Humans and wildlife species face a "dismal" future - including the possibility of mass extinction - if measures are not taken within the next 20 years to arrest "powerful assaults on biodiversity", scientists have warned.
Human overpopulation and over-consumption by the wealthiest in society are driving factors behind the destruction of species on planet Earth, which is having a negative impact on ecosystems, according to researchers.
The grim warning, published in the Proceedings of the National Academy of Sciences journal, states that the hidden rate of species population decreases mean "Earth's sixth mass extinction episode has proceeded further than most assume".
The report, involving scientists at both Stanford and Mexico City universities, found the current rate of vertebrate extinction during the last century was two species a year - this was compared with two species every 100 years over the last two million years.
They warned the estimates were likely to be "conservative", with "several" species of mammal now endangered despite being at "relatively safe" levels at the turn of the millennium.
The report said: "As much as 50% of the number of animal individuals that once shared Earth with us are already gone, as are billions of populations.
"We emphasise that the sixth mass extinction is already here and the window for effective action is very short, probably two or three decades at most.
"All signs point to ever more powerful assaults on biodiversity in the next two decades, painting a dismal picture of the future of life, including human life."
Scientists said the loss of animals from the planet would "promote cascading catastrophic effects on ecosystems", including plants and other wildlife.
The report added: "The resulting biological annihilation obviously will also have serious ecological, economic, and social consequences.
"Humanity will eventually pay a very high price for the decimation of the only assemblage of life that we know of in the universe."
The report was based on analysis of 27,600 mammals, reptiles and amphibians, and cited double-digit decreases in the populations of species such as African lion, which has seen a 43% drop since 1993.
Researchers did not state how long it predicted the human race to survive, but said there was scope to "address the decay of biodiversity."
The report added: "The strong focus among scientists on species extinctions, however, conveys a common impression that Earth's biota (animal and plant life) is not dramatically threatened, or is just slowly entering an episode of major biodiversity loss that need not generate deep concern now.
"Thus, there might be sufficient time to address the decay of biodiversity later, or to develop technologies for 'de-extinction'."