Giant tortoises were common on Mauritius, Réunion, Rodrigues and the Seychelles granitic islands into the seventeenth century. They were easy to catch, survived months in a ship's hold without food, and were taken in enormous numbers as fresh meat. Every one of those island lineages was wiped out.
Aldabra survived because it is hard to land on and has almost no fresh water. Its tortoise population, estimated at around one hundred thousand, is by a wide margin the largest of any giant tortoise anywhere, and the atoll is a UNESCO World Heritage site partly on that basis.
Density there is high enough that the tortoises shape the vegetation. Sustained grazing maintains a low, dwarfed sward known as tortoise turf, made up of plants that flower and set seed close to the ground where cropping cannot reach them.