Ecosystem, Food Chain, Pyramids and the Nitrogen Cycle

What to remember

• An ecosystem is a community of living things (biotic) interacting with their non-living surroundings (abiotic); energy flows in one direction, while materials are recycled.
• Only about 10% of the energy passes from one trophic level to the next (Lindeman's ten percent law), so food chains are short and energy pyramids are always upright.
• Nitrogen moves in a cycle through fixation, nitrification, assimilation, ammonification and denitrification, with bacteria doing the key steps.

Ecology and ecosystem

Ecology is the study of the relationship between organisms and their environment. The word ecosystem was given by A. G. Tansley. A habitat is the place where an organism lives. A population is all the individuals of one species in an area. A community is all the populations living together. An ecosystem is the community plus its non-living environment.

Components of an ecosystem

• Abiotic: sunlight, temperature, water, air, soil, minerals.
• Biotic:
• Producers (autotrophs): green plants, algae and some bacteria. They make food by photosynthesis.
• Consumers (heterotrophs): animals. Primary consumers eat plants (herbivores: goat, cow, grasshopper). Secondary consumers eat herbivores (carnivores: frog, snake). Tertiary consumers eat other carnivores (hawk, lion). Omnivores eat both plants and animals (human, crow).
• Decomposers: bacteria and fungi. They break down dead plants and animals, return minerals to the soil and keep the cycle going. They are called nature's cleaners.

Ecosystems may be natural (forest, grassland, desert, pond, river, sea) or artificial (crop field, aquarium, garden). A small pond is a good example for study because its parts are easy to see.

Food chain and food web

A food chain is a straight line of who eats whom. Each step is a trophic level.

• Grass → grasshopper → frog → snake → eagle.
• Phytoplankton → zooplankton → small fish → large fish.

Types of food chains:

• Grazing food chain: starts with green plants (producers). Most food chains in a land or water ecosystem are of this type.
• Detritus food chain: starts with dead organic matter eaten by decomposers and detritus feeders (dead leaves → earthworm → bird).
• Parasitic food chain: involves parasites (for example plant → insect → bird → parasite on the bird).

A food web is a network of many food chains linked together. It is more stable than a single chain. If one species is lost, the others may use another food source. A single food chain breaks if one link is removed.

A food chain rarely has more than four or five links, because energy is lost at each step.

Energy flow

The sun is the ultimate source of energy for all ecosystems. Producers capture only a small part of the sunlight. Energy flows in one direction: sun → producers → consumers → decomposers. It is not recycled. At each trophic level most energy is lost as heat in respiration and in other activities. According to the ten percent law (Lindeman, 1942), only about 10% of the energy of one level is available to the next level.

Example: if grass has 10,000 units of energy, grasshoppers receive about 1,000, frogs about 100, snakes about 10 and eagles about 1. So top carnivores are few in number. Remember: energy flows (one way), matter cycles.

Ecological pyramids

A pyramid shows the number, biomass or energy at each trophic level. Producers are at the base and top carnivores at the top.

Biomagnification: harmful non-biodegradable chemicals such as DDT and mercury get more concentrated at each higher trophic level, so top carnivores are affected the most. This is why birds of prey are harmed by pesticides.

Biogeochemical cycles

Matter such as water, carbon and nitrogen moves between living things and the environment in cycles.

Water cycle: evaporation, transpiration, condensation, precipitation, run-off and groundwater.

Carbon cycle: plants take in carbon dioxide in photosynthesis. Respiration, decomposition and burning of fuels return it to the air. The ocean and forests act as storage of carbon.

Nitrogen cycle: about 78% of air is nitrogen, but plants and animals cannot use gaseous nitrogen directly. It has to be changed to useful forms.

Farmers use legumes in crop rotation and green manure to put nitrogen into the soil. Nitrogen fertilisers (urea) are added, but excess can pollute water. Biofertilisers such as Rhizobium culture and Azolla (a fern with blue-green algae, used in paddy fields) are eco-friendly.

Ecosystem in action: simple examples

Pond: producers are algae and aquatic plants; consumers are insect larvae, small fish and frogs; decomposers are the bacteria in mud. Forest: very high biodiversity, and decomposers recycle fallen leaves. Grassland: grass, grasshoppers, rodents and snakes, hawks. Mangroves: coastal ecosystem of salt-tolerant trees that protect against storms and are nurseries for fish and prawns; the Coringa mangroves on the Godavari delta in Andhra Pradesh are well known.

An ecosystem can be damaged by removal of species, pollution and over-use. Conservation of food webs is vital; for example, a loss of snakes may cause a rise in rats.

Teaching angle: let students make a food chain with cards or arrows, a bottle (closed) ecosystem, and a food web string game. Ask them to give local examples (paddy field, pond, mango orchard).

Relationships between organisms

Predators control the number of prey, so they help to keep the balance of nature. If all predators were removed, the prey population would grow too much and eat up its food.

Why ecosystems matter

Ecosystems give us food, water, wood, medicines and clean air. Forests hold soil in place and help rain. Wetlands store flood water and clean it. Soil organisms keep the soil fertile. Because each link in a food web depends on the others, damage to one species can affect many others.

Population and balance

A population can grow only until food, water and space run short. The largest number that an environment can support is its carrying capacity. Birth and immigration raise the number; death and emigration lower it. When one species increases or falls suddenly, the balance of the community changes. For example, when farmers use poison to kill rats, the owls and snakes that eat them also suffer. This is why a balanced ecosystem is better protected by natural controls than by chemicals.

Quick pond example with numbers

Suppose algae in a pond hold 5,000 units of energy. By the ten percent law, small fish that eat them get about 500 units, and bigger fish that eat the small fish get about 50. A fish-eating bird would receive about 5. This shows why a bird at the top needs a large area to feed and why a food chain has few levels. Use this type of simple calculation when teaching the topic.

Exam traps

• Energy flows in one direction; nutrients cycle.
• Food chain (single line) is not food web (many links).
• Pyramid of energy is always upright; pyramids of number and biomass can be inverted.
• Rhizobium (symbiotic, in legume root nodules) is not Azotobacter (free-living).
• Nitrification (ammonia to nitrate) is not denitrification (nitrate to nitrogen gas).
• Decomposers are not consumers; they return minerals to the soil.
• Biomagnification is greatest at the top trophic level.
• Ten percent law is about energy, not about numbers.

One-liners

• 1. The word "ecosystem" was proposed by Tansley.
• 2. Green plants are producers.
• 3. Bacteria and fungi are decomposers.
• 4. The ten percent law was given by Lindeman.
• 5. Rhizobium lives in the root nodules of legumes.
• 6. Nitrosomonas converts ammonia into nitrite.
• 7. Nitrobacter converts nitrite into nitrate.
• 8. Pseudomonas is a denitrifying bacterium.
• 9. Azolla is a biofertiliser used in paddy fields.
• 10. DDT shows biomagnification.
• 11. Sunlight is the source of energy of an ecosystem.
• 12. A tree ecosystem has an inverted pyramid of numbers.

Practice questions

1. The term 'ecosystem' was proposed by

   1. Charles Elton
   2. R. Lindeman
   3. Ernst Haeckel
   4. A. G. Tansley
   Answer

   D. A. G. Tansley

   Tansley coined the term in the 1930s.

2. Which of the following is a producer?

   1. Earthworm
   2. Green algae
   3. Mushroom
   4. Frog
   Answer

   B. Green algae

   Algae make food by photosynthesis.

3. Which organisms are called decomposers?

   1. Herbivores
   2. Green plants
   3. Birds of prey
   4. Bacteria and fungi
   Answer

   D. Bacteria and fungi

   They break down dead matter and return minerals to the soil.

4. In the food chain grass → grasshopper → frog → snake, the secondary consumer is the

   1. grasshopper
   2. grass
   3. frog
   4. snake
   Answer

   C. frog

   The grasshopper is primary; the frog is secondary.

5. The ultimate source of energy in most ecosystems is the

   1. wind
   2. water
   3. soil
   4. sun
   Answer

   D. sun

   Sunlight is captured by producers.

6. Which pyramid is always upright?

   1. Pyramid of numbers
   2. Pyramid of energy
   3. Pyramid of biomass
   4. Pyramid of habitat
   Answer

   B. Pyramid of energy

   Energy is lost at every level.

7. Rhizobium bacteria are found in the

   1. roots of grasses
   2. root nodules of legumes
   3. stems of palms
   4. leaves of cereals
   Answer

   B. root nodules of legumes

   They fix nitrogen in symbiosis with legumes.

8. The conversion of nitrite into nitrate is carried out by

   1. Nitrosomonas
   2. Pseudomonas
   3. Rhizobium
   4. Nitrobacter
   Answer

   D. Nitrobacter

   Nitrosomonas makes nitrite from ammonia; Nitrobacter makes nitrate.

9. Denitrification is carried out by

   1. Nitrobacter
   2. Anabaena
   3. Pseudomonas
   4. Azotobacter
   Answer

   C. Pseudomonas

   It returns nitrogen gas to the air.

10. Which of the following is a free-living nitrogen-fixing bacterium?

    1. Nitrobacter
    2. Rhizobium
    3. Azotobacter
    4. Pseudomonas
    Answer

    C. Azotobacter

    Rhizobium is symbiotic; Azotobacter lives freely in the soil.

11. Which of the following is a blue-green alga that fixes nitrogen?

    1. Chlamydomonas
    2. Spirogyra
    3. Ulothrix
    4. Nostoc
    Answer

    D. Nostoc

    Nostoc and Anabaena are cyanobacteria.

12. Azolla is used in paddy fields as a

    1. biofertiliser
    2. growth hormone
    3. pesticide
    4. weedicide
    Answer

    A. biofertiliser

    It harbours a nitrogen-fixing blue-green alga.

13. Which of the following is an omnivore?

    1. Cow
    2. Crow
    3. Lion
    4. Goat
    Answer

    B. Crow

    Crows eat both plant and animal food.

14. Which of the following is a detritus food chain?

    1. Grass → deer → tiger
    2. Plant → caterpillar → bird
    3. Dead leaves → earthworm → bird
    4. Phytoplankton → fish → heron
    Answer

    C. Dead leaves → earthworm → bird

    It begins with dead organic matter.

15. Why is the number of top carnivores in a forest very small?

    1. They do not need food
    2. Energy available decreases at each trophic level
    3. They eat only plants
    4. Producers are few
    Answer

    B. Energy available decreases at each trophic level

    Only a small amount of energy reaches the top.

16. A pyramid of numbers is inverted in a

    1. grassland
    2. tree ecosystem
    3. paddy field
    4. pond with fish only
    Answer

    B. tree ecosystem

    One tree supports many insects.

17. A pyramid of biomass is inverted in a

    1. pond or sea
    2. grassland
    3. desert
    4. forest
    Answer

    A. pond or sea

    Small phytoplankton have less total mass than the animals eating them.

18. The harmful chemical DDT is found in very high amounts in the body of the top carnivore due to

    1. pollination
    2. nitrification
    3. photosynthesis
    4. biomagnification
    Answer

    D. biomagnification

    Non-biodegradable chemicals build up along the food chain.

19. If 20,000 units of energy are in producers, the energy available to the secondary consumer by the ten percent law is about

    1. 200 units
    2. 20 units
    3. 10,000 units
    4. 2,000 units
    Answer

    A. 200 units

    20,000 → 2,000 (primary) → 200 (secondary).

20. A student wants to show that the food web is more stable than a food chain. The best reason is that in a food web

    1. energy is not lost
    2. there are no decomposers
    3. an animal can use another food source if one is lost
    4. only plants are present
    Answer

    C. an animal can use another food source if one is lost

    Many links give alternative food.

21. Cuscuta (dodder) growing on a host plant is an example of

    1. mutualism
    2. commensalism
    3. predation
    4. parasitism
    Answer

    D. parasitism

    It takes food from the host and harms it.

22. A lichen shows the relationship of

    1. mutualism
    2. competition
    3. predation
    4. parasitism
    Answer

    A. mutualism

    Alga makes food and fungus gives protection and water.

23. An orchid growing on a mango tree gains support without harming the tree. This is

    1. parasitism
    2. predation
    3. mutualism
    4. commensalism
    Answer

    D. commensalism

    One partner benefits and the other is unaffected.

24. Which gas forms about 78% of the air and must be 'fixed' before use by plants?

    1. Argon
    2. Nitrogen
    3. Oxygen
    4. Carbon dioxide
    Answer

    B. Nitrogen

    Plants cannot use gaseous nitrogen directly.

25. Lightning helps the nitrogen cycle by

    1. killing decomposers
    2. breaking nitrates
    3. converting nitrogen into oxides that reach the soil
    4. adding oxygen to soil
    Answer

    C. converting nitrogen into oxides that reach the soil

    Oxides dissolve in rain and form nitrates.

26. Which step of the nitrogen cycle turns dead matter and animal waste into ammonia?

    1. Denitrification
    2. Assimilation
    3. Ammonification
    4. Nitrification
    Answer

    C. Ammonification

    Decomposers carry it out.

27. In which of the following is energy transfer between trophic levels shown correctly?

    1. Herbivore → producer → carnivore
    2. Producer → herbivore → carnivore
    3. Carnivore → herbivore → producer
    4. Decomposer → producer → herbivore
    Answer

    B. Producer → herbivore → carnivore

    Energy passes from lower to higher trophic levels.

28. Which of the following coastal ecosystems of Andhra Pradesh is famous for mangroves?

    1. Papikonda
    2. Rollapadu
    3. Kolleru
    4. Coringa
    Answer

    D. Coringa

    Coringa is on the Godavari delta.

29. Mutualism is shown by Rhizobium and a legume because

    1. both organisms benefit
    2. only the bacterium benefits
    3. only the plant benefits
    4. the plant is harmed
    Answer

    A. both organisms benefit

    The bacterium gets food and the plant gets nitrogen compounds.

30. Which of the following sets contains only biotic components?

    1. Humidity, fungi, rain
    2. Fungi, plants, bacteria
    3. Rock, air, algae
    4. Soil, water, sunlight
    Answer

    B. Fungi, plants, bacteria

    Others contain abiotic factors.

31. Statements: 1. Energy flows in one direction in an ecosystem. 2. Energy is recycled by decomposers.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Energy is lost as heat and is not recycled; nutrients cycle.

32. Statements: 1. Pyramid of energy can be inverted. 2. Pyramid of numbers is always upright.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    D. Neither 1 nor 2

    The energy pyramid is never inverted, and the number pyramid can be inverted (tree).

33. Statements: 1. Rhizobium is a symbiotic nitrogen fixer. 2. Azotobacter lives freely in soil.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

34. Statements: 1. Nitrifying bacteria change ammonia to nitrates. 2. Denitrifying bacteria add nitrates to soil.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Denitrification removes nitrates and returns N₂ to the air.

35. Statements: 1. Decomposers are producers of food. 2. Decomposers return minerals to the soil.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Decomposers break down dead matter; they do not make food.

36. Statements: 1. A food web is a network of food chains. 2. Removal of one species can affect many others.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

37. Statements: 1. Ten percent law was proposed by Lindeman. 2. It applies to the transfer of matter such as DDT.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    It concerns energy transfer.

38. Statements: 1. Biomagnification is highest at the producer level. 2. DDT is biodegradable.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    D. Neither 1 nor 2

    Biomagnification is highest at the top and DDT is non-biodegradable.

39. Statements: 1. Predators help to keep prey numbers in balance. 2. A tapeworm is a predator.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    A tapeworm is a parasite.

40. Statements: 1. The sun is the source of energy for the ecosystem. 2. Decomposers are the first trophic level.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Producers form the first trophic level.

41. Statements: 1. Legume crops can improve soil nitrogen. 2. Excess nitrogen fertiliser can pollute water.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both statements are correct.

42. Statements: 1. Nostoc fixes nitrogen. 2. Nitrobacter changes ammonia to nitrite.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Nitrobacter changes nitrite to nitrate; Nitrosomonas changes ammonia to nitrite.

43. Statements: 1. Grazing food chain starts with green plants. 2. Detritus food chain starts with dead matter.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

44. The sequence phytoplankton → zooplankton → small fish → large fish is a

    1. parasitic food chain
    2. food web
    3. detritus food chain on land
    4. grazing food chain of an aquatic ecosystem
    Answer

    D. grazing food chain of an aquatic ecosystem

    It begins with producers and is a single line.