Unit 13
Ecology.
Food chains and their energy, predators and prey, the carbon and nitrogen cycles, biomes, and the microbes in our gut.
Before this unit
- Energy in living things
- Exponents and logarithms Mathematics
The unit, lesson by lesson
Lessons
8 lessons · 46 practice questions · about 3 hours
Food chains
Producers, consumers and trophic levels in a lake’s food chain, the arrows that show where the energy flows, and why energy flows one way and leaves as heat while matter goes round in a cycle.
The ten per cent rule
How much energy reaches each level, when every link multiplies by the same share again. Why a small change in efficiency changes the top a great deal, why food chains are short, and which pyramid always stands upright.
Predator and prey
Foxes and rabbits on one meadow rise and fall in a cycle: four quarters of a loop round a balance point, the foxes lagging behind the rabbits, and the surprise in the averages: good years for rabbits raise foxes, and better hunters stay few.
The carbon cycle
Carbon passes from the air to the grass, the rabbit, the dead leaves and the soil, and back to the air. What plants build themselves from, how much they breathe back, how the decomposers close the cycle, and what happens when they are too slow or too fast.
The nitrogen cycle
The air is full of nitrogen that plants cannot use. Bacteria in clover’s nodules fix it, the decomposers free it from dead leaves, other bacteria turn it into nitrate, and the rain washes some away: a cycle that leaks, and what happens when rot is too slow or too fast.
Biomes and climate
Two numbers, temperature and rain, predict what landscape will grow: rain against the air’s thirst, the growing season trees need, and a mountain climb that passes through every biome in a few thousand metres.
Islands of forest
Orangutans in a forest cut into patches: less forest feeds fewer apes, small isolated groups die out even while their forest stands, the risk of extinction is a chance rather than a sentence, and wildlife corridors join the patches into one population.
Give and take
An ecosystem inside the large intestine: microbes that ferment fibre we cannot digest and give back fatty acids, a fence of mucus that wears thin when they go hungry, antibiotics that make room for opportunists, and the kinds of symbiosis, from mutualism to parasitism.