IB Biology HLEnergy & Matter in EcosystemsPaper 1 & 2~10 min read
Ecosystems as Open Systems
A pond is not sealed inside a glass box. Birds fly in, leaves blow out, sunlight pours in all day. That is what “open system” means, and it is the idea the whole of this topic is built on.
📚 What you need to know
An ecosystem is a group of organisms interacting with each other and with the non-living parts of their environment.
Ecosystems contain biotic (living) and abiotic (non-living) components, and vary hugely in size and complexity.
Ecosystems are open systems: both energy and matter can enter and leave.
In a closed system, energy can enter and leave but matter is only recycled inside.
Most organisms stay put for life, so ecosystems are largely self-contained even though they are open.
The Sun is the initial energy source for most food chains, with exceptions such as deep-sea vents and caves.
NOS: laws describe patterns; theories explain them. “The Sun is the initial source of energy for food chains” is a useful generalisation.
What an ecosystem actually is
Start with the definition, because examiners mark it word by word.
Definition
An ecosystem is a group of organisms interacting with each other and with the non-living parts of the environment.
The important word is and. A community is just the living things. An ecosystem is the living things plus the water, rock, light and air they interact with. Drop the non-living half and you have defined a community by mistake.
Component
What it means
Examples in an ocean ecosystem
Biotic
The living parts, and anything that came from something living
Fish, crabs, corals, algae, seagrass, bacteria
Abiotic
The physical and chemical parts that were never alive
Salinity, pH, temperature, light intensity, mineral availability
The abiotic components are not just background scenery. They decide which organisms can live there, because they supply habitat, nutrients and the conditions organisms need to survive and reproduce. Change the salinity of a rock pool and the community changes with it.
Size and complexity vary enormously
Size: a garden pond is an ecosystem. So is the open ocean. There is no minimum or maximum.
Complexity: a desert has few species and simple feeding links. A tropical rainforest has thousands of species and a tangled web of them.
Each step up adds more organisms. The final step is the one students forget: it adds the non-living surroundings.
Open systems: things get in and out
An open system is one where both energy and matter can cross the boundary in either direction. Every ecosystem is open.
How energy gets in and out
Energy enters as sunlight, and then flows through the ecosystem as stored chemical energy in the tissues of organisms.
Energy also enters inside the tissues of any organism that migrates in – a bird arriving carries chemical energy with it.
When that individual leaves again, the stored energy leaves with it.
How matter gets in and out
Matter enters when an organism arrives, in the form of all the molecules in its cells and tissues.
Matter leaves when an organism leaves, or when material is physically carried away – dead plant matter washed off a beach by waves, or timber cut and removed from a forest.
Energy behaves the same way in both. What changes is whether atoms can cross the boundary.
Open, but still mostly self-contained. Most organisms stay inside one ecosystem for their whole lives, and when they die the matter and energy in their tissues is recycled within that ecosystem. So although the boundary leaks, an ecosystem still behaves as a largely self-contained unit.
Notice how the Earth manages to be closed while every ecosystem on it is open. Matter shuffles between ecosystems all the time – it just never leaves the planet. That single sentence answers a lot of exam questions.
Sunlight as the source of energy
The Sun is the initial source of energy for most food chains. Trace the energy in your lunch back far enough and you reach a leaf.
Light energy from the Sun is converted by producers into chemical energy stored in plant tissues, during photosynthesis.
That chemical energy passes to primary consumers when they eat the plant.
It then passes to secondary consumers when the primary consumers are eaten, and so on up the chain.
The exceptions
Food chains in deep-sea volcanic vents and underground caves get no light at all. They start with bacteria that release energy from chemical reactions instead.
Even in caves the picture is not always clean – water flowing in can carry energy that originally came from sunlight at the surface.
If a question says “explain why the Sun is described as the initial source of energy for most food chains”, the mark is hiding in the word most. Name the vent or cave exception and you have it.
NOS: laws describe, theories explain
This is a Nature of Science point and it is easy marks if you learn the distinction.
Term
What it does
Example
Law
A generalised principle that describes a pattern seen in nature. Built from observation. Does not try to say why.
The first law of thermodynamics: energy cannot be created or destroyed
Theory
Goes further and explains the phenomenon that has been observed
Systems theory, which explains how open and closed systems behave
Both can be used to make predictions, for example about what happens to a system when something changes.
Useful generalisation. “The Sun is the initial source of energy for food chains” is a useful generalisation: it is true for most food chains, and it is worth stating for that reason, even though a few food chains do not rely on the Sun at all.
Worked examples
WE 1
Define ecosystem and identify components
Define the term ecosystem and state one biotic and one abiotic component of a rock pool. (3 marks)
Part 1: the definition
An ecosystem is a group of organisms interacting with each other and with the non-living parts of the environment.
Part 2: biotic
Seaweed (or crabs, limpets, algae) – a living component.
Part 3: abiotic
Salinity (or temperature, pH, light intensity) – never alive.
Community + non-living surroundings = ecosystemdo not stop at “organisms interacting with each other” – that is a community, and it drops a mark
WE 2
Explain why an ecosystem is an open system
Explain why a woodland is described as an open system. (4 marks)
Point 1: what open means
In an open system both energy and matter can enter and leave.
Point 2: energy
Energy enters as sunlight and is trapped by producers; it also enters and leaves stored in the tissues of animals that migrate.
Point 3: matter
Matter enters when organisms arrive and leaves when they leave, or when material such as timber or leaf litter is carried out.
Point 4: the qualifier
Most organisms stay inside for life and are recycled there, so the woodland is still largely self-contained.
Energy in and out, matter in and outgive an example of each crossing, not just the definition – the examples are usually two of the marks
WE 3
Distinguish between a law and a theory
Using an example, distinguish between a scientific law and a scientific theory. (2 marks)
Point 1: law
A law describes a pattern observed in nature but does not explain it, e.g. the first law of thermodynamics states energy cannot be created or destroyed.
Point 2: theory
A theory explains observed phenomena, e.g. systems theory explains the behaviour of open and closed systems.
Laws say what happens. Theories say why.“distinguish” means give both sides in the same sentence structure – describe vs explain
💡 Exam tips
Learn the ecosystem definition exactly, including the non-living half.
Open system = energy and matter cross. Closed = energy only. Say which one crosses, do not just name the type.
Use the phrase largely self-contained if a question asks why ecosystems behave as units.
Say the Sun is the initial source for most food chains, and know the vent and cave exceptions.
For NOS marks: laws describe, theories explain, both predict.
⚠ Common mistakes
Defining an ecosystem as “all the organisms in an area”. That is a community. The abiotic part is what makes it an ecosystem.
Saying ecosystems are closed because nutrients are recycled. Recycling happens, but organisms and material still cross the boundary, so they are open.
Saying energy is recycled. Energy flows through and is lost as heat. Matter is what gets recycled.
Calling abiotic factors “dead things”. Dead leaves were once alive, so they are biotic. Abiotic means never alive.
Writing that all food chains start with the Sun. Vent and cave food chains start with chemical energy.
Mixing up law and theory. A theory is not “a law that is not proven yet” – it does a different job.
Up next: Energy Flow in Ecosystems – how that trapped sunlight actually moves from one organism to the next, and what those arrows in a food chain really mean.
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