IB DP Environmental Systems and Societies Topic 2: Ecology – Revision Notes

The full syllabus guide allocates Topic 2 more teaching hours (22 SL / 35 HL) than any other topic – it is the largest topic in the syllabus at both levels. These notes cover Topic 2 – Ecology, alongside the Topic 1 Foundations revision notes already on the site.

Why Topic 1 must come first

Topic 2 is where ESS’s three foundational concepts – perspectives, systems and sustainability – get their first serious application. An ecosystem is explicitly treated as a system: energy and matter flow through it, and disturbing one part has consequences that propagate through the whole, via positive and negative feedback. Revising Topic 2 without a secure grip on Topic 1’s systems vocabulary (inputs, outputs, storages, transfers, feedback) produces answers that describe ecological facts without the conceptual framing the specification expects.

Core content to hold securely

The public subject brief gives Topic 2’s name and hours but not its subtopic list, so the four headings below are Marlbridge’s outline of core ecology content, not the IB’s itemised subtopics.

• Ecosystem structure – trophic levels, food chains and food webs, and how energy is transferred (and lost) between them.
• Energy flow – the key quantitative idea in ecology: energy transfer between trophic levels is inefficient, with most energy lost as heat at each stage, which is why food chains rarely extend beyond four or five trophic levels.
• Nutrient cycling – how matter, unlike energy, cycles rather than flows in one direction through an ecosystem (for example, the carbon or nitrogen cycle).
• Ecosystem change – succession, and how ecosystems respond to and recover from disturbance, connecting directly back to Topic 1’s systems concept of resilience.

Worked example: applying “systems thinking” to energy flow

A question asks why removing a top predator can affect an entire ecosystem, not just the predator’s immediate prey.

Systems concept:   an ecosystem is an open system with interconnected
                    storages (trophic levels) and transfers (energy
                    flow) -- Topic 1's foundational idea
Direct effect:      the predator's prey population may increase
                    without the predator controlling its numbers
Indirect effect:    the prey species may then over-consume ITS food
                    source, depleting a resource further down the food
                    chain -- a "trophic cascade" that propagates
                    through the whole system
Conclusion:         because the ecosystem is a system of connected
                    parts, a change at one trophic level produces
                    effects that were not the direct target of the
                    original disturbance

Explicitly using the word “system” and identifying the cascade of effects – not just the single direct effect – is what distinguishes a strong ESS answer from a purely descriptive one.

Sustainability and Topic 2

Topic 1’s third concept, sustainability, applies directly here: human activity that disrupts nutrient cycling or removes species from a food web can undermine an ecosystem’s long-term capacity to support both itself and the human communities that depend on it. A strong Topic 2 answer connects ecological disruption explicitly to sustainability, rather than describing ecosystem damage as an isolated scientific fact disconnected from the course’s wider evaluative framework.

HL depth beyond the SL core

HL students study Topic 2 for thirteen additional hours beyond SL (22 to 35 hours) – one of the largest HL uplifts of any topic in the syllabus – typically extending into more detailed quantitative treatment of energy flow (such as calculating ecological efficiency between trophic levels) and more complex food web scenarios involving multiple interacting cascades. HL students should not assume the extra content is simply “more of the same” — practise the quantitative skills specifically, since Paper 1’s data-response questions can require calculation, not just description.

Exam traps

• Describing a food chain or food web without connecting it to the energy-flow concept of inefficient transfer between trophic levels.
• Treating energy flow and nutrient cycling as the same process, when they are fundamentally different: energy flows one way and is ultimately lost as heat; nutrients cycle and are reused.
• Explaining an ecosystem disturbance’s direct effect only, without tracing the indirect, cascading consequences a systems-based answer is expected to identify.
• Forgetting to link Topic 2 content back to Topic 1’s vocabulary (perspectives, systems, sustainability), which the specification says should be “revisited throughout the course.”

Linking to the individual investigation

Because the internally assessed individual investigation draws on the experimental programme’s fieldwork skills, Topic 2’s content (food webs, energy flow, nutrient cycling) is a common and practical source of investigable questions — for example, comparing species diversity or biomass across two contrasting habitats. Where an investigation draws on ecology content, ground the data collection and analysis explicitly in this topic’s vocabulary (trophic level, energy transfer efficiency) to demonstrate genuine syllabus-linked understanding rather than a generic fieldwork exercise.

Self-test

1. Why do food chains rarely extend beyond four or five trophic levels?
2. What is the key difference between how energy and nutrients move through an ecosystem?
3. What is a trophic cascade?
4. Which Topic 1 concept is most directly applied in Topic 2’s content?
5. Why is describing only the direct effect of an ecosystem disturbance an incomplete ESS answer?

Answers: 1. Because energy transfer between trophic levels is inefficient, with most energy lost as heat at each stage, leaving too little energy to support further trophic levels beyond four or five. 2. Energy flows through an ecosystem in one direction and is ultimately lost as heat; nutrients cycle and are reused repeatedly within the system. 3. A chain of indirect effects that spreads through an ecosystem’s trophic levels after a disturbance to one part of it, such as the removal of a top predator. 4. Systems — an ecosystem is explicitly treated as a system with interconnected storages and transfers, subject to feedback. 5. Because a systems-based answer is expected to trace the indirect, cascading consequences of a disturbance, not stop at the first, most obvious effect.