Mass extinction events induced by catastrophic natural phenomena have occurred five times in the history of life on Earth. Many scientists now are warning that a Sixth Mass Extinction catastrophe is unfolding, this time triggered entirely by human activities.

Despite the mounting evidence that this ominous occurrence is taking place all around us, not everyone agrees.

"Drastically increased rates of species extinctions and declining abundances of many animal and plant populations are well documented, yet some deny that these phenomena amount to mass extinction," bioscientist Robert Cowie from the University of Hawai'i at Mānoa, said.

"This denial is based on a highly biased assessment of the crisis which focuses on mammals and birds and ignores invertebrates, which of course constitute the great majority of biodiversity."

In a new study, Cowie and his colleagues aim to disprove the deniers by focusing on the decline of invertebrates, which receive far less attention than vertebrates in discussions of biodiversity loss - even in the prestigious IUCN Red List of Threatened Species, arguably the world's most comprehensive record of species extinctions, but biased towards birds, mammals, and amphibians.

Extrapolating from estimates for land snails and slugs, Cowie and co-authors calculated that Earth could have lost between 7.5% and 13% of the two million known species since 1500 - a startling 150,000 to 260,000 species

Unfortunately, in addition to denial of science on a variety of subjects, some people also dispute that the Sixth Extinction has begun, according to the new study.

The researchers are unsure whether these dangerous trends can be reversed. They do, however, point out that ignoring the crisis or neglecting to act on it is abrogating our moral obligation, and they urge scientists and conservationists to continue to raise awareness of the crisis and to nurture the "innate human appreciation" of biodiversity.

Various conservation projects for particular charismatic animals have been successful in combating the situation. However, these activities will not be able to target all species, nor will they be able to alter the overall trend of species loss. Nonetheless, it is critical to maintain such efforts, to nurture awe for nature, and, most importantly, to catalog biodiversity before it disappears.

Dedicated conservation biologists and conservation organizations are doing what they can, focusing primarily on threatened birds and animals, among which some species may be saved from extinction, according to the study.

The findings are reported in the journal Biological Reviews.