After K-Pg, some plankton populations crashed without bouncing back. Why can a species go extinct even with some survivors still alive?

Answer: Too few left to recover

Too few left to recover ✓ — Right — sparse populations face an extra danger called an extinction threshold. Below a critical density, finding mates, maintaining genetic diversity, and resisting random die-offs all fail. K-Pg recovery models needed this concept because pure 'low biomass' models predict bounce-back; reality showed permanent loss for many species despite leftover individuals.

Light came back too slowly — Light returning slowly was a real K-Pg condition, but it doesn't explain why some species never recovered AFTER light returned. Permanent extinction needs a mechanism that bites even in good times — and sparse populations stay vulnerable long after the original cause is gone.

Predators ate every survivor — Predators eating every survivor is rare in plankton dynamics. The food chain collapsed broadly — most predators were starving too. What actually kills small surviving populations is demographic randomness, not targeted hunting; the extinction is statistical, not by tooth.