Ecosystem

Q1 Fill in the blanks

Ans. (a) Plants are called as producers because they fix carbon dioxide.

(b) In an ecosystem dominated by trees, the pyramid (of numbers) is inverted type.

(d) Common detritivores in our ecosystem are an earthworm.

(e) The major reservoir of carbon on Earth is oceans.

Q2. Which one of the following has the largest population in a food chain?

Ans. Decomposers

Q3The second trophic level in a lake is

Ans. Zooplankton

Q4 Secondary producers are

Ans. Herbivores

Q5 What is the percentage of photosynthetically active radiation (PAR) in the incident solar radiation?

Ans. 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

Ans. (a) Producers serve as a source of food in a grazing food chain and constitute the first trophic level and serve to incorporate nutrients into the body of living organisms whereas detritus food chain begins with detritivores that feed on the dead and decaying matter and serves to release the nutrients back into the atmosphere.

(b) Production refers to the rate of synthesis of organic matter by primary producers through the process of photosynthesis while decomposition refers to the breakdown of complex substances into simpler ones and is carried out by heterotrophic decomposers.

(c) Number and biomass of producers are largest and gradually decreases while moving from primary consumers to top consumers; it gives upright pyramid while Inverted pyramids exhibit lowest quantity of selected attribute at producer level.

(e) Litter constitutes both biodegradable and nonbiodegradable waste while detritus is the only biodegradable waste.

(f) Primary productivity refers to the total organic matter produced by primary producers per unit area per unit time while secondary productivity refers to rate conversion of food molecules into biomass at consumer level per unit area per unit time.

Q7 Describe the components of an ecosystem.

Ans. An ecosystem consists of two components - abiotic and biotic.

Abiotic component includes the physical environmental factors, e.g., the water, soil, wind, sunlight, etc.

Biotic component includes:

(i) Primary producers or the autotrophs, e.g., plants, phytoplankton, some algae, etc., that can use sunlight to make food.

(ii) Primary consumers feed on the producers, the plants. The primary consumers are all herbivores. Some common herbivores are insects, birds and mammals in terrestrial ecosystem and molluscs in the aquatic ecosystem.

(iii) Secondary consumers are animals which eat plant-eating animals. The consumers that feed on these herbivores are the primary carnivores (though secondary consumers), e.g., spiders, beetles and birds.

(iv) Tertiary consumers eat secondary consumers. Animals that depend on the primary carnivores for food are labelled as secondary carnivores, e.g., owl, eagle and fox.

(v) The decomposers are the fungi, bacteria and other small organisms that break down the complex organic matter into inorganic substances like carbon dioxide, water and nutrients.

(vi) Detritivores e.g., earthworm, slugs, crabs and insects that break down detritus such as leaves, bark, flowers and dead remains of animals, including faecal matter into smaller particles.

Q8 Define ecological pyramids and describe with examples, pyramids of number and biomass.

Ans. Ecological pyramids are graphical representations of the relationship between organisms of different trophic levels that can be expressed in terms of number, biomass or energy.

In most ecosystems, the pyramid of number is upright, i.e., producers are more in number than the herbivores and herbivores are more in number than the carnivores. But, the pyramid may be inverted as in a forest ecosystem, where the number of insects (primary consumers) is greater than the number of trees (producers).

The pyramid of biomass is also upright, generally, as the biomass of producers is more than the biomass of herbivores and that of herbivores is more than the biomass of carnivores. But, it is inverted in many ecosystems like in sea ecosystem, where the biomass of fishes (primary consumers) far exceeds that of phytoplankton (producers).

Q9 What is primary productivity? Give brief description of factors that affect primary productivity?

Ans. Primary productivity refers to the total organic matter produced by primary producers per unit area per unit time. It is affected by nutrient availability, types and number of primary producers present in an ecosystem, temperature, sunlight, water, precipitation etc.

Q10 Define decomposition and describe the processes and products of decomposition.

Ans. Decomposition is the process that involves the breakdown of complex organic matter or biomass from the body of dead plants and animals with the help of decomposers into inorganic raw materials such as carbon dioxide, water, and other nutrients.

The various processes involved in decomposition are as follows :

i) Fragmentation- It involves the breakdown of detritus into smaller pieces by the action of detritivores such as earthworms.

ii) Leaching- It is a process where the water soluble nutrients go down into the soil layers and get locked as unavailable salts.

iii) Catabolism- It is a process in which bacteria and fungi degrade detritus through various enzymes into smaller pieces.

iv) Humification- The next step is humification which leads to the formation of a dark-coloured colloidal substance called humus, which acts as reservoir of nutrients for plants.

v) Mineralization- The humus is further degraded by the action of microbes, which finally leads to the release of inorganic nutrients into the soil. This process of releasing inorganic nutrients from the humus is known as mineralization.

Q11 Give an account of energy flow in an ecosystem.

Ans. Photosynthesis fixes carbon from the abiotic environment and incorporates into the biological compounds of producers; food chain transfers the fixed carbon to different trophic levels. Plants absorb 2% of sun energy for photosynthesis.

With each trophic level, only 10% energy is transferred and 90% is lost as heat in respiration. The longer the food chains, the lesser is the energy transfer efficiency. To increase the energy transfer efficiency, food chains must be smaller and consumers should directly feed on producers.

Q12 Write important features of a sedimentary cycle in an ecosystem.

Ans. Phosphorus cycles is an example of the sedimentary nutrient cycle. The natural reservoir of phosphorus is Earth's crust. The rock contains phosphorus in the form of phosphates. By weathering and soil erosion, phosphates enter streams, rivers and then to oceans.

- Sedimentary cycles are very slow. They take a long time to complete their circulation and are considered as a less perfect cycle. This is because, during recycling, the nutrient element may get locked in the reservoir pool, thereby taking a long time to come out and continue circulation. Thus it usually goes out of circulation for a long time.

- With great movements of the crustal plates, the sea floor is uplifted and phosphates become exposed on the drained land surfaces. From here, weathering over long periods of time releases phosphates.

- From rocks, the minute amount of these phosphates dissolve in soil and are absorbed by the roots of the plants.

- Herbivores and other animals obtain this element from plants.

- The waste products and the dead organisms are decomposed by phosphate-solubilising bacteria releasing phosphorus.

Q13 Outline salient features of carbon cycling in an ecosystem.

Ans. The carbon cycle is a gaseous nutrient cycle.

The major part of carbon is found dissolved in oceans. This oceanic reservoir regulates the amount of carbon dioxide in the atmosphere. Fossil fuels also represent a reservoir of carbon.

Carbon cycling occurs through atmosphere, ocean and through living and dead organisms.

Carbon is fixed in the biosphere through photosynthesis by autotrophs and is released back into the atmosphere as CO2 through:

(i) Respiratory activities of the producers and consumers.

(ii) Breakdown activities of decomposers.

(iii) Forest fire and combustion of organic matter.

(iv) Volcanic activity

(v) Rapid deforestation

(vi) Burning of wood and fossil fuel.

Human activities have influenced the carbon cycle by significantly increasing the rate of release of carbon dioxide into the atmosphere.