* Habitat loss: The destruction or fragmentation of an animal's natural habitat can reduce its population size.
* Hunting and poaching: Hunting and poaching can directly reduce the population size of a species.
* Disease: Disease can spread quickly through a population and cause a decline in numbers.
* Climate change: Climate change can have a number of negative effects on animal populations, such as altering their habitat or reducing their food supply.
* Natural predators: Natural predators can prey on a species and reduce its population size.
Based on these factors, it is difficult to say definitively which animal species would have the least population size. However, some species that are facing significant threats to their populations include:
* African elephants: African elephants are poached for their ivory, and their populations have declined significantly in recent years.
* Black rhinoceros: Black rhinoceros are also poached for their horns, and their populations have declined significantly in recent years.
* Mountain gorillas: Mountain gorillas are threatened by habitat loss and poaching, and their populations are estimated to be less than 1,000.
* Giant pandas: Giant pandas are threatened by habitat loss and poaching, and their populations are estimated to be less than 2,000.
* Snow leopards: Snow leopards are threatened by habitat loss, poaching, and climate change, and their populations are estimated to be less than 5,000.