Exam practice

Decomposers break down dead bodies and wastes into simple minerals that return to the soil and are used again by plants. Without them, dead plants and animals would pile up and the environment would become dirty; the minerals would stay locked in the dead bodies, the soil would become poor, plants couldn’t grow, and the animals that depend on the plants would suffer. So the cycle of materials, and the whole ecosystem, would be disturbed.

Too many insects eat too much grass, so the grass (the producers) is destroyed faster than it can grow. Then other grass-eaters, such as rabbits and deer, go hungry, and in time the insects themselves run short of food and die off. The soil, without grass, may wash away. The balance between producers and consumers is upset.

At each step of a food chain only about a tenth of the energy is passed on; the rest is used by the living things to live and is lost as heat. So a great many producers are needed to supply enough energy for a smaller number of primary consumers, and those for still fewer secondary consumers. That is why producers are the most numerous, and top consumers the fewest.

In mutualism, each partner gives the other something it needs. For example, in a lichen the alga makes food for the fungus and the fungus gives the alga water and shelter; a sunbird gets nectar and the flower gets pollinated. Since each gets something useful from the other, both benefit.

In a balanced ecosystem, producers, consumers and decomposers are in the right numbers. Plants make enough food and oxygen and take up carbon dioxide; consumers keep each other’s numbers in check; decomposers clean away dead bodies and wastes and return minerals to the soil. So the air stays clean, the soil fertile and the water pure, and the environment stays fresh.

1. Air: gives oxygen for respiration and carbon dioxide for photosynthesis; nitrogen (as nitrates) for proteins and chlorophyll; water vapor for the water cycle.
2. Solar energy: light for photosynthesis; heat that sets the temperature and climate and drives the water cycle.
3. Water: needed for photosynthesis and for all the chemical reactions of life; most of every living body is water.
4. Soil: gives plants water and minerals (nitrogen, phosphorus, potassium) and is home to many living things.
5. Temperature and humidity: decide which plants and animals can live in a place.

Decomposers (bacteria and fungi) break down the dead bodies and wastes of plants and animals into simple substances. These minerals return to the soil (or the mud of a pond) and are taken up again by plants, so materials are recycled. They also clean the environment of dead matter, so they are called nature’s scavengers. Without them, nutrients would run out and dead bodies would pile up, and the ecosystem would break down.

A grassland ecosystem is the ecosystem of open land covered with grasses. Its abiotic parts are sunlight, air, water, soil and minerals. Its biotic parts are: producers, the grasses and herbs; primary consumers, such as grasshoppers, rabbits and deer; secondary consumers, such as frogs, insect-eating birds and foxes; tertiary consumers, such as snakes, hawks, leopards and tigers; and decomposers, bacteria and fungi in the soil. Draw the grassland part of Figure 6.5, with a food chain such as grass → grasshopper → frog → snake → hawk.

At first, the frogs eat more insects and other small animals, so their numbers fall. The plants and algae those insects ate may then grow more. With more frogs to eat, the animals that eat frogs (snakes, herons) increase. Soon the frogs run short of food, and with more predators hunting them, their numbers fall again. Then the insects recover. In the end, the numbers swing back towards a balance, but for a while the whole pond is disturbed.

The mice were part of a food web, not only rice-eaters. Snakes, owls and other predators that used to eat mice now had to eat other animals instead, such as frogs and insect-eating birds. With fewer frogs and birds, the insect pests that they used to eat (grasshoppers, stem borers) multiplied and damaged far more rice than the mice ever did. (If he used poison, it may also have killed the useful predators that ate poisoned mice.) For example: rice → grasshopper → frog → snake, and rice → mouse → snake. Remove the mice, the snakes eat more frogs, the grasshoppers multiply, and the rice suffers. Killing one link upsets the balance of the whole ecosystem. Try it in the food web widget.

In a pond, energy passes like this: phytoplankton (algae) → zooplankton (Daphnia) → small fish → large fish → heron, or algae → tadpole → water beetle → frog → snake. Draw boxes in a line with arrows pointing to each eater, and label the levels: producer, primary consumer, secondary consumer, tertiary consumer (and top consumer). The phytoplankton make food from sunlight; each animal gets its energy by eating the one before it; and when they die, bacteria and fungi in the mud decompose them, returning minerals to the water for the algae.

• Producers make the food and oxygen for all the others. If they decrease (for example, a forest is cut down), herbivores starve, then carnivores, and less carbon dioxide is taken up from the air.
• Consumers keep each other’s numbers in check. If the predators are killed (tigers, snakes), herbivores increase and overgraze the plants; if herbivores increase (insects), crops and grass are destroyed.
• Decomposers return minerals to the soil. Without them, dead matter piles up and plants run short of nutrients.
• The environment (soil, water, air) supplies the raw materials; if it is polluted, all the living things suffer.

So all four parts depend on each other, and a change in one passes on to all the others.

1. Mutualism: both benefit. Lichen (alga and fungus); sunbird and flower; bacteria in root nodules.
2. Commensalism: one benefits, the other is neither helped nor harmed. Cattle egret and buffalo; orchid on a tree; barnacles on a whale.
3. Parasitism: the parasite benefits, the host is harmed. Mosquito, flea, louse (outside); tapeworm, roundworm (inside); akashbeli on a hedge.
4. Competition: both lose, struggling for the same limited thing. Weeds and rice; snow leopard and wolf for blue sheep.
5. Predation: the predator kills and eats the prey. Owl and mouse; leopard and deer.

• i.Mutualism, between each bee and the flower: the bee gets nectar, and the flower gets its pollen carried to another flower. (The honeybee and the bumblebee, wanting the same nectar, also compete with each other.)
• ii.Commensalism: the spider gets a place for its web; the tree is not affected.
• iii.Commensalism: the swallow gets a safe place to nest; the house is not affected.
• iv.Parasitism (ectoparasites): they benefit and the animals are harmed.
• v.Parasitism (endoparasites): they take food from the host, which is harmed.

• i.Producer: grass. Consumers: grasshopper (primary), frog (secondary), snake (tertiary) and eagle (top consumer).
• ii.By photosynthesis: in their green leaves, using the energy of sunlight, they turn carbon dioxide from the air and water from the soil into food (glucose), giving out oxygen.
• iii.Only about a tenth of the energy passes on at each step; the rest is used up in living and lost as heat. So a large amount of grass is needed to feed fewer grasshoppers, which feed fewer frogs, and so on. The producers have to be the most numerous.
• iv.When the grass, grasshoppers, frogs, snakes and eagles die, bacteria and fungi in the soil decompose their bodies and wastes into minerals, which the grass takes up again. They recycle nutrients and keep the grassland clean.
• v.A balanced ecosystem keeps the numbers of every living thing steady; keeps the air, water and soil clean and fertile; gives us food, fuel, medicines and clean water; prevents pests from multiplying; and stays healthy for the future.