How do we know that a species has definitely gone extinct (as opposed to just being extremely rare/elusive)?
In: Biology
We don’t know for sure. The Coelacanth is a fish that was declared extinct 66 million years ago but was rediscovered in 1938. Local fisherman knew of the fish earlier but didn’t know it was “extinct”.
“A species is extinct when there is no reasonable doubt that the last remaining individual of that species has died.”
[Source](https://www.nationalgeographic.org/encyclopedia/endangered-species/)
The short answer is that we don’t. Consider the [Coelacanth](https://en.wikipedia.org/wiki/Coelacanth).
> Coelacanths were thought to have become extinct in the Late Cretaceous, around 66 million years ago, but were rediscovered in 1938 off the coast of South Africa.
Capture/recapture experiments can help. Say you capture 10 rhinos and spraypaint a dot on them. Release them, come back a year later and capture 10 rhinos again. Only 1 of them has spraypaint, which means that you capture 10% of the population, and the population is 100.
You can use that tool and some other fancy predictive modeling to track a population over time and be able to accurately estimate the population even when it gets very small.
There is never any way to know for sure, though.
There are a few others like Leopard that are in danger from lack of genetic diversity.
And I remember there are several others that are at a dead end from not adapted to habitat which changed faster than they could adapt.
Quick shout out to ‘Extinct or Alive’, a show on Animal Planet that shows this exact question and in my opinion is really well done.
Basically, when we don’t see any members of the species for a while. They could be extinct, or they could be good at hiding. The Zanzibar leopard is an example that is or was thought to be extinct, but there is some evidence to suggest they may still be alive and just really good at living around humans without being seen.
I know that when you’re investigating how many different species there are in an enviroment, you can use [rarefaction](https://en.m.wikipedia.org/wiki/Rarefaction_(ecology)) in order to determine whether or not you have sampled enough of the enviroment to get an accurate look of that area’s richness. I’m not an expert, but I would imagine that a similar thing can be done with a specific species in order to tell how likely it is to be extinct
As everyone else has said, we don’t. One of the poster child cases for captive breeding programs is the [black footed ferret](https://en.m.wikipedia.org/wiki/Black-footed_ferret) which was thought to be extinct until a population of 18 was discovered. Now the population is over 1000 in multiple locations to help protect them from complete extinction.
More recently, [the widely thought extinct Vietnamese mouse deer](https://www.cnn.com/2019/11/11/asia/mouse-deer-vietnam-chevrotain-rediscovered-scn/index.html) was captured on game cameras by biologists after having not seen it for 30 years.
Once the numbers in viable remaining habitat fall below the minimum needed for genetic diversity to reproduce to a sustainable population is basically extinct.
Even if a few breeding pairs remain, the inbreeding will eventually make them sterile,…or vulnerable to disease, or predators.
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