When a species becomes extinct, it's gone for good: the last surviving member has died.

Earth is currently experiencing the worst wave of species die-offs -- a mass extinction of plants and animals -- since the dinosaurs 65 million years ago, according to the Center for Biological Diversity. Scientists say that nearly all of the thousands of currently threatened species -- mammals, birds, fish, reptiles, amphibians, plants and invertebrates -- are at risk because of human activities, including habitat loss, introduction of non-natives, and the effects of climate change.

This list is based on the priorities of the World Wildlife Fund, a conservation organization that focuses on protection of species that are critical to their larger environment, and the The IUCN's Redlist, a comprehensive inventory of the conservation status of species around the world.

Here are 33 animals that are at risk of disappearing from the planet forever.

This article was written by a staff member of TheStreet.