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Large animals like elephants, rhinoceros, polar bears and countless other species worldwide face the highest rate of decline. Reuters

The planet's biodiversity may be reaching a tipping point and we could be in the early days of a sixth mass biological extinction event.

A scientific review of data in Science says the loss and decline of species this time is mostly from human actions. Previous extinctions were driven by natural transformations or disasters like asteroid strikes.

There was a time, in the Pleistocene epoch, some tens of thousands of years ago only, when mammoths, terror birds, giant tortoises, and sabre-toothed cats, as well as giant ground sloths (some of which reached 7 metres in height) and glyptodonts (which resembled car-sized armadillos), roamed the planet freely. They have largely disappeared due to natural changes or catastrophes.

But since 1500, more than 320 terrestrial vertebrates have become extinct. Populations of the remaining species show a 25% average decline. The situation is similar for invertebrate animal life.

Lead author Rodolfo Dirzo, a professor of biology at Stanford, designates the current situation an era of "Anthropocene defaunation."

Reversing the increasing rate of global biodiversity losses may not be possible without embracing intensive, and sometimes controversial, forms of threatened species management, say scientists. Among these, the foremost is what they term as "conservation translocation" or movement of species to re-establish new populations.

Across vertebrates, 16% to 33% of all species are estimated to be globally threatened or endangered. Large animals like elephants, rhinoceros, polar bears and countless other species worldwide face the highest rate of decline, a trend that matches previous extinction events.

Although these species represent a relatively low percentage of the animals at risk, their loss would have trickle-down effects that can affect other species including humans.

For instance, previous experiments conducted in Kenya have isolated patches of land from these large animals to observe how an ecosystem reacts to the removal of its largest species.

Grass and shrubs increase and the rate of soil compaction decreases. Seeds and shelter become more easily available, and the risk of predation drops.

Consequently, the number of rodents doubles -- and so does the abundance of the disease-carrying ectoparasites that they harbour. It all begins with humans whose high density leads to defaunation.

The human population has doubled in the past 35 years; in the same period, the number of invertebrate animals -- such as beetles, butterflies, spiders and worms -- has decreased by 45%.

As with larger animals, the loss is driven primarily by loss of habitat and global climate disruption. And the result again boomerangs on humans. Pollination is affected, as also nutrient cycling and organic decomposition.

New Wilderness Areas
In another related paper, Professor Philip Seddon of the University of Otago and his co-authors suggest that creating "wildness" rather than restoring "wilderness" is the most practical way forward.

One controversial form of translocation called "conservation introductions" is where species are brought into areas outside of their historic range for conservation benefit.

Like the release of exotic species of giant tortoise to restore the grazing functions and seed dispersal lost through tortoise extinctions on islands in the Indian Ocean.

It also involves assisted colonisation, where species are moved outside their range to prevent extinction due to threats in their native habitat. Examples of this include moving native birds, such as kakapo to predator-free offshore islands.

There has been a long-pending plan to translocate Asiatic lions from the Gir sanctuary in India's western state of Gujarat to Kuno Palpur sanctuary in central Madhya Pradesh.

While the latest census figures put their number at 411, up from 359 in 2005, wildlife experts believe the lions have outgrown the Gir forest and a single epidemic or calamity could wipe out the entire population.