Hunting and trapping, excessive use of pesticides, and water pollution are all reasons for the decline of wild animals, but the main reason according to biologists and zoologists is large-scale development by the human race.
在「鄭成功開台史」中被形容為「糜鹿處處」的水鹿，如今就只有二千公尺以上深山，才可能驚鴻一瞥。對於只吃某一種食物、住一種樹上，或移動能力差、要求禦蔽程度高的動物，往往也是環境開發的「先烈」。