Despite what many of those who advocate meat-eating would like to believe, humans do not sit at the top of the food chain. In any event, it’s a food web rather than a chain, due to the many complex interactions involved.
An article commenting on our position in the food web was published in the Proceedings of the National Academy of Sciences (PNAS) in late 2013. 
According to the head of the research team, Sylvain Bonhommeau of the French Research Institute for Exploitation of the Sea in Sète, “We are closer to herbivore than carnivore. . . . It changes the preconception of being top predator.” 
The article considered the trophic level of different species and nations. Trophic levels “represent a synthetic metric of species’ diet, which describe the composition of food consumed and enables comparisons of diets between species”.
A species’ trophic level is calculated as the average of trophic levels of food items in its diet, weighted by quantity, plus one.
If an animal were to eat nothing but cows, its trophic level would be 3, calculated as the sum of 2 (the cow’s trophic level as referred to below) and 1. The trophic level of another animal that were to only eat that animal would be 4, and so on.
Plants and other “primary producers”, such as phytoplankton, have a trophic level of 1. A species that consumes only plants, such as a cow or elephant, has a trophic level of 2. The trophic level of apex predators, such as polar bears and killer whales is 5.5.
The researchers reported that the global median human trophic level (HTL) in 2009 was 2.21, representing a 3 percent increase since 1961. The authors said, “In the global food web, we discover that humans are similar to anchovy or pigs and cannot be considered apex predators”.
Here’s how the rankings of a few species can be depicted, without attempting to display the complex interactions involved:
Figure 1: Some examples of trophic levels
A major concern in terms of the environment and the rights of animals is the increasing overall human trophic level, driven largely by growing levels of meat consumption in China and India. The authors stated, “With economic growth, these countries are gaining the ability to support the human preference for high meat diets”.
Figure 2: Trends in human trophic level (1961-2009)
Since 1960, we have seen a reduction in the percentage of plants in the human diet and a corresponding increase in the percentage of terrestrial and marine animals.
Figure 3: Percentage of plants and animals in the human diet
Some climate change implications
Animal agriculture is a key contributor to increasing concentrations of greenhouse gases such as carbon dioxide, methane and nitrous oxide. Land clearing for livestock grazing and feedcrop production, in addition to releasing massive amounts of carbon, has reduced the biosphere’s ability to draw down existing carbon. According to leading climate scientist, Dr James Hansen, we must reduce atmospheric concentrations of carbon dioxide to around 350 ppm (parts per million) if we are to overcome the threat of climate change. Massive reforestation and restoration of soil carbon is required in order to achieve that target.  In April, 2014, carbon dioxide concentrations reached 401.9 ppm. 
It seems ironic that China is contributing to the problem by increasing its meat consumption. The Chinese leadership would surely understand the extreme dangers posed by climate change, including a potential loss of dry-season water flows into key river systems due to the potential loss of glaciers.
Climate change author, David Spratt, has stated :
“Taken together with those on the neighbouring Tibetan plateau, the Himalayan–Hindu Kush glaciers represent the largest body of ice on the planet outside the polar regions, feeding Asia’s great river systems, including the Indus, Ganges, Brahmaputra, Salween, Mekong, Yangtze and Huang He. The basins of these rivers are home to over a billion people from Pakistan to China. The Himalayas supply as much as 70 per cent of the summer flow in the Ganges and 50–60 per cent of the dry-season flow in other major rivers. In China, 23 per cent of the population lives in the western regions, where glacial melt provides the principal dry season water source. The implications of the loss of the Himalayan ice sheet are global and mind numbing, but such a calamity rarely rates a mention in Australia.”
Australia seems happy to help China to satisfy its growing taste for red meat by expanding its exports. 
The existence of critical environmental externalities in beef production means that the Chinese and other consumers of Australian meat are paying a fraction of the product’s true cost.
Meanwhile, the Chinese maintain a population of nearly 500 million pigs, which is just under half the global population. . Those pigs consume enormous amounts of soy from overseas, including soy grown in the Amazon and cerrado regions of South America. Both regions contain massive stores of carbon, which are released through land clearing for feedcrop production (including soy) and livestock grazing. 
Figure 4: Soybean Production, Consumption and Imports in China 1964-2011
China’s projected soy bean imports for 2014/15 are 72 million tonnes. The second-ranked importer is the European Union, with 12.5 million tonnes. 
With domestic production of 12 million tonnes, China’s total consumption in 2014/15 is 84 million tonnes, up from approximately 70 million tonnes in 2011 (including imports of 59 million tonnes).
Only around 10 percent of the soybeans used in China are consumed directly as food by humans. The other 90 percent are crushed, separating the oil and meal, with the latter widely used in animal feed rations. 
Some health implications
The PNAS paper categorised countries into five groups:
Health concerns have been a key driver of HTL reductions in countries within Groups 4 and 5.
In Group 4, “the nutrition transition has reached a point where health problems associated with high fat and meat diets (i.e., high HTLs) have led to changes in policy and government-run education programs that encourage these populations to shift to more plant-based diets”.
The reductions in HTLs within Scandinavian countries (Group 5) “is due to government policies promoting healthier diets”.
Rising meat consumption in China and India is likely to lead to a marked increase in rates of diseases and conditions such as heart disease, certain cancers, obesity and diabetes. 
According to the American Dietetic Association, well-planned plant-based diets are appropriate for individuals during all stages of the life cycle. 
As such, the world’s human population could aim for a trophic level of 2, with critical environmental and health benefits, not to mention the reduction in animal exploitation and cruelty.
For Australian and New Zealand readers, you should be aware that The Medical Journal of Australia has reported: “In contrast to the United States . . . Food Standards Australia New Zealand permits only a limited number of foods to be fortified with vitamin B12. This includes selected soy milks, yeast spread, and vegetarian meat analogues such as soy-based burgers and sausages.”  Vitamin B12 was once more readily available than at present to those on a plant-based diet without fortification or supplementation, in a manner that was far more natural than the forced breeding practices and ecosystem destruction that characterise the animal agriculture sector, past and present. 
Overall global livestock production is proceeding at unsustainable levels, with no sign of slowing down. If we wish to retain a habitable planet, we must urgently address the issue of diet in addition to fossil fuels.
The time to act is now!
Footnote: None of the material contained in this article should be construed as representing medical, health, nutritional, dietary or similar advice.
Author: Paul Mahony (also on Twitter, Slideshare, and Sribd).
 Bonhommeau, S., Dubroca, L., Le Pape, O., Barde, J., Kaplan, D.M., Chassot, E., Nieblas, A.E., “Eating up the world’s food web and the human trophic level”, Proc. Natl Acad. Sci. USA http://www.pnas.org/cgi/doi/10.1073/pnas.1305827110 (2013)
 Hoag, H., “Humans are becoming more carnivorous”, Nature, 2nd Dec, 2013, doi:10.1038/nature.2013.14282, http://www.nature.com/news/humans-are-becoming-more-carnivorous-1.14282
 Hansen, J; Sato, M; Kharecha, P; Beerling, D; Berner, R; Masson-Delmotte, V; Pagani, M; Raymo, M; Royer, D.L.; and Zachos, J.C. “Target Atmospheric CO2: Where Should Humanity Aim?”, 2008. http://www.columbia.edu/~jeh1/2008/TargetCO2_20080407.pdf
 Earth System Research Laboratory, Global Monitoring Division, Up-to-date weekly average CO2 at Mauna Loa, Week beginning on May 4, 2014 (401.9 ppm), http://www.esrl.noaa.gov/gmd/ccgg/trends/weekly.html
 David Spratt,“Global Warming – No more business as usual: This is an emergency!”, Environmental Activists’ Conference 2008: Climate Emergency – No More Business as Usual, 10 October, 2008, reproduced in Links International Journal of Socialist Renewal, http://links.org.au/node/683
 Binsted, T., “Australia poised to benefit from China’s beef demand”, The Age, 24 April, 2014, http://www.theage.com.au/business/australia-poised-to-benefit-from-chinas-beef-demand-20140424-375pt.html
 FAOSTAT, Live Animals, 2012, http://faostat.fao.org/site/573/DesktopDefault.aspx?PageID=573#ancor, accessed 12 May, 2014. (Actual number: 471,875,000 of a global population of 966,170,968)
 Brown, L.R., “Full Planet, Empty Plates: The New Geopolitics of Food Scarcity, Chapter 9, China and the Soybean Challenge”, Earth Policy Institute, 6 November, 2013, http://www.earthpolicy.org/books/fpep/fpepch9
 United States Department of Agriculture Foreign Agricultural Service Approved by the World Agricultural Outlook Board/USDA Circular Series, “Oilseeds: World Markets and Trade”, May 2014, http://apps.fas.usda.gov/psdonline/circulars/oilseeds.pdf
 Mahony, P., “If you thinks it’s healthy to eat animals, perhaps you should think again”, 12th February, 2013, http://terrastendo.net/2013/02/12/if-you-think-its-healthy-to-eat-animals-perhaps-you-should-think-again/
 Craig, W.J., Mangels, A.R., American Dietetic Association, “Position of the American Dietetic Association: vegetarian diets.”, J Am Diet Assoc. 2009 Jul;109(7):1266-82, http://www.ncbi.nlm.nih.gov/pubmed/19562864
 Zeuschner, C.L. et al., “Vitamin B12 and vegetarian diets”, MJA Open 2012; 1 Suppl 2: 27-32, 4 June 2012, https://www.mja.com.au/open/2012/1/2/vitamin-b12-and-vegetarian-diets
 Capps, A., “B12: A Magic Pill, or Veganism’s Achilles Heel?”, Free from Harm, 11 April, 2014, http://freefromharm.org/health-nutrition/b12-magic-pill-veganisms-achilles-heel/
Figure 1 – Prepared by author
Figure 2 – Bonhommeau, S. et al., op. cit., Figure 1 (A)
Figure 3 – ibid., Supporting Information, Figure 4
Figure 4 – Brown, L.R., op. cit., Figure 9–1 based on data from USDA, Production, Supply, and Distribution, electronic database, at www.fas.usda.gov/psdonline, updated 10 May 2012; D. H. Baker, “D.E. (Gene) Becker and the Evolution of the Corn-Soybean Meal Diet for Pigs,” Illinois Swine Research Reports (2003), pp. 101-04; Jack Cook, An Introduction to Hog Feeding Spreads (Chicago: Chicago Mercantile Exchange, 2009), p. 3.