Ecosystems, Year 9: Living together

Competition between organisms

HintTwo robins, one garden, and not enough worms for both.

The whyA habitat holds only so much food, water, light and space. When two living things need the same thing and there is not enough for both, each gets less, and the one that is better at getting it is more likely to survive and breed.

Light

HintThink what a thick roof of leaves blocks out by midsummer.

The whyPlants compete for light, water, mineral ions and space. Once the canopy closes, very little sunlight reaches the woodland floor, so bluebells do most of their photosynthesis early in the year and store the food in their bulbs.

Mates

HintIt happens only in the breeding season, which is called the rut.

The whyAnimals compete for food, water, territory and mates. During the rut the strongest stags win the chance to breed with a group of hinds, so the features that made them successful are passed on to more calves.

The blossom would not be pollinated

HintAn apple only starts to grow after something has been carried from flower to flower.

The whyApple trees rely on insects to carry pollen between flowers, and the bees rely on the flowers for nectar and pollen. Each depends on the other, which is why losing pollinators threatens many of the food crops people eat.

interdependence

HintStart with a word for relying on something and add a prefix meaning "between".

The whyNo species lives alone. Plants need pollinators and seed-spreaders, animals need plants or other animals for food, and almost everything needs decomposers to recycle nutrients. Because of these links, a change to one population spreads through the food web to others.

rises

HintFewer hunters means fewer of the hunted are caught before they can have young.

The whyThis is one half of the predator–prey cycle. With fewer foxes, more rabbits live long enough to reproduce and their numbers climb. The extra rabbits then feed more foxes, the fox population recovers, and the cycle turns again.

Predators need time to breed after their food supply rises

HintA well-fed vixen must still raise a litter before the fox count goes up.

The whyMore prey does not instantly make more predators. The extra food lets more predators survive and raise more young, and that takes a breeding season or longer. So the predator line follows the prey line, always a step behind.

It takes many prey to feed each predator

HintThink about what a single fox gets through in a year.

The whyOnly a small part of the energy in the rabbits ends up as new fox, so a habitat can support far fewer predators than prey. That is why the prey line sits higher on the graph even when both populations are at a low point.

As prey become scarce, predators starve and their numbers drop first

HintAsk what happens to the hunters once meals get hard to find.

The whyWhen prey are hard to find, fewer predators survive and fewer young are raised. With less hunting, the remaining prey breed and recover. The two populations hold each other in check, which is why the graph shows repeating waves rather than one line crashing to zero.

respiration; combustion

HintOne happens inside every living cell; the other needs a flame.

The whyPhotosynthesis takes carbon dioxide out of the air and builds it into sugars. Respiration by plants, animals and microbes breaks those sugars down again and releases the carbon dioxide, and burning wood or fossil fuels does the same thing much faster.

The animal eats the plant

HintCarbon is built into the sugars, fats and proteins of every leaf and seed.

The whyPlants take carbon from the air and build it into carbohydrates, fats and proteins. When an animal feeds, those carbon compounds become part of its own body, and they pass on again when it is eaten in turn. Carbon moves along food chains as food.

Up — it still respires but cannot photosynthesise in the dark

HintOne of the plant's two gas-swapping processes needs light; the other never stops.

The whyPlants respire all the time, just as animals do, releasing carbon dioxide. In daylight photosynthesis takes in far more carbon dioxide than respiration gives out, so the plant removes it overall. In the dark only respiration is left.

fossil

HintThe same word describes the preserved bones of dinosaurs.

The whyThe carbon in these fuels was taken from the air by photosynthesis long ago and then locked away underground. Burning them releases that stored carbon as carbon dioxide far faster than plants can take it back, which is why the level in the atmosphere is rising.