To understand how the richness of the mammal kingdom has changed we need a metric that captures a range of different animals and is comparable over ti
To understand how the richness of the mammal kingdom has changed we need a metric that captures a range of different animals and is comparable over time. We could look at their abundance – the number of individuals we have – but this is not ideal. We would be counting every species equally, from a mouse to an elephant and this metric would therefore an ecosystem taken over by the smallest mammals look much richer than one in which bigger mammals roam: if the world’s mouse populations multiplied and multiplied – maybe even to the detriment of other animals – then this abundance metric might suggest that these ecosystems were thriving.
Instead, ecologists often use the metric biomass. This means that each animal is measured in tonnes of carbon, the fundamental building block of life.1 Biomass gives us a measure of the total biological productivity of an ecosystem. It also gives more weight to larger animals at higher levels of the ecological ‘pyramid’: these rely on well-functioning bases below them.
I have reconstructed the long-term estimates of mammal terrestrial biomass from 100,000 BC through to today from various scientific sources.2 This means biomass from marine mammals – mainly whales – is not included. The story of whaling is an important one that I cover separately here. This change in wild land mammals is shown in the chart. When I say ‘wild mammals’ from this point, I’m talking about our metric of biomass.
If we go back to around 100,000 years ago – a time when there were very few early humans and only in Africa – all of the wild land mammals on Earth summed up to around 20 million tonnes of carbon. This is shown as the first column in the chart. The mammoths, and European lions, and ground sloths were all part of this.
By around 10,000 years ago we see a huge decline of wild mammals. This is shown in the second column. It’s hard to give a precise estimate of the size of these losses millennia ago, but they were large: likely in the range of 25% to 50%.3
It wasn’t just that we lost a lot of mammals. It was almost exclusively the world’s largest mammals that vanished. This big decline of mammals is referred to as the Quaternary Megafauna Extinction (QME). The QME led to the extinction of more than 178 of the world’s large mammals (‘megafauna’).
Many researchers have grappled with the question of what caused the QME. Most evidence now points towards humans as the primary driver.4 I look at this evidence in much more detail in a related article. Most of this human impact came through hunting. There might also have been smaller local impacts through fire and other changes to natural landscapes. You can trace the timing of mammal extinctions by following human expansion across the world’s continents. When our ancestors arrived in Europe the European megafauna went extinct; when they arrived in North America the mammoths went extinct; then down to South America, the same.
What’s most shocking is how few humans were responsible for this large-scale destruction of wildlife. There were likely fewer than 5 million people in the world.5 Around half the population of London today.6
A global population half the size of London helped drive tens to hundreds of the world’s largest mammals to extinction. The per capita impact of our hunter-gatherer ancestors was huge.
The romantic idea that our hunter-gatherer ancestors lived in harmony with nature is deeply flawed. Humans have never been ‘in balance’ with nature. Trace the footsteps of these tiny populations of the past and you will find extinction after extinction after extinction.