A food or trophic chain is a graphic representation of the multiple connections that exist, in terms of the consumption interactions between the different species that are part of a community.
Trophic chains vary widely, depending on the ecosystem studied and are made up of the different trophic levels that exist there. The base of each network is formed by the primary producers. These are capable of photosynthesis, capturing solar energy.
Successive levels of the chain are made up of heterotrophic organisms. Herbivores consume the plants, and these are consumed by carnivores.
Many times the relationships in the network are not totally linear, since in some cases, the animals have extensive diets. A carnivore, for example, can feed on carnivores and herbivores.
One of the most outstanding characteristics of food chains is the inefficiency with which energy passes from one level to another. Much of this is lost in the form of heat, and only about 10% passes. For this reason, food chains cannot be extended and multilevel.
Where does the energy come from?
All the activities that organisms carry out require energy – from movement, whether by water, land or air, to the transport of a molecule, at the cell level.
All this energy comes from the sun . The solar energy that is constantly radiating to the planet earth, is transformed into chemical reactions that feed life.
In this way, the most basic molecules that allow life are obtained from the environment in the form of nutrients. In contrast to chemical nutrients, which are conserved.
Therefore, there are two basic laws that govern the flow of energy in ecosystems. The first establishes that energy passes from one community to another in two ecosystems through a continuous flow that goes in only one direction. It is necessary to replace the energy of the solar source.
The second law states that nutrients continuously go through cycles and are used repeatedly within the same ecosystem, and also between them.
Both laws modulate the passage of energy and shape the complex interactions network that exists between populations, between communities and between these biological entities with their abiotic environment.
Elements that make it up
In a very general way, organic beings are classified according to the way in which they obtain energy to develop, maintain and reproduce, into autotrophs and heterotrophs.
The first group, the autotrophs, includes individuals who are capable of taking solar energy and transforming it into chemical energy stored in organic molecules.
In other words, autotrophs do not need to consume food to survive, since they are capable of generating it. They are also often referred to as “producers”.
The best known group of autotrophic organisms are plants. However, there are also other groups, such as algae and some bacteria. These have all the metabolic machinery necessary to carry out photosynthesis processes.
The sun, the source of energy that powers the earth, works by merging hydrogen atoms to form helium atoms, releasing huge amounts of energy in the process.
Only a small fraction of this energy reaches the earth, as electromagnetic waves of heat, light, and ultraviolet radiation.
In quantitative terms, a large part of the energy that reaches the earth is reflected by the atmosphere , clouds and the earth’s surface.
After this absorption event, approximately 1% of solar energy remains available. Of this quantity that manages to reach the earth, plants and other organisms manage to capture 3%.
The second group is made up of heterotrophic organisms. They are not capable of photosynthesis, and must actively seek their food. Therefore, in the context of food chains, they are called consumers. We will see later how they are classified.
The energy that the individual producers managed to store is at the disposal of other organisms that make up the community.
There are organisms that, similarly, make up the “threads” of the trophic chains. These are the decomposers or debris eaters.
Decomposers are made up of a heterogeneous group of small animals and protists that live in environments where frequent waste accumulates, such as leaves that fall to the ground and corpses.
Among the most outstanding organisms we find: earthworms, mites, myriapods, protists, insects, crustaceans known as mealybugs, nematodes and even vultures. With the exception of this flying vertebrate, the rest of the organisms are quite common in waste deposits.
Its role in the ecosystem consists of extracting the energy stored in dead organic matter , excreting it in a more advanced state of decomposition.These products serve as food for other decomposing organisms. Like mushrooms, mainly.
The decomposing action of these agents is essential in all ecosystems. If we eliminated all the decomposers, we would have an abrupt accumulation of corpses and other matter.
Besides that the nutrients stored in these bodies would be lost, the soil could not be nourished. Thus, damage to the quality of the soil would cause a drastic decrease in plant life, ending the level of primary production.
In food chains, energy passes from one level to another. Each of the mentioned categories constitutes a trophic level. The first is made up of all the great diversity of producers (plants of all kinds, cyanobacteria, among others).
Consumers, on the other hand, occupy several trophic levels. Those that feed exclusively on plants form the second trophic level and are called primary consumers. Examples of this are all herbivorous animals.
The secondary consumers are made up of carnivores – animals that eat meat. These are predators and their prey are, mainly, the primary consumers.
Finally, there is another level formed by tertiary consumers. Includes groups of carnivorous animals whose prey are other carnivorous animals belonging to secondary consumers.
Food chains are graphic elements that seek to describe the relationships of species in a biological community, in terms of their diet. In didactic terms, this network exposes “who feeds on what or who”.
Each ecosystem presents a unique food web, and drastically different from what we could find in another type of ecosystem. Generally, food chains tend to be more complicated in aquatic ecosystems than in terrestrial ones.
Food webs are not linear
We should not expect to find a linear network of interactions, since in nature it is extremely difficult to precisely define the boundaries between primary, secondary and tertiary consumers.
The result of this pattern of interactions will be a network with multiple connections between the members of the system.
For example, some bears, rodents and even us humans are “omnivores”, which means that the feeding range is wide. In fact, the Latin term means “who eat everything.”
Thus, this group of animals can behave in some cases as a primary consumer, and later as a secondary consumer, or vice versa.
Moving on to the next level, carnivores generally feed on herbivores, or other carnivores. Therefore, they would be classified as secondary and tertiary consumers.
To exemplify the previous relationship, we can use owls. These animals are secondary consumers when they feed on small herbivorous rodents. But, when they consume insectivorous mammals, it is considered tertiary consumer.
There are extreme cases that tend to complicate the network even more, for example, carnivorous plants. Although they are producers, they are also classified as consumers, depending on the prey. If it were a spider, it would become a secondary producer and consumer.
Transfer of energy
Transfer of energy to producers
The passage of energy from one trophic level to the next is a highly inefficient event. This goes hand in hand with the law of thermodynamics which states that the use of energy is never totally efficient.
To illustrate the transfer of energy, let’s take as an example an event in everyday life: the burning of gasoline by our car. In this process, 75% of the energy released is lost in the form of heat.
We can extrapolate the same model to living beings. When ATP bonds are broken for use in muscle contraction, heat is generated as part of the process. This is a general pattern in the cell, all biochemical reactions produce small amounts of heat.
Transfer of energy between the other levels
Similarly, the transfer of energy from one trophic level to another is done with considerably low efficiency. When a herbivore consumes a plant, only part of the energy captured by the autotroph can pass to the animal.
In the process, the plant used some of the energy to grow and a significant portion was lost as heat. In addition, part of the energy from the sun was used to build molecules that are not digestible or usable by the herbivore, such as cellulose.
Continuing with the same example, the energy that the herbivore acquired thanks to the consumption of the plant will be divided into multiple events within the organism.
Part of this will be used to build the parts of the animal, for example the exoskeleton, if it is an arthropod. In the same way as in the previous levels, a large percentage is lost thermally.
The third trophic level comprises the individuals that will consume our hypothetical arthropod above. The same energy logic that we have applied to the two upper levels also applies to this level: much of the energy is lost as heat. This feature limits the length the chain can take.
A trophic pyramid is a particular way of graphically representing the relationships that we have discussed in the previous sections, no longer as a network of connections, but by grouping the different levels into steps of a pyramid.
It has the peculiarity of incorporating the relative size of each trophic level as each rectangle in the pyramid.
At the base, primary producers are represented, and as we move up the graph, the rest of the levels appear in ascending order: primary, secondary and tertiary consumers.
According to the calculations carried out, each step is about ten times higher if we compare it with the upper one. These calculations are derived from the well-known 10% rule, since the passage from one level to the other implies an energy transformation close to that value.
For example, if the level of energy stored as biomass is 20,000 kilocalories per square meter per year, in the upper level it will be 2,000, in the next 200, and so on until reaching the quaternary consumers.
The energy that is not used by the metabolic processes of organisms, represents the discarded organic matter, or biomass that is stored in the soil.
Types of trophic pyramids
There are different types of pyramids, depending on what is represented in it. It can be done in terms of biomass, energy (as in the example mentioned), production, number of organisms, among others.
A typical freshwater aquatic food chain begins with the vast amount of green algae that live there. This level represents the primary producer.
The primary consumer in our hypothetical example will be mollusks. Secondary consumers include species of fish that feed on molluscs. For example, the slimy sculpted species ( Cottus cognatus ).
The last level is made up of tertiary consumers. In this case, the slimy sculpting is consumed by a species of salmon: king salmon or Oncorhynchus tshawytscha .
If we will see it from the perspective of the network, at the initial level of producers we should take into account, in addition to green algae, all diatoms, blue-green algae, and others.
Thus, many more elements are incorporated (species of crustaceans, rotifers and multiple species of fish) to form an interconnected network.
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