Features of Tropical Rainforests (Edexcel GCSE Geography A): Revision Note
Exam code: 1GA0
Biotic & Abiotic Characteristics of Tropical Rainforests
All ecosystems consist of biotic and abiotic elements that are distinct to that ecosystem
Biotic and Abiotic Components of Tropical Rainforests (TRF)
Biotic | Abiotic |
|---|---|
Plants: the warm and humid climate provides perfect conditions for plant growth | Climate: consistently warm, very wet, and humid all year round, with no seasons |
Animals: the wide range of plant species | Soils are low in nutrients due to leaching and rapid uptake of nutrients by plants. Soils are usually red in colour due to high levels of iron |
Humans: traditional communities survive through hunting and gathering using the rainforest plants and animals in a sustainable way | Water levels are high which increases the rate of chemical weathering of the bedrock and leaching of nutrients from the soil |
Interdependence in Tropical Rainforests
Within an ecosystem, the different biotic components depend on each other and the abiotic components for survival (food, water and shelter)
This is known as interdependence
Interdependence in TRFs
Any change in one component will affect the remaining components
Deforestation leads to the loss of habitats and reduction in biodiversity
Two important processes allow ecosystems to interact with each other
Nutrient cycling
Energy flow
Nutrients are essential for organisms to grow and survive
The nutrient cycle moves these nutrients between the biotic and abiotic components within the ecosystem
For example, deciduous trees lose their leaves
These fall to the ground, decompose, and release their stored nutrients back into the soil
The tree roots will absorb these nutrients and use them again for growing
Nutrients are also lost from the cycle through surface run-off and leaching
Nutrients are also be added through precipitation
Gersmehl model
This is used to show how nutrients within the ecosystem:
Enter and exit
Where they are stored
How they are transferred
The model is proportional to size of the store and flow of nutrients
The larger the circle, the larger the store
The thicker the arrow, the more nutrients are in the transfer pathway
The longer the arrow, the faster the rate of flow
Gersmehl model of TRF
In TRFs, biomass is the main store and there is a rapid transfer between stores and the environment, which leaves the soil relatively infertile
Transfer within the nutrient cycle
Soil (abiotic) is formed from minerals and particles from weathered rock (abiotic), dead plants, and animals (biotic)
Soil, water and solar energy (abiotic) then provide nutrients and moisture for plants (biotic) to grow
As part of photosynthesis, plants take in CO2 and release oxygen, balancing gases in the atmosphere
The plants (biotic) are then eaten by animals (biotic), who also eat each other
When they die, the nutrients are then returned to the soil (abiotic) through decomposition
If one component of this cycle is changed, for example, by vegetation being cut down, all the other components will be affected
This leads to:
Less nutrients are added to the soil through decomposition
There will be less food and fewer habitats so the number of animal species will decrease
The soil is exposed to erosion from the sun and rain
CO2 levels will increase
Plants will transpire less, resulting in less precipitation
Energy flows
In ecosystems, food chains are responsible for passing energy around
Simplified food chain
A network of food chains is called a food web
Food webs are made up of levels that describe the position of an organism within the web
These levels are called trophic levels and include
Primary producers (plants) get their energy directly from the sun
Primary consumers (herbivores) eat the primary producers
Secondary consumers (carnivores) then eat the primary consumers
Tertiary consumers (predators) eat the secondary consumers
Quaternary consumers are the apex predators, such as humans, jaguars, harpy eagles, etc.
Not every food chain or web has 5 trophic levels; most are just 4 levels
Trophic levels
Not all the energy is passed on because:
Not everything gets eaten (bones)
Energy is lost immediately (respiration and heat)
Energy is lost through waste production
When an organism dies, it is eaten by microbes and the nutrients are recycled
Loss of energy through trophic levels
TRFs contain more species of animals than any other ecosystem on earth, and so food webs are complex
TRFs food web usually only has 4 trophic levels
Simplified TRF food web
Worked Example
Explain the nutrient cycle of a tropical rainforest.
(4)
Answer:
The biomass store is the largest in a tropical rainforest (TRF) because of the high rate of biodiversity in the system (1).
The soil store is small because the uptake of nutrients by plants is high. Also, there are high levels of leaching due to more rainfall in TRF (1).
The litter store is also small in a TRF, as the rate of decomposition is high because of humidity (1).
The transfer of nutrients is fast between stores due to the wet and warm climate and high levels of biodiversity, meaning that transfer is more likely in a TRF (1).
Rainforest Biodiversity & Adaptations
Tropical rainforests contain the highest biodiversity of plants and animals on Earth
Estimates range from over 50% to 80% of the world's plant and animal species
One 10km2 area can contain up to 1,500 flowering plants, 750 species of trees, 400 species of birds and 150 species of butterflies
Why biodiversity is high
High constant temperatures of 26°C to 30°C
Around 2000mm of rainfall a year
High levels of humidity
Good levels of sunlight all year
Constant growing season
High levels nutrients
Adaptation of biodiversity
The tropical rainforest has five distinct layers
Ground layer (0m)
Shrub layer (3–4 m)
Under canopy (15m)
Canopy (30m)
Emergents (45-55m)
Typical structure of a tropical rainforest
As there is variation within forest's climate conditions, plants and animals have had to adapt in order to survive
Adaptations of Plants and Animals in a Tropical Rainforest
Plants | Animals |
|---|---|
Waxy leaves with drip tips: These ensure that rainwater runs off, preventing rotting | Many animals have adapted to living in the canopy, where there is plenty of food |
Buttress roots: These large roots provide stability for trees that can grow up to 40 meters in height | Parrots, macaws and toucans have powerful beaks to break open nuts |
Trees have large crowns (where they absorb sunlight) with very few branches | Monkeys: Monkeys have evolved with strong grips and long tails for balancing to collect fruit and nuts from the tall main canopy |
Epiphytes are plants that grow on the trunks and branches of trees. They get nutrients from the air, rain or debris blown around the plant | Sloth: Algae grow in the fur of the sloth, helping to camouflage it |
Straight, smooth trunks: To reduce the number of epiphytes using the tree | Animals hunt at night when they have more energy and it is cooler |
Fungi have adapted to take nutrients from dead organic matter in the litter layer | Animals learn to swim or have webbed feet because of the many rivers in TRF |
Evergreen appearance because of the constant growing season, even though many trees are deciduous and lose their leaves | Some animals have a good sense of smell or hearing because of low light levels on the forest floor |
Tree roots are shallow and spread horizontally to gain nutrients from the top layer of soil | Many animals are camouflaged to avoid predators and to blend in with the surroundings, such as geckos |
Worked Example
Explain two ways in which plants have adapted to living in a tropical rainforest.
(4)
Guidance:
With this type of question, one mark is given for identifying the adaptation, and the second mark is given for explaining the adaptation
Remember that the key term here is plant adaptation
Writing about animal adaptation will not earn any marks
Answer:
Trees grow straight, tall trunks (1) to outcompete other species for sunlight (1)
Epiphytes grow on the trunks and branches of trees (1). They get nutrients from the air, rain, or debris blown around the plant and not from the tree (1)
Any other suitable response about plant adaptation
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