1. 'lower than' is wrong because 'lower than' does not mean the same as 'less than' in this context. Population is not lower than some number...it is less than some number.
2. As we all know by now, 'less' is a uncountable modifier and 'fewer' is a countable modifier. Eventhough the sentence reads 'less than 200' (a specific number), its important to note that we do not know the exact number below 200 and hence uncountable. Also, 'fewer breeding females' - is countable because they are only a few and is a countable number.
3. Now for the big one...why 'of'. Typically it should be noun+preposition. Here population + of, which is right. Also a prepositional phrase (a phrase starting with a preposition, here, 'of less than....' does some additional good things. What is it?
The whole phrase, takes on a modifying role, acting as an adjective or an adverb, locating something in time and space, modifying a noun, or telling when or where or under what conditions something happened. Hence the need of 'of'. D misses of and becomes 'off beat' if you may will.
Paul and other veterans comment on my theory pls.