Western Lowland gorilla populations have dropped sixty percent in the past twenty-five years due to poaching, habitat loss, and, mainly, disease. Their numbers are expected to keep dropping, and the species could soon disappear:

A species whose population has declined sharply and is at risk of disappearing is classified as endangered. The passage says Western Lowland gorilla numbers have fallen by 60% and could soon disappear, so “endangered species” is the best choice. “Extinct” means no individuals remain, while “threatened” generally describes a species at risk of becoming endangered.

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