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Unlock This CourseImagine a pond. There's water, mud, sunlight, fish, frogs, algae and bacteria all living and working together. That whole system the living things AND the non-living environment they depend on is called an ecosystem. Ecosystems can be tiny (like a puddle) or absolutely massive (like the Amazon rainforest). What makes something an ecosystem is that all the parts interact with each other.
Key Definitions:
An ecosystem works a bit like a city. Some people grow food (producers), some people eat the food (consumers) and some people clean up the waste (decomposers). Take away any one group and the whole city starts to fall apart. Ecosystems are exactly the same!
Every ecosystem has two types of components. Get these two words into your head they come up constantly in iGCSE Environmental Management.
Biotic means anything that is alive or was once alive. This includes all the plants, animals, fungi and microorganisms in the ecosystem. Biotic factors affect each other for example, if the number of rabbits increases, the fox population might increase too because there's more food available.
Abiotic means anything that is non-living. These are the physical and chemical conditions of the environment. Abiotic factors have a huge influence on which organisms can survive in a particular place.
The Sahara has extreme abiotic conditions very little rainfall (under 25mm per year), scorching daytime temperatures (up to 50°C) and poor sandy soil. These abiotic factors mean only specially adapted biotic components can survive there, like camels, scorpions and cacti-like succulents. Change the abiotic conditions and the whole community of living things would change too.
Living things in an ecosystem can be sorted into groups based on how they get their energy. This is one of the most important ideas in the whole topic.
Producers make their own food using energy from the sun through photosynthesis. They are always the starting point of any food chain. Without producers, nothing else in the ecosystem could survive. On land, producers are mostly green plants and trees. In water, they are algae and phytoplankton.
Grasses, trees, shrubs, mosses, ferns. The Amazon rainforest has over 40,000 plant species acting as producers.
Algae, phytoplankton, seagrasses, water lilies. Phytoplankton produce around 50% of all the oxygen on Earth!
Carbon dioxide + water + sunlight → glucose + oxygen. This glucose is stored energy that gets passed up the food chain.
Consumers cannot make their own food they have to eat other organisms to get energy. They are split into different levels called trophic levels.
Here's something really important only about 10% of energy is passed on from one trophic level to the next. The rest is lost as heat, used for movement, or lost in waste. This is why food chains rarely have more than 4 or 5 levels and why there are always far more producers than top predators in any ecosystem.
Decomposers are the unsung heroes of every ecosystem. They break down dead organisms and waste material, releasing nutrients back into the soil. Without decomposers, dead material would pile up and the nutrients locked inside would never be recycled. The whole ecosystem would grind to a halt.
A food chain shows the flow of energy from one organism to the next in a simple straight line. Arrows show the direction energy moves always from food to feeder.
Grass → Zebra → Lion → Vulture
The grass is the producer. The zebra is the primary consumer. The lion is the secondary consumer. The vulture is the tertiary consumer (and also a scavenger). Decomposers break down whatever is left.
But real ecosystems are much more complicated than a single food chain. Most animals eat more than one thing and most organisms are eaten by more than one predator. When you link all the food chains in an ecosystem together, you get a food web.
The Great Barrier Reef in Australia is one of the most complex ecosystems on Earth. Phytoplankton and algae are the main producers. Zooplankton, small fish and sea urchins are primary consumers. Larger fish like parrotfish and wrasse are secondary consumers. Reef sharks and moray eels sit near the top. Crown-of-thorns starfish eat coral polyps directly. When one part of this web is disrupted for example, when overfishing removes too many sharks the whole web can collapse. This is called a trophic cascade.
Ecosystems don't just need energy they need nutrients too. Unlike energy, nutrients are recycled over and over again. The most important nutrient cycles you need to know are the carbon cycle and the nitrogen cycle.
Carbon moves through ecosystems constantly. Plants absorb CO₂ from the air during photosynthesis. Animals eat plants and release CO₂ through respiration. When organisms die, decomposers release carbon back into the soil and atmosphere. Burning fossil fuels adds extra carbon to the atmosphere, which is why it's a major cause of climate change.
Nitrogen is essential for making proteins. The air is 78% nitrogen, but most living things can't use it directly. Nitrogen-fixing bacteria (some live in the roots of legume plants like peas and beans) convert nitrogen gas into a form plants can absorb. When organisms die, decomposers release nitrogen back into the soil. Farmers add fertilisers to replace nitrogen lost from fields.
Rainforests are among the richest ecosystems on Earth
The tropical rainforest is the most biodiverse ecosystem on Earth and a brilliant example of how all ecosystem components work together.
Tropical rainforests are found near the equator (Brazil, Congo, Indonesia). They receive over 2,000mm of rainfall per year, have temperatures of 25–30°C year-round and get strong sunlight. These ideal abiotic conditions support an enormous variety of life.
Towering emergent trees, canopy trees, understorey shrubs, ground-level ferns and mosses. The Amazon alone has over 390 billion individual trees.
Insects, sloths, monkeys, toucans, jaguars, anacondas, poison dart frogs. Each species occupies a specific niche in the ecosystem.
Warm, moist conditions mean decomposers work incredibly fast. Dead leaves on the rainforest floor can be broken down in just a few weeks.
Here's a surprising fact despite all that lush growth, rainforest soil is actually quite nutrient-poor. This is because decomposers work so fast that nutrients are immediately taken up by plant roots before they can build up in the soil. The nutrients are locked in the living biomass, not the soil. This is why when rainforests are cleared for farming, the soil quickly becomes useless the nutrient cycle has been broken.