What is the least populous surviving species? By “populous” I mean counting the number of individuals. By “surviving” I mean very likely to survive for the foreseeable future, so not threatened, endangered, extinct, etc. I’m most curious about animals, but plants are fine as well.
So you’re asking how small a population can get before it’s considered threatened?
I can’t name you a specific species, but the concept is “minimum viable population.”
“An MVP of 500 to 1,000 has often been given as an average for terrestrial vertebrates when inbreeding or genetic variability is ignored. When inbreeding effects are included, estimates of MVP for many species are in the thousands.”
There is no way to answer this definitively. Whether a species is considered to be “threatened” under the Red List criteria of the International Union for the Conservation of Nature depends on a complex combination of factors, including population size, historical decreases in population, range size, and threats.
“Threatened” includes the categories of Critically Endangered, Endangered, and Vulnerable.
Any species with a population of fewer than 1,000 mature individuals is automatically considered to be at least Vulnerable and hence threatened. So this would be a minimum cut off to not be considered threatened. In practice, many species with populations of fewer than 10,000 individuals will be considered threatened if they are in decline.
There would be few species with populations of just over 1,000 that would not be considered threatened. It would be a chore looking them up.
Also, I’d like information about the world’s tallest dwarf.
I’m asking what species that isn’t endangered (etc) has the fewest individuals. For example, from random Googling, whited-tailed deer have a population of something like 30 million and are no danger of extinction. Are there any non-threatened species with fewer individuals?
I’m curious what animal (or plant) species are likely to be around for a long time, but are comparatively rare now. So species that have a limited geographic distribution, but common within that area. Or species that have a wide geographic distribution, but an extremely low-density population.
Colibri: yeah, I know it’d be a chore, that’s why I’m asking. 
I’m not expecting a definitive answer, but rather “anecdotal” comments of the sort “oh, how about X. xyz, they’re doing well even though there’s not many of them”.
I’ve found the Wikipedia page: Least-concern species - Wikipedia which does list candidate species for my query. But it’s more fun to get educated guesses here than reading Wikipedia pages.
Well, maybe. But then there’s the Whooping Crane,
15 adults in 1938. Now over 600.
And the Peregrine falcon, once on the edge, now Ok.
VIRGINIA NORTHERN FLYING SQUIRREL: once only 10 individuals, now 1100.
There are very few Wyoming toads:Wyoming toad - Wikipedia
There are 115 of Hawaiian Crows. Hawaiian crow - Wikipedia
Sumatran rhinoceroses-. Fewer than 100 individuals remain.
Javan rhinoceros= 50.
Northern white rhinoceros= 3
I’m not sure what your point is. Those examples vary wildly, and none of them really have anything to do with MVP, which is based on statistical probabilities. Having a population below the MVP doesn’t mean a species will go extinct, just that it is likely non-viable without intensive conservation programs. The Northern White Rhino is far below its MVP, which means it almost certainly would not survive without intensive management (which it is receiving). Peregrine Falcons, even though they were reduced in the past, never hit anything close to their MVP, and have rebounded on their own after pesticides were banned.
Considering climate change, habit destruction, trophy hunting, poaching, etc., I’m guessing a figure damn close to “none.”
George …
The last Pinta tortoise (from the Galapagos Islands) … functionally extinct as he is the only remaining Pinta tortoise alive …
With the exception of human commensals like rats and pigeons, of course.
With these exceptions, it is unfortunately true that probably a majority of vertebrate species are declining in population.
I hate to break it to you, but Lonesome George died over five years ago. 
NOOOOOOOOOOOOOoooooooooooooo…
Was genetic information from George preserved? Is there another tortoise species that’s closely-enough related for its eggs to be used as a surrogate? Is the genetic information in a male, in that species, sufficient to also produce a female? In other words, is it possible to bring the Pinta tortoise back from extinction?
But Pleonast said species that are comparatively rare now, and most human commensals ain’t. But species with limited numbers and/or restricted habitats, like lions and tigers and bears? Screwed.
Perhaps some cave inhabiting species, like the Protea in Slovenia or some fish species in Mexico. I’ve seen recently a documentary about a species of fish living in an aquifer in the U.S., with a population of less than a hundred individuals. Im sure Colibri can tell more about it.
I just found out: “Stygofauna” in Wikipedia.
See the link in my post. There is more information here. Turtles with significant Pinta ancestry have been found on nearby Wolf island and could be used to breed back the species.
Sex determination in turtles usually depends on the temperature the eggs are incubated at rather than sex chromosomes, so eggs can produce either sex.
Well, the first half was pointing out that some species went well below “500 to 1,000” individuals and survived.
The next half was pointing out some species on the edge, which I did not make clear.
As I said, that’s not relevant to MVP. From the link in post 3:
And:
Therefore, the fact that a species currently has a population lower than its MVP but still survives at present is irrelevant. The MVP is a statistical model that predicts the survival of such populations in the wild in the future. The MVP is a tool for conservation planning. It is not a deterministic prediction of which species will survive or not.
I thought it might be something like that. Alternately, even in species where sex is genetic, such things might still be possible: In mammals, for instance, one could take the chromosomes for a male, duplicate the X and throw out the Y, and get the chromosomes for a female. On the other hand, you couldn’t do this with a male bird, because in birds, it’s the females who are heterozygous and the males homozygous.