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I'm not sure why it's so difficult to understand. Eating lower down the trophic levels is energy efficient, and the energy level is proportional to environmental destruction, water use, and pollution. This is especially relevant if you have a large population to maintain (food security), which is the case for humans.

Eating up the world’s food web and the human trophic level | PNAS

Abstract

Trophic levels are critical for synthesizing species’ diets, depicting energy pathways, understanding food web dynamics and ecosystem functioning, and monitoring ecosystem health. Specifically, trophic levels describe the position of species in a food web, from primary producers to apex predators (range, 1–5). Small differences in trophic level can reflect large differences in diet. Although trophic levels are among the most basic information collected for animals in ecosystems, a human trophic level (HTL) has never been defined. Here, we find a global HTL of 2.21, i.e., the trophic level of anchoveta. This value has increased with time, consistent with the global trend toward diets higher in meat. National HTLs ranging between 2.04 and 2.57 reflect a broad diversity of diet, although cluster analysis of countries with similar dietary trends reveals only five major groups. We find significant links between socio-economic and environmental indicators and global dietary trends. We demonstrate that the HTL is a synthetic index to monitor human diets and provides a baseline to compare diets between countries.

This first estimate of HTL at 2.21, i.e., a trophic level similar to anchoveta and pigs, quantifies the position of humans in the food web and challenges the perception of humans as top predators (2). Humans dominate ecosystems through changes in land use, biogeochemical cycling, biodiversity, and climate (11, 13, 14). It is not sufficient to separate humans from analyses of ecosystem processes, because there are no remaining ecosystems outside of human influence (15). Thus, investigations of ecosystems, without accounting for the presence of humans, are incomplete (13). There is a variety of other ecological indicators based on trophic ecology theory or diets, e.g., the omnivory index, that may also prove useful in assessing the impact of humans in the functioning of ecosystems. However, a first estimate of an HTL gives us a basic tool that places humans as components of the ecosystem and assists in further comprehending energy pathways, the impact of human resource use, and the structure and functioning of ecosystems.

The global increase in HTL is consistent with the nutrition transition that is expected to continue for several decades (16, 17) from plant-based diets toward diets higher in meat and dairy consumption (18–22). This 0.15 increase in HTL from 1961 to 2009 is mainly due to the increased consumption of fat and meat (SI Appendix, Figs. S5–S8), as opposed to a shift toward the consumption of species with higher trophic levels. In fact, we find that the mean trophic level of terrestrial animals that are consumed by humans has only slightly increased (by 0.01 or 0.5%) due to the higher proportion of pork and poultry in the diet (SI Appendix, Fig. S11_A_), whereas that of marine animals has decreased markedly from 2.88 in 1961 to 2.69 in 2009 (SI Appendix, Fig. S11_B_). This decline in the trophic levels of marine food items in human diets is consistent with the global decline in the mean trophic level of marine fisheries catches. This decline has been related to the consequences of fishing pressures on marine predators (23), although changes in the characteristics of fisheries over time may also influence this trend (24).

The global convergence in HTL is consistent with the convergence in diet structure between countries with diverse levels of development (18, 19), and in agreement with previous studies of the FAO (17, 25). Globalization and economic development facilitate the access to diverse foodstuffs and can enhance the rate of this convergence (18, 26). For India, China, and countries in groups 1–3, HTLs are low and rising. With economic growth, these countries are gaining the ability to support the human preference for high meat diets (18, 19, 26). For countries 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 [i.e., lower their HTL; SI Appendix, Figs. S4–S8 (18, 20, 22)]. Similarly, countries with high initial HTLs (i.e., group 5) show decreasing trends with time (Fig. 3). For Scandinavian countries, this decline is due to government policies promoting healthier diets (18, 22). For example, in 2011, Sweden consumed historically high levels of meat due to low market prices, leading the Swedish government into discussions of a Pigovian tax to reduce this consumption (27). Changes in diet in Mauritania (decreased meat and dairy consumption) and Mongolia (increased proportion of vegetables) are linked to increased urbanization and economic development and decreased nomadism.