Energy Pyramids and Efficiency
As energy moves throughout a food web, the overall amount of energy decreases as it moves from lower to higher trophic levels. Producers contain the most energy, and typically have the highest amount of biomass and/or number of organisms within an ecosystem. Primary consumers have less total energy than producers, because much of the energy they consume is used up by their metabolism and released as heat when one organism eats another. Energy continues to decrease at each successive level, with the result that each higher trophic level typically has less biomass and fewer organisms than the one below.
Energy Pyramids
These relationships can be illustrated through energy pyramids, as the image below shows:
Energy Efficiency and Trophic Levels
In terms of energy, only about 10% of kilocalories (food calories) are passed between each trophic level. This means that primary consumers only get 10% of the available energy from primary consumers. Secondary consumers only receive 10% of the available energy from primary consumers, or 1% of the total from producers. Tertiary consumers only receive 0.1% of the total energy converted from solar energy by primary producers. This means that for every 1000 kilocalories of energy produced by primary producers, a tertiary consumer will only receive 1 calorie!
This means that the bottom of the food chain - primary producers - is where the most efficient consumption of resources happens. At this trophic level, energy input toward food production is lowest and biomass is highest.
If we apply these relationships to human food consumption, this means that eating meat is highly inefficient in terms of the amount of energy, because the available energy has been cut by 90%. Take beef as an example: grain must first be fed to the cattle, then the cattle is consumed by humans. If a cow eats 800 kg of grain, that is reduced to only 80 kg of meat. This means that eating animal products requires much more energy input per calorie produced than eating plant products.
Put another way, if 1000 people could be fed by eating an herbivorous, plant-based diet (i.e. eating as primary consumers), only 100 people could be fed the same number of calories eating a carnivorous diet (i.e. eating as secondary consumers). The energy efficiency of food production has major implications for global food security and the amount of land used to produce food. Far more people can be fed from the same amount of land if they are eating as primary consumers rather than as secondary consumers. Thus, meat consumption has a far higher footprint in terms of the amount of land necessary to sustain it than plant consumption.