Food hierarchy and food web
Ecological trophic interactions are better represented by food webs rather than simple hierarchical relationships. As a consequence, the concepts of primary/secondary/tertiary/... consumers sometimes poorly apply to reality.
Obligate and Optional
Many species are able to feed on a various source of nutrients. As a consequence, ...
I'm going to focus on the why do carnivores exist part of this question, which should be extendible to answer why humans eat meat.
Let's start a thought experiment in which we only allow the consumption of vegetation (plants). In the simple system, we have our plants (lettuce) and our herbivores (rabbits). The lettuces have a constant population size and ...
First of all, apart from primary producers, it can sometimes be difficult to assign an exact position or role to organisms in a tropic web or net since trophic levels might be diffuse or tangled. Most obviously, omnivorism is defined as organisms that eat from different food types, which in practice often means they eat from several trophic levels. Besides ...
Ecological communities are groups of multiple (2+) species of either plants, animals and/or other organisms that interact in a unique habitat. More accurately (from Wikipedia and corroborated by Begon et al. (2006)), we can define an ecological community as an:
assemblage or association of populations of two or more different species ...
A keystone species is a plant or animal that plays a unique and
crucial role in the way an ecosystem functions. Without keystone
species, the ecosystem would be dramatically different or cease to
The sea otter is an example of a keystone species in the Pacific
First, just because people can survive on a vegan diet doesn't mean all other animals can function as herbivores. Members of the cat family, for example, have evolved as obligate carnivores.
According to some researchers, the evolution of carnivory is associated with the Cambrian Explosion (Oxygen Brought Earliest Carnivores to Life).
But, as MG_MD ...
@Artem, it's been a while! Good to see you back on Biology.SE.
I doubt that any rigorous terminology exist to describe these "type-0 heterotrophs" living on "non-organismic" organic compounds.
You might be assuming that early organic matters can naturally be categorized as being fundamentally alive vs fundamentally non-living. It is more a question of ...
To follow up with Remi.b's answer, there is an example of an organism that is a combination of producer + primary consumer + secondary consumer:
Bladderwort (Utricularia spp.) is a carnivorous plant that photosynthesizes and is known to eat insects, fish and tadpoles. In addition, research by Marianne Peroutka suggests these plants also eat algae. (see ...
I am not sure I fully understand where is your misunderstanding. I think your whole misunderstanding boils down to the representation of food chain. Actually, let's talk about food web as the name gives an intuition of what is happening that is more correct than the term food chain.
The conceptual view of the food web in a pyramid is super misleading. This ...
A food web is a representation of who eats whom.
A qualitative food web provides for every pair of vertices just the information whether or not one feeds on the other.
This can be represented e.g. by some directed graph with a set of vertices $V$ and a set of arrows $A$ between the vertices. An arrow has the form
$$a: v_1\rightarrow v_2$$
for some ...
Cannibalism can be found across most animal groups. For some groups of organisms (e.g., spiders, fishes) cannibalism can be common within a single generation. Sometimes it will be between siblings, but more often with other members of their species.
As @heracho pointed out, from a network/food web/flow-diagram perspective, yes, cannibalism is repressed as a ...
A very good resource can be found here:
If you focus on soil analysis this resource could be helpful:
I could also recommend the book
with which I worked extensively. It ...