Which is the weakest species? I shall refer you to the great Quora article by Abdullah T on this subject. He has written very nicely. He claims the dung beetle is the strongest species and that bacteria are the weakest of them all (although some argue that humans are the weakest of them all). Dung beetles can pull over 1100 times their own weight, which is comparable to a human pulling behind him six double decker busses.

Bacteria are the weakest life form because they have no defense system and can be easily killed.

So it seems we humans are in a class with bacteria in being the weakest on Earth.Humans are considered the weakest species because, despite our brains and collective achievements, we are quite defenseless left naked and alone in nature in a place like, say, Africa among the creatures that live there.