IB DP Environmental Systems and Societies Topic 2: Ecology

This guide covers Topic 2 – Ecology, for IB Diploma Programme Environmental Systems and Societies, first assessment 2026. 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, reflecting how central it is to the course.

Where this fits in the syllabus

Topic 2 is where ESS’s three foundational concepts from Topic 1 — perspectives, systems and sustainability — get their first serious application to real ecological content. 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. Studying 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 — the Topic 1 Foundations study guide should be treated as a genuine prerequisite, not optional background.

Core ecology content (Marlbridge outline)

The public subject brief this guide is sourced from publishes only Topic 2’s name and teaching hours; as the full syllabus guide notes, the subtopic breakdown for Topics 2-8 sits in the IB’s subject guide, not in the brief. The headings below are therefore Marlbridge’s own outline of core ecology content, not the IB’s itemised subtopic list – check your school’s copy of the subject guide for the official subtopics.

• Ecosystem structure — trophic levels, food chains and food webs, and how energy is transferred (and lost) between successive trophic levels
• 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, illustrated through named cycles such as 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

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.

How to approach it

The single most important distinction to hold precisely is that energy flows one way through an ecosystem and is ultimately lost as heat, while nutrients cycle and are reused — treating these as the same kind of process is one of the most common conceptual errors in this topic. Build every explanation of ecosystem disturbance around tracing indirect, cascading consequences rather than stopping at the first, most obvious effect: a systems-based ESS answer is explicitly expected to show how a change at one trophic level produces effects elsewhere in the system that were not the direct target of the original disturbance. Sustainability, Topic 1’s third concept, applies directly here too — human activity that disrupts nutrient cycling or removes a 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, so a strong answer connects ecological disruption explicitly to sustainability rather than describing ecosystem damage as an isolated scientific fact. HL students specifically should practise the quantitative skills (ecological efficiency calculations) rather than assuming the extra HL hours are simply more descriptive content, since Paper 1’s data-response questions can require calculation as well as explanation.

Worked example: tracing a trophic cascade

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, applied here

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 own
                   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 full cascade of effects, not just the single direct effect, is what distinguishes a strong ESS answer from a purely descriptive one.

Common mistakes

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 interchangeable processes, when they are fundamentally different. 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 expects to be revisited throughout the course. For HL students, treating the additional teaching hours as purely descriptive and under-practising the quantitative energy-flow calculations that can appear in Paper 1.

Quick revision checklist

• Keep energy flow (one-directional, lost as heat) and nutrient cycling (reused, cyclical) precisely distinct.
• Practise tracing a full trophic cascade from a single disturbance, not just its first, direct effect.
• Explicitly connect ecosystem disruption to sustainability in evaluative answers.
• Revisit Topic 1’s systems vocabulary (storages, transfers, feedback) when writing about Topic 2 content.
• HL: practise ecological efficiency calculations specifically, not only descriptive content.

Official syllabus

International Baccalaureate Organization, Diploma Programme Subject Brief – Sciences: Environmental systems and societies, first assessment 2026 – the same source already cited by the full syllabus guide, which allocates Topic 2 its 22 SL / 35 HL teaching hours. Verified 2026-09-06.