[lutusp]

> ... but it seems curious that this event wouldn't have affected all living creatures universally.

That's easy to explain based on the fact that some species tolerate temperature change better than others. Remember that the Toba event took place just as an Ice Age was beginning, so there were multiple stressors, affecting many species. The survivors were those that (a) could take shelter against the sudden cold, and/or (b) were naturally more resistant to the cold.

A similar partition happened during the asteroid-initiated dinosaur extinction event 65 million years ago -- some species were better able to deal with the sudden cooling of the global climate. Like the little mammals, who could take shelter underground, and from which we eventually sprang.

> Would plants and aquatic species expess such a bottleneck differently ...

Yes. Some of them are able to tolerate freezing conditions that would kill most land animals. As just one example, there are species of frog that can freeze solid, then recover when things thaw out.

https://www.units.muohio.edu/cryolab/projects/woodfrogfreezi...