This thread has a million directions, like spilled milk...Same thing, if you don't know the size of the population then you shouldn't be removing anything from the wild. It doesn't matter if it is one specimen or a thousand, there is a risk that could be the last of them.
For @itiwhetu 's concern about collecting or extinction, if we don't study animals and try to get a handle on what is where, then we have no arguments to save them. No one is going to change logging or farming practices to protect something that isn't there. It's in the interests of people who don't care whether species live or not to not look at them.
Wild Bettas (not splendens) are an example as many species probably were made extinct by human economic activity before anyone ever registered they'd been there. Tell me there are several tiny species that exist nowhere else in my swamp and I will think twice about draining it. Tell me I have an empty swamp and that there's money in drainage, and it's gone.
Now, I have my swamp and I decide there's a market for the tiny species, if I have brains I will sustainably harvest them (after studying their numbers) and maybe help support my family. If I have no brains I'll collect them all and sell them in one shot. If we go away from the question of ignorance and trying to shine a light on diversity over to fish commerce, let's talk freshwater stingrays.
Their spines cause terrible pain if you step on them, and they like to bury themselves in the sand in shallow water. In most places in their Amazonian range, they are shot or speared on sight. They are seen as a dangerous pest, and are locally extinct in a lot of areas. As we go forth and multiply we increase our contact and we tend to kill things that get in our way.
Where they are thriving is in the places where they are collected for the aquarium trade. There, the locals understand they have a valuable resource that can support families that would otherwise be much poorer. I have a friend who is a stingray researcher, and she has spend many hours with locals whose knowledge of the fish runs very deep. The stingray business is still relatively new - just a few decades - and in that time a number of villages have decided to become stewards of a natural resource that supports them. The same pattern repeats itself along all the great tropical rivers and regions where the fish we love are found. Some fish are caught and sold, some fish are caught and eaten, but the numbers are respected because people want their kids to be able to make a living too.