It's a distressing fact that human activities are causing the wide-scale extinction of other species on earth.
Something you may not know, however, is that the fewer animals there are in a given species, the more at risk that species is, and not just because it's easier for a small group to disappear than a large group. Another danger is inbreeding.
Inbreeding is the term used when animals mate with their relatives. All animals, including humans, inherit half their genes from their mothers and half from their fathers. Within certain limits, it's good for offspring to have a father whose genetic makeup is significantly different from its mother's. It's kind of like poker. Reshuffling the deck makes it more likely you'll draw a good hand by chance. If the deck were only fifteen cards, each of which appeared twice, the chances of getting a good hand would be much lower.
Similarly, if you mix two animals that have close genetic makeups, the offspring won't be as healthy as the offspring of two genetically dissimilar parents. That's because many diseases are caused by mutant genes. If you inherit a mutant gene from your mother, you can use the non-mutant version inherited from your father as a backup. If both mother and father had mutant versions of the gene, however, disease will result. Inbreeding makes it much more likely that two mutant copies of a gene will show up.
You can see the problem with endangered species. With fewer than about five hundred members, the gene pool is too restricted. Inbreeding is more likely. The following generations become progressively less healthy, eventually dying out.