Why can a plankton species die out after a food shortage leaves only a few survivors, even once food returns?

Answer: Too few are left: the scattered survivors cannot breed fast enough to outpace ordinary chance losses

Too few are left: the scattered survivors cannot breed fast enough to outpace ordinary chance losses ✓ — A population that has fallen to a few scattered individuals can keep shrinking even in good conditions. Species that reproduce sexually struggle to find mates or to get eggs fertilized when partners are rare, the small group loses genetic variety, and a run of ordinary bad luck, such as a few individuals drifting into the wrong water, can remove the last of them. Ecologists call this an extinction threshold: below a certain density the population cannot replace its losses, so the species disappears without any single final blow.

The old food shortage returns and kills the weakened survivors — A second shortage would certainly hurt, so this is a reasonable guess. But the question is about a species that fails after food has come back and stayed. In that case the cause is not a returning famine but the survivors' own scarcity: the population is too thin to rebuild, however good the conditions are.

Predators hunt down the last survivors one by one — Predation happens, and the last few can be eaten. But a species can vanish even where nothing hunts it, because a handful of scattered survivors reproduce too slowly to replace routine losses. The failure is one of numbers and chance, not of a targeted hunt.