Clean air, water and soil to grow food are necessities of life. So are diverse plant and animal populations. But as the human population continues to increase, animal numbers are falling. There’s a strong correlation. A comprehensive report from the World Wildlife Federation and the Zoological Society of London found that wild animal populations dropped by 58 per cent between 1970 and 2012, and will likely reach a 67 per cent drop by 2020 if nothing is done to prevent the decline.