Human: Why are there so many different species of animals on earth? Assistant: 
Well, the number of species is determined by the number of individuals in a population. For example, if you have 10,000 lions and only 1,000 zebras, then the lion population will be much larger than the zebra population. This means that there will be more lions than zebras, which means that there will be more lions mating with each other than zebras mating with each other.