Humans have destroyed 83 percent of wild mammals and reduced the total biomass of the biosphere to half of its pre-human value

Desdemona Despair

By Damian Carrington
21 May 2018

(The Guardian) – Humankind is revealed as simultaneously insignificant and utterly dominant in the grand scheme of life on Earth by a groundbreaking new assessment of all life on the planet.

The world’s 7.6 billion people represent just 0.01% of all living things, according to the study. Yet since the dawn of civilisation, humanity has caused the loss of 83% of all wild mammals and half of plants, while livestock kept by humans abounds.

The new work is the first comprehensive estimate of the weight of every class of living creature and overturns some long-held assumptions. Bacteria are indeed a major life form – 13% of everything – but plants overshadow everything, representing 82% of all living matter. All other creatures, from insects to fungi, to fish and animals, make up just 5% of the world’s biomass.

Another surprise is that the teeming life revealed in the oceans by the recent BBC television series Blue Planet II turns out to represent just 1% of all biomass. The vast majority of life is land-based and a large chunk – an eighth – is bacteria buried deep below the surface.

“I was shocked to find there wasn’t already a comprehensive, holistic estimate of all the different components of biomass,” said Prof Ron Milo, at the Weizmann Institute of Science in Israel, who led the work, published in the Proceedings of the National Academy of Sciences.

“I would hope this gives people a perspective on the very dominant role that humanity now plays on Earth,” he said, adding that he now chooses to eat less meat due to the huge environmental impact of livestock. […]

The destruction of wild habitat for farming, logging and development has resulted in the start of what many scientists consider the sixth mass extinction of life to occur in the Earth’s four billion year history. About half the Earth’s animals are thought to have been lost in the last 50 years.

But comparison of the new estimates with those for the time before humans became farmers and the industrial revolution began reveal the full extent of the huge decline. Just one-sixth of wild mammals, from mice to elephants, remain, surprising even the scientists. In the oceans, three centuries of whaling has left just a fifth of marine mammals in the oceans. [more]

Humans just 0.01% of all life but have destroyed 83% of wild mammals – study

ABSTRACT: A census of the biomass on Earth is key for understanding the structure and dynamics of the biosphere. However, a global, quantitative view of how the biomass of different taxa compare with one another is still lacking. Here, we assemble the overall biomass composition of the biosphere, establishing a census of the ≈550 gigatons of carbon (Gt C) of biomass distributed among all of the kingdoms of life. We find that the kingdoms of life concentrate at different locations on the planet; plants (≈450 Gt C, the dominant kingdom) are primarily terrestrial, whereas animals (≈2 Gt C) are mainly marine, and bacteria (≈70 Gt C) and archaea (≈7 Gt C) are predominantly located in deep subsurface environments. We show that terrestrial biomass is about two orders of magnitude higher than marine biomass and estimate a total of ≈6 Gt C of marine biota, doubling the previous estimated quantity. Our analysis reveals that the global marine biomass pyramid contains more consumers than producers, thus increasing the scope of previous observations on inverse food pyramids. Finally, we highlight that the mass of humans is an order of magnitude higher than that of all wild mammals combined and report the historical impact of humanity on the global biomass of prominent taxa, including mammals, fish, and plants.[…]

Pre-human biomass [Appendix]

The impact of humans on the distribution of biomass on Earth has begun long before present times. For example, a large extinction event, the Quaternary Megafauna Extinction, which occurred 50,000-7,000 years ago, is at least partially explained by human hunting and habitat alteration (175). This extinction claimed a half of megafaunal species (defined as species weighing over 100 lb, i.e., 44 kg). We estimate the total wild mammal biomass before the Quaternary Megafauna Extinction event and compare it with present day values. The overall biomass of wild mammals is dominated by large mammals, i.e., megafauna (173). Moreover, the estimated megafauna biomass before the Quaternary Megafauna Extinction event is dominated by mammals (175). Therefore, the biomass values of megafauna given below are a good approximation to to the overall biomass of wild mammals.

The global biomass of terrestrial megafauna before the Quaternary Megafauna Extinction event is estimated to have been around ≈0.02 Gt C (175). The global biomass of marine wild mammals before human exploitation is estimated at ≈0.02 Gt C (178). The total wild mammal biomass before the Quaternary Megafauna Extinction event of ≈0.04 Gt C is about 6-fold higher than the ≈0.007 Gt C of extant wild mammal biomass. We cannot currently derive the uncertainty associated with the change in wild mammal biomass before and after human civilization, as we do not have a projection for the uncertainty associated with the pre-human wild land mammal biomass. We hope further research will help better assess the certainty of estimates regarding the biomass of pre-human wild land mammals. As for the pre-human marine wild mammal biomass, Christensen (178) reports a 95% confidence interval of ≈1.3-fold.

Regarding the change in wild marine mammal biomass, Christensen reports a range of 2.4-fold to 7-fold. At the same time that the biomass of wild megafauna collapsed, the biomass of humans gradually increased over the same period. Since the industrial revolution we have witnessed an exponential increase in human population, as well as a rapid increase in the domesticated livestock biomass. Today, the biomass of livestock (≈0.1 Gt) is an order of magnitude larger than that of all the terrestrial wild megafauna before the Quaternary Megafauna Extinction. Even the biomass of humans alone (≈0.05 Gt) is around twice the size of the biomass of all wild megafauna before the Quaternary Megafauna Extinction event.

In addition to the influence humans had on mammal biomass, the expansion of human population in recent centuries has dramatically changed many habitats. One prominent impact humans have had on wild populations was caused by fishing. In order to estimate the relation between current fish biomass and pre-human fish biomass, we rely on models of fishery ecology (291), in conjunction with data on the current state of fisheries. A recent study (292), surveyed the status of fisheries which account for ≈80% of the global fish catch, and estimated that fish biomass is ≈120% of the biomass which will ensure maximum sustainable yields (see Table S12 in Costello et al). Costello et al. puts current fisheries biomass at ≈0.84 Gt wet weight (292), or ≈0.13 Gt C, which is on par with previous estimates for the non-mesopelagic fish biomass (161). Using a database of published landings data and stock assessment biomass estimates, Thorson et al. (293) estimate that the biomass of fish at the maximum sustainable yield represent ≈40% of the biomass the population would have reached in case of no fishing. From these studies, we estimate that before human civilization started fishing, the biomass of fisheries was ≈2-fold larger than current values (1/0.4×1/1.2). If we take estimates for present day fisheries biomass at ≈0.13 Gt C, this translated to ≈0.27 Gt C of fisheries before human civilization (link to full calculation). Assuming that the biomass of mesopelagic fish was not influenced significantly by fishing, we estimate the total global biomass of fish before human civilization across all depths at≈0.8 Gt C.

Many of the impacts humans had on the biomass of wild populations are hard to quantify, as accurate estimates of the standing stocks of fauna before human development are missing. For plants, however, some estimates are available. The biomass of plants is dominated by trees. Estimates put the global biomass of trees before human civilization at around twice its current value (294). As plants are the dominant fraction of global biomass, this means that humans have reduced the total biomass of the biosphere to about half of its pre-human value. [more]

The biomass distribution on Earth