4.2: Humans reshaped the planet’s biomass
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Figure \(\PageIndex{1}\). Impact of human civilization on mammal biomass. Biomass of wild mammals, livestock (dominated by cattle), and humans before human civilization (BP, Before Present) and at the present time. [Schematic after Bar-On et al. ref [4]; Open access article distributed under CC BY-NC-ND 4.0 license]
Comparison of current global biomass with pre-human values (which are very difficult to estimate accurately) allows us to assess the impact of humans on the biosphere (Figure \(\PageIndex{1}\)):
- Human activity contributed to the Quaternary megafauna extinction between ≈50,000 and ≈3,000 years ago, which took away about half of the large (>40 kg) terrestrial mammal species [33]. The biomass of wild terrestrial mammals prior to this extinction period was estimated by Barnosky [29] to be ≈0.02 Gt C. The current biomass of wild terrestrial mammals would be about seven times lower, at ≈0.003 Gt C.
- Intense whaling and exploitation of other marine mammals have resulted in an approximately fivefold decrease in overall marine mammal biomass (from ≈0.02 Gt C to ≈0.004 Gt C [34]).
- While the total biomass of wild mammals (marine and terrestrial) has decreased by about a factor of 6, the total mammal mass increased approximately fourfold, from ≈0.04 Gt C to ≈0.17 Gt C due to the vast increase in the biomass of humanity and its associated livestock.
Human activity has also impacted global vertebrate stocks, with a decrease of ≈0.1 Gt C in total fish biomass, an amount similar to the total biomass remaining in fisheries and tto the gain in the total mammal biomass due to livestock husbandry.
Figure \(\PageIndex{2}\). Agriculture reshaped plant biomass. A, Daintree Rainforest, Queensland, Australia, the world’s oldest continuously surviving rainforest [Photo © Robert Linsdell from St. Andrews, Canada, CC BY 2.0, via Wikimedia Commons]. B, winter wheat germination at Open Grounds Farm in Beaufort, North Carolina, USA [Soil-Science.info, CC BY 2.0, via Wikimedia Commons]
The impact of human civilization on global biomass has not been limited to mammals but has also profoundly reshaped the total amount of carbon sequestered by plants (Figure \(\PageIndex{2}\)). A global census of the total number of trees [35], as well as a comparison of actual and potential plant biomass [6], suggested that total plant biomass (and, by extension, total biomass on Earth) has been cut roughly in half from its value before the onset of human civilization. The total biomass of plants grown by humans is estimated to be ≈10 Gt C, which is only ≈2% of the total existing plant biomass [6].