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Human vs Livestock vs Wild mammal biomass on Earth

At some time in recent pre-history, there were fewer than 100,000 humans, hunter gatherers all, while  the rest of mammal diversity was wild. Then humans domesticated plants and animals, and together they came to dominate the landscape in a  very short time.

A handy way to measure this is by estimating biomass. This is because small mammals are difficult to count, their numbers change rapidly, and you might also consider that they have less of an impact than larger mammals. Its not a terrible assumption to start with assuming 200kg of mice have a similar impact  as a 200kg antelope, although its very interesting to explore how their impacts differ. For our purposes, biomass gives a pretty relevant picture of the problem.  If you can guess at an acceptable ratio of humans and livestock to wild mammals, what would that look like on a healthy planet earth? 20% humans to 80% EVERYTHING else? Maybe you think 50-50 would be sustainable?

Well if you’re like me, then you would probably be pretty blown away by the idea that humans alone outweigh all the remaining terrestrial mammals on the planet by about seven times! And because prey always outweighs their food, the livestock bred to feed mankind weighs double that again. This is more clearly demonstrated in the graph below. Luckily we are omnivores, as the food for pure carnivores usually outweighs the carnivore by a factor of around ten to ensure sustainable harvest (otherwise both populations usually crash).

This outrageous imbalance is a pretty dire picture of the state of the worlds remaining ecosystems, and I hope it gives you some pause when you imagine the BBC wildlife images of vast plains of wildebeest and caribou as being anything substantial.

Botswana has one o the largest populations of elephants in Africa, and so the total picture looks pretty good, with wild biomass accounting for one quarter of the mammal biomass in the country, but considering that until very recently that picture was more like 99% wild animals and 1 % Kalahari Bushman, you can imagine that the effect on the ecosystem is serious.



Background to the calculations: Canadian economist and mathematician Vaclav Smil has calculated the best estimates for the biomass of terrestrial mammals on planet earth, dividing them into human, livestock and remaining wild mammals, and the picture is pretty stark. The calculations are surprisingly easy to do, since we do know a lot about the number of people, average mass of a human over the last century, and also the number of larger livestock on the planet. Since even a few larger animals make such a large contribution to the biomass than hundreds of thousands of smaller mammals, the livestock estimate is quite accurate. Our knowledge of cattle is great despite our lack of knowledge of small stock numbers, and the same goes for wild mammals : we have a good handle on the upper limits for numbers of elephants, wildebeest, deer, kangaroos etc, and mouse and meerkat populations have little impact.

I demonstrated this clearly on a smaller scale with the Botswana figures (lower panel), where we know the human population, the cattle population, and the elephant population very well. Playing with large variation in estimates of small stock and smaller antelope species did little to alter the final picture.


Global Source:Harvesting the Biosphere: The Human Impact by Vaclav Smil

Botswana Sources: Department of National Parks and Wildlife, Botswana Census Bureau, Botswana Meat Corporation

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