Several organizations around the world have attempted to create graphical representations of what we should eat -- sometimes called "food pyramids" (current USDA version is a "food plate"). At first, it looks like this should work pretty well since food percentages are roughly 65/25/15 (carbohydrates/fats/proteins). Unfortunately, when this translates to actual food, most food is composed of a combination of nutritional elements. All fats are not considered to be of the same benefit and the effect of carbohydrates varies immensely depending on included fiber and other nutritional building blocks. Thus, it is difficult to use a pyramid to represent food needs. People still will often think of these food pyramids.
However, there are true food pyramids based on the needs and abundance of life on the planet. These are sometimes called trophic pyramids or energy pyramids. At the foundation level of these pyramids exists life that uses the energy from the sun (directly or indirectly) to manufacture food and body. On the land, these organisms are broadly called plants. In the sea, they are broadly called plankton -- although phytoplankton are the specific ones which are able to perform photosynthesis (creation from light).
These foundation foods (or primary producers) are eaten by the "higher layers" of the ecological food pyramids. It is possible for any organism to make use of them directly. For example, whales may feed on krill which are considered to be plankton (although they, in turn, make use of phytoplankton). In general, the lowest level are directly consumed by the next most abundant form of life. In a food pyramid, the next "level" can be determined by either number or function. Another way of putting it would be to think of a cartoon depiction of a very small fish being eaten by a small fish eaten by a medium fish eaten by a giant fish.
The organisms of the next level are called primary consumers. Primary consumers eat primary producers. So, herbivores are a general class of primary consumers. Although we usually think of mammals as herbivores, insects may be herbivores and worms might be considered to be herbivores.
The following level, sometimes called secondary consumers, may be either omnivores or carnivores. That is, they may eat a combination of producers (plants/phytoplankton) and primary consumers (or other secondary consumers) -- or they may be strictly carnivores that eat only other consumers. The "highest" level (in terms of the pyramid) eats only consumers.
These concepts are discussed in various ways -- food chains, food webs, ecological chains, and so forth. In whatever way they are approached, there are producers, primary consumers, secondary consumers, and tertiary consumers. The primary producers are always at the base.
The base determines the overall capacity of the entire pyramid. Thus, it is enormously important to protect that base. In the sea and on land, the largest threat is pollution although global climate change will certainly affect it in various ways. Note, however, that pollution can be either an aspect of waste ("garbage", runoff from managed land, etc.) or deliberate (even if accidental) contamination by oil and chemical spills and use of various chemicals within the food and non-food production chains.
As omnivores, humans have the capacity to shift their herbivore/carnivore balance -- they can be primarily meat eaters or primarily plant eaters. A shift towards the lower levels can allow more food to be available for all.
