Q1. 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_________.
(e) The major reservoir of carbon on earth is_________.
Answer:

(a) Plants are called as producers (or autotrophs) because they fix carbon dioxide.
(b) In an ecosystem dominated by trees, the pyramid (of numbers) is of inverted type.
(c) In aquatic ecosystems, the limiting factor for productivity is light/PAR/nutrients
(d) Common detritivores in our ecosystem are earthworms.
(e) The major reservoir of carbon on earth is the hydrosphere (oceans)

Q2. 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

Q3. The second trophic level in a lake is:
(a) Phytoplankton
(b) Zooplankton
(c) Benthos
(d) Fishes 
Answer: (b) Zooplankton

Q4. Secondary producers are:
(a) Herbivores
(b) Producers
(c) Carnivores
(d) None of the above
Answer: (d) None of the above

Q5. What is the percentage of photosynthetically active radiation in the incident solar radiation?
(a) 100%
(b) 50 %
(c) 1-5%
(d) 2-10% 
Answer: (b) 50%

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
Answer:

(a) Grazing Food Chain and Detritus Food Chain

1. The grazing food chain begins with green plants (producers), whereas the detritus food chain begins with dead organic matter (detritus).
2. In the grazing food chain, the source of energy is solar energy captured by producers, whereas in the detritus food chain, it is chemical energy from dead matter.
3. The grazing food chain mainly involves producers, herbivores, and carnivores, while the detritus food chain involves decomposers such as fungi and bacteria.
4. The grazing food chain is dominant in aquatic ecosystems, whereas the detritus food chain is dominant in terrestrial ecosystems.

(b) Production and Decomposition

1. Production refers to the formation of biomass or organic matter during photosynthesis, whereas decomposition refers to the breakdown of complex organic matter into inorganic forms.
2. In production, energy is captured and stored in organic compounds, while in decomposition, energy is released and nutrients are recycled.
3. Production is carried out by autotrophs (producers), whereas decomposition is carried out by decomposers such as bacteria and fungi.

(c) Upright Pyramid and Inverted Pyramid

1. An upright pyramid has a broad base with more producers, whereas an inverted pyramid has a narrow base with fewer producers.
2. In an upright pyramid, producers are more numerous and have greater biomass, while in an inverted pyramid, consumers may exceed producers in number or biomass.
3. Grassland ecosystems usually show an upright pyramid, whereas aquatic ecosystems (where zooplankton > phytoplankton) show an inverted pyramid.

(d) Food Chain and Food Web

1. A food chain is a simple and linear sequence of organisms, whereas a food web is a complex network of several interconnected food chains.
2. In a food chain, the dependency is direct (e.g., grass → goat → man), while in a food web, organisms have multiple feeding relationships.
3. The food chain gives a simplified view of energy flow, whereas the food web shows a realistic representation of how energy flows in ecosystems.

(e) Litter and Detritus

1. Litter refers to leaves, twigs, bark, and other plant parts that have fallen to the ground and are partially decomposed by fungi and bacteria, whereas detritus is the raw material for decomposition, including dead plant and animal remains and faecal matter.
2. Litter is mainly plant-based and found on the soil surface, while detritus, on the other hand, includes both plant and animal remains that form the base of the detritus food chain.
3. Litter is an initial stage of decomposition, whereas detritus represents the entire pool of dead organic matter ready for further breakdown.

(f) Primary Productivity and Secondary Productivity

1. Primary productivity is the rate at which producers (plants and autotrophs) convert solar energy into organic matter through photosynthesis, whereas secondary productivity is the rate at which consumers (heterotrophs) produce new biomass by consuming organic matter.

2. Primary productivity is measured as Gross Primary Productivity (GPP) and Net Primary Productivity (NPP), while secondary productivity, on the other hand, is measured as the rate of food energy assimilation by consumers.
3. Primary productivity depends on photosynthetic activity and availability of sunlight, whereas secondary productivity depends on food availability and efficiency of energy transfer.

Q7. Describe the components of an ecosystem.
Answer: An ecosystem is a functional unit of nature where living organisms interact with each other and with the physical environment. It has two main components: abiotic and biotic.

Abiotic components include non-living factors such as air, water, soil, sunlight, temperature, and climate. These factors determine the types of organisms that can survive in a particular habitat.

Biotic components include all living organisms. Producers like green plants and algae make food through photosynthesis. Consumers such as herbivores, carnivores, and omnivores depend on others for food. Decomposers like fungi and bacteria break down dead matter and recycle nutrients back into the environment.

Q8. Define ecological pyramids and describe with examples, pyramids of number and biomass.
Answer: Ecological pyramids are graphical representations that show the relationship between organisms at different trophic levels in terms of number, biomass, or energy. They illustrate how these quantities change from producers to top consumers.

The pyramid of number represents the number of organisms present at each trophic level. For example, in a grassland ecosystem, the pyramid is upright because numerous grasses support fewer herbivores such as rabbits, which in turn support a smaller number of top carnivores like hawks.

The pyramid of biomass represents the total dry weight or mass of organisms at each level. For example, in a marine ecosystem, the pyramid is inverted because a small biomass of phytoplankton supports a much larger biomass of zooplankton and fish.

Q9. What is primary productivity? Give a brief description of factors that affect primary productivity.
Answer: Primary productivity is the rate at which plants produce biomass or organic matter per unit area over a given time through photosynthesis. It includes Gross Primary Productivity (GPP), which is the total rate of photosynthesis, and Net Primary Productivity (NPP), which is the portion of biomass available to consumers after subtracting respiration losses.

Primary productivity is influenced by several factors such as the type of plant species, availability of nutrients, and environmental conditions including light, temperature, and water. The photosynthetic efficiency of plants also plays a major role in determining overall productivity.

Q10. Define decomposition and describe the processes and products of decomposition.
Answer: Decomposition is the biological process through which decomposers break down complex organic matter, or detritus, into simpler inorganic substances such as carbon dioxide, water, and nutrients. It mainly occurs in the presence of oxygen.

The process involves several steps.

1. Fragmentation occurs when detritivores like earthworms break detritus into smaller particles.
2. Leaching allows soluble nutrients to move down into deeper soil layers.
3. Catabolism involves the enzymatic breakdown of organic matter into simpler compounds.
4. Humification results in the formation of humus, a dark, stable material that stores nutrients.
5. Mineralisation releases inorganic nutrients back into the soil.

The main products of decomposition are carbon dioxide, water, inorganic nutrients, and humus.

Q11. Give an account of energy flow in an ecosystem.
Answer: Energy flow in an ecosystem is unidirectional, starting from the sun, the primary energy source. Plants capture about 2–10% of photosynthetically active radiation (PAR) and convert it into chemical energy through photosynthesis. This energy passes from producers to herbivores and then to carnivores, with only about 10% transferred to the next level, while the rest is lost as heat according to the second law of thermodynamics.

Decomposers recycle nutrients by breaking down dead organic matter, ensuring the continuous functioning of the ecosystem. Energy flows through both grazing and detritus food chains, which interlink to form food webs. Since energy is lost at each trophic level, the pyramid of energy is always upright.