Is there a possibility that animals more "recently" extinct animals (Tasmanian Tigers, Passenger Pigeons, river dolphins, etc) still actually existing in small populations somewhere? Or even older extinct animals such as the Dodo. How does an animal gain the title "extinct"?
You ask a really good question! It’s hard to define exactly when animals are extinct. I’m going to quote this Slate article, but it’s worth reading the entire thing for a much more nuanced view:
“The World Conservation Union will label a species extinct only if “there is no reasonable doubt that the last individual has died.” In general, scientists must now show that repeated efforts to survey a species’ known habitat failed to turn up any individual sightings or evidence of its continued survival.”
Some animals, like passenger pigeons, we’re pretty sure are gone because their migratory behavior made them very easy to observe. Others, not as much - there’s a great book called Carnivorous Nights: On the Trail of the Tasmanian Tiger that details the people who are still searching for Tasmanian tigers, hoping they retreated to the really inhospitable terrain in the center of the island.
For the most part, scientists are very careful to prove that animals are extinct before declaring them so - but they’re not always right. Black Footed Ferrets were thought to have been extinct… and then someone’s dog came back with a freshly killed one, and led us back to the last surviving population… and now we’ve got more than 300 reintroduced to the wild and a highly successful conservation program for them. It doesn’t happen often and isn’t something to put a lot weight on hoping for, but sometimes miracles do happen.