Insects survived outside of the ark, the water was full of debris of rotting trees, plants, corpses etc., so they had lots to thrive on. Fish and water mammals survived outside too.
If you need a viable population of some thousands, how come it has been possible to re-establish animal populations based on only a handfull of individuals? In many cases, they have managed to gather the 2, 3 4 or so living individuals of a kind, make them mate and after a few years they have been successfully re-introduced on their island or in their region and now are as safe as any animal population can be these days.
I happen to believe in the flood story - but not that it was global. Huge and quite possibly having worldwide effects, but the flood itself local. There was not enough time for human or animal population to spread as fast as it must have in order to be as plentiful as it must have been during the days of Abraham etc.