• Option B is correct. Pair 3 is correctly matched. Pairs 1 and 2 are incorrectly matched.
• Pair 1 is incorrectly matched. Commensalism is a type of biotic interaction in which one species benefits, the other is unaffected.For example, various dung beetles feed upon the dung of animals. Dung beetles benefit from other animals, whereas beetles have no effect on other animals.
• Pair 2 is incorrectly matched. Amensalism is one of the types of biotic interaction in which one species is harmed, the other is unaffected. Example: In equatorial forest, the small plants are in the shade of a large tree.Large tree retards the growth of the small plant as sunlight doesn’t reach to the bottom. The small plant has no effect on the large tree.
• Pair 3 is correctly matched. Competition is a type of biotic interaction in which both species are harmed by the interaction. For Example,if two species live in the same habitat, and there isn’t enough space for both, they both suffer a shortage of space to live and hence two species will compete for space and both are affected adversely.

• Option C is correct.
• Phosphorus, calcium and magnesium circulate by means of the sedimentary cycle. The element involved in the sedimentary cycle normally does not cycle through the atmosphere but follows a basic pattern of flow through erosion, sedimentation, mountain building, volcanic activity and biological transport through the excreta of marine birds.Hence, option 3 and 4 are correct.
• Carbon is circulated through the atmosphere by process of respiration and photosynthesis.Hence, option 1 is incorrect.
• Nitrogen is circulated through the atmosphere by process of nitrogen fixation and denitrification. Hence, option 2 is incorrect.

• Option B is correct.
• Succession occurs when a series of communities replace one another due to large scale destruction either natural or manmade. This process continues – one community replacing another community, until a stable, mature community develops.
• Succession is characterized by increased productivity, the shift of nutrients from’ the reservoirs, increased diversity of organisms with increased niche development, and a gradual increase in the complexity of food webs.Hence, statement 1 and 2 is correct.Statement 3 is incorrect.

• Option B is correct.
• Carrying capacity is the maximum pressure or load that a system can withstand before breaking down. If we exceed the carrying capacity of earth, the population of earth will collapse.Hence,statement 1 is incorrect.
• Development is sustainable as long as an economy is operating within carrying capacity of the environment.Concept of carrying capacity is integral to Sustainable development. Hence,statement 2 is correct.
• If too much resources are extracted or used up than it can afford to give or too much pollutants are discharged into it than it can absorb, then carrying capacity is severely damaged. Once damaged and destroyed beyond repair, it loses its ability to get back to its pure or usable or harmless state.The damage which are reversible and can be repaired does not affect carrying capacity.It is the irreversible damage which affect carrying capacity.Hence,statement 3 is correct.

• Option A is correct.
• Speciation is the process by which new species are formed and evolution is the mechanism by which speciation is brought about.Natural selection determines as to which variation will succeed and survive. Those variations that enable a species to survive in the struggle for existence are encouraged and promoted. Thus natural selection is responsible for speciation.Hence,statement 1 is correct.
• A species comprises many populations. Often different populations of a species remain isolated due to some geographic barrier such as mountain, ocean, river, etc.Geographic isolation occurs when a physical barrier develops between two populations of a species.The most common way a population undergoes speciation is by geographic isolation.Hence,statement 2 is correct.
• The population then becomes separated into two completely isolated populations by a barrier which prevents their interbreeding and gene exchange.Hence the new species become completely different from parent species and are unable to interbreed with parent species too.Hence,statement 3 is incorrect.

• Option D is correct.
• Ecosystems are capable of maintaining their state of equilibrium. They can regulate their own species structure and functional processes. This capacity of the ecosystem of self regulation is known as homeostasis. Hence, option D is correct.
• For example, in a pond ecosystem if the population of zooplankton increased, they would consume large numbers of the phytoplankton and as a result soon zooplankton would have a short supply of food for them. As the number of zooplankton is reduced because of starvation, the phytoplankton population starts increasing. After some time the population size of zooplankton also increases and this process continues at all the trophic levels of the food chain.
• Ecotopes are the smallest ecologically-distinct landscape features in a landscape mapping and classification system.
• Biocapacity is the capacity of an area to provide resources and absorb wastes.
• Xerosere is a plant succession which is limited by water availability. It includes the different stages in a xerarch succession. Xerarch succession of ecological communities
originated in extremely dry situation such as sand deserts, sand dunes, salt deserts, rock deserts etc.

• Option D is correct.
• Detritus food chains start from the dead organic matter to the detritivore organisms which in turn make food for protozoan to carnivores etc.Hence,statement 1 is incorrect.
• Pyramid of energy represents the total amount of energy at each trophic level.The amount of energy decreases at each subsequent trophic level. This is due to two reasons:
1. At each trophic a part of the available energy is lost in respiration or used up in metabolism.
2. A part of energy is lost at each transformation, i.e. when it moves from lower to higher trophic level as heat.
Hence, statement 2 is incorrect.
• Trophic levels in an ecosystem are not linear rather they are interconnected and make a food web. Thus the food web is a network interconnected food chains existing in an ecosystem. One animal may be a member of several different food chains. Food webs are more realistic models of energy flow through an ecosystem.Since in Food web an organism is dependent on multiple sources of food.Therefore, food webs are more stable than Food chain.Hence,statement 3 is correct.

• Option C is correct
• Nitrifying bacteria like Nitrosobacter and Nitrosomonas found in the soil convert ammonia into nitrate.
• Denitrifying bacteria like Pseudomonas and Clostridium living in the soil reduce the soil nitrites and nitrates into nitrogen which is returned back into the atmosphere.
• Nostoc, Azobacter and Cyanobacteria can combine atmospheric or dissolved nitrogen with hydrogen to form ammonia.

• Option D is correct
• A xerophyte is a group of specific plants which have adapted to xeric habitats i.e., these plants occur in the soil which does not have sufficient amount of water.These plants conserve water by following methods:
(i) They are mostly shrubs.
(ii) Leaves absent or reduced in size.Even they contain thorn so as to prevent water loss. (Hence,statement 1 is correct)
(iii) Leaves and stems are succulent, juicy and water storing. (Hence,statement 3 is incorrect)
(iv) Even the stem contains chlorophyll for photosynthesis. (Hence, statement 4 is correct)
(v) Root system well developed spread over a large area. (Hence,statement 2 is incorrect)

• Option B is correct. Only two pairs are correctly matched.
• Neustons are unattached organisms which live at the airwater interface such as floating plants, etc.Hence, pair 1 is incorrectly matched.
• Nekton contains animals which are swimmers. The nektons are relatively large and powerful as they have to overcome the water currents. Hence, pair 2 is correctly matched.
• The benthic organisms are those found living in the bottom of the water mass. Practically every aquatic ecosystem contains well developed benthos.Hence,pair 3 is correctly matched.