Extinction

The question asks for the definition of extinction. This is a fundamental biology term related to species disappearance.

Extinction means the permanent disappearance of every individual of a species from Earth. The word 'permanent' is crucial — once extinct, a species cannot return.

The correct definition must include both permanence and every individual being gone. A species with even one living individual is not yet extinct.

The option 'The permanent disappearance of every individual of a species from Earth' matches the scientific definition exactly. This is the correct answer.

We need the precise scientific definition of extinction, not just a rough description of a declining population.

'When a species has fewer than 50 individuals remaining in the wild' describes a critically endangered species, not extinction. The species still exists.

'When a species can only survive in captivity' describes the IUCN category 'Extinct in the Wild' (EW) — not full extinction. The species still has living individuals.

'A temporary decline in population' describes a recoverable population crash. Extinction is permanent, not temporary.

'The permanent disappearance of every individual of a species from Earth' is correct. The two key elements — permanence and every individual — make this the only accurate definition.

The question asks about the background extinction rate — the slow, natural rate at which species go extinct over geological time.

Scientists estimate the background extinction rate at roughly 1–5 species per year globally. This is the baseline against which current extinction rates are compared.

1–5 species per year is the correct estimate for the natural background extinction rate. Today's rate is thought to be hundreds to thousands of times higher than this.

We need the natural (background) extinction rate — how fast species naturally go extinct without human interference.

50–100 species per year is far too high for a natural baseline and would suggest a mass extinction even without human involvement. This is not the background rate.

500–1,000 species per year is dramatically higher and reflects more closely the current crisis rate driven by human activity, not the natural background.

1 species per decade is far too slow — it implies fewer than 0.1 species per year, which underestimates even the natural background rate.

1–5 species per year is the accepted scientific estimate for the natural background extinction rate. This is the correct answer.