbivore, and a herbivore cannot recover it from its predator. At every step a large part is converted to heat, which is dissipated to the surroundings and is not usable for further biological work. Energy therefore passes through the ecosystem once and leaves it, which is why the system needs a continuous fresh supply from the sun. Nutrients, in contrast, are recycled indefinitely between the organisms and the soil, water, and air.
Describe the process of decomposition in an ecosystem, naming each step, and discuss the conditions that make decomposition fast or slow.
Answer
Raw material: decomposition acts on detritus, the dead remains of plants and animals — fallen leaves, bark, flowers, dead bodies, and faecal matter. Decomposers, chiefly bacteria and fungi, together with the detritivores such as earthworms, mites, and many insects, break this material down into inorganic substances. Fragmentation: detritivores physically break the detritus into much smaller particles. This step does not release nutrients by itself, but it hugely increases the surface area exposed to microbial attack, so everything that follows becomes faster. Leaching: water percolating through the fragmented material dissolves the water-soluble inorganic nutrients and carries them down into the soil, where some become locked up as salts that are not immediately available to plants. Catabolism: bacteria and fungi secrete extracellular enzymes onto the detritus and break the complex organic compounds down into simpler inorganic ones. This is the chemical heart of the process. Humification: the partly decomposed matter is converted into humus, a dark, amorphous, colloidal substance that is highly resistant to further microbial action and therefore decomposes very slowly. Humus acts as a reservoir of nutrients and greatly improves the water-holding capacity and structure of the soil. Mineralisation: some microbes act even on humus, releasing the inorganic nutrients it holds — nitrogen, phosphorus, and others — back into the soil in a form that plant roots can absorb. The cycle of matter is thus closed. Conditions that control the rate. Decomposition is largely an oxygen-requiring process, so it is fastest in well-aerated soil and slows greatly in waterlogged or anaerobic conditions, which is why organic matter accumulates in marshes and peat. A warm and moist environment favours the activity of decomposer microbes, so decomposition is rapid in a tropical forest and slow in cold or very dry regions, where litter piles up on the forest floor. The chemical quality of the detritus matters as much as the climate. Material rich in nitrogen and in water-soluble sugars decomposes quickly, while material rich in lignin from plant cell walls and in chitin from fungal walls and arthropod exoskeletons decomposes very slowly, because few organisms can produce the enzymes needed to break these polymers. Significance: without decomposition, nutrients would remain permanently locked in dead bodies, the soil would be exhausted, and productivity would collapse. Decomposers are therefore the link that makes nutrient cycling, and hence continued primary production, possible.
A group of students surveys a fenced grassland plot beside their school for one full year. They find that the grasses fix 20,000 kJ per square metre in the year, and that the grasses themselves use 8,000 kJ of this in respiration. The plot supports grasshoppers and other insects, which are eaten by small insect-eating birds, and a single kestrel hunts over the plot and takes those birds. They also notice that a thick layer of dead grass and leaf litter builds up in the corner where the ground stays damp, and that this litter breaks down far more slowly than the litter in the sunlit open part of the plot. (a) Calculate the gross and net primary productivity of the plot and state which of the two is available to the grasshoppers. (b) Arrange the organisms into trophic levels and estimate the energy reaching the kestrel. (c) The students are surprised that only one kestrel can be supported. Explain why, using the appropriate law. (d) Suggest why the litter in the damp corner decomposes more slowly, and name the step of decomposition that would be most delayed there.
Answer
(a) Gross primary productivity is the total energy fixed by the grasses, which is 20,000 kJ per square metre per year. Net primary productivity is what remains after respiratory loss, so NPP = 20,000 − 8,000 = 12,000 kJ per square metre per year. It is the net primary productivity of 12,000 kJ, not the gross figure, that is available to the grasshoppers and to the decomposers, because the 8,000 kJ has already been dissipated as heat by the plants themselves. (b) The grasses are the producers and form the first trophic level with 12,000 kJ available. The grasshoppers and other insects are primary consumers at the second trophic level and receive about 10 per cent of that, roughly 1,200 kJ. The insect-eating birds are primary carnivores at the third level with about 120 kJ. The kestrel is a secondary carnivore at the fourth trophic level and receives about 12 kJ per square metre per year. (c) The reason is Lindeman's ten per cent law — only about a tenth of the energy at one trophic level is incorporated into the body of the next, the remainder being lost as respiratory heat and in matter that is never eaten or never assimilated. By the fourth trophic level the 12,000 kJ of the producers has shrunk to about 12 kJ. A large predator needs a great deal of energy for maintenance and hunting, so this small remainder can support only one individual over a wide area. The same principle explains why food chains are limited to three or four links and why top carnivores are always few and are the first to vanish when a habitat shrinks. (d) The damp corner is likely to be waterlogged, and decomposition is largely an oxygen-requiring process, so anaerobic conditions greatly slow the decomposer microbes. Dead grass is also rich in lignin, which resists microbial breakdown. The step most delayed would be catabolism, the enzymatic breakdown of the complex organic compounds by bacteria and fungi, since it is this stage that depends most directly on aerobic microbial activity; as a result the material accumulates instead of being mineralised.
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