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IB DP ESS — Topic 2: Ecology | SL / HL | Paper 1 & Paper 2A
🎯 Learning Intentions By the end of this note, you will be able to:
- Define ecological pyramids, biomass, and bioaccumulation using IB-aligned ESS terminology.
- Explain why energy, numbers, and biomass decrease across successive trophic levels, and the ecological consequences of this pattern.
- Describe the structure and limitations of pyramids of numbers, biomass, and energy in representing trophic relationships.
- Distinguish between bioaccumulation and biomagnification, and explain how each process operates within food chains and food webs.
- Analyse how persistent organic pollutants (POPs) such as DDT and methylmercury move through trophic levels using real-world data and named case studies.
- Evaluate the ecological and human health implications of biomagnification at different trophic levels, with reference to scale, trade-offs, and multiple perspectives.
- Discuss the limitations of ecological pyramids as models of ecosystem structure and function.
Real-World Context & Relevance
In the 1950s and 1960s, populations of bald eagles ( Haliaeetus leucocephalus ) across North America collapsed dramatically. Breeding pairs in the contiguous United States fell from an estimated 100,000 in the pre-colonial era to fewer than 417 nesting pairs by 1963 (US Fish & Wildlife Service). The culprit was not hunting or habitat loss alone — it was a colourless, nearly odourless insecticide called DDT (dichlorodiphenyltrichloroethane), which had been sprayed across vast agricultural landscapes to control mosquitoes and crop pests. Eagles were not being sprayed directly. Yet the chemical was reaching them in lethal concentrations through the food chain, causing eggshell thinning so severe that adults crushed their own eggs during incubation.
This phenomenon — the progressive concentration of a chemical as it moves up through trophic levels — is called biomagnification, and it is one of the most striking demonstrations of why ecological pyramids matter. Understanding how energy, numbers, and biomass are distributed across trophic levels is not merely an academic exercise. It reveals why top predators are disproportionately vulnerable to environmental contamination, why fisheries collapse when apex species are removed, and why human societies — positioned at or near the top of many food chains — face unique exposure risks from industrial pollutants. The story of DDT and the bald eagle is a systems story: a chemical introduced at one level of an ecosystem ripples upward with amplifying consequences.
🌍 Why This Matters Ecological pyramids and biomagnification are not abstract models — they explain why top predators like eagles, orcas, and polar bears accumulate the highest concentrations of industrial pollutants, why human consumption of large predatory fish (tuna, swordfish) carries health advisories, and why the removal of a single trophic level can destabilise entire ecosystems. These concepts underpin fisheries management, toxicology policy, and international agreements such as the Stockholm Convention on Persistent Organic Pollutants (2001).
Core Concepts & Definitions
Before exploring what ecological pyramids reveal about ecosystems, it is worth asking a fundamental question: why does energy diminish as it moves from one organism to another? When a zebra eats grass, it does not convert all of the grass's chemical energy into its own body mass. Much is lost as heat during cellular respiration, some is used for movement and reproduction, and some passes through the digestive system unused. This inefficiency — typically around