Biological annihilation via the ongoing sixth mass extinction signaled by vertebrate population losses and declines (2017) Ceballos et al., Proceedings of the National Acadeemy of Sciences of the USA, DOI: https://doi.org/10.1073/pnas.1704949114

Guest Post by Jonatan Marquez

The Crux

The rate at which species and populations have been going extinct in the last couple of centuries has well and truly earned the title of the planet’s sixth mass extinction event. However, most people rarely realize the severity of the situation. Hearing about the loss of two vertebrate species a year or having the last of some far-off species die out doesn’t see to cause much concern in the general public.

A species extinction is always preceded by population declines and extinctions. Perhaps highlighting the state of natural communities at this level might put the severity of the situation in better context. For example, the Living Planet Index (LPI) estimates that between 1970 and 2012, wildlife abundance has decreased by 58%. This paper focuses on the state and trends of populations of vertebrates by analysing i) the proportion undergoing declines or shrinkages, ii) the global distribution of population reduction events and iii) the general scale of population declines among mammal populations.

How it Works

The study used species data from the International Union for Conservation of Nature (IUCN) to assess the number of vertebrate species (i.e. mammals, birds, reptiles and amphibians) undergoing range contractions or abundance reductions. They also retrieved spatial data to make maps showing the distribution of species richness, declining species and the proportion of species declining by region.

An additional in-depth analysis of population extinctions was applied to 177 mammal species. Their current ranges were compared to their historical ranges to calculate the percentage of area coverage lost.

Did you know: Global Isotherms Global isotherms are the regions (e.g. latitude or altitude) of the globe that lie in a particular temperature range. As the global temperature rises, isotherms shift towards the poles. This means that if, for example Morocco used to be covered by an isotherm that includes areas of up to 22 degrees Celsius, with climate change this isotherm will now envelope Spain, and Morocco will be enveloped by a new isotherm rising from the south. Many species are expected to follow these isotherms so they can stay within their optimal temperature. Therefore, some populations in Spain may decide to move to France, so their populations in Spain would decline. This might sound like populations shifting, not declining, but often barriers between regions trap species (e.g. the Strait of Gibraltar between Morocco and Spain), cause the population to instead shrink and eventually go extinct.

What They Found

There are a huge number of population losses and species declines across the globe. The amount of species loss was generally greater closer to the equator, where species richness is higher too. However, the pattern was not uniform across the globe, nor among species groups. Regions like the Amazon forest or South-East Asia had the greater number of species declining per 10,000 km2 (the map’s cell size), while Madagascar seemed like a hotspot for reptilian extinctions in particular. The distribution of reptiles and amphibian species decreasing differed from that of birds and mammals. On the other hand, the proportion of decreasing species was generally greater towards the North Pole than near the equator, especially for birds.

Problems

Carrying out such a large-scale study has some limitations (e.g. the resolution of the map, the definition of a population used) that may compromise the accuracy of the results. However, the general results and message of the study would not change regardless of these details.

Instead, I found more worrying the emphasis on the negative. Because of climate change, many species or population around the globe are moving their ranges to colder regions (towards the poles, higher latitudes or greater depths). This means that their ranges will shrink on one side but expand on the other, and I did not see them mention these expansions anywhere.

Also, the study shows two types of maps, one with number of species with disappearing or decreasing populations and one with their proportions relative to the number of species in the area. The patterns are very different, and it begs the question, which one is more worrisome.

So What?

Humans do not seem to be able of understanding the gravity of losing two vertebrate species a year because of the large time scale of the issue. It’s a concept we call shifting baselines – the concept that we’ve become used to such extinction and start to perceive it as a norm. Species that once may have littered our surrounding regions may not have been there in our lifetime, so we do not see a problem. So highlighting the rate at which populations are dwindling is expected to put things into a more understandable perspective. Studies such as these show that the problems we may perceive as minor are in fact part of a global trend that is nothing short of catastrophic, after all, it has only happened six times in the earth’s history.