Doyle Rice

USA TODAY

The loss and decline of animals around the world — caused by habitat loss and global climate disruption — mean we're in the midst of a sixth "mass extinction" of life on Earth, according to several studies out Thursday in the journal Science.

One study found that although human population has doubled in the past 35 years, the number of invertebrate animals – such as beetles, butterflies, spiders and worms – has decreased by 45% during that same period.

"We were shocked to find similar losses in invertebrates as with larger animals, as we previously thought invertebrates to be more resilient." said Ben Collen of the U.K.'s University College London, one of the study authors.

Although big, photogenic species, such as tigers, rhinos and pandas, get the bulk of the attention, researchers say it's clear that even the disappearance of the tiniest beetle can significantly change the various ecosystems on which humans depend.

"We tend to think about extinction as loss of a species from the face of Earth, and that's very important, but there's a loss of critical ecosystem functioning in which animals play a central role that we need to pay attention to as well," said lead author Rodolfo Dirzo of Stanford University.

"Habitat destruction will facilitate hunting and poaching, and species will have difficulty in finding refuge given land use change and climatic disruption," added Dirzo.

The study reported that around 322 species have gone extinct over the last five centuries.

Scientists have coined the phrase "anthropocene defaunation" — meaning human-caused animal decline — to describe this apparent mass extinction.

Five times in the history of the Earth, a huge percentage of the planet's life has been wiped out in what are called mass extinctions, typically from collisions with giant meteors.

About 66 million years ago, one well-known extinction killed off the dinosaurs, along with three out of four species on Earth. About 252 million years ago, the "Great Dying" snuffed out about 90% of the world's species.

What's new about this extinction is "that the underlying driving force for this is not a meteorite or a mega-volcanic eruption; it is one species - homo sapiens," said Dirzo.

Overall, scientists estimate that due to all of the past extinctions, about nine out of 10 of all life-forms that have existed on our planet are extinct.

Another article in this week's Science, led by Philip Seddon of the University of Otago in Dunedin, New Zealand, details the way we can reduce this mass extinction by reintroducing animals to wild populations and recolonizing entire populations — such as giant tortoises — to areas in which they've gone extinct.

That study found that "some substantial progress in reversing defaunation is being achieved through the intentional movement of animals to restore populations."

A third report in the journal finds that animals such as gibbons, orangutans and various types of foxes, bears and rhinoceroses have been steadily disappearing from large, protected areas of land around the world.

The papers in this week's Science continue research into the mass extinction; a study this year in Science found that species of plants and animals are becoming extinct at least 1,000 times faster than they did before humans appeared.

Science is published by the American Association for the Advancement of Science, a non-profit science society based in Washington.

Contributing: Associated Press