Here's why:
* Different populations: Woodland caribou are not a single, uniform population. They exist in various herds across their range in North America.
* Varying status: The status of different woodland caribou herds varies. Some are listed as threatened or endangered, while others are considered stable or even increasing.
* Gradual decline: The decline of many woodland caribou herds has been a gradual process over decades, driven by various factors like habitat loss, climate change, and predation.
To be more specific, you can ask:
* When did *specific herds* of woodland caribou become listed as endangered or threatened?
* When did the overall population of woodland caribou start experiencing significant decline?
Providing information about the specific herd or geographic area you're interested in will allow for a more accurate and helpful response.