FOOD WEB: TROPHIC LEVELS, TYPES, TERRESTRIAL AND MARINE - ENVIRONMENT - 2025

The trophic web or food web is the set of food interactions between living beings in an ecosystem. A food web is formed by the intertwining of multiple food chains (linear sequence that goes from the producer to the last consumer).

In a strict sense, trophic networks are not open but end up forming closed cycles where every organism ends up being food for another. This is because decomposers and detritivores end up incorporating the nutrients of any living being into the network.

Food webs. Source: Roddelgado

Within a trophic network, different trophic levels are identified, with the first one consisting of the producers that introduce energy and matter to the system through photosynthesis or chemosynthesis.

Then, these producers serve as food for the so-called primary consumers, who in turn will be consumed by other (secondary) consumers. In addition, other levels of consumers may be present depending on the complexity of the ecosystem.

Additionally, the networks become more complex because there is a significant proportion of omnivorous organisms (they consume animals, plants, fungi). Therefore, these types of organisms can occupy different trophic levels at any given time.

There are different types of trophic networks according to the different ecosystems where they develop and the model used by the researcher. In general terms, we find terrestrial trophic networks and aquatic trophic networks and within the latter freshwater and marine ones.

Similarly in terrestrial networks, each biome has its peculiarities depending on the species that make it up.

Trophic levels

Trophic levels refer to the hierarchy of each node in the trophic web starting from the producer. In this sense, the first trophic level is that of producers, followed by the different levels of consumers. A very particular type of final consumer are detritivores and decomposers.

Trophic levels. Source: Roddelgado

Although the model tends to represent the network as a bottom-up hierarchy, it is really a three-dimensional and unlimited network. In the end the higher level consumers will also be consumed by the detritivores and decomposers.

Likewise, the mineral nutrients released by detritivores and decomposers will be reincorporated into the network by the primary producers.

- Flow of energy and matter

An ecosystem is a complex interaction of abiotic factors (climate, soil, water, air) and biotic factors (living organisms). Matter and energy flow in this ecological system, the primary source of energy being electromagnetic radiation from the Sun.

Another source of energy is the hot springs from the fumaroles of the oceanic abyssal depths. This source feeds very particular trophic networks, only on the seabed.

- Producers

Plants and algae are producing organisms

The producers are all those organisms that obtain their energy from inorganic sources, either solar energy or inorganic chemical elements. These producers constitute the entry point for energy and matter to the food web.

Solar energy and life

The Sun's energy cannot be used by all living organisms for their structural and functional development. Only autotrophic organisms can assimilate it and transform it into assimilable forms for the rest of life on Earth.

This is possible thanks to a biochemical reaction called photosynthesis, activated by solar radiation captured by a specialized pigment (chlorophyll). Using water and atmospheric CO2, photosynthesis transforms solar energy into chemical energy in the form of carbohydrates.

From carbohydrates and using minerals absorbed from the soil, autotrophic organisms can build all their structures and activate their metabolism.

The main autotrophs are the plants, algae and photosynthetic bacteria that constitute the first level of the trophic chain. Therefore, any organism that consumes an autotroph will have access to that chemical form of energy for its own development.

Chemotrophs

The Archean kingdom (unicellular similar to bacteria) includes organisms capable of obtaining energy from the oxidation of inorganic compounds (lithotrophs). For this they do not use sunlight as a primary source of energy, but chemicals.

These substances are obtained, for example, in the deep sea, emitted by the escapes of submarine volcanoes. Likewise, they are autotrophic organisms, and therefore also form part of the base of food chains.

- Primary consumers

This level includes heterotrophic organisms, that is, they are not capable of producing their own food and obtain it by consuming primary producers. Therefore, all herbivores and also organisms that consume chemosynthetic archaea are primary consumers.

Herbivores

Not all plant structures are easy to digest like the fleshy fruits that have evolved to be consumed and help disperse the seeds.

Hervíboro. Source: Larry D. Moore

In this sense, herbivores have adapted to digest fibrous plant tissues through complex digestive systems. In these systems, symbiotic relationships are established with bacteria or protozoa that help the process through fermentation.

Omnivores

Omnivores are consuming organisms capable of behaving as primary, secondary, and even tertiary consumers. That is, they are organisms that consume both food of plant, animal, fungus or bacterial origin.

This category includes the human being, also their relatives the chimpanzees and other animals such as bears. Likewise, many detritivores and decomposers behave strictly as omnivores.

The presence of omnivores, especially at intermediate levels of the networks, make their analysis more complex.

- Secondary consumers

They are those heterotrophic organisms that are not able to directly consume the producers and obtain their energy by consuming the primary consumers. They constitute the carnivores, which ingest and digest the tissues that make up the body of the primary consumers to obtain energy and develop.

Lesser predators

As secondary consumers, especially those organisms that, while feeding on primary consumers, can be the object of consumption. In this case, they will serve as food for larger predators that constitute the category of tertiary consumers.

Insectivorous plants

Dionaea muscipula

Another case that introduces complexity in trophic networks is that of insectivorous plants. These plants are producers insofar as they carry out the photosynthesis process from solar energy, but they are also secondary and tertiary consumers, since they degrade insects.

For example, plant species of the families Droseraceae (genus Drosera) and Sarraceniaceae (genus Heliamphora), grow on the tops of tepuis (tabular sandstone mountains with nitrogen-poor soils). These types of plants have evolved to obtain nitrogen from the bodies of insects and even small frogs.

- Tertiary consumers

They are heterotrophic organisms that feed on other consumers, whether primary or secondary. In the case of omnivores, they also include producers directly in their diet.

Here are the super predators that are organisms capable of predating others, but are not subject to predation. However, at the end of their life cycle they end up being eaten by scavengers, detritivores and decomposers.

Super predators

They are considered to be at the top of the food pyramid, with humans being the main super predator. Almost all food webs have one or more of these super predators such as the lion in the African savanna and the jaguar in the Amazon rainforest.

Carnivores. Source: Luca Galuzzi (Lucag)

In marine ecosystems are sharks and killer whales, while in tropical freshwater ecosystems there are crocodiles and alligators.

Scavengers

Some animals feed on the carcasses of other animals that were not hunted by them. Such is the case of buzzards or vultures, as well as some species of hyenas (the spotted hyena is capable of hunting).

It is therefore about consumers that feed on consumers of any trophic level. Some authors include them in decomposers, while others deny this location because these animals consume large chunks of meat.

In fact, there are some predators that act as scavengers when hunting is scarce, such as big cats and even humans.

Parasites

The different forms of parasitism are also a factor in the complexity of food webs. A bacterium, a fungus or a pathogenic virus consume the parasitized organism and even cause its death and therefore behave like consumers.

- Decomposers or detritivores

It includes the great variety of organisms that contribute to the degradation of organic matter once living beings die. They are heterotrophs that feed on decaying organic matter and include bacteria, fungi, protists, insects, annelids, crabs, and others.

Bacteria and fungi

Although these organisms are not capable of directly ingesting portions of organic matter, they are very efficient decomposers. They do this thanks to the secretion of substances capable of dissolving tissues and then absorbing nutrients.

Detritivores

Detritivore. Source:

These organisms directly consume decaying organic matter to obtain their food. For example, earthworms (Lumbricidae) that process organic matter, the moisture scale (Oniscidea), beetles and many species of crabs.

Types of food web

There are different criteria to classify food webs and in principle there are as many types of food webs as there are ecosystems on Earth.

- According to the dominant medium

A first classification criterion is based on the two main existing media on the planet, which are land and water. In this way, there are terrestrial networks and aquatic networks.

In turn, aquatic networks are differentiated into freshwater and marine; existing in each case different types of networks.

- According to the biological interaction

They can also be differentiated according to the predominant biological interaction, the most common being those based on predation. In these, a predation sequence is generated from the primary producers and their consumption by herbivores.

Parasitism

There are also trophic networks based on parasitism, in which a species normally smaller than the host feeds on it. On the other hand, there are hyperparasites (organisms that parasitize other parasites).

For example, the plant family Loranthaceae groups together hemiparasitic plants. In this case, the plants carry out photosynthesis, but parasitize other plants to obtain water and minerals.

In addition, there are some species of this family that parasitize other plants of the same group and behave as hyperparasites.

- According to the representation model

Food webs are also classified depending on the representation model used. This depends on the interest of the researcher, according to which the model will reflect a certain type of information.

Thus there are source networks, sunken networks, connectivity networks, energy flow networks and functional networks.

Source networks

These models focus on the main source nodes, that is, those that provide the greatest amount of food to the system. In such a way that they represent all the predators that feed on these knots and the amount of food they obtain.

Sunken nets

Unlike the previous model, this one focuses on the knots of predators, representing all their prey and what those prey consume. Thus, while the source web goes from bottom to top in the sequence of trophic levels, the sunken web follows the reverse path.

Connectivity networks

In this case, one starts from the network as a whole and tries to represent all possible food connections in the ecosystem.

Power flow networks

This type of food web model focuses on the quantitative flow of energy through the ecosystem. These are the so-called stoichiometric studies, which establish the amounts of matter and energy that interact in a reaction and measure the product.

Functional networks

Functional networks focus on establishing the weight of each subgroup of nodes in the operation of the system, defining structure and functions. It assumes that not all food interactions that occur in the ecosystem have the same importance for its functional stability.

At the same time, this type of network evaluates how many of the possible trophic connections in an ecosystem actually exist and which nodes provide more or less biomass.

- Evolution of food webs

Finally, a food web can be neo-ecological or paleoecological. In the first case it represents a current food web and in the second a reconstruction of an already extinct web.

Terrestrial food web

In the terrestrial environment there is a great diversity of ecosystems made up of different combinations of species. Therefore, the trophic webs that can be delimited reach an enormous number.

Terrestrial food web. Source: chris 論 (through works of J. Patrick Fischer, C. Schuhmacher, Madprime, Luis Fernández García, Luis Miguel Bugallo Sánchez, chung-tung yeh, Susanne Heyer and Simon Andrews)

It is necessary to bear in mind that the biosphere is a totally interconnected complex system, which is why it is a gigantic food web. However, in order to understand the functioning of nature, the human being defines functional portions of that network.

Thus, it is possible to characterize the trophic web of a tropical forest, a temperate forest, a savannah or a desert, as separate entities.

- Food web of the forest

In a tropical forest the diversity of living organisms is enormous, as well as the microenvironments that are generated in it. Therefore, the food interactions that occur are also very diverse.

Productivity and nutrient cycling

The plant productivity of the tropical forest is high and there is also a high efficiency in the recycling of nutrients. In fact, the highest proportion of nutrients is found in plant biomass and in the litter that covers the soil.

Producers

The largest collection of solar energy by producers in the tropical forest occurs in the upper canopy. However, there are several lower strata that capture the light that manages to filter, including climbers, epiphytes, herbs and ground shrubs.

Primary consumers

Consistent with the above, most of the primary consumers of the forest feed in the forest canopy. There is a great diversity of insects that feed on the leaves of the trees, while birds and fruit bats consume fruits and seeds.

There are also mammals such as monkeys, sloths and squirrels that feed on leaves and fruits.

Secondary consumers

Many birds are insectivores and some insects such as the praying mantis are predators of other herbivorous insects. There are also insectivorous mammals such as the honey bear that consumes ants, in this case both herbivorous and carnivorous.

Jungle ants

One of the most numerous and taxonomically varied groups in the jungles are the ants, although due to their size they go unnoticed.

The different species of ants can behave as primary consumers, feeding on leaves and plant secretions. Other species act as secondary consumers by hunting and feeding on other insects and even larger animals.

Ants Source: Muhammad Mahdi Karim

A prominent case is the legionary ants or marabunta in tropical forests that periodically make up masses of thousands or millions of individuals. These advance together preying on all animals within their reach, mainly insects, although they can consume small vertebrates.

The overflow forest or flooded forest

This type of forest is a clear example of the complexity that the trophic web can reach in the tropical forest. In this case, during the rainy season in the mountain ranges that give rise to the great rivers that cross the forests, floods occur.

The waters of the river penetrate the jungle reaching up to 8 and 10 m in height and in these conditions the freshwater and terrestrial sylvatic trophic networks are integrated.

Thus, there are cases such as the Arapaima gigas fish that is capable of one jump to capture small animals perched on the leaves of the trees.

Tertiary consumers

The great predators of the rainforest are felines, large snakes, as well as crocodiles and alligators. In the case of the rainforest of the American tropics, the jaguar (Panthera onca) and the anaconda (Eunectes murinus) are examples of this.

For its part, in the African jungle there are the leopard, the poisonous black mamba snake (Dendroaspis polylepis) or the African python (Python sebae). And in the case of tropical Asia are the tiger (Panthera tigris) and the reticulated python (Malayopython reticulatus).

There are also predatory birds that occupy the highest trophic level, such as the harpy eagle (Harpia harpyja).

Decomposers

The rainforest floor is an ecosystem unto itself, with a great diversity of organisms. These include various groups such as bacteria, fungi, protists, insects, annelids and mammals that make their burrows there.

Most of these organisms contribute to the decomposition process of organic matter that is reabsorbed by an intricate system of roots and fungi.

The rhizosphere (soil root system) has been found to include so-called mycorrhizal fungi. These fungi establish symbiotic relationships with the roots that provide them with nutrients and the fungi facilitate the absorption of water and minerals by the tree.

- Desert food web

Deserts are low-productivity ecosystems due to their environmental conditions, especially the scarce water supply and extreme temperatures. These environmental conditions condition a scarce vegetation cover, so production is restricted and the fauna present is scarce.

The few plant species such as animals have adapted in their evolutionary process to these conditions. Most animals have nocturnal habits and they spend the day in underground dens to avoid solar radiation.

Producers

In these ecosystems, producers are made up of xerophilous plant species (adapted to drought conditions). In the case of American deserts, cacti are a good example of this and they provide edible fruits consumed by insects, birds and rodents.

Primary consumers

In the desert areas live insects, birds, reptiles and rodents that feed on the few plants that inhabit the desert. In the Sahara desert there are species of herbivores that can go long periods without drinking water.

Dromedary (Camelus dromedarius). Source: Cesar I. Martins from Jundiai, Brazil

Among these are the dromedary (Camelus dromedarius) and the dorcas gazelle (Gazella dorcas).

Secondary consumers

Carnivorous species inhabit the desert that feed on primary consumers. Among these are arachnids such as scorpions that feed on other insects.

Likewise, there are birds of prey such as hawks and owls that capture other birds, rodents and reptiles. There are also poisonous snakes such as the rattlesnake (Crotalus spp.) Whose prey is mainly desert rodents.

In the American deserts among the mammals are the puma (Puma concolor) and the coyote (Canis latrans). While in the Sahara live several species of fox, among them the fennec (Vulpes zerda) and the pale fox (Vulpes pallida).

Tertiary consumer

The Sahara cheetah (Acinonyx jubatus hecki) is the largest predator in this desert, but it is sadly in danger of extinction.

Marine trophic web

Marine food web. Source: chris 論 (through works of J. Patrick Fischer, C. Schuhmacher, Madprime, Luis Fernández García, Luis Miguel Bugallo Sánchez, chung-tung yeh, Susanne Heyer and Simon Andrews)

The diversity of marine environments also determines a great variety of trophic webs. In this case, two types of basic trophic networks stand out: the one based on phytoplankton and the one supported by chemosynthetic archaea.

- Based on phytoplankton

The most characteristic food web of the marine environment is based on the activity of phytoplankton (microscopic photosynthetic organisms that float in the surface layers). From these producers, various food chains are generated that form the complex marine trophic networks.

Producers

Phytoplankton include numerous species of cyanobacteria, protists, and single-celled algae such as diatoms. They are photosynthetic autotrophs that form populations of billions of microscopic individuals.

Phytoplankton (diatoms). Source: Prof. Gordon T. Taylor, Stony Brook University

These are carried away by ocean currents and serve as food for primary consumers. In shallow waters, where sunlight reaches, meadows of algae and even aquatic angiosperms develop.

The producers also serve as food for fish, sea turtles and other organisms that are in turn predated.

Primary consumers

One of the main ones is zooplankton, which are microscopic animals that are also part of plankton and feed on phytoplankton. In addition, other primary consumers are the blue whale, the whale shark, and many fish.

In coral reefs, coral polyps feed on phytoplankton and other organisms feed on polyps. Such is the case of the parrotfish (Scaridae) and the crown-of-thorns star (Acanthaster planci).

Secondary consumers

Among these are a variety of organisms that feed on fish, such as other fish, anemones, snails, crabs, seals, sea lions.

Tertiary consumers

The big marine predators are sharks, especially the larger species like the white shark. Another great predator in the open sea is the killer whale and so are dolphins, with seals being one of the favorite prey of the killer whale, which in turn feed on fish.

Decomposers

The decomposition process is aided by the conditions of the marine environment and the action of bacteria and decomposing worms.

- Based on chemosynthetic archaea

In the hydrothermal vents that are in the oceanic ridges at more than 2,000 m depth there are very peculiar ecosystems. Taking into account that the seabed at these depths are almost deserted, the explosion of life in these areas stands out.

Producers

Sunlight does not reach these depths, therefore the photosynthesis process cannot develop. This is why the food web of these ecosystems is supported by autotrophic organisms that obtain energy from another source.

In this case, they are archaea that are capable of oxidizing inorganic compounds such as sulfur and producing chemical energy. These bacteria find an environment conducive to their massive multiplication thanks to the warm waters of the fumaroles generated by volcanic activity.

Similarly, these fumaroles expel compounds such as sulfur that serve for their chemosynthesis.

Primary consumers

Animals like clams, worms, and other organisms feed on archaea. Likewise, there are very particular symbiotic associations such as that of the gastropod called the squamous foot snail (Crysomallon squamiferum).

This snail depends exclusively on the symbiotic relationship that it establishes with the chemosynthetic archaea that provide it with food.

Secondary consumers

Some deep-sea fish feed on other organisms which in turn consume the chemosynthetic bacteria.

Detritivores

In the deep ocean there are species of fish, worms and other organisms that live on organic debris that precipitate from the surface.

Currents and nutrients

Cold deep currents push nutrients from the seabed to the surface, thus integrating marine food webs.

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