Each tiger, for example, is estimated to need 10 to 100 square kilometres, depending on the quality of the terrain. Secondly, for reasons of disease, genetics and simple accident, no population of wild animals can be considered secure unless it contains around 500 individuals. A thousand is probably more realistic. This means that a reserve that is smaller than, say, Yorkshire, probably cannot maintain a secure population of tigers. There is no wild-breeding population of 500 tigers, and there are precious few spare Yorkshires in Asia.
Tigers are an extreme example but many creatures are similarly placed.
They only hang on because elephants are so long-lived. It is a world-wide problem. Even Yellowstone, the biggest park in the US outside Alaska, is believed too small to maintain a viable population of grizzly bears. In war, animals in reserves merely become easy meat. The last P re David's Deer in China were slaughtered in the Emperor's garden in the Boxer rebellion.