☀️ Climate drives growth
Constant heat, plenty of rain and strong sunlight mean plants can grow all year. More plants means more food and more homes for animals.
The Tropical Rainforest Ecosystem » Rainforest Life: Nutrient Cycle and Adaptations
What you'll learn this session
Study time: 30 minutes
Cambridge spec: 3.3.4
- How nutrients cycle quickly between biomass, litter and soil in the rainforest
- How the climate, plants and animals depend on each other
- How rainforest plants adapt to survive
- How rainforest animals adapt to survive
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Unlock This CourseA forest that feeds itself
You already know the rainforest is hot and wet all year and that its soils are thin. So how can such a huge, green forest grow on poor soil? The answer is the nutrient cycle: nutrients are passed round and round very quickly, so they hardly ever sit in the soil.
Think of a shop where every pound is spent as soon as it arrives. The money moves fast, but there is never much in the till. The rainforest works in the same way.
Key terms:
- Nutrient cycle: the movement of nutrients between living things, dead material and the soil.
- Biomass: the total mass of living things, mainly the trees and other plants.
- Decomposition: the breakdown of dead material, by fungi and bacteria, which releases nutrients.
- Interdependence: living things relying on each other to survive.
The fast nutrient cycle
Nutrients are held in three stores: biomass, litter and soil. The cycle moves them round in a loop.
- Biomass store: the trees and other living things hold most of the nutrients.
- Litter store: leaves, fruit and branches fall to the forest floor all year.
- Soil store: litter is broken down and a small amount of nutrients enters the soil.
- Shallow roots take the nutrients straight back up into the biomass.
The cycle is fast because the heat and moisture are ideal for fungi and bacteria. Litter rots within weeks, whereas in a cooler climate it takes far longer. The nutrients are taken up by roots almost at once, so most of them are stored in the biomass, not in the soil.
| Store | Rainforest (made-up %) | Cooler forest (made-up %) |
|---|---|---|
| Biomass | 70 | 40 |
| Litter | 2 | 20 |
| Soil | 28 | 40 |
The numbers are made up, but the pattern is real: in the rainforest the biomass store is the biggest and the litter store is very small, because litter is used up so quickly.
Climate, plants and animals depend on each other
Remember, abiotic factors are the non-living parts, such as heat, rain and soil. Biotic factors are the living parts, such as plants and animals. In the rainforest they are closely linked: the abiotic factors drive the growth of plants, and the plants feed the animals.
🌿 Plants feed everything
Plants are the producers at the start of every food chain. Leaves feed insects and monkeys, and these feed snakes, birds and jaguars.
🐝 Pollinators
Bees, butterflies, bats and hummingbirds carry pollen between flowers, so the plants can make seeds. In return they get nectar.
🐦 Seed dispersers
Toucans, monkeys and fruit bats eat fruit and drop the seeds far from the parent tree. This gives young trees space to grow.
A good named example is the Brazil nut tree of the Amazon. It is pollinated by large bees, and its seeds are opened and spread by a rodent called the agouti. Take away the bees or the agouti and the tree struggles to reproduce.
Food chains link it all together. A simple one is: leaf, caterpillar, tree frog, snake, eagle. Dead animals and plants then return nutrients to the cycle. If one link is lost, the others suffer. That is interdependence.
How plants adapt
An adaptation is a feature that helps a plant or animal survive in its environment. Rainforest plants face heavy rain, shade and a constant fight for light.
| Adaptation | What it is | How it helps |
|---|---|---|
| Buttress roots | Huge, wide roots that spread out from the base of tall trees | They support a very tall tree in thin, shallow soil and take in nutrients from the surface |
| Drip tips | Leaves with a pointed end | Rain runs off quickly, so leaves do not stay wet and grow algae or fungus |
| Lianas | Woody climbing vines | They climb up tree trunks to reach the sunlight without having to build a thick trunk |
| Epiphytes | Plants such as orchids and bromeliads that grow on branches | They live high up in the light and take water from the rain and air, not the soil |
| Thin bark | Smooth, thin bark on trunks | There is no cold to keep out, and water runs off easily |
| Leaves that turn to the sun | Leaves that angle towards the light | They catch as much sunlight as possible, even in the shade |
How animals adapt
🐆 Camouflage
The jaguar's spotted coat blends into dappled light. The sloth grows green algae in its fur, so it looks like part of the tree.
🐒 Climbing and gliding
Spider monkeys use a gripping tail to climb. Flying lizards and flying squirrels glide between trees to avoid the dangerous forest floor.
🦉 Nocturnal habits
Bats and owl monkeys are active at night. This avoids the heat and daytime predators, and it spreads the competition for food.
🍏 Specialist diets
The toucan's long beak reaches fruit on thin branches. Hummingbirds have long beaks for nectar. Many species eat only one type of food, so they do not compete with others.
Common mistakes
- Saying nutrients are stored in the soil. Most are in the biomass.
- Saying decomposition is fast because it is wet. It is fast because it is hot and wet.
- Listing an adaptation without saying how it helps. Always add "so that".
- Mixing up lianas (climbers rooted in the ground) and epiphytes (plants that grow on other plants).
Exam-style question
Fig. 1.1 shows made-up data for nutrient stores in two forests.
| Store | Forest A (%) | Forest B (%) |
|---|---|---|
| Biomass | 68 | 35 |
| Litter | 2 | 25 |
| Soil | 30 | 40 |
Fig. 1.1
(a) Define the term biomass. [1 mark]
(b) Identify which forest is the tropical rainforest. Give one reason for your answer. [2 marks]
(c) Describe one adaptation of rainforest animals. [2 marks]
(d) Explain how the climate helps the nutrient cycle to work quickly in the rainforest. [4 marks]
(e) Explain how plants and animals in the rainforest depend on each other. [6 marks]
Model answer
(a) The total mass of living things, such as plants (1).
(b) Forest A (1), because most nutrients are stored in the biomass (68%) and very little in the litter (2%) (1).
(c) The sloth has green algae in its fur (1), so it is camouflaged against the trees (1). Accept another correct adaptation with its use.
(d) It is hot all year (1) and wet all year (1). This is ideal for fungi and bacteria (1), so litter decomposes within weeks and nutrients are released quickly for roots to take up (1).
(e) Level 3 (5-6 marks): a developed, linked explanation with a named example, for example: Plants are producers that begin the food chain, so leaves and fruit feed insects, monkeys and birds. Toucans and monkeys eat fruit and drop the seeds far from the tree, which helps new trees to grow. Bees and bats carry pollen between flowers so plants can make seeds. In the Amazon, the Brazil nut tree needs bees to pollinate it and the agouti to spread its seeds. If one species is lost, the others are affected.
Level 2 (3-4 marks): several correct points about pollination, seed dispersal or food chains, with limited linking. Level 1 (1-2 marks): one or two simple points, such as "animals eat plants".
Exam tip
In a six-mark "Explain" question, use at least one named example, such as the Brazil nut tree, and link each point with "so" or "because".