Opinion: there have been five mass extinctions in the past and there is much evidence, including the disappearance of several species, to suggest the sixth may already be happening

We are at the beginnings of a mass extinction event. For most of us this is a hard concept to grasp. There are millions of species on earth today, more than at any time  in the earth’s history so where is the evidence for a mass extinction event?

There have been five mass extinctions in the past, each happening over a timescale of hundreds of thousands of years and being followed by intervals of millions of years before another catastrophic change in environmental conditions causes up to half of existing species to disappear. These events are clearly marked in the fossil record.

The most well-known of the mass extinctions events, that of the dinosaurs, was brought about by the impact of a meteor and subsequent alteration in the environmental conditions on a planet wide scale. With no meteor impacts and no network of erupting volcanoes to disrupt the atmosphere, where do we look for evidence that an extinction crisis is underway?

In the battle between dinosaurs and meteors, there was only one winner

The evidence that science points to for the sixth mass extinction event comes from a variety of sources. First is the obvious extinction of species. In evolutionary terms, there are always species going extinct and, of course, new species evolving. This is the background rate of extinction. A 2015 paper by Gerardo Ceballos et al assesses the current extinction rates for a range of vertebrate species and estimates current extinction rates are up to 100 times the background rate.

They conclude that "our calculations very likely underestimate the severity of the extinction crisis because our aim was to place a realistic 'lower bound' on humanity’s impact on biodiversity. Therefore, although biologists cannot say precisely how many species there are, or exactly how many have gone extinct in any time interval, we can confidently conclude that modern extinction rates are exceptionally high, that they are increasing, and that they suggest a mass extinction underway"

Extinction is generally hard to spot hence why the authors are cautious with their estimates. But over the past century, the disappearance of several species has been witnessed in either zoos or at locations familiar to scientists. Pinta Island tortoise Lonesome George was the last of his species and lived on the Galapagos Islands until his death in 1989. The last Passenger pigeon, Martha,  died in Chicago Zoo in the early 20th century. Just fifty years earlier, accounts of the Passenger pigeons told stories of flocks of millions of birds passing day and night for several days and the population was estimated to be in the region of four billion birds. Hunting and habitat loss certainly drove a decline in numbers, but this was a bird of huge communal groups and once the boundary was passed, extinction was inevitable.

Lonesome George says hello

These last individuals have been termed endlings. Dolly Jorgensen explores the derivation and use of the word. With the death of that individual (the endling), the species as a whole is extinct. And while there are the higher profile endlings there are clearly more and more endlings which will die unnoticed and undocumented in the wild.

Working with extinction rates alone it can be hard to see how great the declines in biodiversity are. If we examine the animal populations other evidence of biodiversity loss emerges. The World Wildlife Fund (WWF) Living Planet Report 2016 states that "population sizes of vertebrate species have, on average, dropped by more than half in little more than 40 years". That’s a fairly dramatic loss in numbers. In the same time, the human population has grown from about four billion to 7.3 billion.

A sumatran orangutan at the Sumatran Orangutan Conservation Programme's rehabilitation centre in Indonesia. Photo: Ulet Ifansasti/Getty Images

An article by Gerardo Ceballos, Paul R. Ehrlich and Rodolfo Dirzo says it all in the title: Biological annihilation via the ongoing sixth mass extinction signaled by vertebrate population losses and declines. In their study, 40 percent of the 177 mammal species they surveyed had experienced severe population decline.

It's not just large animal populations in decline and declines are being recorded in what are protected areas. Just last year, a study of flying insects in protected sites in Germany found a decline in biomass of 72 percent over 27 years and this decline was up to 82 percent in summer months. From the study, it is not clear what the cause of these drastic declines are. However, given the role that insects play in ecosystems, the loss in biomass must have consequences for insect feeding birds and mammals, the pollination of plants and the general functioning of ecosystems.

One of the less than 150 corncrakes present in Ireland

In Ireland, we have seen how changes in farming practices, such as a move from saving hay to silage, has led to serious declines in breeding populations of corncrake. The National Parks and Wildlife Service 2015 report  documents the decline in corncrake numbers from over 4,000 in the 1970s to a low of 150 in recent years. An ongoing programme of support for farmers to manage grasslands in a "corncrake friendly’ manner and monitoring of the population reflects the value put on the bird in Ireland. Globally, the corncrake is in a healthy state with populations of over two million corncrakes in Russia, but the species is threatened in Ireland. If the corncrake has almost disappeared from the Irish landscape, what other species, mammals, birds, insects and plants have been impacted?

If animal populations continue this decline across many speciesm what sort of world are we heading into? Biotic homogenization: a few winners replacing many losers in the next mass extinction is the aptly titled article by Michael L. McKinney and Julie L. Lockwood which suggests a world where only species adapted to a human modified world will thrive.

This homogenized world is described by Stephen Meyers in The End of the Wild as one where there are three types of species. The weedy species are plants, animals and other organisms that thrive in continually disturbed humane dominated environments - these are the organisms that thrive in our cities, cultivated lands and in an environment of rapidly changing climates. Relic species do not thrive in human dominated environments and these species require regular restocking and management to survive outside zoos.

From RTÉ Radio One's Countrywide, Anita Donaghy of Birdwatch Ireland discusses the Irish curlew, another species of wetland bird under threat

One step beyond this are the ghost species, these are on the brink of extinction. Meyers notes that there are 10,000 tigers living as pets in the United States and less than 7,000 living in the wild. While pessimistic ("the extinction crisis is over - we have lost"), Meyers still sees merit in the development of natural area trusts, NATS, very large areas of protected lands that allow a broad range of species to thrive.

As with climate change, scientists have been pointing to biodiversity loss for a number of decades now. The Millennium Ecosystem Assessment, an international assessment of ecosystem health across the globe, has a comprehensive report on biodiversity loss and actions needed to address it. Again, as with climate change the solutions can only be implemented with strong political will and politicians will only act when voters demand they do.

The views expressed here are those of the author and do not represent or reflect the views of RTÉ