Common mistakes · ESS · 9 min read
12 IB ESS mistakes that cost marks, and how to fix each one
The errors that turn a solid ESS answer into a mediocre one, each with an answer that loses marks next to one that earns them.
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The most common IB ESS mistakes are vague answers ("pollution is bad"), mixed-up ecology definitions (biomass vs productivity, energy vs matter, positive vs negative feedback) and evaluations that argue only one side. None of them is about not knowing the content; they are about how precisely you write it down, which makes them fixable with practice. Below are twelve, each with an answer that loses marks next to one that earns them.
Mistakes in how you write answers
1. "Pollution is bad" answers
General statements about harm score little because mark schemes reward specific steps. Name the pollutant, its source, the pathway and the effect, in order.
Loses marks"Fertiliser run-off pollutes the river and kills the fish."
Earns marks"Nitrates and phosphates from fertiliser wash into the river and cause an algal bloom. When the algae die, decomposing bacteria use up dissolved oxygen (high biochemical oxygen demand), so oxygen levels fall and fish die."
The second answer has several separate, markable steps; the first has one at best.
2. Case studies with no specifics
"For example, in some countries overfishing caused a collapse" is not an example. An examiner needs a place, a time and ideally a number.
Loses marks"Overfishing has caused fish stocks to collapse in many places."
Earns marks"Canada declared a moratorium on the northern cod fishery off Newfoundland in 1992, after decades of catches above the stock's sustainable yield left too few adult fish to replace the population."
Build a one-line card for each case study (place, date, number, which topic it serves) and review it with spaced repetition; Anki for IB shows how.
3. Environmental value systems described but not applied
Knowing that an ecocentric view puts nature first is AO1. Marks in Paper 2 and in the IA's Strategy criterion come from using the value systems to explain a real disagreement.
Loses marks"Ecocentrists value nature. Technocentrists believe technology can solve problems. Anthropocentrists put humans first."
Earns marks"For a proposed dam that would flood a forested valley, an ecocentric stakeholder would oppose it because the forest has value in itself whatever its use to people; a technocentric stakeholder would support it as a source of low-carbon electricity and argue that fish passes and replanting can limit the damage."
4. Evaluating only one side
"Evaluate", "discuss" and "to what extent" all need strengths and limitations and a conclusion. An answer that only lists advantages has not done what was asked, however many advantages it lists. (See IB command terms explained for the full set.)
Loses marks"A carbon tax is effective because it makes polluting more expensive, encourages renewable energy and raises money for the government."
Earns marks"A carbon tax puts a price on an externality, so firms cut emissions where it is cheapest, and the revenue can fund renewables. However, it can fall hardest on low-income households, and if neighbouring countries do not tax carbon, production may simply move there. It is effective if the rate is high enough and paired with measures that protect poorer households and trade."
Mistakes with ecology concepts
5. Confusing biomass with productivity, and GPP with NPP
Biomass is an amount: the mass of living material present, in g m⁻². Productivity is a rate: how fast new biomass or energy is produced, in g m⁻² yr⁻¹ or kJ m⁻² yr⁻¹. Gross primary productivity (GPP) is the total fixed by photosynthesis; net primary productivity (NPP) is what is left after the producers' own respiration: NPP = GPP − R. The same pattern applies to consumers: gross secondary productivity = food eaten − faecal loss, and net secondary productivity = GSP − R.
Loses marks"Tropical rainforests have the highest productivity because they have the most biomass."
Earns marks"Tropical rainforests have high NPP because high temperatures, high rainfall and strong sunlight all year allow fast photosynthesis. Their large biomass is the result of that productivity accumulating over a long time, not its cause."
A high-biomass system can have modest productivity, and a low-biomass one can be very productive: phytoplankton have little standing biomass at any moment but turn over rapidly.
6. Saying energy is recycled
Energy flows through ecosystems; matter cycles. Energy enters as sunlight, is passed along food chains as chemical energy and leaves as heat from respiration. It is not destroyed and it is not reused. Matter (carbon, nitrogen, phosphorus) is returned to the soil, water and air and used again.
Loses marks"Decomposers recycle the energy in dead leaves back to the plants."
Earns marks"Decomposers release nutrients such as nitrates and phosphates from dead leaves into the soil, where plants take them up again. The chemical energy in the leaves is used by the decomposers and lost to the environment as heat during respiration."
The "10% rule" is a rough average, not a law: transfer efficiencies vary widely between ecosystems and trophic levels. If a question gives figures, calculate the efficiency from them.
7. Mixing up positive and negative feedback
Positive and negative describe what the loop does, not whether it is good. Negative feedback counteracts a change and keeps a system near equilibrium (more prey leads to more predators, which reduces the prey). Positive feedback amplifies a change and moves the system away from where it started, which is how tipping points are crossed.
Loses marks"Melting sea ice is negative feedback because it is bad for the climate."
Earns marks"Melting sea ice is positive feedback: less ice lowers albedo, so the darker ocean absorbs more solar radiation, which raises temperature and melts more ice, amplifying the original warming."
8. Getting sustainable yield wrong
Sustainable yield is the amount of a renewable resource that can be harvested per unit time without reducing the natural capital, in effect the natural increase over that period. Two errors are common: treating it as "as much as you can catch", and assuming maximum sustainable yield (MSY) is harvested from the largest population. In a logistic growth model the population adds the most individuals per year at about half its carrying capacity, so that is where MSY sits.
Loses marks"Fishing at the maximum sustainable yield means the stock can never be overfished."
Earns marks"MSY is the largest catch that could in theory be taken each year indefinitely, from a stock kept near half its carrying capacity. In practice stock estimates are uncertain and recruitment varies from year to year, so a quota set exactly at MSY can still deplete the stock; a precautionary quota is set below it."
9. Mixing up biodiversity terms
Species richness is the number of species. Evenness is how evenly individuals are spread across those species. Species diversity combines both. Habitat diversity is the range of habitats in an area, and genetic diversity is the range of genetic variation within a species or population. More individuals does not mean more biodiversity.
Loses marks"Site B has higher biodiversity because 200 organisms were found there and only 80 at site A."
Earns marks"Both sites have 6 species, so species richness is equal, but site B's Simpson's reciprocal index is lower because one species makes up most of its individuals. Site A's individuals are spread more evenly, so site A has the higher species diversity."
10. Confusing ozone depletion with the greenhouse effect
These are separate problems with different causes. Stratospheric ozone absorbs incoming ultraviolet radiation; ozone-depleting substances such as CFCs release chlorine that breaks ozone down, so more UV-B reaches the surface (skin cancer, cataracts, damage to phytoplankton). The greenhouse effect is about outgoing infrared radiation being absorbed and re-emitted by gases such as CO₂ and methane. The ozone hole does not let heat in. The link that does exist: CFCs are also potent greenhouse gases.
Loses marks"The hole in the ozone layer lets more heat in, which causes global warming."
Earns marks"Ozone depletion increases the UV-B reaching the surface, a risk to health and to producers. Global warming comes from greenhouse gases absorbing infrared radiation emitted by the Earth. They are separate processes, although CFCs contribute to both."
Mistakes with data and the case study
11. Misreading graphs and data
Before you write anything about a graph, check the axes, the units and the scale. Then describe the trend with numbers from the graph, including any change in direction. The classic trap is reading a rate as an amount.
Loses marks"The population growth rate fell from 2.0% to 1.1% per year, so the population decreased."
Earns marks"The growth rate fell from 2.0% to 1.1% per year but stayed positive, so the population kept increasing, only more slowly."
Also watch for logarithmic axes, axes that do not start at zero, and claims that a correlation proves a cause. Two lines rising together are evidence of a link, not proof of one.
12. Ignoring the case-study material in Paper 1
The old course supplied a resource booklet; the new Paper 1 gives you a range of data on one unseen case. Either way, the mistake is the same: answering from general knowledge when the question points you to a figure. If the question says "using the data" or names a figure, it expects you to quote from it, and an answer that doesn't is unlikely to reach full marks.
Loses marks"Tourism damages coral reefs because of pollution and boats."
Earns marks"Figure 2 shows live coral cover fell from 45% to 20% at the two sites nearest the new resort, but changed little at the distant control site. This suggests a local cause such as sewage discharge or anchor damage, rather than regional warming, which would affect all sites."
Annotate every figure before you start answering, and treat each figure as a source of evidence you will need somewhere. The ESS exam format guide has a step-by-step routine for the unseen case study.
How do you get a 7 in ESS?
There is no shortcut, but these habits are what turn knowing the content into marks:
- Answer the command term. Describe gets numbers, explain gets reasons, evaluate gets both sides and a judgement.
- Write separate points. One idea per sentence, with each step of a process stated, because each step can be a mark (see how IB mark schemes work).
- Know a handful of case studies precisely, rather than many vaguely.
- Check definitions word by word. Rate vs amount, energy vs matter, richness vs diversity. Small words carry the marks.
- Practise with mark schemes and log your errors. If the same mistake appears three times, it goes on a list you read before every practice paper. Why you're not improving explains the method.
Doing the IA? Several of these mistakes, especially vague issues and one-sided evaluation, cost marks there too; see the ESS IA guide.
Questions students ask
Do I need to memorise formulas for ESS?
Yes, the ones you use. The ESS guide does not mention a data booklet, so know NPP = GPP − R, gross secondary productivity = food eaten − faecal loss, NSP = GSP − R, Simpson's reciprocal index and the Lincoln index. Ask your teacher what, if anything, is supplied in your exam.
How many case studies do I need for ESS?
There is no official number. A useful target is one or two per topic that you can describe with a place, a date and at least one number, plus, at HL, an example for each of the law, economics and ethics lenses.
Why do I lose marks in ESS when I know the content?
Usually because the answer is accurate but vague: no numbers from the data, no named example, or a one-sided response to an evaluate or to-what-extent question. Mark schemes reward specific, separate points, so precision earns more than length.
Try one: ESS
In a grassland, producers have a gross primary productivity of 25,000 kJ m⁻² yr⁻¹ and lose 15,000 kJ m⁻² yr⁻¹ in respiration. The primary consumers eat 2,000 kJ m⁻² yr⁻¹ of plant material, lose 1,200 kJ m⁻² yr⁻¹ in faeces and 600 kJ m⁻² yr⁻¹ in respiration. What is the net secondary productivity of the primary consumers?
- 200 kJ m⁻² yr⁻¹
- 800 kJ m⁻² yr⁻¹
- 1,400 kJ m⁻² yr⁻¹
- 10,000 kJ m⁻² yr⁻¹
Show the answer and why
Answer: A
Gross secondary productivity is food eaten minus faecal loss: 2,000 − 1,200 = 800 kJ m⁻² yr⁻¹. Net secondary productivity is GSP minus respiration: 800 − 600 = 200 kJ m⁻² yr⁻¹. Option B stops at GSP and forgets respiration. Option C subtracts respiration but forgets the faeces, which were never absorbed. Option D is the producers' NPP (25,000 − 15,000), the right formula at the wrong trophic level.