Earth could be on the verge of its sixth great extinction, according to a new study, led by a Stanford University professor. Life on our planet contains more diversity than at any previous time in geological history. However, a new meta-study of research, directed by an international team of scientists, states life may have reached a tipping point. The investigators point to evidence we may be experiencing the first days of a great extinction.

The study points out that 322 species of terrestrial vertabrates have become extinct since the year 1500. Populations of surviving species are declining by an average of 25 percent, according to investigators. Invertebrate species could be in even more danger, as two-thirds of monitored populations have experienced an average 45 percent decline in numbers. During that same time, human population has doubled.

Five great extinctions have taken place on Earth so far. The first of these, the End-Ordovician mass extinction, took place between 445 and 440 million years ago. The most severe, known as The Great Dying, took place 252 million years ago, wiping out 97 percent of species. This was triggered by a massive volcanic eruption in what is now Siberia.

"Much remains unknown about this 'Anthropocene defaunation;' these knowledge gaps hinder our capacity to predict and limit defaunation impacts. Clearly, however, defaunation is both a pervasive component of the planet's sixth mass extinction and also a major driver of global ecological change," researchers wrote in an article announcing their study.

Worldwide, between 16 and 33 percent of all vertabrates are classified as either threned or endangered. Large animals are suffering the greatest population losses, which the team states also occurred during earlier extinction events. This is because larger animals require more room than smaller animals on which to live, and reproduce at a slower rate. Humans also target many larger animals for hunting and capture, providing additional challenges for these animals.

According to researchers, the disappearance of large animals, called megafauna, from the environment can result in the proliferation of smaller creatures, like rodents. These tiny creatures can carry parasites and diseases that could harm humans.

"Where human density is high, you get high rates of defaunation, high incidence of rodents, and thus high levels of pathogens, which increases the risks of disease transmission," Rodolfo Dirzo, biology professor at Stanford,  said.

If we are entering the sixth great extinction, it would be the first time one of these events was triggered by the actions of a single species - in this case, humans.

Investigation of biodiversity and the possibility of a sixth great extinction was profiled in the journal Science

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