Directions : In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as :
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of assertion.
(c) If assertion is true but reason is false.
(d) If assertion is false but reason is true.
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Assertion : Amount of organic matter synthesised by producers per unit time and per unit area during the process of photosynthesis is referred to as net primary productivity.
Reason : Primary productivity is usually high and sustained throughout the year in tropical areas due to abundance of sunlight.
(d) The total amount of organic matter synthesised by producers per unit time and per unit area in the process of photosynthesis is called as gross primary productivity (GPP). Primary productivity is high and sustained throughout the year in tropical areas while it is limited in temperate areas by cold climate and small growing period. Maximum sunlight is available in tropics whereas poles receive minimum sunlight.Â
Assertion : Oceans are a low productivity ecosystems despite occupying about 70% of the earth’s surface.
Reason : In aquatic ecosystems, productivity is limited by light which decreases with increasing water depth.
(a) Productivity levels of an ecosystem depend upon plant species inhabiting a particular area, their photosynthetic capacity, availability of nutrients, sunlight, moisture and a variety of other environmental factors. The annual net primary productivity of the whole biosphere is approx. 170 billion tons (dry weight) of organic matter. Of this, despite occupying about \(70 \%\) of earth’s surface, oceans contribute only \(32 \%\) of the total productivity ( 55 billion tons out of 170 billion tons). Thus, oceans are a low productivity ecosystems. It is because in oceans, productivity is limited by light which decreases with increasing water depth.
Assertion : Temperature and soil moisture are the important climatic factors that regulate the process of decomposition.
Reason : Warm and moist environment favours decomposition whereas low temperature and anaerobiosis inhibit decomposition.
(b) Decomposition is largely an oxygen-requiring process. The rate of decomposition is controlled by chemical composition of detritus and climatic factors. Temperature and soil moisture are the most important climatic factors that regulate decomposition through their effects on the activities of soil microbes. Warm and moist environment favour decomposition whereas low temperature and anaerobiosis (due to excessive moisture) inhibit decomposition resulting in build up of organic materials.
Assertion : The loss of biologically useful energy as heat with every energy transfer in a food chain is a consequence of the second law of thermodynamics.
Reason : Energy does not remain trapped permanently in any organism, it is either passed on to higher trophic level or becomes available to detritivores and decomposers after the organism dies.
(b)
According to second law of thermodynamics, every activity involving energy transformation is accompanied by dissipation of energy. In other words, the loss of biologically useful energy as heat with every energy transfer in a food chain is a consequence of the second law of thermodynamics. Every time energy is transferred from one place to another or transformed from one kind to another, some of it is converted into heat. An organism transfers chemical energy from glucose or fatty acids to ATP (cellular respiration) and then to the chemical bonds of new molecules (during molecular synthesis), and transforms chemical energy into active transport of molecules, muscle contractions, and a variety of other activities, which are essential to life. Because every living organism continuously converts chemical energy into heat, there is always a loss of chemical energy with each step in a food chain.
Energy does not remain trapped permanently in any organism. – It is either passed on to the higher trophic level or becomes available to detrivores and decomposers after the organism dies. Death of organism is the beginning of the detritus food chain/web.
Assertion : Herbivores are also called as key industry animals because they convert plant matter into animal matter.
Reason : Decomposers play a pivotal role in the ecosystem and they indirectly support the producers.
(b) According to Elton (1939), herbivorous animals are key industry animals as they convert plant matter into animal matter and all other consumers’ (higher animals’) lives are dependent upon these primary consumers. Decomposers (also called as transformers) transform organic compounds into simple inorganic compounds. Saprophytic fungi and bacteria belong to this category. They act upon dead bodies of plants and animals and decompose them to their elemental stage. These in turn can be used by producers for their growth and photosynthetic activity. In this way, decomposers indirectly support the producers and play a pivotal role in ecosystem.
Assertion : In a marine aquatic ecosystem, the biomass of phytoplanktons (producers) at any given time, is lower than the biomass of zooplanktons (primary consumers).
Reason : Phytoplanktons are consumed almost as rapidly as they are formed and thus have shorter life spans.
(a) In a marine aquatic ecosystem, the biomass of phytoplanktons (producers) is smaller than that of zooplanktons (primary consumers) and the biomass of zooplanktons is smaller than that of secondary consumers. This results in the inverted pyramid of biomass in an aquatic ecosystem. This shape is the consequence of the very short life spans of phytoplanktons, which are consumed almost as rapidly as they are formed.
Assertion : Aquatic herbivores are usually more productive as compared to terrestrial herbivores.
Reason : Phytoplanktons achieve faster growth rate and are more nutritious to heterotrophs than their terrestrial counterparts due to their small size and lack of structural tissues.
(a) Most plant material is not consumed or is not digested by terrestrial herbivores, because plants have so much structural material. Most algal material (phytoplanktons) is consumed and digested by aquatic herbivores, because algae have very little structural material. Moreover, phytoplanktons turn over much more rapidly than terrestrial plants, so biomass at any given time is low as compared to the productivity. Phytoplanktons grow and reproduce rapidly, so a small mass can have a faster rate of primary production. In contrast, terrestrial primary producers grow and reproduce slowly. Thus, terrestrial herbivores are less productive than aquatic herbivores.
Assertion : During an ecological succession, the number of species increases, the community biomass increases and the community’s ratio of respiration to photosynthesis also increases.
Reason : At climax community of an ecological succession, the rate of photosynthesis of a community is almost equal to the rate of respiration.
(b) If both assertion and reason are true but reason is not the correct explanation of assertion
Assertion: Crustose lichens, in a lithosere, secrete organic acids causing the weathering of rocks so that minerals essential for proper growth of lichens are released.
Reason : Early colonists in a primary succession are usually lichens, which suggests that colonisation is easier when an organism has a mutualistic association.
(b) The pioneer community in a lithosere (biotic succession on bare rock) is constituted by lichens. The pioneer lichens are usually crustose lichens such as .Graphis, Rhizocarpon, etc. Lichens can tolerate desiccation. They produce organic acids which cause weathering of rocks so that minerals essential for proper growth of lichens are released. Lichens hold the fine particles of rock and initiate soil formation. It paves the way for growth of next serai community. Lichens are dual organisms which contain a permanent symbiotic association of a fungus and an alga. Early colonisation by lichens suggests that colonisation is easier when the organism has mutualistic association.
Assertion : Primary succession occurs over a primarily bare area where there was no living matter from the very beginning.
Reason : During primary succession, reproductive structures of the previous occupants give rise to a new seral community as soon as the conditions become favourable.
(c) Primary succession (or prisere) is the biotic succession that occurs on a previously sterile or primarily bare area. Newly exposed sea floor, igneous rocks, sand dunes, new cooled lava sediments or newly submerged areas are some of the examples of primary bare area. Soil and humus is absent at the beginning of primary succession and reproductive structures of any previous community are absent. Thus, the environment is very hostile for pioneer community and primary succession takes a long time for completion.
Assertion : Secondary succession always involves a predictable sequence of species and ends up with the same climax community as existed prior to the disturbance.
Reason: A pond can be considered as a self-sustained ecosystem as it possesses all the structural and functional components which work as a unit and show all the functional aspects of the ecosystem.
(d) Secondary succession does not always involve a predictable sequence of species and does not always end up with the same climax community as existed prior to the disturbance. Chance factors may play a role when species not normally a part of the succession happen to colonise, and so alter the normal sequence. Succession will not follow a predictable sequence if the climax community was a relict of past climatic and soil conditions. Instead, a new form of community that is more appropriate to the new conditions appears after destruction of the relict community. Similarly, where human activities have modified the climate or soil or have eliminated potential colonists by destroying adjacent communities, the same climax community may not reappear. Pond is a self-sustained ecosystem present in a shallow water body. It has all the structural components (biotic and abiotic) which work as a unit and show all the functional aspects of the ecosystem.
Assertion : The process of nitrification involves the decomposition of proteins of dead plants and animals, and nitrogenous wastes like urea, uric acid, etc. of animals to ammonia.
Reason : Nitrogen cycle is a gaseous cycle.
(d) The process of decomposition of proteins of dead plants and animals, and nitrogenous wastes like urea, uric acid etc., of animals to ammonia is referred to as ammonification. Ammonification is a part of nitrogen cycle. Nitrogen cycle is a type of gaseous nutrient cycle.
Assertion : Sedimentary nutrient cycles of phosphorus, calcium, magnesium, are considered as imperfect cycles.
Reason : These cycles get more easily disrupted by local disturbances as the bulk of material remains in the relatively inactive and immobile reservoir on the earth’s crust.
(a) Sedimentary nutrient cycles, e.g., phosphorus, calcium, magnesium are less perfect (or imperfect) as they get more easily disrupted by local disturbances as the bulk of material remains in the relatively inactive and immobile reservoir on the earth’s crust (lithosphere). On the other hand, gaseous nutrient cycles are rapid and more perfect as compared to sedimentary cycles. It is because in gaseous nutrient cycles, e.g., carbon, hydrogen, oxygen, nitrogen, nutrients remain in circulation more or less uniformly. In these cycles, nutrients self-adjust rather quickly because of large reservoir pool (atmosphere or hydrosphere).
Assertion : In nature, the recycling of carbon is essentially a self regulating feedback system.
Reason : The reservoir pool of carbon consists of free \(\mathrm{CO}_2\) in the atmosphere.
(c) Biogeochemical cycle of carbon is a gaseous cycle. Its cycling pool consists of \(6 \times 10^{14} \mathrm{~kg}(29 \%)\) of free \(\mathrm{CO}_2\) in the atmosphere and \(1.45 \times 10^{15} \mathrm{~kg}(71 \%)\) of dissolved \(\mathrm{CO}_2\) in the oceans. Oceans also regulate the amount of \(\mathrm{CO}_2\) in the atmosphere. Reservoir pool of carbon is lithosphere. Lithosphere contains \(2.8 \times 10^{21} \mathrm{~kg}\) of carbon. Natural exchange between lithosphere and hydrosphere or atmosphere is a very slow process. Major exchange in carbon cycle occurs between organisms and the atmosphere or hydrosphere. This cycling is a self-regulated feedback system but has recently been upset due to rapid deforestation and increasing combustion of fossil fuels.
Assertion : Phosphorus cycle is an imperfect cycle as a sufficient amount of phosphorus combines with \(\mathrm{Al}^{3+}\), \(\mathrm{Fe}^{2+}\) and \(\mathrm{Ca}^{2+}\) to form insoluble and unavailable salts.
Reason : Phosphate circulates in abiotic environment in lithosphere as well as in atmosphere.
(c) Phosphorus cycle is an example of sedimentary cycle. Greatest reservoir of phosphate in the world is relatively insoluble ferric and calcium phosphate in rocks. Cyclic pool is soil for terrestrial ecosystems (lithosphere) and water for aquatic ecosystems (hydrosphere). Phosphate circulates in the abiotic environment in lithosphere as well as hydrosphere. Atmosphere or gaseous cycle is absent. Inside soil some phosphorus is lost through leaching. Similarly, a sufficient amount of phosphorus combines with calcium, iron or aluminum and becomes insoluble. It settles down at the bottom of lake or ocean as sediment. Bone and teeth may also remain undegraded. Such phosphorus becomes part of lithosphere. It is released after a very long interval when the rocks containing them are exposed to weathering agencies or are mined. Thus, phosphorus cycle is an imperfect cycle.
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