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IB DP ESS — Topic 2: Ecology (HL Extension) | HL | Paper 1 & Paper 2A
🎯 Learning Intentions By the end of this note, you will be able to:
- Define cladistics, clade, and cladogram using IB-aligned ESS terminology, and distinguish these from traditional Linnaean classification.
- Explain how cladograms are constructed using shared derived characteristics and how they represent evolutionary relationships among species.
- Distinguish between the fundamental niche and the realized niche of a species, and explain the ecological mechanisms that cause them to differ.
- Analyse how interspecific competition, predation, and environmental gradients compress a species' realized niche relative to its fundamental niche.
- Describe the key features of r-selected and K-selected life history strategies, including reproductive rate, parental investment, and population dynamics.
- Evaluate the ecological significance of life history trade-offs in the context of population stability, extinction risk, and conservation management.
- Discuss how cladistics, niche theory, and life history strategies together inform our understanding of biodiversity, species interactions, and ecosystem function.
Real-World Context & Relevance
In 2019, the IPBES Global Assessment reported that approximately one million species face extinction, many within decades — a rate estimated to be tens to hundreds of times higher than the natural background rate (IPBES, "Global Assessment Report on Biodiversity and Ecosystem Services," 2019). To understand why certain species are more vulnerable than others, ecologists must look beyond simple species counts and examine the evolutionary relationships between organisms, the ecological space they occupy, and the reproductive strategies they employ. These three dimensions — phylogenetic history, niche, and life history — are not independent: they are deeply intertwined threads in the fabric of biodiversity.
Consider the African cheetah ( Acinonyx jubatus ), which has a fundamental niche spanning vast open savanna ecosystems but a realized niche compressed by competition with lions and hyenas into marginal habitats and off-peak hunting times. Its K-selected life history — slow reproduction, small litter sizes, high parental investment — means that population recovery after decline is painfully slow. Meanwhile, cladistic analysis reveals that the cheetah's closest relatives are not other large African cats but the puma and jaguarundi of the Americas, a phylogenetic insight that has reshaped conservation genetics. Understanding these three ecological frameworks together gives conservation biologists the tools to predict which species are most at risk and why.
🌍 Why This Matters Cladistics reveals evolutionary relationships that inform conservation prioritisation; niche theory explains how species coexist and what happens when that coexistence is disrupted by invasive species or habitat change; and life history strategies determine how quickly populations can recover from disturbance. Together, these frameworks are foundational to predicting and managing biodiversity loss in a rapidly changing world.
Core Concepts & Definitions
For most of the history of biology, organisms were classified by overall physical similarity — a system that grouped dolphins with fish and bats with birds. But physical similarity can be deeply misleading: dolphins and fish share a streamlined body shape not because they share a recent common ancestor, but because they independently evolved similar solutions to moving through water. This phenomenon, called convergent evolution, reveals a fundamental problem with classification based on appearance alone. In the twentieth century, the German entomologist Willi Hennig proposed a radically different approach: classify organisms exclusively by their shared evolutionary history, using only characteristics inherited from a common ancestor.
📖 Key Definition — Cladistics Cladistics is a method of biological classification that groups organisms based on shared derived characteristics (synapomorphies) inherited from a common ancestor. A clade is a group consisting of an ancestor and all of its descendants. A cladogram is a branching diagram that represents the hypothesised