"That's why we, along with conservation and science colleagues around the world, are calling for the most ambitious worldwide agreement yet - a new global deal for nature and people - to bend the curve of biodiversity loss".
Almost 20 percent of the Amazon rainforest, the world's largest, has disappeared in five decades.
Measured by weight, or biomass, wild animals today only account for four percent of mammals on Earth, with humans (36 percent) and livestock (60 percent) making up the rest.
The population decline of the species is most pronounced in the tropics with South and Central America and the Caribbean suffering "the most dramatic decline with an 89 per cent loss compared to 1970".
The staggering loss is reflected in the organization's Living Planet Index, which tracks global biodiversity by measuring the abundance of mammals, reptiles, birds and other species.
"For the 2018 report, 319 new species have been added to the Living Planet Index database", Colby Loucks, deputy goal lead and senior director at the WWF Wildlife Conservation Program, told weather.com.
The report highlights that over recent decades, human activity has also severely impacted the habitats and natural resources wildlife and humanity depend on such as oceans, forests, coral reefs, wetlands and mangroves.
The Paris Agreement, negotiated under the United Nations convention on climate change, also set a clear target: global warming must be held to "well below" 2 deg C, and 1.5 deg C if possible.
The strongly worded report claims that humanity has thus far "failed the natural world" with declines in freshwater ecosystems especially pronounced.
With the world set to review progress on sustainable development and conserving biodiversity under United Nations agreements by 2020, there is a window of opportunity in the next two years, the group argued.
Unless humanity collectively pulled together now the situation would only worsen - to the detriment of humanity, the director general of the leading conservation organisation said. Barrett said we need a new global deal for people and the environment, and this is our last chance to do this right. And it's not just climate change.
According to the forecast, WWF, 2020 will be critical, as will be announced the results of measures taken in the framework of the Paris climate agreement. Globally, nature provides services worth around $125 trillion a year, while also helping ensure the supply of fresh air, clean water, food, energy, medicines and more. "There can not be a healthy, happy, and prosperous future for people on a planet with a destabilized climate, depleted oceans and rivers, degraded land, and empty forests, all stripped of biodiversity, the web of life that sustains us all".
The authors urged the 200 member countries of the Convention on Biological Diversity to come up with a set of global goals to protect animal species when they meet next month in Egypt.
On average, there would be an average fall in the populations of species of 40%. The United States is one of the largest drivers of those declines.