Chapter 12: Interactions within Ecosystems (Summary Sheet)

Part A · The ecosystem and the organisms in it

1 Ecosystem, Community, Habitat, Population

Term	What it names
Population	All the organisms of one species in a particular habitat
Community	All the populations of all the species in one habitat, interacting — living things only
Habitat	The physical surroundings an organism lives in — the non-living conditions only
Ecosystem	A community of organisms interacting with one another and with the physical environment they occupy — both halves required

Three checks settle the question: how many species? (one → population) · are non-living things included? (organisms only → community; water, light, soil or air too → ecosystem) · conditions on their own? (→ habitat).
Interactions make an area a working system — feeding, competing, sheltering, breaking down dead bodies.

2 The Physical Factors, and How We Measure Them

Name these: light intensity, temperature, water, air (oxygen), mineral salts, acidity or alkalinity (pH), and salinity in water.
They are measurable and vary from place to place: a data logger with probes records pH, temperature and light intensity over time.
The physical side decides which plants can grow there — and so which animals can be supported.

3 Interdependence — Four Ways Organisms Depend on One Another

Way	How it works
Food	Energy and nutrients pass from one organism to another when one feeds on another
Shelter and support	Organisms live and grow on other organisms — a seedling surviving only in the shade of a larger plant
Transport and dispersal	Animals carry fruit and seeds away and drop them, so a plant appears where it could not have reached alone
Return of nutrients	Decomposers break down dead organisms and waste, releasing nutrients plants take up again

Disturb one part and the effect travels through the community — nothing stands alone.

4 Feeding Groups, and What Each Consumer Eats

Group	What it does
Producer	Makes its own food, capturing light energy in photosynthesis and storing it as chemical potential energy
Consumer	Cannot make its own food; takes in the bodies of other organisms for energy
Decomposer	Feeds on dead organisms and waste, breaking them down and releasing the nutrients inside — bacteria and fungi

By diet: herbivore — plants only (water snail on algae) · carnivore — animals only (dragonfly nymph on tadpoles) · omnivore — both (tilapia).

5 Relationships Between Organisms in a Community

Relationship	What happens
Predator – prey	The predator hunts and kills; the prey is eaten. Predators have sharp senses, claws or speed; prey escape by camouflage, a hard shell or living in groups.
Competition	Two or more organisms need the same resource — food, water, shelter, light or mineral salts — and there is not enough to go round. It limits population size even with no predator present.
Mutualism	Both benefit. A bee feeds on a flower’s nectar; as it moves from flower to flower it carries pollen, so the flower can make seeds.
Parasitism	The parasite lives on or in a host and gains from it while the host is harmed — a tapeworm in the human gut takes the host's digested food.

Part B · Energy and nutrients in a community

6 Food Chains: Energy in a Straight Line

A food chain traces how food and the energy in it pass from one organism to the next. Our pond chain: hydrilla → mayfly nymph → dragonfly nymph → pond heron — each arrow starts at the organism eaten and points to the one that eats it.

Trophic level	What occupies it	In our pond chain
First	Producer	Hydrilla
Second	Primary consumer (herbivore)	Mayfly nymph
Third	Secondary consumer (carnivore)	Dragonfly nymph
Fourth	Tertiary consumer (top carnivore)	Pond heron

Only about 10% of the energy in one level becomes body material in the level above; the rest leaves as heat from respiration, in movement and in waste. That is why chains are short.
Exam tip: start with a producer, point every arrow from the eaten to the eater, label the levels, and leave decomposers out — they feed on every level at once.

7 Food Webs: Reading the Whole Community

A food web is the whole network of food chains in one habitat drawn together. Few animals eat only one kind of food, so one chain tells only part of the story.
Read it in order: find the producers (nothing points at them) → follow the arrows for consumers with a varied diet → note that one organism can hold more than one trophic level (the tilapia) → count the levels on the longest route.

The question asks	How to answer it
Which is a producer?	Name the organism with no arrow pointing at it
How many trophic levels?	Count the organisms on the longest route to a top carnivore
What if a population changes?	Name it, then work outward one arrow at a time: fewer predators → fewer deaths; more food → more births; less food → more deaths or movement away. Give the second and third links.

8 Energy Flow and Ecological Pyramids

Energy enters almost every ecosystem as light from the Sun, is captured by producers in photosynthesis, then passes along the chain as things are eaten.
An ecological pyramid shows, level by level, the energy, the number of organisms or the amount of living material at each trophic level.

Trophic level	Organism (1 m² of pond, 1 year)	Energy stored
1st — producer	Algae and pondweed	50 000 J
2nd — primary consumer	Tadpoles and water snails	5 000 J
3rd — secondary consumer	Dragonfly nymphs	500 J
4th — tertiary consumer	Small fish	50 J

The pyramid of energy is never inverted — energy can only decrease upward.
A pyramid of numbers can mislead: one rain tree may feed some 15 000 aphids, about 900 ladybirds and a few mynas.

9 Nutrients Are Recycled, Energy Is Not

Feature	Energy	Nutrients (mineral salts)
Direction	One way — in as light, along the chain once	A loop — back to the soil or water, taken up again
At the end	Released as heat, so light must keep arriving	Nitrates and phosphates re-enter the soil, ready for plant roots
Used again?	No	Yes, repeatedly

Because decomposers (bacteria and fungi) feed on dead organisms and waste — breaking them down outside their own bodies and taking in what they need — the inorganic nutrients, including the carbon and nitrogen compounds locked up in dead matter, go back into the soil, water or air and can be used by living things again.
Take decomposers out of an ecosystem and those nutrients stay locked in the dead matter: the soil loses its fertility and producers run short of the raw materials they need.

Part C · Survival, change and people

10 Adaptations: Structural and Behavioural

An adaptation is an inherited feature that raises an organism’s chance of surviving and reproducing in its habitat — the outcome of natural selection, not a choice the organism makes.
Two types only: structural — a body part — and behavioural — an action.

Type	Our example	How it helps survival
Structural	Overlapping plates of hard keratin on the pangolin	Too tough for a predator’s teeth to pierce, so it survives an attack and breeds
Behavioural	The common Asian toad digging backwards into damp soil in a dry spell	Stops water being lost through its thin skin, so it lives through the dry months

Answer shape: trait → the challenge it answers → the benefit to survival and reproduction.

11 When Conditions Change, Adaptations Can Fail

An adaptation works only against the conditions that produced it. Change temperature, rainfall, food supply or the mix of species — and the same feature can become a handicap.

Condition that changed	The adaptation, and why it now fails	Result for the population
Warmer winters, so there is little snow on the ground	A snow hare grows a white winter coat that hides it against snow. With little snow, the white coat stands out against brown ground.	Predators spot it far more easily, so more hares are caught → the population shrinks.
A long dry spell, or land drained for building	The Asian toad survives dry months by digging into damp soil, which stops water escaping through its thin skin. There is no damp soil left.	The toad loses water faster than it replaces it, so more of them die before the wet season comes back.

A population with little genetic variation may hold no individual suited to the new conditions, so it can fall sharply or disappear.
More variation → a better chance that some individuals match the new conditions → they survive and reproduce and pass the traits on: natural selection happening in front of us. That is why biodiversity is worth protecting.
Answer shape: name the factor that changed → name the adaptation that no longer matches → the consequence → how the population changes over the following generations.

12 How Human Activity Changes Ecosystems

Impact	What happens, and the consequence
Nutrient pollution	Fertiliser and sewage → algae bloom → decomposers use up the oxygen → fish suffocate.
Habitat loss	Clearing, draining or building removes shelter, nesting sites and food; what is left is too small or too far apart to recover.
Over-harvesting	Taken faster than they reproduce, so numbers fall below the level needed to sustain the population.
Introduced species	A foreign organism spreads because nothing limits it, then out-competes, eats or infects natives.
Pollutants	Heavy metals and pesticides poison tissues; oil coats feathers and gills; plastic is eaten as food.

13 Technology and the Modern Lifestyle

Technology / lifestyle	Impact on the environment
Motor vehicles	Petrol and diesel engines release carbon dioxide and nitrogen oxides → global warming and acid rain; roads fragment habitats.
Electricity and industry	Burning coal, oil and gas for power releases the same gases — every extra appliance adds to it.
Farming technology	Machinery, fertilisers and pesticides raise yields but bring nutrient pollution, kill non-target insects and compact soil.
Fishing technology	Sonar, trawlers and long lines catch more than the population can replace → over-harvesting.
Modern lifestyle	Single-use plastic, food waste, fast fashion and e-waste make more waste and use more energy per person; plastic is eaten as food.

Evaluate means weigh both sides: the benefit (food, transport, medicine, comfort) against the harm (pollution, habitat loss, climate change), then say how the harm can be cut.

14 Climate Change, Biodiversity and Sustainable Living

Warming shifts temperature and rainfall, so conditions change faster than organisms can adapt: species that cannot move or adjust decline, and the community an area can support changes.
Biodiversity — the variety of living things — gives a community more traits that might suit the new conditions, which is why conserving the environment matters.
Conservation works at every level: nature reserves and marine parks set land aside by law, wildlife corridors let organisms move between patches, and replanting native species rebuilds lost communities.
Sustainable living means taking what we need without using it up or damaging it for future generations: reuse and recycle, use energy efficiently, buy local food, compost waste so nutrients return to the soil, and keep catches within quotas and closed seasons.

15 Cultures That Practise Sustainable Living

Many cultures have long lived sustainably through how they use the land, water and living things around them.

Practice	How it keeps the resource
Community-managed irrigation (Ifugao rice terraces, Bali’s subak)	Water is shared by agreed rules and channelled to every field.
Returning crop waste and manure to the soil	Nutrients go back into the ground instead of being bought, so the soil keeps producing.
Rotational farming with a fallow period	The land rests and regrows its nutrients before it is planted again.
No-take areas and closed seasons (Pacific island tabu grounds)	Fish are left to breed in a protected patch, so the catch stays possible.
Controlled burning (Aboriginal fire-stick farming)	Small cool fires clear dead growth and bring back fresh pasture, instead of one huge fire.

Answer shape: name the practice → what it does to the environment → how it keeps the resource for the next generation.

16 Say It Like This

The question asks	The phrase that earns the mark
Define an ecosystem	“A community of organisms interacting with one another and with the physical environment they occupy.”
Population or community?	“All the river snails of one species” = population; “all the populations of all the species” = community.
Why so few trophic levels?	“Only about 10% of the energy passes to the next level; the rest is lost as heat, in movement and in waste, so too little is left for another level.”
Is energy recycled?	“No — energy flows one way and is lost as heat, so light must keep arriving; nutrients are the ones recycled.”
How do nutrients return?	“Decomposers break down dead organisms and waste, releasing mineral salts that plant roots take up again.”
How does an adaptation help?	“The trait [name it] answers [name the challenge], so the organism is more likely to survive and reproduce.”
Why conserve biodiversity?	“More species and variation give a better chance that some individuals suit the new conditions, so the community goes on.”