Whether you focus on mammals, birds, reptiles or amphibians – land vertebrates are in rapid decline everywhere around us, illustrating a general decline of Life on Earth – and a prelude to the Holocene-Anthropocene Mass Extinction, that is being underestimated in speed and severity.

The reason of this underestimation: demography – the decline of species populations. This is not always counted in when assessing the global ecological crisis, but if you do, you get to see that the actual trend is much worse than most people think…

Really a very interesting but also sobering way to look at extinction risk: demography – the decline of populations, expressed geographically. (Left is species richness, centre is species decline, right is number of species that are experiencing population losses – that’s an interesting category to look at…

When ecologists talk about demography they don’t mean human demography (although that times consumption is of course the driver of it all) – but population demography for all of Earth’s biodiversity.

And oddly enough – when studying extinction risk – that tool (counting the numbers, not just individuals, also the total number of populations) tends to get overlooked, we learn from a specialised research team, consisting of ecologists Gerardo Ceballos of the National Autonomous University of Mexico and Paul Ehrlich and Rodolfo Dirzo of Stanford University.

In May this year they published a study in PNAS under a very dire title (‘Biological annihilation via the ongoing sixth mass extinction signaled by vertebrate population losses and declines’) and it they signal there’s an “extremely high population decay in vertebrates” going on – even among common species.

These common species, like the birds you see everyday in your own backyard – species you tend to take for granted – are often ignored when assessing the severity and speed of the unfolding Holocene-Anthropocene Mass Extinction, sometimes referred to as the Sixth Mass Extinction [a classification we personally don’t like so much – Earth’s geologically history is not that tightly organised].

Two important terms: population extinctions – and intraspecific biodiversity

But what’s possibly more important than simply monitoring the absolute numbers of individual species [arduous as that may be], is monitoring general demographic health of species.

That’s because almost all species on Earth live in different, often separated communities – that are independent populations. And as general numbers within species decline, there is widespread ‘population extinction’ going on – also of species that are overall still considered to be common and unthreatened.

Extinctions of smaller population should be extrapolated as a general warning for all species affected, the authors state – as the current mass extinction is in effect one giant downward trend, one that tends to get underestimated, when only actual species extinctions are counted:

“Roughly a third (8,851/27,600) of all land vertebrate species examined are experiencing declines and local population losses of a considerable magnitude. Such proportion of decreasing species varies, depending on the taxonomic group, from 30% or more in the case of mammals, birds, and reptiles, to 15% in the case of amphibians. Furthermore, of the decreasing species, many are now considered endangered. Beyond that, roughly 30% of all decreasing species are still sufficiently common that they are considered of “low concern” by IUCN, rather than “endangered.” That so many common species are decreasing is a strong sign of the seriousness of the overall contemporary biological extinction episode.”

These smaller populations often harbour an important part of the genetic diversity of species – so population extinctions should also be linked to something else: the decline of intraspecific biodiversity – another important way to quantify the mass extinction, and something that’s equally undesirable as the loss of numbers of species, the usual definition.

Only way to stop extinction wave is ending human overconsumption – ‘window of action is 2, 3 decades at most’

The relative role that climate change plays in the unfolding Holocene-Anthropocene Mass Extinction? The authors of the PNAS study pile all extinction drivers together, stating they act synergistically – and act on a philosophical and powerful note:

“The likelihood of this rapid defaunation lies in the proximate causes of population extinctions: habitat conversion, climate disruption, overexploitation, toxification, species invasions, disease, and (potentially) large-scale nuclear war — all tied to one another in complex patterns and usually reinforcing each other’s impacts.”

“Much less frequently mentioned are, however, the ultimate drivers of those immediate causes of biotic destruction, namely, human overpopulation and continued population growth, and overconsumption, especially by the rich. These drivers, all of which trace to the fiction that perpetual growth can occur on a finite planet, are themselves increasing rapidly. Thus, we emphasize that the sixth mass extinction is already here and the window for effective action is very short, probably two or three decades at most. All signs point to ever more powerful assaults on biodiversity in the next two decades, painting a dismal picture of the future of life, including human life.”

© Rolf Schuttenhelm | www.bitsofscience.org