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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 population, population density, natality, mortality, immigration, and emigration using IB-aligned ESS terminology.
- Explain the difference between exponential (J-curve) and logistic (S-curve) population growth, including the role of carrying capacity.
- Describe the factors that limit population growth, distinguishing between density-dependent and density-independent limiting factors.
- Analyse population growth curves and demographic data using ESS analysis parameters including scale, uncertainty, and sustainability.
- Explain how predator–prey relationships create oscillating population cycles and feedback loops within ecosystems.
- Describe the demographic transition model (DTM) and its relationship to human population growth.
- Evaluate the ecological and social implications of human population growth, including resource consumption and carrying capacity at a global scale.
- Discuss contrasting worldviews (ecocentric, technocentric, anthropocentric) in relation to human population management and resource use.
Real-World Context & Relevance
In 1950, the global human population stood at approximately 2.5 billion. By November 2022, the United Nations confirmed it had surpassed 8 billion — a more than threefold increase in just seven decades. This unprecedented growth rate is not simply a demographic curiosity; it is one of the most consequential ecological events in Earth's history. The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) Global Assessment (2019) identified the expansion of human land use — driven largely by population growth and rising consumption — as the single greatest driver of biodiversity loss on the planet.
Yet population dynamics are not unique to humans. In 1944, 29 reindeer were introduced to St. Matthew Island in the Bering Sea, a remote and previously uninhabited landmass. With no predators and abundant lichen, the population exploded to approximately 6,000 individuals by 1963. Then, in the winter of 1963–64, the population collapsed catastrophically to just 42 animals. The lichen — the primary food source — had been consumed far beyond its capacity to regenerate. This real-world case encapsulates the core tension at the heart of population dynamics: the relationship between population growth, resource availability, and the limits imposed by the environment.
🌍 Why This Matters Understanding population dynamics is fundamental to managing fisheries, conserving endangered species, planning sustainable cities, and evaluating whether Earth's ecosystems can continue to support human civilisation. Every conservation strategy, every fisheries quota, and every urban planning decision rests on an understanding of how populations grow, stabilise, and collapse.
Core Concepts & Definitions
Before we can understand how populations change over time, we need a precise vocabulary for describing them. A population is not simply a collection of organisms — it is a dynamic system with inputs, outputs, and internal feedbacks. The size of any population at any moment is the result of four interacting processes: births, deaths, immigration, and emigration. Understanding these processes and how they interact with