Exercises
Fill in the blanks.
(a) Plants are called as_________because they fix carbon dioxide.
(b) In an ecosystem dominated by trees, the pyramid (of numbers) is_________type.
(c) In aquatic ecosystems, the limiting factor for the productivity is_________.
(d) Common detritivores in our ecosystem are_________.
Answer
(e) The major reservoir of carbon on earth is_________.
Ans. (a) producers (autotrophs)
(b) inverted (a single large tree supports a very large number of insects)
(c) light (its availability decreases with depth of water)
(d) earthworms
(e) oceans
Which one of the following has the largest population in a food chain?
(a) Producers
(b) Primary consumers
(c) Secondary consumers
(d) Decomposers
Answer
(d) Decomposers. Decomposers (bacteria and fungi) are microscopic and present in enormous numbers, far exceeding the number of organisms at any other level.
Q3. The second trophic level in a lake is
(a) Phytoplankton
(b) Zooplankton
(c) Benthos
(d) Fishes
Ans. (b) Zooplankton. They feed on phytoplankton (the producers, first trophic level) and are the primary consumers.
Q4. Secondary producers are
(a) Herbivores
(b) Producers
(c) Carnivores
(d) None of the above
Ans. (a) Herbivores. Secondary productivity is the rate of formation of new organic matter by consumers; herbivores are the first to convert plant matter into animal biomass, so they are the secondary producers.
What is the percentage of photosynthetically active radiation (PAR) in the incident solar radiation?
(a) 100%
(b) 50 %
(c) 1-5%
(d) 2-10%
Answer
(b) 50%. Less than 50% of the incident solar radiation is PAR; of this, plants capture only 2–10%.
Q6. Distinguish between
(a) Grazing food chain and detritus food chain
(b) Production and decomposition
(c) Upright and inverted pyramid
(d) Food chain and Food web
(e) Litter and detritus
(f) Primary and secondary productivity
Ans. (a) Grazing food chain (GFC) starts with living green plants (producers) and passes to herbivores and carnivores, e.g. grass → goat → man; it is the major conduit of energy in aquatic ecosystems. Detritus food chain (DFC) starts with dead organic matter (detritus) and involves decomposers (bacteria, fungi) and detritivores; in terrestrial ecosystems a much larger fraction of energy flows through it.
(b) Production is the synthesis of organic matter from inorganic raw materials by producers during photosynthesis, using solar energy (anabolic). Decomposition is the breakdown of complex organic matter of detritus into CO₂, water and inorganic nutrients by decomposers (catabolic), releasing nutrients for reuse.
(c) Upright pyramid: the number, biomass or energy is largest at the producer level and decreases at each higher trophic level, e.g. pyramid of numbers in a grassland; the energy pyramid is always upright. Inverted pyramid: the base is smaller than the higher levels, e.g. pyramid of numbers of a tree with insects and birds, or the pyramid of biomass in the sea (small standing crop of phytoplankton supporting a larger standing crop of zooplankton/fishes).
(d) A food chain is a single linear sequence of organisms through which food and energy pass, each organism eating the one before it. A food web is a network of many interconnected food chains (including links between GFC and DFC, and omnivores), giving alternative food pathways and greater stability.
(e) Litter is the dead plant material (leaves, twigs, bark, flowers) lying on the ground surface. Detritus is the whole of the dead organic matter – dead remains of plants and animals and faecal matter, including litter – that forms the raw material for decomposition.
(f) Primary productivity is the rate of production of biomass (organic matter) by producers during photosynthesis per unit area per unit time (GPP and NPP). Secondary productivity is the rate of formation of new organic matter by consumers.
Describe the components of an ecosystem.
Answer
An ecosystem has two components:
(i) Abiotic components: non-living factors – water, air, soil, minerals, dissolved inorganic and organic substances, and climatic factors such as solar input, temperature cycles and day length that regulate the rate of functioning.
(ii) Biotic components: (a) producers (autotrophs) – green plants, phytoplankton and algae that convert inorganic materials into organic food using sunlight; (b) consumers (heterotrophs) – herbivores (primary consumers), primary carnivores (secondary consumers) and higher carnivores; (c) decomposers – bacteria, fungi and flagellates that break down dead matter and return nutrients.
Example – a pond: water, dissolved substances and bottom soil are abiotic; phytoplankton, algae and floating, submerged and marginal plants are producers; zooplankton, free-swimming and bottom-dwelling animals are consumers; fungi, bacteria and flagellates at the bottom are decomposers.
Structurally, an ecosystem shows species composition and stratification (vertical layering, e.g. trees, shrubs, herbs). Functionally, the components work together in productivity, decomposition, energy flow and nutrient cycling.
Define ecological pyramids and describe with examples, pyramids of number and biomass.
Answer
An ecological pyramid is a graphical representation of the relationship between organisms at successive trophic levels in terms of number, biomass or energy. Its base represents the producers and the apex the top consumers.
Pyramid of number: shows the number of individuals at each trophic level. In a grassland it is upright – nearly 6 million plants support fewer herbivores, still fewer primary carnivores and only about 3 top carnivores. In a tree ecosystem it is inverted – one tree supports many insects, which support fewer birds.
Pyramid of biomass: shows the total dry weight of living matter (standing crop) at each trophic level. In terrestrial ecosystems it is upright, showing a sharp decrease in biomass at higher trophic levels. In the sea it is generally inverted because a small standing crop of phytoplankton (which multiplies very fast) supports a larger standing crop of zooplankton and fishes.
What is primary productivity? Give brief description of factors that affect primary productivity.
Answer
Primary productivity is the rate of production of biomass or organic matter per unit area over a period of time by plants during photosynthesis, expressed as g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹. Gross primary productivity (GPP) is the total rate of production; net primary productivity NPP = GPP − R (respiration losses), which is available to heterotrophs.
Factors affecting it: (i) the plant species inhabiting the area and their photosynthetic capacity; (ii) environmental factors – sunlight (in water, light falls with depth), temperature and moisture/rainfall; (iii) availability of nutrients (e.g. nitrogen, phosphorus) and water. Hence productivity varies in different ecosystems; oceans, though covering about 70% of the surface, produce only 55 of the 170 billion tons of annual NPP.
Define decomposition and describe the processes and products of decomposition.
Answer
Decomposition is the breakdown of complex organic matter (detritus) into inorganic substances such as CO₂, water and nutrients by decomposers.
Processes (operating simultaneously): (i) Fragmentation – detritivores like earthworms break detritus into smaller particles. (ii) Leaching – water-soluble inorganic nutrients move down into the soil horizon and get precipitated as unavailable salts. (iii) Catabolism – bacterial and fungal enzymes degrade detritus into simpler inorganic substances. (iv) Humification – formation of humus, a dark, amorphous, colloidal substance highly resistant to microbial action that decomposes very slowly and acts as a reservoir of nutrients. (v) Mineralisation – further degradation of humus by microbes releasing inorganic nutrients.
Products: CO₂, water, inorganic nutrients (minerals) and humus.
Decomposition is largely oxygen-requiring. It is faster in warm, moist conditions and with detritus rich in nitrogen and sugars; it is slower with detritus rich in lignin and chitin, at low temperature and under anaerobic conditions.
Give an account of energy flow in an ecosystem.
Answer
(i) The sun is the only source of energy for all ecosystems (except deep-sea hydrothermal ones). Less than 50% of incident solar radiation is PAR, and plants capture only 2–10% of PAR; this sustains the entire living world.
(ii) Producers convert solar energy into chemical energy in food. This passes to primary consumers (herbivores), then to secondary consumers (primary carnivores) and to higher carnivores along food chains/webs.
(iii) Energy flow is unidirectional – from sun
⇒ producers
⇒ consumers; it is not returned to the sun.
(iv) At each trophic level, much energy is used in respiration and lost as heat (second law of thermodynamics), so the amount of energy decreases at successive levels. Following the 10 per cent law, only about 10% of energy is transferred to the next trophic level, which limits the number of trophic levels in a grazing food chain.
(v) Dead organisms and wastes become detritus, and their energy flows to decomposers through the detritus food chain; in terrestrial ecosystems most energy flows through the DFC.
(vi) Because energy is lost at every step, the pyramid of energy is always upright.
Important Extra Questions
What is stratification? Give an example.
Answer
Vertical distribution of different species at different levels. In a forest, trees occupy the top layer, shrubs the second and herbs and grasses the bottom layers.
Why is measurement of biomass in dry weight more accurate?
Answer
Water content varies with season and conditions, so fresh weight fluctuates; dry weight reflects the actual organic matter.
What is standing crop?
Answer
The mass of living material at a trophic level at a particular time, measured as biomass or number per unit area.
How many trophic levels can a sparrow occupy?
Answer
At least two: it is a primary consumer when it eats seeds and fruits, and a secondary consumer when it eats insects and worms.
Why can the pyramid of energy never be inverted?
Answer
Because some energy is always lost as heat when it flows from one trophic level to the next, so each higher level always has less energy than the one below.