Ecosystem Explorer

The richest place on the poorest soil

Tropical rainforest covers roughly 6 per cent of the land surface of the Earth and contains more than half of all known species. It also grows on some of the poorest soil in the world. Dig down in the Amazon and you find a thin layer of dark material over deeply weathered, nutrient-starved ground, and farmers who clear rainforest usually find the land exhausted within a few seasons. That sounds contradictory and is not. Almost all the nutrients in a rainforest are not in the soil at all: they are in the living vegetation. Leaves fall year-round into constant heat and moisture, decomposers break them down within weeks rather than years, and the nutrients released are taken straight back up by the dense mat of roots waiting at the surface. The cycle is furiously fast and almost nothing is left sitting in storage. Cut the trees down and you have removed the store itself, which is why the fertility vanishes with the forest. That is what an ecosystem is: not a list of animals but a system of components and the flows between them. Living things, called biotic, and non-living conditions, called abiotic, linked so tightly that a change in one part travels through the rest. This module covers those components, the cycle that moves nutrients between them, a small-scale UK example you can use in an exam, and why the great biomes sit where they do. Throughout it asks you to do the thing the mark scheme rewards most: follow a change all the way through rather than stopping at the first link.

Five ecosystem terms

The first two define the system, the next two are the roles within it, and the last is the model that ties the whole topic together.

Trace the whole chain

A farmer spreads fertiliser on a field beside a pond, and heavy rain follows. Put the six stages of what happens next in order.

• Rain washes dissolved nutrients off the field and into the water
• Algae, no longer short of nutrients, multiply rapidly across the surface
• The surface layer blocks the light, so the plants rooted below can no longer photosynthesise
• Those plants die, and so eventually does the algal bloom itself
• Bacteria multiply to break down the mass of dead material
• Their respiration strips the dissolved oxygen from the water, and the fish and invertebrates suffocate

Why does rainforest soil stay poor?

The most species-rich ecosystem on land grows on notably infertile soil. Which answer explains that?

• The nutrients are held in the vegetation rather than in the ground. Constant heat and moisture let decomposers break down fallen material within weeks, and the dense surface root mat absorbs what is released almost immediately, so nutrients spend very little time in the soil where they could accumulate or be measured. Heavy rainfall also washes soluble nutrients downwards out of reach. The forest is not growing on rich ground; it is recycling the same nutrients through itself very fast.
• The underlying rock in the tropics contains fewer nutrients than rock elsewhere.
• Rainforest trees have unusually deep roots that reach nutrients far below the soil.
• There are very few decomposers in a rainforest, so dead material never breaks down.

Same three stores, three very different shapes

The nutrient cycle model uses the same three stores everywhere, and the sizes tell you what kind of place you are looking at. Read the rows across and the reason is always the same two abiotic conditions: warmth and moisture, which set how fast decomposition runs.

Label the nutrient cycle

The three circles are the stores of a nutrient cycle, drawn at different sizes to show how much each holds, and the arrows are the transfers running between them. Tap a label, then tap the pin it belongs to. Two of the labels name real parts of a nutrient cycle that this diagram does not show.

Match each pond dweller to its role

Follow it all the way round

The specification note for this topic names one skill above all others: when you are asked about interdependence, trace a change through the whole web or cycle, not just the first link. Most answers stop far too early. Asked what happens if a predator is removed, they write that its prey will increase, and stop. That is one link, and it is worth one mark. A full answer keeps going, and it goes in more than one direction. The prey increases, so the plants it eats are grazed harder and decline, so the other animals that depended on those plants decline too; meanwhile the scavengers and decomposers that fed on the predator's leftovers lose a food source. Then it comes back round: with the plants reduced, the prey population eventually crashes from starvation. Two or three steps and a loop back will reach the top band where one step will not. Two more habits. Cross between biotic and abiotic. The strongest links in these answers are usually the ones that leave the list of living things altogether: light, oxygen and temperature are what carry an effect from one part of a pond to another, and an answer that only ever talks about who eats whom has missed half the system. And name the components properly. Producer, consumer and decomposer are technical terms with specific meanings, and decomposers are the ones most often forgotten, even though they are what makes the cycle a cycle rather than a one-way journey. On command words: "describe the distribution" wants where biomes are, using latitude. "Explain the distribution" wants the climatic reasons, and those come straight from the circulation model. "Explain interdependence" wants the chain, followed properly.

Complete the ecosystem facts

An ecosystem is made of living components, called _____, and non-living conditions such as light, temperature and oxygen. Every food chain begins with a _____, which captures energy from sunlight, and it is completed by decomposers, which return nutrients from dead material to the soil. Those nutrients move between three stores: the living vegetation, the _____ lying on the ground, and the soil. In tropical rainforest almost all of the nutrients are held in the vegetation, because constant heat and moisture mean decomposition takes _____ rather than years, while on the tundra the cold means dead material accumulates instead. Rainforest covers around _____ per cent of the land surface yet holds over half of all known species.

Spot the three accurate ecosystem claims

Select the THREE statements that are true.

• The strongest interdependence answers often follow a link out of the living community altogether, because conditions rather than creatures are frequently what move an effect from one corner of a pond to another.
• On the tundra most nutrients sit in undecomposed litter rather than in the vegetation or the soil, because cold slows decomposition so far that dead material accumulates instead of breaking down.
• Cleared rainforest land is often exhausted within a few seasons, because the nutrients were held in the trees rather than in the ground, so removing the trees removes the store.
• Rainforest supports so much life because it grows on exceptionally fertile soil.
• When a predator is removed from a food web, the only significant effect is that its prey becomes more numerous.
• Decomposers are an optional extra in an ecosystem rather than a necessary component.

A 6-mark interdependence answer, marked

Using a small-scale UK ecosystem, explain how a change to one component can affect the whole ecosystem. [6 marks] In a UK freshwater pond the producers are pondweed and algae, the primary consumers include water snails and tadpoles, secondary consumers include dragonfly larvae and newts, a heron feeds at the top, and bacteria in the silt act as decomposers. Suppose fertiliser is washed off the surrounding farmland into the water after heavy rain. The nutrients allow algae to multiply rapidly across the surface, and because that layer blocks the sunlight, the pondweed rooted below can no longer photosynthesise and dies. This removes both a food supply for the snails and tadpoles and, more importantly, a source of dissolved oxygen. When the algal bloom itself dies back, bacteria multiply to decompose the large mass of dead material, and their respiration consumes much of the oxygen that remains, so fish and invertebrates suffocate. The effect then travels upwards through the web, because the dragonfly larvae and newts that fed on those invertebrates lose their food supply, and the heron, which fed on several chains at once, abandons the pond altogether. The change has therefore reached every level of the community without a single organism being removed directly, and it did so largely through an abiotic link, the availability of light and then of oxygen, rather than through predation. Recovery depends on the nutrient input stopping and on oxygen levels returning, which can take years. What earned the marks: a named small-scale ecosystem with its components identified by role; a change traced through several linked steps rather than one; the effect carried by abiotic conditions, light and oxygen, as well as by feeding relationships; and a closing point about recovery, which shows the system is being treated as a system.

Find the two abiotic links

Five steps in a chain of effects through a pond. Two of them work through non-living conditions rather than through one organism eating another. Tap those TWO.

• A layer of algae across the surface stops sunlight reaching the plants rooted below.
• Water snails, having lost the weed they grazed on, become far less numerous.
• Bacteria decomposing the dead material use up most of the dissolved oxygen in the water.
• The heron stops visiting because there are no longer enough fish to make the journey worthwhile.
• Dragonfly larvae decline as the small invertebrates they hunt disappear.

Build the rainforest soil sentence

Assemble a sentence that resolves the apparent paradox properly.

Four changes, four systems

Each stage describes a change to one component. Work out what happens to the rest.

• Pike are removed from a pond by anglers. A student writes that the small fish the pike ate will become more numerous, and stops there.
• A row of trees is planted along the southern edge of a pond, heavily shading the water for most of the day.
• A farmer clears a patch of tropical rainforest and is told the soil beneath it must be excellent, given what was growing there.
• Vehicle tracks cut across tundra vegetation are still visible decades later, while similar damage in a temperate wood has grown over within a few years.

Explain the distribution

Describe the global distribution of large-scale ecosystems and explain the reasons for it. [9 marks]

• Describe the pattern using latitude, naming at least three biomes and where each one sits
• Explain the reasons through climate, and specifically through temperature falling away from the equator and rainfall varying with rising and sinking air
• Link the desert belt and the equatorial forest belt to the circulation that produces them, rather than treating their positions as coincidence
• Use a link phrase such as "which means that" so each climatic condition is turned into a reason for the vegetation found there
• If you have room, add what the nutrient cycle looks like in one of your biomes, since it shows you understand the ecosystem rather than only the map