Species–Area Calculator
The species–area relationship: how species richness scales with the area of habitat, and how many species are expected to be lost when habitat shrinks — the arithmetic behind extinction estimates from deforestation.
Across islands, reserves and habitat patches, the number of species rises with area as a power law, S = c × A^z, with z typically 0.
How the species–area calculator works
Across islands, reserves and habitat patches, the number of species rises with area as a power law, S = c × A^z, with z typically 0.2–0.35 (0.25 is the classic value). Because of the exponent, halving the area does not halve the species: it removes about 16% of them at z = 0.25, and a 90% loss of habitat removes about 44%. Enter the species and area you know and the area you are asking about.
Formula: S₂ = S₁ × (A₂ ÷ A₁)^z; fraction retained = (A₂ ÷ A₁)^z
Worked examples
| Inputs | Species expected | Note |
|---|---|---|
| 200 species in 1,000 → 100 | 112 | 112 species — 44% lost |
| Halving the area | 168 | 168 — 16% lost |
| A larger island | 112 | 112 species |
FAQFrequently asked questions
Where does the power law come from?
Bigger areas hold more habitats and larger populations that persist through bad years; the relationship was first fitted to island floras a century ago and holds across scales from moss patches to continents.
What z should I use?
About 0.25 for islands and isolated reserves; 0.12–0.2 for patches within a continuous landscape, where species can move between them; 0.3–0.4 for remote oceanic islands. The choice matters: a 90% habitat loss removes 44% of species at z = 0.25 but only 29% at 0.15.
Is the loss immediate?
No — the calculation gives the number the shrunken area can support in the long run. Species above that line survive for a while as an “extinction debt” before dwindling, which is why losses lag deforestation by decades.
Does it work for planning reserves?
It is the basis of the SLOSS debate — single large or several small — and of targets like protecting 30% of habitat. One large reserve holds more species than several small ones of the same total area if the small ones share species.
Where these figures come from
- NCBI Bookshelf — Molecular Biology of the Cell — growth kinetics and molecular conventions
- Wellcome Sanger Institute — a leading UK genomics institute
Last checked: September 2026. Formulas are the standard textbook forms; assumptions are stated on each page because they are where these models break.