What causes wildlife reduction?

Primarily, interactions with humans, either directly or indirectly.

Directly:

Hunting of animals for food and/or to obtain parts of an animal (IE, tusks of elephants, the horns of rhinoceros, polar bears because of their fur, etc.), including recreational hunting;

Indirectly:

Loss of habitat due to human interference and/extinction of other species (including plants) which animals need for food (IE giant panda) or reproduction.