ESS SL Paper 1: Answering Systems and Biodiversity Questions
Use the material in front of you: name the flow between two storages, check which form of a diversity index you have, and judge the main threat from evidence.
On an ESS systems or biodiversity question, the strongest answer is the one that uses the figure or data in front of you: it names the storage an arrow leaves and the storage it enters, or reads a diversity value in the setting the question describes, instead of repeating a textbook definition. Environmental systems and societies counts towards both the individuals and societies group and the sciences group of the IB Diploma (ibo.org, DP passing criteria), and these questions reflect that mix: they ask for a scientific reading of a diagram or data set and, often, a judgement about human pressure on it. This post covers three question families for SL candidates: reading an unseen system diagram, interpreting biodiversity data, and justifying the main threat to an ecosystem, with a short revision routine for each. Topic numbers differ between syllabus editions and textbooks, so we name the ideas rather than the numbers.
Reading an unseen system diagram: boundary, storages, flows
When a question gives you a system diagram you have not seen before, read it in a fixed order before you write anything.
- Boundary. What is inside the system and what is outside? Inputs and outputs cross the boundary; everything else is internal.
- Storages. The boxes or reservoirs where energy or matter is held.
- Flows. The arrows between storages, and whether each is a transfer (a change of location, such as run-off carrying nitrate from soil to a river) or a transformation (a change of form, state or energy, such as photosynthesis turning carbon dioxide and water into glucose).
- Feedback. Any path of arrows that leads back to where it started. Decide whether it counteracts the change (negative feedback) or amplifies it (positive feedback).
Then write in the diagram's own terms. "Carbon moves from the atmosphere to biomass" is a definition. "The arrow labelled photosynthesis moves carbon from the atmosphere storage to the vegetation storage, so a fall in forest cover shrinks this flow" uses the diagram and says what changes. The second sentence names the flow, both storages and the direction, and that is the habit to practise.
For drawing and labelling your own models, see IB ESS systems diagrams: stocks, flows and feedback labels; this post is about reading the diagram you are given.
Biodiversity data: richness, evenness and what an index value means
Biodiversity questions often give you counts of species at one or more sites. Before you interpret them, be clear about which aspect of diversity the numbers show. Species richness is the number of species present. Evenness is how equally individuals are spread across those species. Two sites can have the same richness and very different evenness: a meadow where one grass makes up almost every individual is less diverse than one where the same species occur in similar numbers.
Diversity indices such as Simpson's combine richness and evenness in one number, but the index is written in more than one form. In some forms a higher value means greater diversity; in others, which measure the chance that two individuals picked at random belong to the same species, a higher value means lower diversity. Check which form your course and the question use before you say what the number means. Getting the direction backwards turns a correct calculation into a wrong conclusion.
Comparisons only hold when the data were collected in comparable ways. If two sites were sampled with different methods, different effort or in different seasons, say so: that limitation belongs in any evaluation of the data.
Threats to biodiversity: naming the pressure the evidence supports
A second family of questions asks which pressure is driving a loss of biodiversity in a given place. The usual candidates are habitat loss, fragmentation, pollution, invasive species, overexploitation and climate change, and a real ecosystem often faces several at once. The answer is not a list of all of them. It is a judgement about which one the evidence points to most strongly, with the evidence named.
Build it in this order. Name the pressure you judge to be the main driver. Support it with evidence from the material, ideally from more than one source, such as a falling count in a table and a passage describing new drainage. Then deal with the strongest alternative: say why the evidence fits it less well, or under what conditions it might matter more. That last part is what turns a list into a judgement.
Common pitfalls and how to avoid them
- Defining instead of using. Writing out the definition of a flow or of biodiversity does not answer a question about this diagram or these data. Use the term on a named feature of the material.
- Reversing the index. Reading a diversity index in the wrong direction. Check the form before you interpret.
- Losing the setting. Writing "this shows low diversity" without naming the site, the community or the pressure the question describes.
- Listing every threat. Naming all the pressures you know instead of judging which one the evidence supports.
- Ignoring how the data were collected. Comparing sites without checking that the sampling was comparable.
Linking the systems idea to the biodiversity evidence
The strongest sentences in these answers do three things at once: they name a concept, point to a feature of the material, and link the two. "The positive feedback in the diagram, where melting ice reduces albedo, accelerates the temperature rise shown in the data table" names the concept (positive feedback), points to the material (the diagram and the table) and links them with a cause. The same pattern works for biodiversity: "Nutrient run-off from the farmland described in the passage is a flow into the lake storage, which fits the fall in invertebrate species shown for the downstream site."
How far to take each sentence depends on the command term: a describe question needs the feature, an explain question needs the cause, and evaluate needs a judgement with a counter-point. The verbs themselves are covered in describe vs explain in IB ESS; the point here is that the verb tells you how much of the systems reasoning to write out.
Working through a diversity index calculation
When a question asks you to calculate an index from the data, show your working in three parts: the formula with the values from the table substituted in, the arithmetic and the result, and a sentence that says what the result means for the site in the question. Showing the substitution lets a reader follow your method even if the arithmetic slips.
The interpretation sentence is the part that is easiest to leave vague. "This shows high diversity" is true of many sites. A better sentence names the site, uses the value and links it to the setting: "Site A's index is higher than Site B's, so, in the form of the index used here, the community at Site A is more diverse, which fits the description of Site B as drained farmland." Write the interpretation in the terms the question uses.
The same three parts work for other calculations from biodiversity data, such as percentage cover, species richness or the proportion of introduced species. For the statistical tools ESS uses, such as standard deviation and correlation, see statistical tools in IB ESS.
| Question family | What to read first | What the answer shows | Common slip |
|---|---|---|---|
| System diagram | Boundary, storages, flows, feedback | The named flow, both storages and the direction of change | Defining the flow instead of locating it |
| Diversity index | Which form of the index is used | Substitution, result, interpretation in the setting | Reading the index in the wrong direction |
| Richness and evenness | What each column of the data counts | Which aspect of diversity differs between sites | Treating richness as the whole of diversity |
| Threat to biodiversity | Each source's evidence on each pressure | Main pressure, evidence, strongest alternative | Listing every threat without a judgement |
A revision routine for systems and biodiversity questions
Revise these questions by practising the reading, not by rereading notes. A simple weekly routine:
- Diagram reading. Take a system diagram from your textbook, cover its labels, and write one sentence per arrow that names the flow, the storage it leaves, the storage it enters and whether it is a transfer or a transformation.
- Index direction. Work one diversity index calculation from a past or textbook question, and write the interpretation sentence before you look at the answer.
- Threat judgement. Take a short case from your notes or the news, choose the main pressure on its biodiversity, and write the evidence and the strongest alternative.
- Timed practice. At regular intervals, answer a set of past-paper questions under the timings in the current subject guide and check your answers against the markscheme.
After each timed attempt, pick out the few habits that cost you most, such as a missing storage name, a reversed index or an interpretation with no setting, and work on those before the next attempt.
What a strong answer looks like on the page
A strong answer to a systems or biodiversity question is anchored, specific and short. Anchored: every claim points to something in the diagram, table or passage. Specific: it names the storage, the species, the site or the pressure rather than speaking in general. Short: it says what the command term asks for and stops, because extra sentences on the same point add length without adding evidence.
It also stays calm when the material is unfamiliar. An ecosystem you have never studied is still made of storages and flows, and its data still show richness and evenness. Reading in the same order every time (boundary, storages, flows, feedback; then what the data count and which form of the index applies) is what makes an unfamiliar question manageable.
Conclusion and next steps
Systems and biodiversity questions come down to reading the material in front of you: locate the flow between named storages, check which form of a diversity index you are using before you interpret it, and judge the main threat from the evidence rather than listing them all. The vocabulary is what you work with; the answer is what you do with it on the diagram or data you are given.
IB Courses offers one-to-one online ESS SL lessons of 90 minutes, as well as small groups of up to six students. In lessons we practise these three question families on past-paper material and check each answer against the markscheme.