First it's climate change, which has been a leading cause of the significant decline in animals of all types - fish, birds, mammals, reptiles and amphibians. The World Wildlife Foundation (WWF) just published its 2016 Living Planet Report. It found a 58 percent overall decline in vertebrate populations from 1970 to 2012, the latest year with available data. If things go along this way, the world could lose more than two-thirds of wildlife by 2020. Of course, we humans have also had a large effect on the matter through habitat loss, overexploitation and pollution.