One of the stock challenges to a global flood is biogeography. How did animals disperse from Armenia to their present locations? I'd simply point out that that's not a problem unique to young-earth creationism. It's a problem for secular science as well:
And don't even get me started on the weird history of biogeography. The weird thing in biogeography are the disjunctions - places where very similar species are separated by an ocean. Sometimes the species are on islands, and sometimes on separate continents. One explanation was vicariance - animals and plants got their modern distribution on land masses that are no longer there. In Darwin's day, this was the favorite explanation of a guy named Edward Forbes. He speculated that land bridges used to connect continents (like Europe and North America) so that species now separated by oceans used to have a much larger range on land that sank into the ocean. Then Darwin argued that Forbes was wrong and instead championed the occasional lucky dispersal across oceans to account for these disjunctions. Darwin even did experiments like floating seeds in saltwater to see how long they could go and still germinate. Then came plate tectonics and suddenly vicariance got some new life. There weren't land bridges, but the continents used to be all connected. Then plate tectonics and biogeography developed to the point where scientists decided that many disjunctions were much younger than the continental split, and so we're back to the occasional lucky dispersal as Darwin hypothesized. Today it's sort of a mix. Vicariance and dispersal are both invoked depending on the situation. I could go on and on. Madagascar is fascinating case study. You should look it up some time.