You cannot run an experiment on the Amazon. So this page covers the two things you can do instead: shrink an ecosystem down to something that fits in a jar, and put a number on how much of a real one has been lost.
📚 What you need to know
A mesocosm is an experimental container in which a naturally occurring ecosystem is simulated.
Mesocosms let you control every factor except the one being studied, and need a control mesocosm alongside.
You should be able to describe how to set up a terrestrial and an aquatic mesocosm.
NOS: mesocosm work must follow IB experimental guidelines on the use of animals.
Deforestation threatens Amazon stability by changing temperature and rainfall, and could push it past a tipping point.
You must be able to calculate percentage change to express the extent of ecosystem loss.
What a mesocosm is
The stability of an ecosystem can be investigated using a model ecosystem, and the name for that model is a mesocosm.
Definition
A mesocosm is an experimental container in which a naturally occurring ecosystem is simulated.
Mesocosms are used to study how an ecosystem responds to changes in specific factors – nutrient levels, light levels and so on. The reason they are useful is that, unlike a real ecosystem, it is possible to control all of the factors other than the variable being studied.
They can be built at very different sizes for very different purposes:
Water tanks on land, to study the effect of sewage pollution on ponds or lakes.
Underwater enclosures in coastal waters or lakes, to study the effect of temperature change or dissolved carbon dioxide on ocean ecosystems.
Large greenhouse-like buildings planted with trees to replicate a rainforest, to investigate the passage of carbon through that ecosystem.
The honest weakness. Mesocosm experiments can be considered unrealistic, precisely because of their enclosed nature and the level of control that can be achieved – the thing that makes them useful is also the thing that makes them artificial. Realism can be improved by designing larger mesocosms that share more features with a real ecosystem, for example one big enough to allow mixing of layers of water in an ocean mesocosm.
Designing a mesocosm in the lab
Small mesocosms can be built in a school laboratory. Whichever type you build, the same design decisions come up, and each one has a reason attached.
Decision
Why
The container should be transparent
So that sunlight can reach the producers inside
Autotrophs must be included
So light energy can be converted into chemical energy inside the mesocosm
Small primary consumers only, such as zooplankton or small invertebrates
And only if the mesocosm is large enough to support them
No secondary consumers
There is not enough energy in the food chain to sustain them for long, and it could be considered unethical to let the primary consumers be eaten this way
Sealed rather than open
A sealed system prevents organisms and substances entering or leaving, so it is more controlled and more useful experimentally
Once the mesocosm is set up, a known factor is altered to assess its effect – different light levels, different temperatures and so on. And to assess the impact of changing that one factor, a control mesocosm must be set up at the same time.
The control
A control mesocosm is exactly the same as the experimental one, except that the altered variable is not changed. Its purpose is to demonstrate that any change observed is due to the altered factor and not some other factor.
Notice what is missing from both: predators. There is not enough energy in a jar to sustain them, and it would not be ethical to try.
🧩 Building a terrestrial mesocosm
Drainage material such as gravel goes in the bottom of a clear container.
A layer of charcoal on top of the drainage layer, which helps prevent the growth of mould.
A layer of sphagnum moss or filter paper on top of the charcoal, to provide separation between the base layers and the organic matter above.
A layer of soil or compost above the separation layer, providing organic material and micro-organisms to aid nutrient cycling.
Plant slow-growing producers such as healthy mosses and ferns in the growth medium.
Water the growth medium, then seal the container with a lid.
Place the container in a light location and make sure the temperature is stable.
One point about watering that is easy to get wrong: the mesocosm may need watering while it establishes, but you should avoid excessive watering. Once it has stabilised, the plants release enough water vapour to maintain moisture levels themselves.
🧩 Building an aquatic mesocosm
The base layer should be organic substrate from the bottom of a lake or pond, which provides naturally occurring nutrients and microorganisms.
Add lake or pond water, which contains the required microscopic organisms and avoids the chemicals present in tap water.
Add healthy aquatic plants to produce carbohydrates and to oxygenate the water.
Small aquatic organisms such as water fleas or water snails may be added, but no more than the mesocosm can support, and primary consumers only.
Place the container in a light location and make sure the temperature is stable.
NOS: the ethics of using animals
The IB policy on animals in schools says that investigations should only involve animals where no alternative options are available, that any investigation involving animals should not be cruel, and that it should include measures that remove potential causes of animal distress.
Applied to a mesocosm, that usually means removing animals entirely. This is the most ethical approach, because not all mesocosms need animals in order to be sustainable – producers and decomposers alone will do.
If animals are genuinely required, the guidelines point towards including only a limited number of herbivorous animals in a carefully controlled environment. In practice: enough food available, the mesocosm not too hot or too cold, the investigation not running for too long, and the animals returned to their natural environment at the end.
If an exam asks you to design a mesocosm investigation, saying “I would not include animals, because the investigation can be carried out without them” is a strong answer, not a lazy one. It shows you have applied the guideline rather than recited it.
Deforestation of the Amazon
The Amazon rainforest is the standard example of human activities endangering ecosystem sustainability, and the mechanism is not the obvious one. Cutting trees down does not just remove trees. It changes the climate of the forest itself.
Here is the chain. Transpiration releases water vapour into the air above the rainforest. That has a cooling effect, which in turn affects air movement and rainfall. So changing the number of trees carrying out transpiration changes local temperature and rainfall.
And temperature and rainfall are exactly the factors that control the rates of photosynthesis and nutrient cycling – two of the four stability requirements from the previous page. Deforestation therefore has a knock-on effect on the ecosystem’s ability to support itself at all.
Scientists are concerned the Amazon could reach a tipping point beyond which it is no longer stable: so many trees removed that temperature and rainfall patterns change significantly, and climatic factors move beyond the tolerance levels of some species. The honest position, which is worth stating in an answer, is that there is real uncertainty about how much rainforest would have to be lost for this to happen – so we do not know how close the Amazon is to losing stability.
Calculating percentage change
You could be asked to put a number on ecosystem loss. The calculation is percentage change, and the only thing that catches people out is which value goes on the bottom.
The formula
percentage loss of rainforest = (change in rainforest area ÷ original rainforest area) × 100
The change in forest cover is found by subtracting the final forest cover from the initial forest cover. The denominator is always the original figure, never the final one.
Dividing by the area remaining instead of the original gives 25 %, which is the single most common error on this calculation.
Worked examples
WE 1
Calculate percentage loss
An area of rainforest covered 2 500 000 hectares in 1990. By 2020 the area of forest cover was 1 900 000 hectares. Calculate the percentage loss of rainforest. (2 marks)
Step 1: find the change2 500 000 − 1 900 000 = 600 000 haStep 2: divide by the original and multiply by 100(600 000 ÷ 2 500 000) × 100 = 24 %24 % lossthe denominator is the 1990 figure – using 1 900 000 gives 31.6 %, which is wrong
WE 2
Justify a mesocosm design
A student sets up a sealed aquatic mesocosm to investigate the effect of light intensity on an ecosystem. Explain why a control mesocosm is needed, and why no fish should be included. (3 marks)
Point 1: the control
A control mesocosm is set up that is identical in every way except the light intensity, which is left unchanged.
Point 2: why
This demonstrates that any change observed is due to the altered factor and not some other factor.
Point 3: no fish
Fish are secondary consumers, so there would not be enough energy in the food chain to sustain them, and it could be considered unethical to include them.
One variable changed, everything else identical, no predators“to make it a fair test” is not enough on its own – say what the control demonstrates
WE 3
Explain a threat to stability
Explain how deforestation could reduce the stability of the Amazon rainforest ecosystem. (4 marks)
Point 1: transpiration
Trees release water vapour into the air above the forest by transpiration, which has a cooling effect.
Point 2: the climatic change
Fewer trees means less transpiration, so local temperature and rainfall patterns change.
Point 3: the knock-on effect
Temperature and rainfall affect the rates of photosynthesis and nutrient cycling, so the forest becomes less able to support itself.
Point 4: the tipping point
If climatic factors move beyond the tolerance levels of some species, the ecosystem may pass a tipping point beyond which it is no longer stable.
Removing trees changes the forest’s own climateadd that there is uncertainty over how much loss would trigger this – examiners reward the caveat
💡 Exam tips
Define a mesocosm as an experimental container simulating a naturally occurring ecosystem.
Always mention the control mesocosm in any design question.
Justify each layer of a terrestrial mesocosm – the marks are for the reasons, not the list.
Say primary consumers only, and give both reasons: energy and ethics.
In percentage change, divide by the original value.
For the Amazon, route your answer through transpiration to temperature and rainfall.
⚠ Common mistakes
Dividing by the final value. Percentage change always uses the original as the denominator.
Using tap water in an aquatic mesocosm. It lacks the organisms and contains chemicals.
Including a predator to make it “realistic”. There is not enough energy, and it is unethical.
Forgetting the control. Without it you cannot attribute any change to the altered factor.
Saying deforestation only removes trees. The key effect is on temperature and rainfall.
Claiming we know when the Amazon will collapse. The amount of loss required is uncertain.
Up next: Keystone Species – what happens to a stable ecosystem when you remove one particular species, and why some species matter far more than their numbers suggest.
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