NEET 2025 MCQ with Answer Explanation : “Ecosystem”.

Here below given some basic mcq's of ecosystem with answer which is explained in details. Lets check one by one. Question 1:Which of the following is considered a primary consumer in an ecosystem?(A) Lion(B) Cow(C) Eagle(D) Snake Answer: (B) CowExplanation:Primary consumers are herbivores that directly depend on producers (plants) for their food. In this case, a cow feeds on grass and other plants, making it a primary consumer. Lions and eagles are carnivores (secondary or tertiary consumers), while snakes often feed on small animals, making them secondary or tertiary consumers depending on the food chain. Question 2:Which of the following statements is true regarding ecological pyramids?(A) Pyramids of number are always upright.(B) Pyramids of energy are always upright.(C) Pyramids of biomass are always inverted.(D) Pyramids of number are always inverted. Answer: (B) Pyramids of energy are always upright.Explanation:Ecological pyramids represent the number, biomass, or energy at different trophic levels. The pyramid of energy is always upright because energy decreases at each successive trophic level due to the loss of energy in the form of heat during respiration and other metabolic activities. Pyramids of number and biomass can be either upright or inverted depending on the ecosystem. Question 3:Which of the following contributes the most to the total energy flow in an ecosystem?(A) Herbivores(B) Carnivores(C) Primary producers(D) Decomposers Answer: (C) Primary producersExplanation:Primary producers, such as plants, algae, and some bacteria, form the base of the energy pyramid by converting solar energy into chemical energy through photosynthesis. This energy is then transferred to herbivores (primary consumers) and subsequently to higher trophic levels. The majority of the ecosystem’s energy originates from primary producers. Question 4:The primary productivity of an ecosystem is related to:(A) Gross primary productivity(B) Net primary productivity(C) Secondary productivity(D) Both (A) and (B) Answer: (D) Both (A) and (B)Explanation:Primary productivity refers to the rate at which energy is converted by autotrophs (producers) into organic substances. Gross primary productivity (GPP) is the total amount of energy captured via photosynthesis. Net primary productivity (NPP) is the energy left after autotrophs have met their own energy needs (i.e., GPP minus the energy used for respiration). Both GPP and NPP contribute to the understanding of primary productivity in an ecosystem. Question 5:Which of the following is an example of a detritivore?(A) Earthworm(B) Snake(C) Eagle(D) Tiger Answer: (A) EarthwormExplanation:Detritivores are organisms that feed on dead and decaying organic matter, helping to break it down and recycle nutrients. Earthworms, by consuming decaying matter in soil, are classic examples of detritivores. Snakes, eagles, and tigers are carnivores and do not feed on dead matter directly. Question 6:What is the role of decomposers in an ecosystem?(A) Fix atmospheric nitrogen(B) Convert organic material into inorganic nutrients(C) Absorb sunlight for photosynthesis(D) Help in pollination Answer: (B) Convert organic material into inorganic nutrientsExplanation:Decomposers, such as bacteria and fungi, break down dead organisms and organic waste into simpler inorganic compounds like nitrogen, phosphorus, and carbon. These nutrients are then recycled back into the ecosystem, allowing primary producers to reuse them. Question 7:The concept of "Ecological Niche" refers to:(A) The number of species in an ecosystem(B) The role a species plays in its ecosystem(C) The physical space a species occupies(D) The food chain in which a species participates Answer: (B) The role a species plays in its ecosystemExplanation:An ecological niche is the role or function a species performs within its ecosystem, including how it interacts with other species, its habitat, and its feeding habits. It encompasses both the species' physical environment and its interactions with other organisms. Question 8:In an aquatic ecosystem, the major primary producers are:(A) Phytoplankton(B) Zooplankton(C) Small fish(D) Fungi Answer: (A) PhytoplanktonExplanation:Phytoplankton are microscopic, photosynthetic organisms that float in the water column and are the primary producers in aquatic ecosystems. They capture sunlight and convert it into chemical energy, supporting the entire aquatic food web. Question 9:Which of the following is not a type of ecological pyramid?(A) Pyramid of energy(B) Pyramid of biomass(C) Pyramid of numbers(D) Pyramid of decomposition Answer: (D) Pyramid of decompositionExplanation:Ecological pyramids are used to represent the relative amounts of energy, biomass, or number of organisms at each trophic level in an ecosystem. There is no such thing as a pyramid of decomposition. The recognized pyramids are those of energy, biomass, and numbers. Question 10:Which one of the following is an example of primary succession?(A) Forest growing in an abandoned agricultural field(B) Growth of lichens on a bare rock(C) Regrowth of a forest after a wildfire(D) Recovery of a coral reef after bleaching Answer: (B) Growth of lichens on a bare rockExplanation:Primary succession occurs in areas that have not previously supported life, such as bare rock surfaces. Lichens are pioneer species that begin the process of soil formation, which eventually allows other plant species to establish themselves. The other examples relate to secondary succession. NEET & CET Aspirant Get Daily MCQ and update on Telegram : Click Here NEET & CET Aspirant Get Daily MCQ and Update on Whatsapp : Click Here Amazon Loot Deals : Click Here Question 11:The term "standing crop" refers to:(A) The total biomass of living organisms in a specific trophic level at a given time(B) The rate of energy flow in an ecosystem(C) The reproductive capacity of a population(D) The total number of species in an ecosystem Answer: (A) The total biomass of living organisms in a specific trophic level at a given timeExplanation:Standing crop refers to the total biomass present in a given trophic level at a particular time. It reflects the amount of living material available in the ecosystem at that specific moment. Question 12:In a typical pyramid of energy, what percentage of energy is transferred from one trophic level to the next?(A) 1%(B) 10%(C) 30%(D) 50% Answer: (B) 10%Explanation:According to the 10% law, only about 10% of the energy at one trophic level is passed on to the next trophic level, while the remaining 90% is lost as heat, metabolism, and other biological processes. Question 13:Which type of biome is characterized by high temperatures and heavy rainfall throughout the year?(A) Desert(B) Tundra(C) Tropical rainforest(D) Grassland Answer: (C) Tropical rainforestExplanation:Tropical rainforests are characterized by consistently high temperatures and heavy rainfall year-round. These biomes are home to diverse species and exhibit complex and dense vegetation due to the favorable conditions for plant growth. Question 14:The relationship between a bee and a flowering plant is an example of:(A) Parasitism(B) Mutualism(C) Commensalism(D) Competition Answer: (B) MutualismExplanation:In mutualism, both species benefit from the relationship. A bee obtains nectar (food) from a flowering plant, and in return, the bee helps in pollination, which is essential for the plant’s reproduction. Both the bee and the plant gain from this interaction. Question 15:Which of the following nutrient cycles does not have an atmospheric component?(A) Carbon cycle(B) Nitrogen cycle(C) Phosphorus cycle(D) Water cycle Answer: (C) Phosphorus cycleExplanation:The phosphorus cycle lacks a significant atmospheric component because phosphorus does not typically exist as a gas under normal environmental conditions. It primarily cycles through rocks, soil, water, and living organisms. Question 16:Which of the following processes in the carbon cycle is primarily responsible for removing CO₂ from the atmosphere?(A) Respiration(B) Photosynthesis(C) Decomposition(D) Combustion Answer: (B) PhotosynthesisExplanation:Photosynthesis, carried out by plants and other autotrophs, is the primary process that removes carbon dioxide (CO₂) from the atmosphere. During photosynthesis, CO₂ is converted into organic compounds such as glucose, which can be used for growth and energy storage. Question 17:Which trophic level contains the highest amount of energy in an ecosystem?(A) Producers(B) Primary consumers(C) Secondary consumers(D) Tertiary consumers Answer: (A) ProducersExplanation:Producers, such as plants, contain the highest amount of energy because they convert solar energy into chemical energy through photosynthesis. This energy is then transferred to consumers at higher trophic levels, but with decreasing amounts due to energy loss at each step. Question 18:What does "keystone species" refer to in an ecosystem?(A) The most abundant species in an ecosystem(B) A species that plays a critical role in maintaining the structure of an ecosystem(C) A species at the top of the food chain(D) A species that is highly susceptible to extinction Answer: (B) A species that plays a critical role in maintaining the structure of an ecosystemExplanation:A keystone species has a disproportionately large impact on its ecosystem relative to its abundance. It plays a critical role in maintaining the structure, diversity, and balance of the ecosystem, and its removal can lead to significant changes in the ecosystem's composition. Question 19:What does the term "biomagnification" refer to?(A) Increase in population size(B) Accumulation of pollutants in organisms at higher trophic levels(C) Transfer of energy between trophic levels(D) Growth of plant biomass Answer: (B) Accumulation of pollutants in organisms at higher trophic levelsExplanation:Biomagnification refers to the process by which certain toxic substances (such as pesticides or heavy metals) become more concentrated as they move up the food chain. Organisms at higher trophic levels accumulate more of these toxins, leading to harmful effects. Question 20:Which of the following represents a climax community in ecological succession?(A) A barren rock(B) A mature forest(C) A pond with algae(D) A grassland Answer: (B) A mature forestExplanation:A climax community is the final, stable stage of ecological succession, where the ecosystem has reached equilibrium. A mature forest is an example of a climax community, where species diversity and population sizes are relatively stable over time. Question 21:Which of the following is not a component of the detritus food chain?(A) Fungi(B) Earthworms(C) Herbivores(D) Bacteria Answer: (C) HerbivoresExplanation:The detritus food chain consists of organisms that feed on dead organic matter (detritus), such as fungi, earthworms, and bacteria. Herbivores, on the other hand, are part of the grazing food chain because they feed directly on living plants (producers). Question 22:The relationship between a remora fish and a shark, where the remora benefits by attaching itself to the shark for transportation and food scraps, without harming the shark, is an example of:(A) Parasitism(B) Commensalism(C) Mutualism(D) Predation Answer: (B) CommensalismExplanation:Commensalism is a type of symbiotic relationship where one organism benefits, and the other is neither harmed nor helped. In this case, the remora fish benefits from the shark by gaining food and transportation, while the shark is unaffected. Question 23:Secondary productivity in an ecosystem refers to:(A) The total biomass produced by primary producers(B) The biomass produced by herbivores and carnivores(C) The amount of energy captured from the sun(D) The rate of organic matter production by decomposers Answer: (B) The biomass produced by herbivores and carnivoresExplanation:Secondary productivity refers to the generation of biomass by consumers (herbivores, carnivores) through feeding on primary producers or other consumers. It represents the flow of energy from one trophic level to the next in an ecosystem. Question 24:In which of the following biogeochemical cycles is nitrogen fixation a key process?(A) Water cycle(B) Carbon cycle(C) Nitrogen cycle(D) Phosphorus cycle Answer: (C) Nitrogen cycleExplanation:Nitrogen fixation is a process in which atmospheric nitrogen (N₂) is converted into ammonia (NH₃) or related compounds, making it available for uptake by plants. It is a key step in the nitrogen cycle. Question 25:Which of the following ecosystems has the highest net primary productivity?(A) Desert(B) Tropical rainforest(C) Tundra(D) Open ocean Answer: (B) Tropical rainforestExplanation:Tropical rainforests have the highest net primary productivity due to their favorable climate, which supports year-round photosynthesis and high biodiversity. Deserts, tundras, and open oceans have comparatively lower primary productivity. Question 26:In ecological terms, which of the following is an example of a "carbon sink"?(A) Animals(B) Forests(C) Oceans(D) Both (B) and (C) Answer: (D) Both (B) and (C)Explanation:A carbon sink is any system that absorbs more carbon than it releases. Forests and oceans are significant carbon sinks, as forests absorb CO₂ through photosynthesis, and oceans dissolve CO₂ and store it in various forms. Question 27:During nitrogen fixation, which of the following organisms play a major role?(A) Cyanobacteria(B) Fungi(C) Earthworms(D) Phytoplankton Answer: (A) CyanobacteriaExplanation:Cyanobacteria, also known as blue-green algae, are a group of microorganisms that can fix atmospheric nitrogen into a form that plants can use. They play a significant role in the nitrogen cycle. Question 28:Which of the following is an abiotic component of an ecosystem?(A) Algae(B) Bacteria(C) Soil(D) Grass Answer: (C) SoilExplanation:Abiotic components of an ecosystem are non-living factors that influence the environment, such as soil, water, temperature, and light. Algae, bacteria, and grass are living (biotic) components. Question 29:In the pyramid of biomass, which type of ecosystem typically shows an inverted pyramid?(A) Grassland ecosystem(B) Marine ecosystem(C) Forest ecosystem(D) Desert ecosystem Answer: (B) Marine ecosystemExplanation:In a marine ecosystem, the pyramid of biomass is often inverted because the biomass of phytoplankton (producers) is very small compared to the biomass of consumers such as fish. Despite their small biomass, phytoplankton reproduce rapidly and support large populations of consumers. Question 30:Which of the following is an example of an autotroph?(A) Fungi(B) Deer(C) Algae(D) Wolf Answer: (C) AlgaeExplanation:Autotrophs are organisms that produce their own food through processes like photosynthesis. Algae, like plants, are autotrophs because they use sunlight to synthesize their own food. Fungi, deer, and wolves are heterotrophs, relying on other organisms for food. Question 31:Which of the following is true about the "10% law" of energy transfer in an ecosystem?(A) 10% of energy is lost as heat at each trophic level.(B) Only 10% of the energy from one trophic level is passed on to the next.(C) 10% of energy is transferred from the producer level to the consumer level.(D) 10% of the total energy is available to decomposers. Answer: (B) Only 10% of the energy from one trophic level is passed on to the next.Explanation:The 10% law states that only about 10% of the energy at one trophic level is transferred to the next trophic level. The remaining 90% is lost in various forms, primarily as heat through metabolic processes. Question 32:What is the primary factor that limits the length of food chains in ecosystems?(A) Amount of water available(B) Size of organisms at each trophic level(C) Loss of energy between trophic levels(D) Rate of photosynthesis Answer: (C) Loss of energy between trophic levelsExplanation:The loss of energy between trophic levels limits the length of food chains. Because only about 10% of energy is transferred from one trophic level to the next, the amount of energy available decreases at each successive level, restricting the number of trophic levels an ecosystem can support. Question 33:Which term refers to the interaction where one species benefits and the other is unaffected?(A) Parasitism(B) Mutualism(C) Commensalism(D) Competition Answer: (C) CommensalismExplanation:Commensalism is a relationship between two species in which one benefits, while the other is neither helped nor harmed. Examples include barnacles on whales or birds nesting in trees. Question 34:Which of the following organisms is involved in ammonification during the nitrogen cycle?(A) Nitrifying bacteria(B) Denitrifying bacteria(C) Fungi(D) Phytoplankton Answer: (C) FungiExplanation:Ammonification is the process by which organic nitrogen in dead organisms and waste products is converted into ammonia (NH₃) by decomposers like fungi and bacteria. Nitrifying bacteria are involved in nitrification, and denitrifying bacteria in denitrification. Question 35:In a grassland ecosystem, which trophic level would a snake occupy if it feeds on a frog that feeds on insects?(A) Primary consumer(B) Secondary consumer(C) Tertiary consumer(D) Quaternary consumer Answer: (C) Tertiary consumerExplanation:The snake would be a tertiary consumer in this food chain. The grass (producer) is consumed by the insects (primary consumer), which are eaten by the frog (secondary consumer). The snake, feeding on the frog, is a tertiary consumer. Question 36:Which of the following factors does not limit the primary productivity of an ecosystem?(A) Availability of nutrients(B) Solar energy(C) Availability of carbon dioxide(D) Biomass of secondary consumers Answer: (D) Biomass of secondary consumersExplanation:The primary productivity of an ecosystem is limited by factors such as nutrient availability, sunlight, and carbon dioxide for photosynthesis. The biomass of secondary consumers does not directly affect primary productivity, as it refers to consumers rather than producers. Question 37:What is the approximate efficiency of energy transfer from producers to primary consumers?(A) 1%(B) 5%(C) 10%(D) 20% Answer: (C) 10%Explanation:According to the 10% law, about 10% of the energy captured by producers (plants) is transferred to primary consumers (herbivores). The rest is lost as heat or used in metabolic processes. Question 38:Which of the following is an example of an ecosystem service?(A) A beehive producing honey(B) A plant undergoing photosynthesis(C) Nutrient cycling in a forest(D) A deer eating grass Answer: (C) Nutrient cycling in a forestExplanation:Ecosystem services are benefits that humans obtain from ecosystems. Nutrient cycling is an essential ecosystem service that helps maintain soil fertility, supports plant growth, and sustains life. Photosynthesis is a process within the ecosystem, but nutrient cycling broadly benefits ecosystem sustainability. Question 39:Which biogeochemical cycle is responsible for weathering of rocks as an important component?(A) Water cycle(B) Phosphorus cycle(C) Carbon cycle(D) Nitrogen cycle Answer: (B) Phosphorus cycleExplanation:The phosphorus cycle involves the weathering of rocks, which releases phosphate ions into the soil. Unlike the nitrogen and carbon cycles, the phosphorus cycle lacks a significant atmospheric component and is driven by the slow release of phosphorus from rocks. Question 40:Which of the following contributes most to the "greenhouse effect"?(A) Nitrogen(B) Carbon dioxide(C) Oxygen(D) Ozone Answer: (B) Carbon dioxideExplanation:Carbon dioxide (CO₂) is a major greenhouse gas that traps heat in the Earth's atmosphere, contributing to the greenhouse effect and global warming. Although other gases like methane and nitrous oxide are also involved, CO₂ is the primary contributor. Question 41:What is "ecological efficiency"?(A) The ability of an ecosystem to recover after a disturbance(B) The percentage of energy transferred from one trophic level to the next(C) The rate of primary productivity in an ecosystem(D) The biomass produced at each trophic level Answer: (B) The percentage of energy transferred from one trophic level to the nextExplanation:Ecological efficiency refers to the percentage of energy transferred from one trophic level to the next in an ecosystem. Typically, only about 10% of the energy at one level is passed on to the next, while the rest is lost as heat or used in metabolism. Question 42:What is the primary function of nitrifying bacteria in the nitrogen cycle?(A) Fixing atmospheric nitrogen(B) Converting ammonia into nitrites and nitrates(C) Denitrification of nitrates into nitrogen gas(D) Decomposition of dead organisms Answer: (B) Converting ammonia into nitrites and nitratesExplanation:Nitrifying bacteria play a crucial role in the nitrogen cycle by converting ammonia (NH₃) into nitrites (NO₂⁻) and then into nitrates (NO₃⁻), which plants can absorb and use. This process is called nitrification. Question 43:Which of the following is an example of secondary succession?(A) A forest growing in an area after a volcanic eruption(B) The formation of a pond in a barren land(C) Regrowth of vegetation in a forest after a fire(D) Lichens growing on bare rock Answer: (C) Regrowth of vegetation in a forest after a fireExplanation:Secondary succession occurs in areas where a disturbance (like fire, flood, or human activity) has removed the existing vegetation but left the soil intact. The regrowth of vegetation in a forest after a fire is a classic example. Primary succession, on the other hand, occurs in areas without preexisting soil, such as after a volcanic eruption. Question 44:In which of the following ecosystems would you expect to find the highest species diversity?(A) Desert(B) Tundra(C) Tropical rainforest(D) Temperate grassland Answer: (C) Tropical rainforestExplanation:Tropical rainforests have the highest species diversity of any ecosystem due to their warm, moist climate and consistent energy input from sunlight, which supports a wide variety of organisms. Deserts, tundras, and grasslands have less favorable conditions for such biodiversity. Question 45:Which of the following is an example of biological magnification?(A) An increase in the size of fish populations in a lake(B) Accumulation of DDT in the tissues of top-level predators(C) The gradual buildup of minerals in the soil(D) Growth of bacteria in water due to pollution Answer: (B) Accumulation of DDT in the tissues of top-level predatorsExplanation:Biological magnification refers to the increasing concentration of toxic substances, such as DDT or mercury, in organisms at higher trophic levels. As top predators consume contaminated prey, the concentration of these toxins becomes higher in their tissues. Question 46:What is "gross primary productivity" (GPP)?(A) The total amount of energy stored in an ecosystem(B) The total amount of energy captured by photosynthesis in a given time(C) The energy available to primary consumers(D) The amount of energy lost as heat in an ecosystem Answer: (B) The total amount of energy captured by photosynthesis in a given timeExplanation:Gross primary productivity (GPP) is the total amount of energy captured by plants through photosynthesis. It represents the rate at which producers in an ecosystem convert solar energy into chemical energy. Question 47:Which of the following describes net primary productivity (NPP)?(A) Energy lost as heat by plants(B) Energy used by plants for respiration(C) The energy remaining after respiration that is available for growth and reproduction(D) The total energy captured by plants during photosynthesis Answer: (C) The energy remaining after respiration that is available for growth and reproductionExplanation:Net primary productivity (NPP) is the energy that remains after plants have used some of the captured energy for respiration. NPP represents the amount of energy available for growth, reproduction, and consumption by herbivores. Question 48:Which of the following statements about a food web is true?(A) It shows a linear flow of energy from one trophic level to the next.(B) It shows the complex interconnections between multiple food chains in an ecosystem.(C) It involves only producers and consumers.(D) It does not include decomposers. Answer: (B) It shows the complex interconnections between multiple food chains in an ecosystem.Explanation:A food web represents the interconnected food chains within an ecosystem. It shows how different species interact with one another as they transfer energy, including producers, consumers, and decomposers. Question 49:Which of the following organisms is a decomposer in an ecosystem?(A) Grass(B) Fungi(C) Rabbit(D) Eagle Answer: (B) FungiExplanation:Decomposers, such as fungi and bacteria, break down dead organic material, recycling nutrients back into the ecosystem. Grass, rabbits, and eagles are not decomposers; they play different roles in the food chain. Question 50:Which of the following ecosystems is considered the most productive in terms of biomass production?(A) Grassland(B) Desert(C) Coral reefs(D) Arctic tundra Answer: (C) Coral reefsExplanation:Coral reefs are among the most productive ecosystems on Earth in terms of biomass production due to their high biodiversity, efficient nutrient cycling, and the vast array of species that contribute to primary and secondary productivity. Question 51:Which type of ecological succession occurs when an area previously devoid of life becomes colonized by living organisms?(A) Secondary succession(B) Primary succession(C) Autogenic succession(D) Allogenic succession Answer: (B) Primary successionExplanation:Primary succession occurs in areas where no life previously existed, such as newly formed volcanic islands or regions exposed after a glacier retreats. It begins with the colonization of pioneer species like lichens and mosses. Question 52:In a food chain, the amount of energy available to the top predator is usually:(A) Greater than that of primary producers(B) Equal to that of primary producers(C) Less than that of primary producers(D) Equal to that of herbivores Answer: (C) Less than that of primary producersExplanation:The amount of energy available to top predators is significantly less than that available to primary producers due to the loss of energy at each trophic level, as per the 10% law. Most energy is lost as heat, with only about 10% being passed on to the next level. Question 53:Which of the following is a consequence of deforestation on the carbon cycle?(A) Increase in carbon sequestration(B) Increase in CO₂ release into the atmosphere(C) Decrease in atmospheric CO₂(D) Stabilization of carbon levels Answer: (B) Increase in CO₂ release into the atmosphereExplanation:Deforestation leads to the release of stored carbon from trees and plants into the atmosphere as CO₂. This contributes to an increase in atmospheric CO₂ levels, exacerbating the greenhouse effect and climate change. Question 54:Which of the following correctly represents a detritus food chain?(A) Grass → Cow → Lion(B) Algae → Small fish → Large fish(C) Dead leaves → Earthworms → Birds(D) Grass → Insect → Frog Answer: (C) Dead leaves → Earthworms → BirdsExplanation:The detritus food chain involves the breakdown of dead organic matter (detritus) by decomposers and detritivores. In this example, dead leaves (detritus) are consumed by earthworms (detritivores), which are then eaten by birds (secondary consumers). Question 55:Which of the following organisms is a primary consumer?(A) Snake(B) Lion(C) Rabbit(D) Fungi Answer: (C) RabbitExplanation:Primary consumers are herbivores that feed on plants (producers). A rabbit, which feeds on grass and other plants, is a primary consumer. Snakes and lions are secondary and tertiary consumers, respectively, and fungi are decomposers. Question 56:Which of the following ecosystems is known for its high net primary productivity (NPP)?(A) Arctic tundra(B) Open ocean(C) Estuaries(D) Desert Answer: (C) EstuariesExplanation:Estuaries are highly productive ecosystems where freshwater meets saltwater, providing nutrients that support high levels of primary productivity. This makes them among the most productive ecosystems, along with tropical rainforests and coral reefs. Question 57:Which gas is the primary cause of the "greenhouse effect"?(A) Oxygen(B) Nitrogen(C) Carbon dioxide(D) Sulfur dioxide Answer: (C) Carbon dioxideExplanation:Carbon dioxide (CO₂) is the primary greenhouse gas responsible for trapping heat in the Earth's atmosphere, leading to the greenhouse effect and contributing to global warming. Question 58:The term "keystone species" refers to:(A) A species that is the most abundant in an ecosystem(B) A species that plays a crucial role in maintaining the structure of an ecosystem(C) A species that is at the top of the food chain(D) A species that reproduces rapidly Answer: (B) A species that plays a crucial role in maintaining the structure of an ecosystemExplanation:A keystone species is one whose presence and role within an ecosystem have a disproportionately large effect on the other organisms in the system. The removal of a keystone species can lead to significant changes in the ecosystem structure and function. Question 59:In which biome would you expect to find the greatest annual temperature fluctuations?(A) Tropical rainforest(B) Desert(C) Tundra(D) Temperate forest Answer: (B) DesertExplanation:Deserts experience extreme temperature fluctuations, with very hot days and cold nights due to the lack of moisture in the air to moderate temperatures. Tropical rainforests have stable temperatures, while tundras and temperate forests have more moderate fluctuations. Question 60:What is the role of nitrogen-fixing bacteria in the nitrogen cycle?(A) Converting nitrates into nitrogen gas(B) Converting nitrogen gas into ammonia(C) Decomposing organic matter(D) Converting ammonia into nitrates Answer: (B) Converting nitrogen gas into ammoniaExplanation:Nitrogen-fixing bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃), which plants can use. This process is essential for making nitrogen available to living organisms, as most cannot directly use atmospheric nitrogen. Question 61:Which of the following is an abiotic factor in an ecosystem?(A) Trees(B) Birds(C) Temperature(D) Insects Answer: (C) TemperatureExplanation:Abiotic factors are the non-living components of an ecosystem, such as temperature, sunlight, soil, and water. Trees, birds, and insects are biotic (living) components. Question 62:Which of the following is an example of an invasive species?(A) Bengal tiger in the Sundarbans(B) Zebra mussels in the Great Lakes(C) Polar bears in the Arctic(D) Kangaroos in Australia Answer: (B) Zebra mussels in the Great LakesExplanation:Zebra mussels are an invasive species that were introduced to the Great Lakes and have caused significant ecological damage by outcompeting native species and altering habitats. Invasive species are non-native organisms that disrupt the ecosystems they invade. Question 63:Which of the following is a primary abiotic factor that influences the distribution of biomes?(A) Population density(B) Annual rainfall(C) Predation(D) Plant diversity Answer: (B) Annual rainfallExplanation:The primary abiotic factors that influence biome distribution are temperature and annual rainfall. These factors determine the type of vegetation and animal life that can thrive in a particular biome. Question 64:Which of the following best represents the correct order of ecological organization from smallest to largest?(A) Population → Community → Organism → Ecosystem → Biosphere(B) Organism → Population → Community → Ecosystem → Biosphere(C) Community → Organism → Population → Ecosystem → Biosphere(D) Ecosystem → Population → Organism → Community → Biosphere Answer: (B) Organism → Population → Community → Ecosystem → BiosphereExplanation:The correct order of ecological organization is organism (an individual living thing), population (a group of organisms of the same species), community (different species living together), ecosystem (living and non-living components interacting), and biosphere (all ecosystems on Earth). Question 65:Which of the following statements about ecological pyramids is true?(A) Energy pyramids are always upright.(B) Biomass pyramids are always inverted.(C) Numbers pyramids are always upright.(D) Energy pyramids are sometimes inverted. Answer: (A) Energy pyramids are always upright.Explanation:Energy pyramids are always upright because energy decreases at each successive trophic level due to the loss of energy as heat. Biomass pyramids and numbers pyramids may be inverted in some ecosystems, but energy pyramids cannot be inverted. Question 66:Which process in the water cycle involves water vapor being released by plants into the atmosphere?(A) Evaporation(B) Precipitation(C) Transpiration(D) Condensation Answer: (C) TranspirationExplanation:Transpiration is the process by which water vapor is released from the leaves of plants into the atmosphere. It is a crucial part of the water cycle, contributing to the movement of water from the land to the atmosphere. Question 67:Which of the following nutrients is typically the most limiting in terrestrial ecosystems?(A) Phosphorus(B) Nitrogen(C) Potassium(D) Calcium Answer: (B) NitrogenExplanation:Nitrogen is often the most limiting nutrient in terrestrial ecosystems, as it is required in large amounts for plant growth, but its availability is limited in many soils. Plants rely on nitrogen-fixing bacteria and the nitrogen cycle to obtain usable nitrogen. Question 68:The phenomenon of "biological oxygen demand" (BOD) is associated with:(A) The amount of oxygen used by fish for respiration(B) The oxygen requirement of aquatic plants(C) The amount of oxygen consumed by microorganisms during decomposition of organic matter(D) The oxygen content in polluted water Answer: (C) The amount of oxygen consumed by microorganisms during decomposition of organic matterExplanation:Biological oxygen demand (BOD) measures the amount of oxygen required by microorganisms to decompose organic matter in water. High BOD indicates high levels of organic pollution, leading to oxygen depletion in water bodies. Question 69:Which of the following best defines a "biome"?(A) A large geographic area with a distinct climate and specific types of plants and animals(B) A collection of interacting species in a particular environment(C) An area characterized by similar soil types(D) A single species population in an ecosystem Answer: (A) A large geographic area with a distinct climate and specific types of plants and animalsExplanation:A biome is a large geographic region characterized by specific climate conditions and particular plant and animal communities. Examples include deserts, tundras, rainforests, and grasslands. Question 70:Which of the following statements is true regarding secondary succession?(A) It begins on a newly exposed surface, such as lava(B) It occurs after a disturbance that leaves the soil intact(C) It is slower than primary succession(D) It does not involve pioneer species Answer: (B) It occurs after a disturbance that leaves the soil intactExplanation:Secondary succession occurs in areas where the previous community has been disturbed (e.g., by fire, flood, or human activity), but the soil remains intact. It is faster than primary succession, which occurs in lifeless areas such as newly formed volcanic land. Question 71:What is the primary role of detritivores in an ecosystem?(A) They decompose dead organisms(B) They fix nitrogen in the soil(C) They convert sunlight into chemical energy(D) They recycle nutrients by breaking down dead organic matter Answer: (D) They recycle nutrients by breaking down dead organic matterExplanation:Detritivores, such as earthworms and fungi, break down dead organic material and recycle nutrients back into the ecosystem. This process is vital for maintaining nutrient cycles in ecosystems. Question 72:Which of the following factors directly affects the rate of primary productivity in an ecosystem?(A) Number of herbivores(B) Solar energy input(C) Availability of carnivores(D) Population density Answer: (B) Solar energy inputExplanation:Primary productivity is directly influenced by the amount of solar energy available to plants for photosynthesis, along with other factors like nutrient availability and water. Herbivores, carnivores, and population density have an indirect effect on primary productivity. Question 73:Which of the following is an example of a carbon sink?(A) Combustion of fossil fuels(B) Burning of forests(C) Oceans absorbing atmospheric CO₂(D) Volcanic eruptions Answer: (C) Oceans absorbing atmospheric CO₂Explanation:Carbon sinks are natural systems that absorb and store carbon from the atmosphere. Oceans are major carbon sinks, as they absorb large amounts of atmospheric CO₂. Forests and soils also act as carbon sinks by storing carbon in plant biomass and organic matter. Question 74:In an ecosystem, which of the following is a producer?(A) Algae(B) Deer(C) Eagle(D) Snake Answer: (A) AlgaeExplanation:Producers are organisms that produce their own food through photosynthesis. Algae, along with plants, are primary producers in ecosystems. Deer, eagles, and snakes are consumers. Question 75:Which of the following ecosystems has the highest biodiversity?(A) Desert(B) Tropical rainforest(C) Arctic tundra(D) Freshwater lake Answer: (B) Tropical rainforestExplanation:Tropical rainforests have the highest biodiversity of any terrestrial ecosystem due to their warm, stable climate, high levels of rainfall, and abundant energy and nutrient availability. Deserts and tundras have lower biodiversity due to harsher environmental conditions. Question 76:Which of the following processes releases carbon dioxide into the atmosphere?(A) Photosynthesis(B) Respiration(C) Nitrogen fixation(D) Transpiration Answer: (B) RespirationExplanation:Respiration, carried out by both plants and animals, releases carbon dioxide (CO₂) into the atmosphere as a byproduct of breaking down glucose for energy. Photosynthesis absorbs CO₂, while nitrogen fixation and transpiration do not involve CO₂ release. Question 77:What is the role of mycorrhizae in ecosystems?(A) Nitrogen fixation(B) Helping plants absorb water and nutrients(C) Decomposing dead organic matter(D) Transpiration Answer: (B) Helping plants absorb water and nutrientsExplanation:Mycorrhizae are symbiotic associations between fungi and plant roots. They enhance the plant’s ability to absorb water and essential nutrients, such as phosphorus, from the soil, thus benefiting plant growth and productivity. Question 78:Which of the following ecosystems is considered a "carbon sink"?(A) Coral reefs(B) Urban areas(C) Forests(D) Agricultural fields Answer: (C) ForestsExplanation:Forests act as carbon sinks because trees absorb CO₂ during photosynthesis and store carbon in their biomass. Urban areas and agricultural fields generally do not serve as significant carbon sinks. Question 79:Which of the following human activities is the leading cause of deforestation?(A) Reforestation programs(B) Industrialization(C) Slash-and-burn agriculture(D) Wildlife conservation Answer: (C) Slash-and-burn agricultureExplanation:Slash-and-burn agriculture is one of the major causes of deforestation, especially in tropical regions. This practice involves clearing forests to create farmland, leading to habitat destruction, loss of biodiversity, and disruption of carbon and water cycles. Question 80:Which of the following is an example of a density-independent factor affecting population growth?(A) Competition for food(B) Predation(C) Drought(D) Parasitism Answer: (C) DroughtExplanation:Density-independent factors, like drought, affect population size regardless of the population's density. In contrast, density-dependent factors (such as competition, predation, and parasitism) have a greater impact as population density increases. Question 81:Which of the following pollutants causes "eutrophication" in water bodies?(A) Pesticides(B) Nitrates and phosphates(C) Sulfur dioxide(D) Carbon monoxide Answer: (B) Nitrates and phosphatesExplanation:Eutrophication is caused by the excessive input of nutrients like nitrates and phosphates into water bodies, leading to algal blooms. This depletes oxygen in the water, harming aquatic life. Question 82:Which of the following statements is true regarding the ozone layer?(A) It absorbs harmful infrared radiation from the sun(B) It is found in the troposphere(C) It protects the Earth from harmful ultraviolet (UV) radiation(D) It is being depleted by carbon dioxide emissions Answer: (C) It protects the Earth from harmful ultraviolet (UV) radiationExplanation:The ozone layer, located in the stratosphere, absorbs most of the sun’s harmful ultraviolet (UV) radiation, protecting living organisms on Earth from its damaging effects. The ozone layer is primarily being depleted by chlorofluorocarbons (CFCs), not CO₂ emissions. Question 83:In an aquatic ecosystem, the base of the food web consists of:(A) Zooplankton(B) Small fish(C) Phytoplankton(D) Crustaceans Answer: (C) PhytoplanktonExplanation:Phytoplankton are the primary producers in aquatic ecosystems, forming the base of the food web. They produce energy through photosynthesis and support the higher trophic levels, including zooplankton, fish, and other aquatic organisms. Question 84:Which of the following organisms would most likely occupy the highest trophic level in a food chain?(A) Grass(B) Insects(C) Deer(D) Lion Answer: (D) LionExplanation:Lions are apex predators, occupying the highest trophic level in a food chain. They are tertiary consumers that feed on primary and secondary consumers, such as herbivores like deer. Question 85:Which of the following pollutants is primarily responsible for acid rain?(A) Carbon dioxide(B) Nitrogen oxides and sulfur dioxide(C) Methane(D) Ozone Answer: (B) Nitrogen oxides and sulfur dioxideExplanation:Acid rain is caused by the release of nitrogen oxides (NOx) and sulfur dioxide (SO₂) into the atmosphere. These gases react with water vapor to form nitric acid and sulfuric acid, which then fall as acid rain, damaging ecosystems. Question 86:In the process of biomagnification, which of the following substances is most likely to accumulate in the highest concentration at the top of the food chain?(A) Water(B) Oxygen(C) Pesticides like DDT(D) Carbon dioxide Answer: (C) Pesticides like DDTExplanation:Biomagnification is the process where toxic substances like pesticides (e.g., DDT) accumulate in increasing concentrations at each trophic level of a food chain. Top predators accumulate the highest concentrations of these harmful substances. Question 87:Which of the following ecosystems has the lowest net primary productivity (NPP)?(A) Desert(B) Tropical rainforest(C) Coral reef(D) Grassland Answer: (A) DesertExplanation:Deserts have the lowest net primary productivity due to the scarcity of water and limited vegetation. Tropical rainforests and coral reefs have high NPP due to abundant resources and favorable environmental conditions. Question 88:Which of the following organisms is an example of a primary producer?(A) Grass(B) Rabbit(C) Fox(D) Earthworm Answer: (A) GrassExplanation:Primary producers, like grass, use photosynthesis to convert sunlight into energy, forming the base of the food chain. Rabbits are primary consumers, foxes are secondary consumers, and earthworms are decomposers. Question 89:Which of the following is an example of a renewable resource?(A) Natural gas(B) Coal(C) Wind energy(D) Petroleum Answer: (C) Wind energyExplanation:Wind energy is a renewable resource because it is continuously replenished by natural processes. In contrast, natural gas, coal, and petroleum are non-renewable resources, as they take millions of years to form and are depleted faster than they can be replaced. Question 90:The term "ecological footprint" refers to:(A) The impact of human activity on ecosystems(B) The area of land needed to support a population's resource use and waste generation(C) The total amount of energy consumed by a community(D) The biological productivity of an ecosystem Answer: (B) The area of land needed to support a population's resource use and waste generationExplanation:An ecological footprint measures the amount of land and water required to support a person or population’s consumption of resources and absorption of waste. It helps assess the sustainability of resource use. Question 91:The concept of "carrying capacity" refers to:(A) The maximum number of species an ecosystem can support(B) The number of prey animals in an ecosystem(C) The maximum number of individuals of a species that an environment can sustain indefinitely(D) The reproductive rate of a species Answer: (C) The maximum number of individuals of a species that an environment can sustain indefinitelyExplanation:Carrying capacity is the maximum population size of a species that an environment can support indefinitely, given the available resources such as food, water, and habitat space. Question 92:Which of the following is a major cause of biodiversity loss?(A) Habitat destruction(B) Conservation programs(C) Sustainable farming practices(D) Ecotourism Answer: (A) Habitat destructionExplanation:Habitat destruction, caused by activities such as deforestation, urbanization, and agriculture, is a major cause of biodiversity loss. It leads to the displacement and extinction of species as their natural environments are disrupted or destroyed. Question 93:Which of the following is an example of mutualism?(A) Bees pollinating flowers(B) A lion hunting a deer(C) A tapeworm living in a human intestine(D) A bird nesting in a tree Answer: (A) Bees pollinating flowersExplanation:Mutualism is a symbiotic relationship where both organisms benefit. Bees pollinate flowers while collecting nectar, which benefits both the bees (as food) and the flowers (as pollination). The other options represent other types of relationships, such as predation or parasitism. Question 94:Which of the following is a secondary consumer in a food chain?(A) Grasshopper(B) Deer(C) Frog(D) Algae Answer: (C) FrogExplanation:Secondary consumers feed on primary consumers (herbivores). Frogs, which feed on insects, are secondary consumers. Grasshoppers are primary consumers, deer are also primary consumers, and algae are primary producers. Question 95:What role do lichens play in primary succession?(A) They accelerate soil erosion(B) They are the first organisms to colonize bare rock and begin the process of soil formation(C) They are secondary consumers(D) They prevent the growth of other plants Answer: (B) They are the first organisms to colonize bare rock and begin the process of soil formationExplanation:Lichens are pioneer species in primary succession. They can colonize bare rock, secrete acids that break down the rock, and begin the process of soil formation, allowing other plants to eventually grow. Question 96:Which of the following ecosystems is primarily characterized by its permafrost?(A) Tropical rainforest(B) Desert(C) Tundra(D) Grassland Answer: (C) TundraExplanation:The tundra biome is characterized by its permafrost, a layer of permanently frozen soil. This biome has very cold temperatures, limited vegetation, and a short growing season. Question 97:Which of the following organisms is a tertiary consumer?(A) Grass(B) Rabbit(C) Hawk(D) Cow Answer: (C) HawkExplanation:A hawk is a tertiary consumer because it feeds on secondary consumers, such as small mammals or birds, which are themselves carnivores or omnivores. Grass and rabbits are part of the lower trophic levels, and cows are herbivores (primary consumers). Question 98:Which of the following gases is primarily responsible for the depletion of the ozone layer?(A) Methane(B) Chlorofluorocarbons (CFCs)(C) Carbon dioxide(D) Nitrous oxide Answer: (B) Chlorofluorocarbons (CFCs)Explanation:CFCs are the main chemicals responsible for the depletion of the ozone layer. They break down ozone molecules in the stratosphere, leading to the formation of the ozone hole. Question 99:The term "trophic level" refers to:(A) The number of organisms in a population(B) The position of an organism in the food chain(C) The total biomass in an ecosystem(D) The rate of energy transfer in an ecosystem Answer: (B) The position of an organism in the food chainExplanation:A trophic level represents the position an organism occupies in a food chain, such as producers (first level), herbivores (second level), and carnivores (higher levels). Question 100:Which of the following best describes the greenhouse effect?(A) The trapping of heat in Earth's atmosphere by greenhouse gases(B) The reflection of solar radiation back into space(C) The loss of heat from Earth's surface(D) The depletion of the ozone layer Answer: (A) The trapping of heat in Earth's atmosphere by greenhouse gasesExplanation:The greenhouse effect refers to the process by which greenhouse gases, such as carbon dioxide and methane, trap heat in the Earth's atmosphere, warming the planet. This is essential for maintaining life-supporting temperatures but can lead to global warming when excessive.

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