Organisms and Their Environment: Food Chains, Food Webs, Energy Flow, Nutrient Cycles and Population Growth

1. A food chain is shown.

grass → grasshopper → frog → snake → eagle

Which organism is the tertiary consumer?

A grasshopper
B frog
C snake
D eagle

2. In a food chain, arrows show

A the direction of movement of minerals only
B the direction of energy transfer
C the direction predators move when hunting
D the direction carbon dioxide moves during respiration

3. Which organism is always found at the first trophic level in a food chain?

A herbivore
B carnivore
C producer
D decomposer

4. A food chain has five trophic levels. If the producer contains 50 000 kJ of energy and 10% is transferred at each stage, how much energy reaches the fourth trophic level?

A 50 kJ
B 500 kJ
C 5000 kJ
D 50 000 kJ

5. Which statement best explains why food chains are usually short?

A producers cannot photosynthesise after being eaten
B energy is lost at each trophic level, so little remains for higher levels
C carnivores cannot digest protein
D decomposers remove all oxygen from ecosystems

6. Which process transfers carbon from producers to primary consumers?

A feeding
B photosynthesis
C decay only
D nitrification

7. Which process removes carbon dioxide from the atmosphere?

A respiration
B combustion
C photosynthesis
D decomposition

8. Which process adds carbon dioxide to the atmosphere?

A photosynthesis by green plants
B respiration by animals
C absorption of nitrate ions by roots
D evaporation of water from leaves

9. Which organisms convert dead organic matter into simpler substances?

A producers
B decomposers
C herbivores
D top carnivores

10. Which row correctly matches organism and role?

A grass: primary consumer
B rabbit: producer
C fox: secondary consumer
D fungus: tertiary consumer only

11. A pyramid of biomass is usually more useful than a pyramid of numbers because it

A shows the mass of living material at each trophic level
B always has the same shape in aquatic ecosystems
C counts only animals and ignores plants
D shows the exact age of every organism

12. One oak tree supports 500 caterpillars, which support 20 birds.
    Which pyramid is most likely to be inverted at the base?

A pyramid of biomass
B pyramid of numbers
C pyramid of energy
D pyramid of carbon dioxide

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

13. Which pyramid can never be inverted in a stable ecosystem?

A pyramid of numbers
B pyramid of biomass
C pyramid of energy
D pyramid of organism size

14. Why is some energy lost between trophic levels?

A all food eaten is converted into biomass
B energy is lost in faeces, movement, respiration and heat transfer
C energy is created by decomposers and stored in rocks
D consumers convert oxygen directly into glucose

15. A caterpillar eats 2000 kJ of leaf material. It uses 1200 kJ in respiration and loses 500 kJ in faeces.
    How much energy is available as new biomass for the next trophic level?

A 300 kJ
B 500 kJ
C 700 kJ
D 1700 kJ

16. In the same example, what percentage of the eaten energy becomes caterpillar biomass?

A 15%
B 25%
C 60%
D 85%

17. Which statement about biomass is correct?

A it is the total dry mass of living organisms in a given area
B it is the number of different species only
C it is the mass of water inside cells only
D it is the total oxygen released by producers

18. Why is dry mass used when measuring biomass?

A water content varies and can give misleading mass values
B dry mass includes more water than fresh mass
C living organisms cannot be weighed when alive
D dry mass measures only carbon dioxide

19. A plant uses light energy to make glucose. Only a small percentage of light energy is stored in plant biomass.
    Which reason best explains this?

A all wavelengths of light are absorbed equally
B some light is reflected, transmitted or not used by chlorophyll
C plants do not respire during daylight
D glucose cannot be converted into biomass

20. Which food chain is correctly ordered?

A hawk → snake → mouse → grass
B grass → mouse → snake → hawk
C mouse → grass → hawk → snake
D grass → hawk → snake → mouse

21. In the food web below, arrows show energy transfer.

grass → rabbit → fox
grass → grasshopper → frog → fox
berries → bird → fox
grasshopper → bird

Which organisms are secondary consumers?

A grass and berries
B rabbit and grasshopper
C frog and bird
D fox only

22. In the same food web, what is the most likely effect if grasshopper numbers fall greatly?

A frog numbers may decrease
B grass numbers must become zero
C foxes must immediately become extinct
D berry production must stop

23. Which term describes all the organisms of one species living in one area at one time?

A community
B ecosystem
C population
D habitat

24. Which term describes all the populations of different species living and interacting in one area?

A community
B population
C niche
D trophic level

25. Which term includes living organisms and their physical environment interacting together?

A ecosystem
B genotype
C species
D organ system

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

26. Which statement best describes a habitat?

A the role an organism plays in its ecosystem
B the place where an organism lives
C all organisms feeding at the same trophic level
D the number of individuals in a population

27. Which statement best describes a niche?

A the physical place only where an organism sleeps
B the role of an organism in its ecosystem, including feeding and interactions
C the total biomass of producers only
D the genetic code of one species

28. Which factor is density-dependent?

A earthquake
B temperature of a desert at midday
C spread of disease in a crowded population
D volcanic eruption

29. Which factor is density-independent?

A competition for food
B spread of infectious disease
C predation depending on prey abundance
D drought

30. A population grows rapidly at first, then growth slows and finally levels off.
    What is the best explanation?

A carrying capacity has been reached due to limiting factors
B all organisms stop respiring
C producers become consumers
D mutation stops reproduction permanently

31. Which term describes the maximum population size that an environment can support over time?

A trophic level
B carrying capacity
C decomposer capacity
D mutation rate

32. In a sigmoid population growth curve, the stationary phase occurs because

A birth rate equals death rate approximately
B death rate is always zero
C birth rate becomes infinite
D all individuals become genetically identical

33. Which factor is most likely to limit a population of herbivores?

A availability of food plants
B amount of nitrogen gas in the air only
C number of chloroplasts in carnivores
D oxygen concentration inside xylem

34. A predator population increases shortly after the prey population increases.
    Why is there usually a delay?

A predators need time to reproduce after more food becomes available
B prey must first become producers
C predators cannot respire while prey are abundant
D prey immediately stop breeding when predators appear

35. In a predator-prey relationship, why might prey numbers rise after predator numbers fall?

A fewer prey are eaten
B all prey become predators
C producers disappear
D prey stop competing for food completely

36. A student estimates plant population using quadrats. The mean number of plants per 0.25 m² quadrat is 12.
    The field area is 200 m².
    What is the estimated plant population?

A 600
B 960
C 2400
D 9600

37. Which method gives the least biased estimate of plant distribution in a field?

A placing quadrats only where plants are most common
B placing quadrats randomly
C placing quadrats only near the path
D placing one quadrat and assuming it represents the whole field

38. A student uses a transect from a pond edge to dry ground.
    What is this method best for studying?

A changes in species distribution along an environmental gradient
B random inheritance of sex chromosomes
C digestion of starch by amylase
D rate of oxygen uptake in respiration

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

39. A population of beetles is estimated using capture-mark-release-recapture.

First capture: 80 beetles marked
Second capture: 100 beetles caught
Marked in second capture: 20 beetles

What is the estimated population size?

A 100
B 200
C 400
D 800

40. Which assumption is needed for a valid capture-mark-release-recapture estimate?

A marked animals mix back randomly with the population
B marked animals cannot be recaptured
C all animals die between samples
D marks must make animals easier for predators to see

41. Which event would make capture-mark-release-recapture overestimate population size?

A marked animals become trap-shy and are less likely to be recaptured
B marked animals are recaptured in exactly the expected proportion
C no births, deaths, immigration or emigration occur
D the mark does not affect survival

42. Which process converts atmospheric nitrogen gas into compounds plants can use?

A nitrogen fixation
B denitrification
C transpiration
D photosynthesis

43. Which organisms carry out nitrogen fixation in root nodules of leguminous plants?

A nitrifying bacteria
B nitrogen-fixing bacteria
C denitrifying bacteria
D yeast cells

44. Which process converts ammonium ions into nitrates?

A nitrification
B denitrification
C carbon fixation
D evaporation

45. Which process converts nitrates into nitrogen gas?

A nitrification
B nitrogen fixation
C denitrification
D feeding

46. Which process releases ammonium compounds from dead organisms and waste?

A decomposition
B photosynthesis
C transpiration
D pollination

47. Which statement about nitrogen is correct?

A plants absorb nitrogen gas directly through stomata to make proteins
B plants absorb nitrate ions and use them to make amino acids and proteins
C animals absorb nitrates from soil through skin
D decomposers convert nitrates directly into glucose

48. Which sequence correctly shows nitrogen moving from soil into an animal?

A nitrates → plant proteins → animal proteins
B nitrogen gas → animal starch → plant glucose
C ammonium ions → oxygen → animal glycogen
D nitrates → bile → carbon dioxide

49. Which process returns carbon from fossil fuels to the atmosphere?

A combustion
B nitrification
C osmosis
D selective breeding

50. Which statement is correct?

A energy is recycled in ecosystems, but nutrients flow in one direction only
B nutrients are recycled in ecosystems, but energy flows through and is eventually lost as heat
C both energy and nutrients are permanently stored in consumers
D decomposers stop nutrient cycling by trapping minerals forever

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

1. Answer: C
   A is wrong because the grasshopper is the primary consumer.
   B is wrong because the frog is the secondary consumer.
   C is correct because the snake eats the secondary consumer, so it is the tertiary consumer.
   D is wrong because the eagle is the quaternary consumer in this chain.

2. Answer: B
   A is wrong because arrows do not show mineral movement only.
   B is correct because arrows in food chains show the direction of energy transfer.
   C is wrong because arrows do not show predator movement.
   D is wrong because carbon dioxide movement is part of the carbon cycle, not food chain arrow meaning.

3. Answer: C
   A is wrong because herbivores are primary consumers.
   B is wrong because carnivores are higher-level consumers.
   C is correct because producers occupy the first trophic level.
   D is wrong because decomposers feed on dead material and are not the first trophic level in a normal grazing food chain.

4. Answer: A
   Producer = 50 000 kJ.
   Second trophic level = 5000 kJ.
   Third trophic level = 500 kJ.
   Fourth trophic level = 50 kJ.
   A is correct.
   B, C and D are too large because they stop the 10% transfer too early.

5. Answer: B
   A is wrong because producers can still photosynthesise as a group if some are eaten.
   B is correct because energy is lost at each trophic level, so little energy remains for higher consumers.
   C is wrong because carnivores can digest protein.
   D is wrong because decomposers recycle nutrients; they do not remove all oxygen.

6. Answer: A
   A is correct because carbon in plant biomass passes to primary consumers when they feed.
   B is wrong because photosynthesis moves carbon from carbon dioxide into producers.
   C is wrong because decay transfers carbon from dead matter.
   D is wrong because nitrification is part of the nitrogen cycle.

7. Answer: C
   A is wrong because respiration adds carbon dioxide to the atmosphere.
   B is wrong because combustion also adds carbon dioxide.
   C is correct because photosynthesis removes carbon dioxide from the atmosphere.
   D is wrong because decomposition releases carbon dioxide.

8. Answer: B
   A is wrong because photosynthesis removes carbon dioxide.
   B is correct because respiration releases carbon dioxide into the atmosphere.
   C is wrong because nitrate absorption is part of nitrogen nutrition.
   D is wrong because evaporation of water from leaves is transpiration.

9. Answer: B
   A is wrong because producers make organic molecules by photosynthesis.
   B is correct because decomposers break down dead organic matter into simpler substances.
   C is wrong because herbivores feed on plants.
   D is wrong because top carnivores feed on other consumers.

10. Answer: C
    A is wrong because grass is a producer.
    B is wrong because rabbits are primary consumers.
    C is correct because a fox feeding on a herbivore is a secondary consumer.
    D is wrong because fungi are decomposers, not tertiary consumers only.

11. Answer: A
    A is correct because a pyramid of biomass shows the mass of living material at each trophic level.
    B is wrong because biomass pyramids can sometimes be unusual in aquatic ecosystems.
    C is wrong because biomass includes plants and animals.
    D is wrong because it does not show age.

12. Answer: B
    A is wrong because one oak tree can have a large biomass, so biomass may still be broad at the base.
    B is correct because one producer supporting many caterpillars gives an inverted pyramid of numbers at the base.
    C is wrong because pyramids of energy are never inverted.
    D is wrong because “pyramid of carbon dioxide” is not the standard ecological pyramid.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

13. Answer: C
    A is wrong because pyramids of numbers can be inverted.
    B is wrong because pyramids of biomass can sometimes be inverted, especially in aquatic ecosystems.
    C is correct because energy is always lost between trophic levels, so a pyramid of energy cannot be inverted in a stable ecosystem.
    D is wrong because organism size is not the standard pyramid used for energy transfer.

14. Answer: B
    A is wrong because not all food eaten becomes biomass.
    B is correct because energy is lost in faeces, movement, respiration and heat transfer.
    C is wrong because decomposers do not create energy.
    D is wrong because consumers do not convert oxygen directly into glucose.

15. Answer: A
    Energy available as new biomass = 2000 – 1200 – 500 = 300 kJ.
    A is correct.
    B is wrong because 500 kJ is faecal loss.
    C is wrong because it ignores faecal loss or respiration wrongly.
    D is wrong because it subtracts only 300 from the total.

16. Answer: A
    Percentage = 300 ÷ 2000 × 100 = 15%.
    A is correct.
    B, C and D are wrong calculation traps.

17. Answer: A
    A is correct because biomass is the total dry mass of living organisms in a given area.
    B is wrong because number of species is biodiversity/species richness.
    C is wrong because biomass is not just water mass.
    D is wrong because oxygen released is linked to photosynthesis rate, not biomass.

18. Answer: A
    A is correct because water content varies, so fresh mass can be misleading.
    B is wrong because dry mass excludes water.
    C is wrong because living organisms can sometimes be weighed, but water content remains the issue.
    D is wrong because dry mass does not measure carbon dioxide.

19. Answer: B
    A is wrong because plants do not absorb all wavelengths equally.
    B is correct because some light is reflected, transmitted or not absorbed by chlorophyll.
    C is wrong because plants respire during daylight too.
    D is wrong because glucose can be converted into biomass such as cellulose, starch, proteins and lipids.

20. Answer: B
    A is wrong because it starts with the top predator.
    B is correct because grass is the producer, mouse is the primary consumer, snake is the secondary consumer and hawk is the tertiary consumer.
    C is wrong because a mouse does not feed on grass after being placed before it.
    D is wrong because hawks do not feed directly on grass in this chain.

21. Answer: C
    A is wrong because grass and berries are producers.
    B is wrong because rabbit and grasshopper are primary consumers.
    C is correct because frog eats grasshopper, and bird can act as a secondary consumer when eating grasshopper.
    D is wrong because fox can be a secondary or higher consumer, but it is not the only secondary consumer.

22. Answer: A
    A is correct because frogs feed on grasshoppers, so frog numbers may fall.
    B is wrong because grass numbers do not have to become zero.
    C is wrong because foxes have other food sources in the web.
    D is wrong because berries are not directly dependent on grasshoppers.

23. Answer: C
    A is wrong because a community includes all populations of different species.
    B is wrong because an ecosystem includes organisms and physical environment.
    C is correct because a population is all organisms of one species in one area at one time.
    D is wrong because habitat is the place an organism lives.

24. Answer: A
    A is correct because a community is all populations of different species living and interacting in one area.
    B is wrong because a population is one species only.
    C is wrong because niche means the role of an organism.
    D is wrong because trophic level is feeding level.

25. Answer: A
    A is correct because an ecosystem includes living organisms and their physical environment interacting together.
    B is wrong because genotype is allele combination.
    C is wrong because species is a group able to breed and produce fertile offspring.
    D is wrong because organ system is a level of organisation in organisms.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

26. Answer: B
    A is wrong because that describes a niche.
    B is correct because a habitat is the place where an organism lives.
    C is wrong because that describes a trophic level.
    D is wrong because that describes population size.

27. Answer: B
    A is wrong because niche is more than just a physical place.
    B is correct because a niche is the role of an organism in its ecosystem, including feeding and interactions.
    C is wrong because producer biomass is not niche.
    D is wrong because genetic code is not niche.

28. Answer: C
    A is wrong because earthquakes affect populations regardless of density.
    B is wrong because desert temperature is density-independent.
    C is correct because disease spreads more easily in crowded populations, so it is density-dependent.
    D is wrong because volcanic eruptions are density-independent.

29. Answer: D
    A is wrong because competition increases with population density.
    B is wrong because infectious disease spread often depends on density.
    C is wrong because predation can depend on prey abundance.
    D is correct because drought affects populations regardless of their density.

30. Answer: A
    A is correct because growth levels off when carrying capacity is reached due to limiting factors.
    B is wrong because organisms continue respiring.
    C is wrong because producers do not become consumers.
    D is wrong because mutation does not permanently stop reproduction.

31. Answer: B
    A is wrong because trophic level is feeding level.
    B is correct because carrying capacity is the maximum population size an environment can support over time.
    C is wrong because decomposer capacity is not the standard term.
    D is wrong because mutation rate is unrelated to maximum population size.

32. Answer: A
    A is correct because in the stationary phase, birth rate approximately equals death rate.
    B is wrong because death rate is not zero.
    C is wrong because birth rate does not become infinite.
    D is wrong because all individuals do not become genetically identical.

33. Answer: A
    A is correct because herbivore populations are often limited by availability of food plants.
    B is wrong because nitrogen gas in air is not directly used by herbivores.
    C is wrong because carnivores do not have chloroplasts.
    D is wrong because xylem oxygen concentration is irrelevant to herbivore population size.

34. Answer: A
    A is correct because predator numbers rise after a delay because predators need time to reproduce when prey becomes more available.
    B is wrong because prey do not become producers.
    C is wrong because predators still respire.
    D is wrong because prey do not immediately stop breeding when predators appear.

35. Answer: A
    A is correct because if predator numbers fall, fewer prey are eaten, so prey numbers may rise.
    B is wrong because prey do not all become predators.
    C is wrong because producers do not disappear because predator numbers fall.
    D is wrong because prey may still compete for food.

36. Answer: D
    Field area = 200 m².
    Each quadrat = 0.25 m².
    Number of quadrats in field = 200 ÷ 0.25 = 800.
    Estimated population = 800 × 12 = 9600.
    D is correct.
    A, B and C are calculation traps.

37. Answer: B
    A is wrong because choosing areas where plants are common causes bias.
    B is correct because random quadrat placement gives a less biased estimate.
    C is wrong because placing quadrats near a path is biased.
    D is wrong because one quadrat is unlikely to represent the whole field accurately.

38. Answer: A
    A is correct because a transect is used to study changes in species distribution along an environmental gradient.
    B is wrong because sex chromosome inheritance is genetics.
    C is wrong because starch digestion is enzyme biology.
    D is wrong because oxygen uptake is respiration investigation.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

39. Answer: C
    Estimated population = first marked × second caught ÷ marked recaptured.
    = 80 × 100 ÷ 20 = 400.
    C is correct.
    A, B and D are calculation traps.

40. Answer: A
    A is correct because marked animals must mix randomly back into the population for the estimate to be valid.
    B is wrong because marked animals must be able to be recaptured.
    C is wrong because deaths between samples make the estimate unreliable.
    D is wrong because marks should not affect survival or predation risk.

41. Answer: A
    A is correct because if marked animals are less likely to be recaptured, the number of marked animals in the second sample is too low, causing an overestimate.
    B is wrong because expected recapture gives a valid estimate.
    C is wrong because no births, deaths, immigration or emigration improves validity.
    D is wrong because marks not affecting survival improves validity.

42. Answer: A
    A is correct because nitrogen fixation converts atmospheric nitrogen gas into compounds plants can use.
    B is wrong because denitrification converts nitrates into nitrogen gas.
    C is wrong because transpiration is water loss from plants.
    D is wrong because photosynthesis fixes carbon dioxide, not nitrogen gas.

43. Answer: B
    A is wrong because nitrifying bacteria convert ammonium ions into nitrates.
    B is correct because nitrogen-fixing bacteria in root nodules convert nitrogen gas into useful nitrogen compounds.
    C is wrong because denitrifying bacteria convert nitrates to nitrogen gas.
    D is wrong because yeast carries out fermentation, not root nodule nitrogen fixation.

44. Answer: A
    A is correct because nitrification converts ammonium ions into nitrates.
    B is wrong because denitrification converts nitrates into nitrogen gas.
    C is wrong because carbon fixation is photosynthesis.
    D is wrong because evaporation is water changing to vapour.

45. Answer: C
    A is wrong because nitrification produces nitrates.
    B is wrong because nitrogen fixation converts nitrogen gas into useful compounds.
    C is correct because denitrification converts nitrates into nitrogen gas.
    D is wrong because feeding transfers organic nitrogen through food chains.

46. Answer: A
    A is correct because decomposition releases ammonium compounds from dead organisms and waste.
    B is wrong because photosynthesis produces glucose from carbon dioxide and water.
    C is wrong because transpiration is water loss.
    D is wrong because pollination is pollen transfer.

47. Answer: B
    A is wrong because plants do not absorb nitrogen gas directly through stomata to make proteins.
    B is correct because plants absorb nitrate ions and use them to make amino acids and proteins.
    C is wrong because animals do not absorb nitrates from soil through skin.
    D is wrong because decomposers break down organic matter; they do not convert nitrates directly into glucose.

48. Answer: A
    A is correct because plants absorb nitrates, make plant proteins, and animals obtain nitrogen by eating plant proteins.
    B is wrong because animals do not make starch from nitrogen gas.
    C is wrong because oxygen is not the step between ammonium ions and animal glycogen.
    D is wrong because bile and carbon dioxide are not the correct nitrogen pathway.

49. Answer: A
    A is correct because combustion of fossil fuels releases carbon dioxide into the atmosphere.
    B is wrong because nitrification is part of the nitrogen cycle.
    C is wrong because osmosis is water movement.
    D is wrong because selective breeding does not release carbon from fossil fuels.

50. Answer: B
    A is wrong because energy is not recycled; it flows through ecosystems and is lost as heat.
    B is correct because nutrients are recycled, while energy flows through food chains and is eventually lost as heat.
    C is wrong because energy and nutrients are not permanently stored in consumers.
    D is wrong because decomposers help recycle nutrients, not stop cycling.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.