This simple process explains so much about the beauty of natural processes. It is so complex, yet so efficient (see here for a short recap on what is a food chain) It is modeled by nature in such a way that even if a few links are out of place, it will continue to function. One way in which it does this by interconnecting multiple food chains into a food web.
Food chains are of two types. The two types differ in where the food chain starts. One starts from plants, while the other starts from dead things. Both are rich sources of nutrients for the ecosystem.
Grazing food chain
This food chain starts from plants. Herbivores (primary consumers) eat plants and gain energy. From herbivores, other carnivores and omnivores gain energy. The starting point of this food chain is always green plants, which fix sunlight into organic matter. Therefore, this food chain directly depends on sunlight and the organic matter fixed by it.
Detritus food chain
The detritus food chain starts from anything that is dead in an ecosystem: a fallen leaf, dead herbivores or dead carnivores. Remains of dead organisms contain organic matter, and is yummy food for many species. Most commonly, it is the microorganisms who are the primary consumers in this food chain. They eat and decompose the dead organic matter into inorganic molecules. These microbes (or other, bigger primary consumers of the detritus food chain like ants, flies, etc.) have their own specialized predators.
The detritus food chain doesn’t start from active fixing of sunlight into organic matter. It can start with any dead matter in the ecosystem. (I agree that the dead matter probably depended on matter produced from photosynthesis, but the relationship is rather indirect.)
The flow of energy in a single food chain; either grazing or detritus, is called the single chain model.
Y-shaped energy flow model in ecosystems
In nature, the grazing food chain and detritus food chain often mix with each other. For example, when a herbivore dies of natural causes, it cannot be eaten by a predator! Instead, it enters the detritus food chain. The Y-shaped model explains how the grazing food chain and detritus food chain connect with each other.
The y-shaped model was originally proposed by H.T Odum (1956), but his brother E.P. Odum (1983) gave a generalized model of this relationship and called it the Universal model of flow of energy.
Consider this example-
In this diagram, the upper food chain of plant—>deer—>lion is a part of the grazing food chain. The lower food chain of plant—>termite—>aardvark is a part of the detritus food chain. However, the top predator in this example, the lion, eats the deer and the aardvark. Also, when the deer and lion die, they get eaten by the termite! This shows the interlinking of the two food chains.
Theoretically, this interlinking of food chains looks like this-
You can see that at every stage, the two food chains are linked. The herbivore can be eaten by the predator of the decomposer. That predator can then be eaten by the top predator. Likewise, another piece of link is the fact that when the herbivore and top predator get decomposed by the decomposers when they die. (The decomposed materials are again used up by plants as nutrients. Hardly anything goes to waste!) In some ecosystems, the decomposers are also eaten by the top predators. For example, mosquito larvae in ponds are eaten by the big fishes living there.
The model is called the Y-shaped model because from the side, it looks like the letter “Y”.
This flow of energy is yet another example of how beautifully built our ecosystems are. It also underlines how changes in even one part of the chain can reverberate right through the ecosystem.
You might also be interested in: The top-down vs bottom-up approach in an ecosystem.