Changes in ocean temperatures and increasing ocean acidification also threaten many marine species, especially corals and mollusks with external shells. Overexploitation from hunting and harvesting also has adversely affected many species. For example, about 20 million tropical fish and 12 million corals are harvested annually for the aquarium trade,
depleting natural populations in some parts of the world.
All these factors have increased the numbers of threatened species. Almost one in four mammal species, including four of the six remaining species of great apes,
and one in eight bird species were considered at significant risk of extinction at the start of the 21st century.
In addition, the World Wildlife Fund noted in a 2016 report that vertebrate populations overall declined by 58 percent between 1970 and 2010.