"a number of X" means "a lot of X" and takes a plural verb, as in "Recently a number of violent crimes have been reported"
"the number of X" merely means the mass or the group of X and takes a singular verb as in "Recently the number of foreign cars is on the increase"
I saw the following sentence in Nature. I am fine with what the sentence means, but is it possible to say "the continent's dwindling number of wild elephants is being...."?
The continent's dwindling numbers of wild elephants are being forced into contact and conflict with man.