Variola Major?
I kid…though this does raise some interesting questions on how you define “surviving,” “species,” and “populous.” I imagine that even a physically small sample of smallpox could well outnumber anything else in the running, if you (somehow) counted individual viruses. And if you actually counted them as “organisms” in the first place!
Updating this thread:
Unfortunately the number is now down to two as the last male died:
White Rhin…oh.
I’d think species that occupy very obscure and uncommon niche habitats which are also far away from human influences would match the criteria… maybe something like bacteria or invertebrates that can only live around deep water hydrothermal vents for example.
I’d think those types of habitat and the numbers of individuals of a given species to be comparatively rare… yet on the other hand they’ve been around for hundreds of thousands of years or more before we even discovered them so they seem to be self-sustaining. Hopefully we don’t visit these areas very often or destroy them with our activities so they’d be likely to persist.
I think the types of species in question would have to occupy places where humans seldom go either due to the hostile environment or the sheer vast expanse, so probably somewhere in or under the oceans. I think pretty much any land species and a lot of pelagic aquatic species are vulnerable to either direct impacts or indirect habitat impacts related to human activity.
The Lord Howe Island Phasmid is sometimes called the rarest insect in the world. It disappeared from Lord Howe Island, off the Australian coast in the Pacific due to human pressure, but a population survived on a volcanic spike called Ball’s Pyramid miles from there. Ball’s Pyramid’s biodiversity consisted of 1 stunted melaleuca bush and 24 phasmids when re-discovered.
Its being bred up well, but I suspect it was on the wrong side of the sustainability line to have survived long-term. It would only need one errant cyclone …
But then perhaps five years later Peru may end up suffering its first ever tree lobster plague, when countless phasmids begin to over-run South America.
Note that is a subspecies, not a full species. Still tragic.
Alas, down to only 2 now. And both female.
The only living male was euthanized yesterday (due to old age & incurable, painful disease). Leaves behind just 2 females, a mother & daughter pair. They did save genetic material (semen?) from the male, that may be used to breed more. But the surviving females are his daughter & grand-daughter, so this would be severe inbreeding.