TROPHIC LEVELS AND THE ORGANISMS THAT COMPOSE THEM (EXAMPLES) - BIOLOGY - 2025

The trophic levels are the set of organisms - or species of organisms - they have the same position in the flow of nutrients and energy in an ecosystem. Generally, there are three main trophic levels: the primary producers, the secondary producers, and the decomposers.

The primary producers are chemosynthetic plants, algae and prokaryotes. Within the consumers there are different levels, herbivores and carnivores. Lastly, decomposers are a large group of fungi and prokaryotes.

Felines are consumers. Source: pixabay.com

In most ecosystems, these different trophic levels intertwine in complex and interdependent food webs. That is, each predator has more than one prey and each prey can be exploited by more than one predator. The plot can be made up of up to 100 different species.

These chains are characterized by being short, since the transfer of energy from one level to another is quite inefficient - only 10% of the energy goes from one level to another, approximately.

The study of trophic levels and how they are assembled in complex food webs is a central theme in the ecology of populations, communities and ecosystems. The interaction between levels and between chains affects the dynamics and persistence of populations and the availability of resources.

Autotroph and heterotroph

To understand what a trophic level is, it is necessary to understand two basic concepts in biology: autotrophs and heterotrophs.

Autotrophs are organisms capable of generating their own "food", using solar energy and the enzymatic and structural machinery necessary to carry out photosynthesis or through chemosynthesis.

Heterotrophs, meanwhile, lack these mechanisms and must actively seek food - just like us humans.

Fungi are often confused with autotrophic organisms (due to their inability to move and way of life superficially similar to plants). However, these organisms are heterotrophic and degrade the nutrients that surround them. Later we will see the role that fungi play in chains.

Trophic levels and their characteristics

Roddelgado

The passage of energy occurs sequentially, through power. In this way, one organism is consumed by another, the latter by a third, and so the system continues. Each of these "links" is what we call a trophic level.

In this way, ecologists distribute organisms based on their main source of nutrition and energy.

Formally, a trophic level comprises all organisms that are in a similar position in terms of energy flow in an ecosystem. There are three categories: producers, consumers, and decomposers. Below we will analyze in detail each of the mentioned levels.

-First trophic level: producers

The first trophic level in the chain is always made up of a primary producer. The identity of these organisms varies depending on the ecosystem. This floor is the one that supports the rest of the trophic levels.

For example, in terrestrial environments the primary producers are different species of plants. In aquatic ecosystems they are algae. Metabolically, the producers can be photosynthetic (the majority) or chemosynthetic.

Using energy from sunlight, photosynthetic organisms synthesize organic compounds that they later incorporate in the cellular respiration process and as building blocks to continue their growth.

As we would expect, these organisms outnumber their consumers. In fact, almost all (99%) of the organic matter in the living world is made up of plants and algae, while heterotrophs occupy only the remaining 1%.

On the other hand, the chemosynthetic primary producers are found mostly in hydrothermal water sources located deep in the ocean - where these prokaryotic organisms are very abundant.

Green world hypothesis

You have surely noticed that most natural ecosystems are green. In fact, a total of 83.10 ¹⁰ tons of carbon is stored in the plant biomass of terrestrial ecosystems - an extraordinarily high number.

This fact seems curious, since there are a very high number of primary consumers who eat plant matter.

According to this hypothesis, herbivores consume little plant matter, since they are controlled by a variety of factors that limit their populations, such as the presence of predators, parasites, and other kinds of diseases. In addition, the plants have toxic chemical agents that prevent consumption.

Calculations made so far estimate that herbivores consume about 17% of producers' total net production each year - the rest is consumed by detritivores.

Now with these numbers in mind, we can conclude that herbivores are not really a noticeable nuisance to plants. However, there are very specific exceptions, where herbivores are capable of eliminating entire populations in a very short time (some pests).

-Second trophic level: consumers

The trophic levels that are above the primary producers are formed by heterotrophic organisms, and depend directly or indirectly on the autotrophic producers. Within the group of consumers we also find several levels.

Primary consumers: herbivores

Energy enters through primary consumers. These are made up of animals that consume plants or algae. In each ecosystem we will find a specific group of animals that make up the level of primary consumers.

One of the most striking characteristics of herbivores is that most of the material is excreted undigested. The energy that is digested goes on to drive the daily activities of the herbivore and another part will be transformed into animal biomass.

The first is often called "loss" by breathing. However, breathing is a vital activity that the animal must perform.

Secondary consumers: carnivores

The next level is made up of secondary consumers or carnivores: animals that feed on other animals. Only a small portion of the herbivore's body is incorporated into the carnivore's body.

Some secondary consumers may have a mixed diet, including both plants and animals in their diet. Therefore, their classification is not usually very clear and they are present in more than one trophic level.

Tertiary and quaternary consumers

Some trophic chains are characterized by tertiary and quaternary consumers, which indicates that they consume secondary and tertiary level animals, respectively.

Detritivores or scavengers

A particular type of consumer is made up of the individuals known as scavengers. This type of feeding is characterized by the consumption of dead prey and not live prey.

The scavenger diet includes detritus: decomposing vegetable portions, such as leaves, roots, branches and trunks or also dead animals, exoskeletons, and skeletons.

-Third trophic level: decomposers

Like the detritivores of the previous group, the organisms of the third trophic level act on decomposing material. However, they are not biological entities that overlap, since the function of each varies profoundly.

The main function of decomposers is the transformation of organic matter into inorganic matter, thus closing the cycle of matter within ecosystems. In this way, the vegetables have matter for their disposal. Those in charge of carrying out this important final work are bacteria and fungi.

Fungi are organisms that secrete enzymes whose substrates are the organic substances that surround them. After enzymatic digestion, fungi can absorb the products for food.

Most decomposers are microscopic agents that we cannot see with the naked eye. However, its importance goes beyond its size, since if we eliminate all the decomposers on the planet, life on earth would cease due to a shortage of ingredients for the formation of new organic substances.

Examples

Meadow

Our first example is focused on a meadow. For practical purposes we will use simple chains in order to demonstrate how the trophic levels are linked and how they vary depending on the ecosystem. However, the reader must take into account that the real chain is more complex and with more participants.

Grass and other plants would form the primary producer level. The various insects that inhabit our hypothetical meadow (for example, a cricket) will be the primary consumers of the grass.

The cricket will be consumed by a secondary consumer, in our example it will be a small rodent. The mouse will in turn be consumed by a tertiary consumer: a snake.

In case the meadow is inhabited by a carnivorous bird, such as eagles or owls, they will consume the mouse and act as quaternary consumers.

Ocean

Now, let's do the same hypothetical reasoning but in an aquatic ecosystem. In the ocean, the primary producer is phytoplankton, which are plant organisms that live dispersed in the water. The latter will be consumed by the primary consumer, zooplankton.

The different species of fish that inhabit the ecosystem will be the secondary consumers.

Tertiary consumers that eat fish could be seals or some other carnivore.

Our chain in the ocean ends with a well-known quaternary consumer: the great white shark, which will feed on the seal of the previous level.

Energy transfer between trophic levels

As a general rule, it has been established that the net energy transfer between each of the trophic levels reaches a maximum efficiency of only 10%, and is popularly known as the “10% rule”. However, within each community this approach can vary considerably.

This means that of the total energy stored by herbivores, for example, it represents only 10% of the total energy that was in the primary producer that they consumed. In the same way, in secondary consumers we find 10% of the energy stored by primary consumers.

If we want to see it in quantitative terms, consider the following example: suppose we have 100 calories of solar energy captured by photosynthetic organisms. Of these, only 10 calories will pass to herbivores, and only 1 to carnivores.

Food chains are not simple

When we think about food chains we could assume that the levels that compose them are arranged in linear sets, perfectly delimited from each other. However, in nature we find that one level interacts with several levels, making the chain look like a network.

Food chains are short

When looking at food chains, we will realize that they are composed of only a few levels - most of five links or less. Some special chains, as in the Antarctic network, have more than seven links.

Therefore, researchers have questioned the existence of few trophic levels. The hypotheses relevant to the subject are the following:

Energy hypothesis

There are two hypotheses to explain this limitation in length. The first is the so-called "energy hypothesis", where the main limitation of the chain is the inefficiency of energy transmission from one level to another. At this point, it is worth remembering the 10% hypothesis mentioned in the previous section.

Following the assumption of the previous hypothesis, we should find that in ecosystems with high primary productivity by the photosynthetic organisms in the area, the chains are longer, since the energy with which it begins is greater.

Dynamic stability hypothesis

The second hypothesis is related to dynamic stability and proposes that the chains are short because they present greater stability than the longer chains. If an abrupt population fluctuation occurs in the lower levels we could find the local extinction or decrease of the upper trophic levels.

In environments that are more prone to environmental variability, higher-level predators should have the plasticity to find new prey. Also, the longer the chain, the more difficult the system will recover.

Evidence

Taking into account the data collected by the researchers, the most likely hypothesis appears to be the energy hypothesis. Through manipulation experiments, it has been concluded that primary productivity proportionally affects the length of the food chain.

References

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