Food chains and food webs — Combined Science 0653 - Biology Revision Notes & Topical Questions

Section 1 of 16Terms in feeding relationships

Term	Definition
Producer	Organism that makes its own organic nutrients, usually using sunlight by photosynthesis
Consumer	Organism that gets its energy by feeding on other organisms
Herbivore	Animal that gets its energy by eating plants
Carnivore	Animal that gets its energy by eating other animals
Decomposer	Organism that gets its energy from dead or waste organic material

Section 2 of 16Food chains

Food chain

Food chain the transfer of energy from one organism to the next, beginning with a producer.

Arrows

• Arrows they point from the organism eaten to the organism that eats it, showing the direction of energy flow.

Source

• Source the original energy is light energy from the Sun.

Consumer order

• Consumer order primary, secondary and tertiary consumers, according to their position in the chain.

Section 3 of 16Example food chain

Position	Organism	Reason
Producer	Grass	Makes its own food by photosynthesis
Primary consumer	Rabbit or mouse	Eats the producer
Secondary consumer	Fox or barn owl	Eats the primary consumer
Tertiary consumer	Eagle	Eats the secondary consumer

• Energy
  • some energy is lost as heat at each step.

Section 4 of 16Food webs

Food web

Food web a network of interconnected food chains.

Realistic

• Realistic animals rarely feed on only one food source.

Interdependence

• Interdependence a change in one population affects others in the web.

Section 5 of 16Human impact on food webs

Overharvesting

• Overharvesting removing too many individuals of a food species.

Introduced species

• Introduced species foreign species brought into a habitat.

Knock-on effects

• Knock-on effects these can last long and affect organisms throughout the chain or web.

Section 6 of 16Why chains are short

Energy loss

• Energy loss energy is lost at every step, so food chains have few levels.

Number of organisms

• Number of organisms there are fewer organisms at higher levels.

Section 7 of 16More from the specification

Carbon cycle perturbation from fossil fuels

• Carbon cycle perturbation from fossil fuels extracting and combusting subterranean fossil fuels rapidly releases ancient locked carbon into atmospheric trophic food webs.

Trophic Feeding Categories

• Organisms in an ecosystem are categorised into primary producers (autotrophs), primary consumers (herbivores), secondary consumers (carnivores), tertiary consumers,
  • and saprotrophic decomposers.

Ecological Pyramids of Numbers versus Biomass

• Pyramids of numbers count individual organisms and can become inverted,
• whereas pyramids of biomass quantify total dry mass at each trophic level and consistently present an upright pyramid profile.

Interdependence within Food Webs

• Disrupting or removing a key population within an intricate food web triggers cascading multi-trophic effects that alter competitor and predator abundance throughout the ecosystem.

Microorganism

• A living thing too small to see without a microscope.

Bacteria

• Single cells with no nucleus.

Carbon dioxide

• Gas that turns limewater cloudy.

Carbon cycle

• The movement of carbon between the air, living things, oceans and rocks.

Fossil fuel

• Coal, oil or gas formed from dead organisms over millions of years.

Trophic level

• Position of an organism in a food chain

Biomass

• Mass of living material, often dry mass

Growth

• Permanent increase in size.

Environment

• The surroundings and conditions an organism lives in.

Ecosystem

• Network of interactions between living organisms and the non-living things around them.

Section 8 of 16Predicting the effects of change

1Predator falls

• Predator falls its prey increase and may then eat too much of their food, which declines.

2Producers die

• Producers die all the consumers are affected.

3Feeding on several organisms

• Feeding on several organisms removing such an organism has a smaller effect than removing the only food of another.

4Wording

• Wording use increase or decrease rather than die out, and give a reason.

Section 9 of 16Effect of removing a food source

Eaten by few animals

• Eaten by few animals the plants it eats increase, and animals relying only on it decrease significantly.

Alternative food

• Alternative food an animal with another food source decreases only slightly.

Section 10 of 16Energy and numbers at higher levels

• Energy loss
  • energy is lost at each level, so there are fewer organisms at higher levels.

Section 11 of 16Pyramids of numbers

Pyramid of numbers

Pyramid of numbers bars whose width shows how many organisms are at each trophic level.

Not always pyramid-shaped

• Not always pyramid-shaped one large organism, such as an oak tree, can support many smaller organisms.

Order

• Order producers at the bottom, then primary, secondary and tertiary consumers.

Labels

• Labels each bar is labelled with its trophic level and drawn to scale on graph paper.

Section 12 of 16Pyramids of biomass

Pyramid of biomass

Pyramid of biomass living matter, often dry mass, at each trophic level.

Shape

• Shape it is always pyramid-shaped because mass decreases up a food chain.

Advantage

• Advantage it takes account of organism size and approximates the energy available at each level.

Section 13 of 16Populations in the web

Populations in the web

Population

Population organisms of one species in the same area at the same time.

Community

Community all populations in an ecosystem.

Ecosystem

Ecosystem the community plus its environment.

Competition

• Competition organisms compete for food, water and living space.

Section 14 of 16More from the specification

Energy transfer through food chains

• Energy enters ecosystems via photosynthetic producers capturing sunlight;
  • only about 10% of energy is transferred between successive trophic levels, with the remainder lost through respiration, excretion,
• and heat.

The carbon cycle and carbon dioxide

• Carbon cycles between living organisms and the abiotic atmosphere through photosynthesis, feeding, respiration, decomposition,
• and combustion of fossil fuels.

The nitrogen cycle in ecosystems

• Nitrogen gas in the atmosphere is converted into nitrates by nitrogen-fixing bacteria,
• which plants absorb to synthesize amino acids and proteins necessary for growth.

Human impact on ecological cycles

• Human activities,
• such as burning fossil fuels and extensive deforestation, elevate atmospheric carbon dioxide levels, disrupting the natural carbon cycle and driving global climate change.

Bacteria

• Single cells with no nucleus.

Carbon cycle

• The movement of carbon between the air, living things, oceans and rocks.

Fossil fuel

• Coal, oil or gas formed from dead organisms over millions of years.

Growth

• Permanent increase in size.

Photosynthesis

• Process in which plants use light energy to make glucose and oxygen from carbon dioxide and water.

Microorganism

• A living thing too small to see without a microscope.

Section 15 of 16Key terms

Term	Meaning
Producer	Organism that makes its own food
Consumer	Organism that feeds on others
Decomposer	Organism feeding on dead or waste material
Food web	Network of interconnected food chains
Herbivore	Animal that eats plants
Carnivore	Animal that eats animals

Term	Meaning
Pyramid of numbers	Bars showing numbers at each trophic level
Pyramid of biomass	Bars showing living mass at each trophic level
Biomass	Mass of living material, often dry mass
Trophic level	Feeding position in a food chain
Interdependence	Organisms depending on each other
Predator	Animal that kills and eats other animals

Section 16 of 16Exam technique

1

Pointing arrows from the eater to the eaten — arrows point from the eaten to the eater.

2

Starting a food chain with a consumer — it starts with a producer.

3

Saying an organism will die out — say its population will decrease.

4

Leaving out the Sun as the original energy source.

1

Saying a species will die out when its food falls — its population decreases.

2

Assuming pyramids of numbers are always pyramid-shaped.

3

Counting the same organism at two trophic levels without reason.

4

Forgetting that predators of the changed organism also change.