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IB DP ESS — Topic 2: Ecology | SL & HL | Paper 1 & Paper 2A/2B
🎯 Learning Intentions By the end of this note, you will be able to:
- Define the terms habitat, population, community, ecosystem, and biome using IB-aligned ESS terminology.
- Distinguish between abiotic and biotic components of an ecosystem and explain how they interact.
- Explain how energy flows and matter cycles through ecosystems, including the roles of producers, consumers, and decomposers.
- Describe the processes of primary and secondary ecological succession and the concept of a climax community.
- Analyse how changes in abiotic factors affect community structure and species distribution at local and global scales.
- Evaluate the role of keystone species and trophic cascades in maintaining ecosystem stability.
- Discuss contrasting worldviews (ecocentric, technocentric, anthropocentric) in relation to habitat conservation and ecosystem management.
- Construct food webs and systems diagrams to represent energy flow and ecological relationships within a named ecosystem.
Real-World Context & Relevance
In 1995, wolves were reintroduced to Yellowstone National Park in the United States after a 70-year absence. What followed astonished ecologists: not only did elk populations change in behaviour and distribution, but riverbanks stabilised, vegetation recovered, and even the physical course of rivers shifted. This phenomenon — a trophic cascade — demonstrated that a single species could restructure an entire ecosystem. The wolves had not merely altered the food web; they had changed the physical landscape itself.
This example illustrates a fundamental truth about ecosystems: they are not simply collections of species living in the same place. They are dynamic, interconnected systems in which every component — living and non-living — influences every other. The 2019 IPBES Global Assessment reported that approximately one million species are currently threatened with extinction, with habitat loss identified as the primary driver. Understanding how communities, habitats, and ecosystems are structured and how they function is therefore not merely an academic exercise — it is the foundation of all conservation, land management, and sustainability decision-making.
🌍 Why This Matters Ecosystems provide services — clean water, food, climate regulation, pollination, and cultural value — that underpin human civilisation. When community structure collapses through habitat loss, invasive species, or overexploitation, these services degrade. Understanding ecological organisation from habitat to biome is essential for designing effective conservation strategies and evaluating sustainability policies at every scale.
Core Concepts & Definitions
To understand how ecosystems function, we must first establish the vocabulary of ecological organisation. Ecologists work across multiple scales — from the microhabitat of a single rock pool to the vast expanse of the boreal forest biome. Each level of organisation has distinct properties that cannot be predicted simply by examining the level below it. This property — known as emergence — is central to systems thinking in ESS.
Habitat, Population, and Community
Consider a coral reef in the Indo-Pacific. Individual clownfish live among the tentacles of sea anemones — this specific physical location, with its particular set of abiotic conditions (temperature, salinity, light penetration, substrate), is the clownfish's habitat. All the clownfish of the same species living in that reef at a given time constitute a