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Species Diversity and Its Measurement
IB DP ESS — Topic 3: Biodiversity and Conservation | SL / HL | Paper 1 & Paper 2A/2B
🎯 Learning Intentions By the end of this note, you will be able to:
- Define species diversity, species richness, and species evenness using IB-aligned ESS terminology.
- Distinguish between species richness and species evenness as components of species diversity.
- Explain how the Simpson's Diversity Index (D) is calculated and what its value indicates about an ecosystem's health.
- Describe the role of species diversity in maintaining ecosystem stability and resilience at local, regional, and global scales.
- Analyse data on species diversity using ESS analysis parameters including scale, reliability, uncertainty, and sustainability.
- Evaluate the strengths and limitations of different methods used to measure and monitor species diversity in the field.
- Discuss contrasting worldviews (ecocentric, anthropocentric, technocentric) in relation to the value of species diversity and conservation priorities.
Real-World Context & Relevance
In 2019, the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) released its landmark Global Assessment Report, warning that approximately one million animal and plant species now face extinction — many within decades. This figure, drawn from analysis of over 15,000 scientific sources, represents the most severe biodiversity crisis in human history. Yet the crisis is not uniform: some ecosystems, such as the tropical rainforests of the Congo Basin and the coral reefs of the Coral Triangle, still harbour extraordinary concentrations of species, while others — intensively farmed monocultures, urban wastelands, and degraded wetlands — support only a fraction of their former biological richness.
What makes this crisis particularly urgent for environmental scientists is that the loss of species is not simply an aesthetic or ethical concern — it has measurable consequences for the functioning of ecosystems on which human societies depend. When the Atlantic cod ( Gadus morhua ) population collapsed off the Grand Banks of Newfoundland in the early 1990s following decades of industrial overfishing, it was not just a single species that disappeared from the ecosystem. The entire food web was restructured: prey species such as shrimp and crab exploded in abundance, while predators that depended on cod declined. The collapse demonstrated, at a real-world scale, that species diversity is not merely a count of what lives in a place — it is a measure of ecological complexity, resilience, and function.
🌍 Why This Matters Understanding how to measure and interpret species diversity gives environmental scientists the tools to detect ecosystem degradation before it becomes irreversible, to prioritise conservation resources, and to evaluate the effectiveness of restoration efforts. Without standardised, quantitative measures of diversity, conservation policy would be based on intuition rather than evidence.
Core Concepts & Definitions
Before we can measure biodiversity, we need to understand what we are actually measuring. When