The combined weight of all the ants in the world is equal to the combined weight of all the humans in the world. Scientists probably know this because they have counted them all individually, they made a guess, or have some calculation they use to determine this fact.
There are approximately 12,000 species of ants on the planet. I consider them a danger because they are so good at working together and if by some freak of nature they became really big I am sure they would kill us all.
There are approximately 7 billion humans on Earth. You can check the current status of this by clicking on this LINK
to the counter.
Problem is that there are 10 thousand trillion ants. Man that is a lot of ants. I guess we better stay friends with them just in case they attack.