As Robert May said in a paper published in Science:
"If some alien version of the Starship Enterprise visited Earth, what might be the visitors' first question? I think it would be: 'How many distinct life forms—species—does your planet have?' Embarrassingly, our best-guess answer would be in the range of 5 to 10 million eukaryotes (never mind the viruses and bacteria), but we could defend numbers exceeding 100 million, or as low as 3 million."
At first, it doesn't seem like a big deal, but the fact that there are so many species that we've yet to discover has real consequences for our ability to understand changes in global biodiversity and the rate of species extinctions. If we don't know that certain species exist, we also don't know that they might have, or will soon, disappear. Some animals will inevitably go extinct before we realize that they existed.
