If you are talking about birds, mammals, and reptiles, I beleive there are about 30,000 species worldwide alive today. Subtract from that those that are water-adapted, mix in the idea speciation occurs when one group splits into two with a loss of information in the gene pool let's estimate how many species would actually have to have been on board. Your guess is as good as mine, 10,000 maybe?
That is not an impossible number. And for every large animal there are many small ones, so that the average size might be close to a chicken. I've seen one chicken house that had that many animals in it, and one family care for four or five such houses. The reptiles can go much longer without care, one good meal can keep a snake going for months, and many animals can hibernate or estivate.
If Noah's ark is a fable, it is NOT because it the science demands it be.
Right...let's say, for the sake of conversation, that the ark did contain only land-based species.
How would Noah & his helpers keep the animals from fighting & killing each other? It seems like quite a lot to ask an ark full of rhinos, gorillas, ostriches, gila monsters, ocelots, & three-toed sloths to play nicely.
And what about the animals that must have been unknown to that region of the world? And how many tons of food would the ark have to hold, not to mention the diversity of each species' diets? How would they have gotten rid of what must have been tons & tons of waste a day?
Science is unnecessary when common sense will do.
Here's a question that I would like to see discussed. If the ark existed and if the entire world was flooded and if all the animals that needed to be out of water were on board, how then did the species get back to where they are today? There are species that are native only to particular islands. How did they get there?
Certainly they didn't walk. Not over water. Assuming the Bible's timeline is correct and the flood occurred only within the last 10,000 years (or even the last 100,000 years) that's not enough time to get the entire planet repopulated to the levels that exist today with the level of complexity and integration that exists today.
The only logical explanation to species dispersment across the world that I've seen, short of God magically making species with no wings fly, is continental drift. The problem with continental drift is that by definition it blows the Noah's ark story to bits due to the length of time it would have taken. To believe in Continental Drift and Noah's Ark would mean placing Noah's Ark back hundreds of millions of years ago.