Biodiversity is measured by richness (how many species) and evenness (how balanced). Habitat loss, invasive species, pollution and overharvesting reduce it; conservation solutions are judged by evidence and trade-offs.
Biodiversity is the variety of life: of genes, of species and of ecosystems. It connects to keystone species and to how ecosystems recover from disturbances. To compare places, ecologists need to measure it.
🔑 Key idea
Species diversity has two parts. Richness is how many species there are. Evenness is how equal their numbers are. Simpson's diversity index combines both: $$D = 1 - \sum p_i^2$$ where $p_i$ is the share of individuals belonging to species i. D runs from 0 (one species) toward 1 (many species in even numbers).
💡 Why it works
$\sum p_i^2$ is the chance that two individuals picked at random belong to the same species. If one species makes up nearly everything, that chance is high and D is low. If many species share the community evenly, two random picks rarely match, so D is high.
✏️ Worked example: Two forests
Forest 1 has 4 tree species, 25 trees each. Forest 2 has the same 4 species: 85, 5, 5 and 5 trees. Compare their Simpson indexes.
Answer: Same richness, but Forest 1 is far more diverse (D = 0.75 against 0.27).
Same four species, very different evenness.
The main threats to biodiversity are well established: habitat loss (the largest), invasive species, pollution, overharvesting and climate change. Solutions involve trade-offs that communities weigh differently, such as protecting land versus using it for farms, homes or jobs, and the costs of controlling invasive species. Scientists contribute evidence on what each option is likely to do; choosing among them is a decision for the people affected.
⚠️ Common mistake
It is natural to count only the number of species. Two forests with ten species each are not equally diverse if one is 91% a single species. Check evenness too: a high Simpson index needs both many species and fairly equal numbers.
You can record what lives near you, a good first step in measuring biodiversity, with Nature Study.
Practice
Pass with 3 right answers, the last 2 in a row. Each right answer earns 2 XP.