A simplified model of a food chain starts from below, with producers: plants, algae and vegetation in general. The abundance of vegetation depends on the availability of natural resources like water and sunshine. The next link is herbivorous or omnivorous consumers: rabbits, sheep and all other organisms that feed mainly with this vegetation. The population of these consumers depends on the amount of vegetation and is also affected by the following link: predators (this is not the formal term; the formal terms are below). Predators can be omnivores or carnivores and usually are in the “top” links in the food chain.
There are other important links that are rarely included in this simplified model of the food chain: the scavengers and decomposers. Scavengers consume the remains of dead animals. The decomposers are organisms such as fungi and bacteria that consume “junk” and make available nutrients that are essential for producers, thus forming a closed cycle. These links are taken into account in food webs but not in food chains. Normally each link includes several species found in the same trophic position (producer, herbivore, carnivore); these positions or links are known as trophic levels.
* There is another type of autótrofo that does not perform photosynthesis. These organisms use another process called chemosynthesis: convert chemical elements (instead of sunlight) into energy. The quimiosintetizadores are not as common but are also primary producers because they produce their own energy.
** Although it can be argued that the human being is a threat to all animals, humans usually included only when they have a significant influence as a consumer in the ecosystem in question.
As in many other cases, the reality is much more complex than the model. In fact, in a real ecosystem chain it looks more like a cycle or a network with many connections between species and levels. However, understanding the food chain is important for several reasons