Have humans damaged the Earth's ecosystems so severely that we're well on our way to the biggest mass extinction since the dinosaurs vanished 66 million years ago? And are we running out of time to reverse the negative impacts of our actions?

Three scientists who have studied extinctions of thousands of species of vertebrates believe so, though others are sceptical of the doomsday-like findings.

A new study published Monday paints a grim picture: The populations of nearly 9,000 vertebrate species, including mammals, such as cheetahs, lions and giraffes, have significantly declined between 1900 and 2015.

Almost 200 species have gone extinct in the last 100 years alone. The study says the losses are indicative of the planet's "ongoing six major extinction events" and has cascading consequences for human life on Earth.

"This is the case of a biological annihilation occurring globally, even if the species these populations belong to are still present somewhere on Earth," Rodolfo Dirzo, the study's co-author and a Stanford University biology professor, said in a news release.

The researchers analysed 27,600 species of birds, amphibians, mammals and reptiles - about half of all known vertebrate species - and found that 8,851 (about 32 percent) have seen declining populations and shrinking areas of habitat.

A more detailed analysis on 177 mammal species found that more than 40 percent have experienced significant drops in population. The findings, the study says, mean that billions of animal populations that once roamed the Earth are now gone.

The authors describe the shrinking population of species as "a massive erosion of the greatest biological diversity in the history of Earth."

"Thus, 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," the authors wrote.

"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."

A few examples: There were only a little more than 7,000 cheetahs in existence last year, and their population may drop another 53 percent over the next 15 years, according to National Geographic. Borneo and Sumatran orangutans have been considered endangered for years mainly because of loss of habitat.

The population of African lions has dropped by more than 40 percent in the last 20 years. West African lions, in particular, are nearing extinction, with only about 400 animals left. Historically, lions roamed southern Europe, the Middle East, northwestern India and most of Africa.

Today, there are only scattered populations in sub-Saharan Africa and a few remnants at Gir Forest National Park in India, according to the study.

The driving force is a steady drumbeat of human activities that result in habitat losses, pollution and climate disruption, among others.

"The massive loss of populations and species reflects our lack of empathy to all the wild species that have been our companions since our origins," the study's lead author, Gerardo Ceballos, an ecology professor at the Universidad Nacional Autónoma de México in Mexico, said in the news release.

"It is a prelude to the disappearance of many more species and the decline of natural systems that make civilisation possible."

Some in the scientific community disagree with the study's grim findings.

Stuart Pimm, head of the conservation ecology at Duke University in North Carolina, said the study unnecessarily raises alarms by saying the Earth is already in the midst of a cataclysmic event. Pimm believes the sixth mass extinction is just beginning, and not well on its way.

"It's a little bit dramatic," Pimm said. "Yes, we are driving species to extinction a thousand times faster than we should. So yes, there is a problem. But on the other hand, telling people that we're all doomed and going to die isn't terribly helpful."

Doug Erwin, curator at the Smithsonian National Museum of Natural History, said placing the ongoing extinctions of animal species in the same playing field as the mass extinction events in history, or the Big Five, amounts to "junk science."

"Many of those making facile comparisons between the current situation and past mass extinctions don't have a clue about the difference in the nature of the data, much less how truly awful the mass extinctions recorded in the marine fossil record actually were," he told The Atlantic last month.

"It is absolutely critical to recognise that I am NOT claiming that humans haven't done great damage to marine and terrestrial, nor that many extinctions have not occurred and more will certainly occur in the near future. But I do think that as scientists we have a responsibility to be accurate about such comparisons."

Others say the study's findings are bleak - and rightly so.

Kieran Suckling, executive director of the Centre for Biological Diversity, said the researchers accurately show that population losses are not just confined to a certain geographic area or within certain species of animals.

"What they show is it's a mass, global phenomenon," Suckling said. "I think they made the case very strongly that we are right now in the sixth extinction, and if we continue the trend we're on, we're going to be looking at 50 to 75 percent of our species lost over the next hundred years."

Noah Greenwald, endangered species director for the Centre for Biological Diversity, agreed with the researchers' conclusion that the window for humans to take action is quickly getting narrow.

"The study is right in raising alarm bells .. especially with our change in climate," Greenwald said. "We really need to protect as much habitat as we can now. Our population continues to expand, our consumption continues to expand. We're going in the wrong direction, quickly."

The concept of a sixth mass extinction is not new, and the study is not the first to make the case that Earth is already in the middle of it.

Two years ago, some of the same researchers argued that species are disappearing at a rate unparalleled since the Cretaceous mass extinction of dinosaurs. The 2015 study found that vertebrate species have been disappearing up to about 100 times faster than the normal rate over the last century.

On a happier note, scientists point to ongoing efforts to save endangered species and their habitats.

"We've dramatically increased the area protected by national parks, increased the area of the oceans that's being protected. We have reduced deforestation rate in the Amazon," Pimm said. "I'm not trying to say that it's all good news, but there's good news out there."

And there's a chance to save endangered species - as long as humans fully commit to it, Suckling said.

"Because once they go on endangered species list, they go from neglect or maybe tacit management to very active, focused efforts to save them. And those work," Suckling said.

"The good news here is that once humans decide to save individual species - and we're quite good at it - we can actually reverse this negative trend."

Concerned citizens can do practical things like planting native plans in their yard. They can also contact their representatives in Congress to show their support for habitat protection, Greenwald said, though he cautioned that the current Congress is "the most anti-endangered species in history."

The Centre for Biological Diversity has tallied 34 pending legislations that would weaken protections for endangered species, Greenwald said.

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