Chapter 14 – Natural Resources – MCQs
Chapter 14 – Natural Resources – MCQs (Part 1: Q1–Q25)
Q1. Which of the following is a non-renewable natural resource?
a) Water
b) Soil
c) Petroleum ✅
d) Forest
Answer: c) Petroleum
Explanation: Petroleum cannot be replenished quickly; it is a non-renewable fossil fuel.
Q2. Which is the primary source of energy for all living organisms?
a) Coal
b) Sun ✅
c) Water
d) Wind
Answer: b) Sun
Explanation: The Sun is the ultimate source of energy, supporting photosynthesis and life on Earth.
Q3. Which gas is necessary for photosynthesis?
a) Oxygen
b) Carbon dioxide ✅
c) Nitrogen
d) Methane
Answer: b) Carbon dioxide
Explanation: Plants absorb CO₂ during photosynthesis to produce glucose and oxygen.
Q4. Which of the following is a renewable resource?
a) Coal
b) Petroleum
c) Forest ✅
d) Minerals
Answer: c) Forest
Explanation: Forests can be renewed naturally if conserved and managed properly.
Q5. Which of the following is a greenhouse gas?
a) Oxygen
b) Carbon dioxide ✅
c) Nitrogen
d) Argon
Answer: b) Carbon dioxide
Explanation: Carbon dioxide traps heat in the atmosphere, causing greenhouse effect.
Q6. Which gas is responsible for ozone layer depletion?
a) Oxygen
b) CFCs ✅
c) Nitrogen
d) Argon
Answer: b) CFCs
Explanation: Chlorofluorocarbons (CFCs) damage ozone by releasing chlorine atoms.
Q7. Which cycle involves movement of water between Earth and atmosphere?
a) Carbon cycle
b) Water cycle ✅
c) Nitrogen cycle
d) Oxygen cycle
Answer: b) Water cycle
Explanation: The water cycle involves evaporation, condensation, and precipitation.
Q8. Which process releases oxygen into the atmosphere?
a) Photosynthesis ✅
b) Respiration
c) Burning fuels
d) Decomposition
Answer: a) Photosynthesis
Explanation: Green plants release oxygen as a by-product during photosynthesis.
Q9. Which of the following is a fossil fuel?
a) Wood
b) Coal ✅
c) Biogas
d) Sunlight
Answer: b) Coal
Explanation: Coal is formed from ancient plant matter buried under Earth’s crust.
Q10. Which gas is essential for respiration in animals?
a) Oxygen ✅
b) Carbon dioxide
c) Nitrogen
d) Argon
Answer: a) Oxygen
Explanation: Oxygen is necessary for aerobic respiration in animals.
Q11. Which cycle maintains balance of oxygen and carbon dioxide in nature?
a) Water cycle
b) Nitrogen cycle
c) Oxygen cycle ✅
d) Sulphur cycle
Answer: c) Oxygen cycle
Explanation: The oxygen cycle involves processes like photosynthesis and respiration, maintaining balance.
Q12. Which of the following is an exhaustible natural resource?
a) Air
b) Coal ✅
c) Water
d) Forests
Answer: b) Coal
Explanation: Coal is limited and cannot be replenished in human lifespan.
Q13. Which part of atmosphere contains ozone layer?
a) Troposphere
b) Stratosphere ✅
c) Mesosphere
d) Thermosphere
Answer: b) Stratosphere
Explanation: Ozone layer lies in stratosphere and protects from harmful UV rays.
Q14. Which process converts nitrogen gas into usable form for plants?
a) Respiration
b) Nitrogen fixation ✅
c) Combustion
d) Photosynthesis
Answer: b) Nitrogen fixation
Explanation: Nitrogen fixation by bacteria converts atmospheric nitrogen into nitrates.
Q15. Which form of water is available for use by living organisms?
a) Ocean water
b) Groundwater ✅
c) Ice caps
d) Sea water
Answer: b) Groundwater
Explanation: Groundwater is fresh and most accessible for plants, animals, and humans.
Q16. Which of the following is a man-made cause of air pollution?
a) Volcanic eruption
b) Forest fire
c) Vehicle emissions ✅
d) Dust storm
Answer: c) Vehicle emissions
Explanation: Burning of fossil fuels in vehicles is a major cause of air pollution.
Q17. Which nutrient is returned to soil by decomposers?
a) Nitrogen ✅
b) Oxygen
c) Argon
d) Helium
Answer: a) Nitrogen
Explanation: Decomposers convert organic waste into nitrogen compounds for soil fertility.
Q18. Which of the following is not a biotic component of environment?
a) Plants
b) Animals
c) Bacteria
d) Soil ✅
Answer: d) Soil
Explanation: Soil is abiotic; plants, animals, and microbes are biotic.
Q19. Which of the following is an abiotic component of ecosystem?
a) Grass
b) Tiger
c) Water ✅
d) Bacteria
Answer: c) Water
Explanation: Water is non-living (abiotic) but essential for ecosystem functioning.
Q20. Which cycle involves decomposition of dead organisms?
a) Nitrogen cycle ✅
b) Oxygen cycle
c) Water cycle
d) Carbon cycle
Answer: a) Nitrogen cycle
Explanation: Nitrogen compounds are released back into soil by decomposers.
Q21. Which of the following is a renewable energy source?
a) Petroleum
b) Natural gas
c) Wind ✅
d) Coal
Answer: c) Wind
Explanation: Wind energy is renewable and does not get exhausted.
Q22. Which gas is released during respiration?
a) Oxygen
b) Carbon dioxide ✅
c) Nitrogen
d) Hydrogen
Answer: b) Carbon dioxide
Explanation: Respiration produces CO₂ as a waste product.
Q23. Which part of Earth has maximum biodiversity?
a) Desert
b) Polar region
c) Tropical rainforest ✅
d) Grassland
Answer: c) Tropical rainforest
Explanation: Tropical rainforests support the greatest biodiversity due to warm and wet climate.
Q24. Which one is not a fossil fuel?
a) Coal
b) Petroleum
c) Natural gas
d) Biogas ✅
Answer: d) Biogas
Explanation: Biogas is a renewable fuel, not a fossil fuel.
Q25. Which process is responsible for formation of clouds?
a) Transpiration
b) Evaporation ✅
c) Respiration
d) Photosynthesis
Answer: b) Evaporation
Explanation: Water evaporates from surface and condenses to form clouds.
Natural Resources – MCQs (Part 2: Q26–Q50)
Q26. Which process returns carbon dioxide to the atmosphere?
a) Photosynthesis
b) Respiration ✅
c) Nitrogen fixation
d) Absorption by plants
Answer: b) Respiration
Explanation: Respiration in plants and animals releases CO₂ into the atmosphere.
Q27. Which is the largest reservoir of water on Earth?
a) Rivers
b) Lakes
c) Oceans ✅
d) Groundwater
Answer: c) Oceans
Explanation: Oceans contain about 97% of Earth’s total water.
Q28. Which component of the atmosphere is used for combustion?
a) Oxygen ✅
b) Nitrogen
c) Argon
d) Carbon dioxide
Answer: a) Oxygen
Explanation: Oxygen supports burning and respiration.
Q29. Which of the following causes acid rain?
a) Oxygen and water
b) SO₂ and NO₂ ✅
c) Nitrogen and oxygen
d) Carbon monoxide
Answer: b) SO₂ and NO₂
Explanation: Sulphur dioxide and nitrogen oxides combine with water to form acids, causing acid rain.
Q30. Which process increases the amount of oxygen in atmosphere?
a) Photosynthesis ✅
b) Respiration
c) Combustion
d) Fermentation
Answer: a) Photosynthesis
Explanation: Photosynthesis by plants releases oxygen as a by-product.
Q31. Which human activity leads to global warming?
a) Deforestation ✅
b) Afforestation
c) Rainwater harvesting
d) Organic farming
Answer: a) Deforestation
Explanation: Cutting trees increases CO₂ levels, leading to global warming.
Q32. Which layer of atmosphere is closest to Earth?
a) Troposphere ✅
b) Stratosphere
c) Mesosphere
d) Thermosphere
Answer: a) Troposphere
Explanation: Troposphere is the lowest atmospheric layer where weather phenomena occur.
Q33. Which natural resource is known as ‘liquid gold’?
a) Water
b) Petroleum ✅
c) Coal
d) Natural gas
Answer: b) Petroleum
Explanation: Petroleum is very valuable and is called liquid gold.
Q34. Which gas forms the major component of air?
a) Oxygen
b) Carbon dioxide
c) Nitrogen ✅
d) Argon
Answer: c) Nitrogen
Explanation: Nitrogen constitutes about 78% of air.
Q35. Which process is not part of the water cycle?
a) Evaporation
b) Condensation
c) Photosynthesis ✅
d) Precipitation
Answer: c) Photosynthesis
Explanation: Photosynthesis is not part of the water cycle; it’s part of the carbon/oxygen cycle.
Q36. Which of the following is a soil pollutant?
a) Humus
b) Pesticides ✅
c) Microorganisms
d) Minerals
Answer: b) Pesticides
Explanation: Excessive pesticides contaminate soil, reducing fertility.
Q37. Which process causes oxygen depletion in water bodies?
a) Eutrophication ✅
b) Condensation
c) Respiration
d) Photosynthesis
Answer: a) Eutrophication
Explanation: Excess nutrients cause algal growth, which reduces oxygen in water.
Q38. Which cycle involves decomposition of organic matter to release carbon?
a) Water cycle
b) Carbon cycle ✅
c) Nitrogen cycle
d) Oxygen cycle
Answer: b) Carbon cycle
Explanation: Carbon is released into atmosphere during decomposition and respiration.
Q39. Which is a renewable source of energy?
a) Coal
b) Petroleum
c) Solar energy ✅
d) Natural gas
Answer: c) Solar energy
Explanation: Solar energy is inexhaustible and renewable.
Q40. Which process reduces groundwater levels?
a) Afforestation
b) Over-pumping of wells ✅
c) Rainwater harvesting
d) Infiltration
Answer: b) Over-pumping of wells
Explanation: Excessive pumping of groundwater reduces its levels.
Q41. Which cycle involves bacteria converting nitrates back into nitrogen gas?
a) Nitrogen fixation
b) Denitrification ✅
c) Nitrification
d) Carbon cycle
Answer: b) Denitrification
Explanation: Denitrifying bacteria convert nitrates into nitrogen gas, releasing it to atmosphere.
Q42. Which is a non-metal that is essential for plant growth?
a) Iron
b) Nitrogen ✅
c) Calcium
d) Magnesium
Answer: b) Nitrogen
Explanation: Nitrogen is a non-metal that forms proteins and nucleic acids in plants.
Q43. Which gas is produced during decomposition of organic waste in landfills?
a) Carbon dioxide
b) Methane ✅
c) Oxygen
d) Nitrogen
Answer: b) Methane
Explanation: Decomposition under anaerobic conditions produces methane.
Q44. Which human activity increases soil erosion?
a) Afforestation
b) Deforestation ✅
c) Crop rotation
d) Mulching
Answer: b) Deforestation
Explanation: Cutting trees exposes soil, leading to erosion.
Q45. Which water pollutant causes ‘blue baby syndrome’?
a) Lead
b) Mercury
c) Nitrates ✅
d) Phosphates
Answer: c) Nitrates
Explanation: High nitrates in drinking water reduce oxygen in infants’ blood, causing the disease.
Q46. Which part of Earth is known as the ‘biosphere’?
a) Atmosphere only
b) Hydrosphere only
c) Lithosphere only
d) All regions where life exists ✅
Answer: d) All regions where life exists
Explanation: Biosphere includes lithosphere, hydrosphere, and atmosphere where organisms live.
Q47. Which human activity contributes to depletion of ozone layer?
a) Vehicle emissions
b) Use of CFCs ✅
c) Burning of wood
d) Deforestation
Answer: b) Use of CFCs
Explanation: Chlorofluorocarbons destroy ozone molecules in stratosphere.
Q48. Which process is responsible for the cooling effect after rainfall?
a) Evaporation ✅
b) Condensation
c) Respiration
d) Photosynthesis
Answer: a) Evaporation
Explanation: Evaporation of rainwater uses heat energy, lowering temperature.
Q49. Which of the following is a mineral resource?
a) Soil
b) Forest
c) Coal
d) Iron ore ✅
Answer: d) Iron ore
Explanation: Iron ore is a mineral extracted from Earth’s crust.
Q50. Which cycle maintains balance of nitrogen in atmosphere?
a) Carbon cycle
b) Nitrogen cycle ✅
c) Water cycle
d) Oxygen cycle
Answer: b) Nitrogen cycle
Explanation: Nitrogen cycle involves fixation, nitrification, and denitrification to maintain nitrogen balance.
Natural Resources – MCQs (Part 3: Q51–Q75)
Q51. Which process helps in replenishment of groundwater?
a) Runoff
b) Infiltration ✅
c) Evaporation
d) Condensation
Answer: b) Infiltration
Explanation: Infiltration allows rainwater to seep into the soil and recharge groundwater.
Q52. Which of the following is not a fossil fuel?
a) Coal
b) Petroleum
c) Natural gas
d) Biogas ✅
Answer: d) Biogas
Explanation: Biogas is a renewable fuel obtained from decomposition of organic waste.
Q53. Which nutrient cycle is most closely linked to proteins and nucleic acids?
a) Carbon cycle
b) Oxygen cycle
c) Nitrogen cycle ✅
d) Water cycle
Answer: c) Nitrogen cycle
Explanation: Nitrogen is a component of proteins and nucleic acids.
Q54. Which factor is responsible for desertification?
a) Overgrazing ✅
b) Afforestation
c) Rainwater harvesting
d) Mulching
Answer: a) Overgrazing
Explanation: Overgrazing removes vegetation cover, leading to soil erosion and desertification.
Q55. Which form of energy is obtained from tides?
a) Geothermal energy
b) Hydroelectric energy
c) Tidal energy ✅
d) Solar energy
Answer: c) Tidal energy
Explanation: Tidal energy is harnessed from rise and fall of ocean tides.
Q56. Which one is not a greenhouse gas?
a) Carbon dioxide
b) Methane
c) Oxygen ✅
d) Nitrous oxide
Answer: c) Oxygen
Explanation: Oxygen does not trap heat; CO₂, CH₄, and N₂O are greenhouse gases.
Q57. Which practice helps in conservation of forests?
a) Deforestation
b) Overgrazing
c) Afforestation ✅
d) Mining
Answer: c) Afforestation
Explanation: Planting more trees helps in forest conservation.
Q58. Which cycle involves conversion of ammonia into nitrates?
a) Nitrogen fixation
b) Nitrification ✅
c) Denitrification
d) Carbon cycle
Answer: b) Nitrification
Explanation: Nitrifying bacteria convert ammonia into nitrates for plants.
Q59. Which energy is obtained from inside the Earth?
a) Tidal energy
b) Geothermal energy ✅
c) Solar energy
d) Nuclear energy
Answer: b) Geothermal energy
Explanation: Geothermal energy is heat energy stored inside Earth’s crust.
Q60. Which natural resource is overexploited by mining?
a) Soil
b) Minerals ✅
c) Forest
d) Water
Answer: b) Minerals
Explanation: Mining depletes mineral resources and also causes land degradation.
Q61. Which gas is used by plants for protein synthesis?
a) Oxygen
b) Nitrogen ✅
c) Carbon dioxide
d) Argon
Answer: b) Nitrogen
Explanation: Plants use nitrogen (absorbed as nitrates) to make proteins.
Q62. Which phenomenon causes global warming?
a) Greenhouse effect ✅
b) Water cycle
c) Ozone depletion
d) Acid rain
Answer: a) Greenhouse effect
Explanation: Excess greenhouse gases trap heat, causing global warming.
Q63. Which of the following is an inexhaustible natural resource?
a) Coal
b) Petroleum
c) Solar energy ✅
d) Natural gas
Answer: c) Solar energy
Explanation: Solar energy is renewable and inexhaustible.
Q64. Which organisms are responsible for nitrogen fixation in soil?
a) Algae
b) Fungi
c) Bacteria ✅
d) Viruses
Answer: c) Bacteria
Explanation: Nitrogen-fixing bacteria (e.g., Rhizobium) convert atmospheric nitrogen into nitrates.
Q65. Which of the following contributes to air pollution?
a) Burning fossil fuels ✅
b) Photosynthesis
c) Rainfall
d) Afforestation
Answer: a) Burning fossil fuels
Explanation: Combustion of coal, petroleum, and gas releases pollutants like CO₂, SO₂, and NO₂.
Q66. Which is the main cause of ozone hole?
a) Carbon dioxide
b) CFCs ✅
c) Oxygen
d) Methane
Answer: b) CFCs
Explanation: Chlorofluorocarbons (CFCs) break down ozone molecules in stratosphere.
Q67. Which cycle involves conversion of organic matter into carbon dioxide?
a) Water cycle
b) Carbon cycle ✅
c) Oxygen cycle
d) Nitrogen cycle
Answer: b) Carbon cycle
Explanation: Decomposition and respiration release carbon dioxide into the atmosphere.
Q68. Which type of soil erosion is caused by wind in arid regions?
a) Gully erosion
b) Sheet erosion
c) Wind erosion ✅
d) Splash erosion
Answer: c) Wind erosion
Explanation: In deserts, strong winds carry away the top fertile soil.
Q69. Which gas is required for combustion and respiration?
a) Oxygen ✅
b) Carbon dioxide
c) Nitrogen
d) Argon
Answer: a) Oxygen
Explanation: Oxygen supports burning and is essential for respiration.
Q70. Which part of Earth contains the hydrosphere?
a) Air
b) Water ✅
c) Land
d) Rocks
Answer: b) Water
Explanation: Hydrosphere includes all water bodies – oceans, rivers, lakes, and groundwater.
Q71. Which process removes excess CO₂ from the atmosphere?
a) Respiration
b) Photosynthesis ✅
c) Combustion
d) Decomposition
Answer: b) Photosynthesis
Explanation: Plants absorb CO₂ during photosynthesis, reducing atmospheric CO₂.
Q72. Which is a man-made resource?
a) Water
b) Forest
c) Dam ✅
d) Air
Answer: c) Dam
Explanation: Dam is a human-made structure for storing water and generating electricity.
Q73. Which natural resource is known as “black gold”?
a) Petroleum
b) Coal ✅
c) Natural gas
d) Iron ore
Answer: b) Coal
Explanation: Coal is called black gold because of its high energy value.
Q74. Which process is responsible for formation of rainwater?
a) Condensation ✅
b) Evaporation
c) Photosynthesis
d) Infiltration
Answer: a) Condensation
Explanation: Water vapor condenses into droplets that fall as rain.
Q75. Which cycle is disturbed due to excessive use of fertilizers?
a) Carbon cycle
b) Nitrogen cycle ✅
c) Oxygen cycle
d) Water cycle
Answer: b) Nitrogen cycle
Explanation: Excessive fertilizers increase nitrates in soil and water, disturbing the nitrogen cycle.
Natural Resources – MCQs (Part 4: Q76–Q100)
Q76. Which process in water cycle brings water back to Earth’s surface?
a) Evaporation
b) Condensation
c) Precipitation ✅
d) Infiltration
Answer: c) Precipitation
Explanation: Rain, snow, or hail brings water back to Earth during precipitation.
Q77. Which of the following is a non-renewable energy resource?
a) Solar energy
b) Wind energy
c) Coal ✅
d) Hydroelectricity
Answer: c) Coal
Explanation: Coal is a fossil fuel that takes millions of years to form.
Q78. Which natural phenomenon maintains Earth’s temperature?
a) Ozone layer
b) Greenhouse effect ✅
c) Acid rain
d) Global warming
Answer: b) Greenhouse effect
Explanation: The greenhouse effect naturally traps heat and keeps Earth warm enough for life.
Q79. Which is an example of inexhaustible natural resource?
a) Petroleum
b) Coal
c) Wind ✅
d) Natural gas
Answer: c) Wind
Explanation: Wind is available in unlimited supply and cannot be exhausted.
Q80. Which pollutant is mainly responsible for smog?
a) Sulphur dioxide ✅
b) Oxygen
c) Nitrogen gas
d) Helium
Answer: a) Sulphur dioxide
Explanation: Smog is formed when SO₂ and other pollutants mix with fog.
Q81. Which gas do plants absorb from air during respiration?
a) Oxygen ✅
b) Nitrogen
c) Carbon dioxide
d) Argon
Answer: a) Oxygen
Explanation: Like animals, plants also use oxygen during respiration.
Q82. Which is the ultimate source of all natural resources?
a) Forest
b) Water
c) Sun ✅
d) Soil
Answer: c) Sun
Explanation: Sun drives all natural cycles and supports life on Earth.
Q83. Which type of soil is most fertile?
a) Sandy soil
b) Clay soil
c) Loamy soil ✅
d) Rocky soil
Answer: c) Loamy soil
Explanation: Loamy soil has balanced sand, silt, and clay, making it best for crops.
Q84. Which is the largest reservoir of fresh water on Earth?
a) Rivers
b) Groundwater
c) Ice caps ✅
d) Lakes
Answer: c) Ice caps
Explanation: Most of Earth’s fresh water is locked in glaciers and ice caps.
Q85. Which natural resource forms humus?
a) Decomposed organic matter ✅
b) Rocks
c) Minerals
d) Water
Answer: a) Decomposed organic matter
Explanation: Humus is formed by decomposition of dead plants and animals.
Q86. Which is the largest carbon reservoir on Earth?
a) Plants
b) Soil
c) Oceans ✅
d) Atmosphere
Answer: c) Oceans
Explanation: Oceans store large amounts of dissolved carbon dioxide.
Q87. Which of the following is not an effect of deforestation?
a) Soil erosion
b) Increase in rainfall ✅
c) Desertification
d) Loss of biodiversity
Answer: b) Increase in rainfall
Explanation: Deforestation reduces rainfall instead of increasing it.
Q88. Which gas is released during photosynthesis?
a) Oxygen ✅
b) Carbon dioxide
c) Nitrogen
d) Methane
Answer: a) Oxygen
Explanation: Plants release oxygen as a by-product of photosynthesis.
Q89. Which form of energy is produced in dams?
a) Nuclear energy
b) Geothermal energy
c) Hydroelectric energy ✅
d) Wind energy
Answer: c) Hydroelectric energy
Explanation: Dams use falling water to generate hydroelectric power.
Q90. Which activity reduces global warming?
a) Burning fossil fuels
b) Deforestation
c) Afforestation ✅
d) Excessive mining
Answer: c) Afforestation
Explanation: Planting trees absorbs CO₂, reducing global warming.
Q91. Which gas is essential for combustion?
a) Oxygen ✅
b) Carbon dioxide
c) Nitrogen
d) Hydrogen
Answer: a) Oxygen
Explanation: Oxygen supports combustion of fuels.
Q92. Which type of pollution is caused by loud noise?
a) Air pollution
b) Water pollution
c) Noise pollution ✅
d) Soil pollution
Answer: c) Noise pollution
Explanation: Excessive sound beyond safe limits causes noise pollution.
Q93. Which part of the nitrogen cycle is carried out by lightning?
a) Nitrogen fixation ✅
b) Denitrification
c) Nitrification
d) Decomposition
Answer: a) Nitrogen fixation
Explanation: Lightning converts atmospheric nitrogen into usable nitrates.
Q94. Which cycle maintains the supply of carbon dioxide and oxygen in nature?
a) Water cycle
b) Carbon cycle ✅
c) Nitrogen cycle
d) Sulphur cycle
Answer: b) Carbon cycle
Explanation: Carbon cycle balances CO₂ and O₂ through respiration and photosynthesis.
Q95. Which of the following is not a renewable resource?
a) Sunlight
b) Wind
c) Coal ✅
d) Water
Answer: c) Coal
Explanation: Coal is non-renewable as it forms over millions of years.
Q96. Which resource is essential for hydropower plants?
a) Coal
b) Wind
c) Water ✅
d) Sunlight
Answer: c) Water
Explanation: Flowing or falling water is used to generate hydroelectric power.
Q97. Which part of air do living organisms not use directly?
a) Oxygen
b) Nitrogen ✅
c) Carbon dioxide
d) Water vapor
Answer: b) Nitrogen
Explanation: Nitrogen is inert and must be fixed into nitrates before use by organisms.
Q98. Which gas causes acid rain along with SO₂?
a) Nitrous oxide ✅
b) Carbon monoxide
c) Oxygen
d) Methane
Answer: a) Nitrous oxide
Explanation: Nitrogen oxides react with water to form nitric acid, causing acid rain.
Q99. Which renewable energy source uses moving air?
a) Solar energy
b) Wind energy ✅
c) Tidal energy
d) Geothermal energy
Answer: b) Wind energy
Explanation: Wind turbines convert kinetic energy of air into electricity.
Q100. Which resource forms the base of all food chains?
a) Plants ✅
b) Animals
c) Fungi
d) Bacteria
Answer: a) Plants
Explanation: Plants are producers that capture solar energy and form the base of food chains.
