what are the criteria for scientist to consider a species extinct? maybe they just didnt search enough, like given the vastness of earth there r probably areas still unexplored?
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They don’t. They assume it is, based on calculation on how often we should be seeing them if they were not extinct. Not sure of the exact threshold, but very often it is more of a “99.99% chance this is extinct” than “yep, that was the last one”. That’s why sometimes we actually can find animals that were previously thought extinct.
We don’t, always. And we are occasionally wrong about it. The [coelacanth](https://en.wikipedia.org/wiki/Coelacanth) is a famous example: it was long believed to be a member of a group that had been extinct since the time of the dinosaurs, but turns out to still have some good-sized populations off the coast of southeast Africa (which, at the time, was a very remote place to go study).
But in practice, a species that is still common enough to maintain a breeding population is usually common enough to find if you’re really really looking for it, and extinction usually isn’t declared unless there have been no observations for a while (a decade or more). The only time we know the moment of extinction is if the animal is known to be otherwise extinct but a (usually captive) [endling](https://en.wikipedia.org/wiki/Endling) is known.
A species is declared extinct in the wild after it isn’t observed on numerous surveys over a long period of time.
There is no exact criteria because some animals are certainly more elusive than others and more difficult to survey. It’s much easier to declare the Mastodon extinct than it would be to declare an obscure species of stick bug extinct.
One is a megafauna giant that dominates the landscape and can be easily recorded from an aerial survey, and the other is a camouflage master that can elude a survey repeatedly.
Sometimes “extinct” species do turn up in another survey, moving them back to “critically endangered” status. Again, usually a small and highly elusive animal that’s hard to track even when they’re doing well.
There is no one way to definitively prove that a species is extinct, but there are several indicators that can suggest that a species is no longer present. For example, if a species has not been seen or detected in its natural habitat for a prolonged period of time, and efforts to locate it have been unsuccessful, it is considered to be possibly extinct. Similarly, if the habitat of a species has been significantly altered or destroyed, and the species is unable to adapt, it may also be considered possibly extinct. In some cases, a species may be declared extinct if there is no reasonable doubt that it is no longer present. This can be a difficult determination to make, and it is often based on a combination of factors, including the length of time the species has been missing, the amount of effort that has been made to locate it, and the availability of suitable habitat.
We have biologists that go out to the places where animal live and count how many of each kind they see. They do it in defined areas, and try to be as precise as possible. How you do it depends on whether you’re counting tiny things like bugs, or big things like giraffes, but there’s ways to get counts.
We watch the counts and compare them over months and years. If the number steadily decreases, and we don’t notice the animals moving to a new habitat, we pay special attention to counting them. We say that they are “threatened”. When the numbers have gone down a lot, we say that they are “endangered”. Eventually, we might not be able to find any more. It’s possible that there’s just a few, and they’re hidden and can’t be counted, but if after looking for them for years you don’t find any, you say “they’re extinct” (or, if there’s some in zoos, “extinct in the wild”).
We’re currently in the throes of a major extinction event called the “Holocene Extinction” or “Anthropocene Extinction”. Because of habitat destruction an pollution, the numbers of animals in the wild has been decreasing every year. At the moment, the number of animals (not species) is currently decreasing by about 50% every 40 years or so, about 1,000x the historical rate, which is why this is considered a major extinction event. Yet, we don’t have a real good handle on how many species go extinct each year as we can’t count them all, and we’re not fully aware of them all; we mostly notice when something big goes extinct, or a population suddenly crashes.
Scientists generally know where a species normally lived, what it fed on, what its habits were, and so on.
So while it can never be 100% conclusively proven a species is extinct, scientists can make a pretty good assessment if some or all of the following are met:
* There’s no evidence of individuals of the species, despite extensive searching in its known habitat and related areas throughout the world. No bones, no droppings, no confirmed sightings.
* There is no evidence of the actions or behaviour of a species. e.g. suppose the species was known to scratch trees in a certain specific way, or have a very distinctive mating call, and there hasn’t been any evidence of any of these things occurring in many years.
* It’s known that the species had a limited habitat, and this has been completely destroyed by human or natural actions.
* It’s known that the species had a limited food source, which is also strongly suspected to be extinct.
* It’s known that the species was actively hunted by humans or introduced predators.
This isn’t to say that it’s *completely impossible* that there may some hidden individuals of the species out there somewhere. Maybe the moved to a new habitat or food source out of desperation. Maybe a few got lucky and found a small inaccessible area. It’s possible.
But what scientists say is, in effect, “Based on everything we knew about these creatures, where they lived, and how they behaved, we find no evidence of them existing any more”
It’s an assumption. If there hasn’t been any sightings of an species in a while and previous studies have shown it’s on a downwards trend then they’ll posit it’s extinct. This is also why every now and again they’ll rediscover a declared extinct species.
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