The trophic level of an organism is the position it occupies in a food web. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the number of steps it takes from the start of the chain.
A food web starts at trophic level 1 with primary producers such as plants, can move to herbivores at level 2, carnivores at level 3 or higher, and typically finish with apex predators at level 4 or 5.
The path along the chain can form either a one-way flow or a food “web”. Ecological communities with higher biodiversity form more complex trophic paths.
What Is Trophic Level?
The trophic level is a step in a nutritive series, or food chain, of an ecosystem. The organisms of a chain are classified into these levels based on their feeding behavior.
The first and lowest level contains the producers, green plants. The plants or their products are consumed by the second-level organisms the herbivores, or plant eaters.
At the third level, primary carnivores, or meat eaters, eat the herbivores; and at the fourth level, secondary carnivores eat the primary carnivores.
These categories are not strictly defined, as many organisms feed on several trophic levels; for example, some carnivores also consume plant materials or carrion and are called omnivores, and some herbivores occasionally consume animal matter.
A separate trophic level, the decomposers or transformers, consists of organisms such as bacteria and fungi that break down dead organisms and waste materials into nutrients usable by the producers.
Trophic structure refers to the partitioning of biomass between different trophic levels. It is controlled chiefly by the biomass of the primary producers.
The primary producers affect the transfer efficiency between trophic levels as they essentially provide the energy and the nutrient inputs. Apart from them, another important factor is the top-down component.
The latter includes the predators. Their consumption suppresses the lower trophic levels. In a way, the predators help the primary producers by controlling or limiting excessive herbivory by predation. They serve as biological control of the lower trophic levels.
Another way by which the predators can promote primary productivity is by intraspecific competition. Both the primary producers and the predators are major factors for regulatory control.
Trophic Level Pyramid
An ecological pyramid is often depicted as a trophic-level pyramid. It is a graphical representation in the shape of a pyramid comprised of plants and animals in a certain ecosystem.
The shape indicates that the bottom trophic level is comprised of organisms that can make their food through available sources from the environment.
They do not feed on other organisms to obtain their nutritional requirements. Thus, they represent the base. This portion of the pyramid is comprised of producers.
As the trophic levels go up, it tapers towards the peak. This pyramid shape depicts the biomass in each trophic level. Biomass is the amount of living or organic matter in an organism.
The base shows the largest biomass and then diminishes in amount as it moves up to the apex. This is the most common structure in ecosystems.
However, there are also instances wherein an inverted pyramid occurs. The latter results when the combined weight of producers is smaller than the combined weight of consumers.
The ecological pyramid is also sometimes referred to as the Energy Pyramid. This is because it similarly depicts the trophic levels’ energy, i.e. the energy flow through the various trophic levels.
The highest energy is often found at the base that producers occupy. As the trophic structures progress to the top, the energy flow is depicted to diminish from the bottom to the top.
Examples of Trophic Level
Primary producers, or ”autotrophs”, are organisms that produce biomass from inorganic compounds. In general, these are photosynthesizing organisms such as plants or algae, which convert energy from the sun, using carbon dioxide and water, into glucose.
This glucose is then stored within the plant as energy, and oxygen, which is released into the atmosphere.
In terrestrial ecosystems, almost all of the primary production comes from vascular plants such as trees, ferns, and flowering plants. In marine ecosystems, algae and seaweed fill the role of primary production.
Some deep-sea primary producers perform oxidization of chemical inorganic compounds instead of using photosynthesis; these organisms are called ”chemoautotrophs”.
Primary consumers are herbivores, that is, animals that are adapted to consuming and digesting plants and algae (autotrophs). Herbivores are generally split into two categories: grazers, such as cows, sheep, and rabbits, whose diets consist of at least 90% of grass, and browsers, such as deer and goats, whose diets consist of at least 90% of tree leaves or twigs.
Primary consumers may also consume other forms of plant material. Many bats, birds, and monkeys eat fruit (frugivores); birds, insects, bats, and arachnids (spiders) eat nectar (nectarivores); and termites and beetles eat wood (xylophages).
In marine ecosystems, primary consumers are zooplankton, tiny crustaceans which feed off photosynthesizing algae known as phytoplankton.
Secondary consumers, at trophic level three, are carnivores and omnivores, which obtain at least part of their nutrients from the tissue of herbivores. This includes animals and carnivorous plants that feed on herbivorous insects (insectivores).
Secondary consumers are usually small animals, fish, and birds such as frogs, weasels, and snakes, although larger apex predators, such as lions and eagles, may consume herbivores, and can also exist within the second trophic level of an ecosystem.
In marine ecosystems, all species that consume zooplankton are secondary consumers; this ranges from jellyfish to small fish such as sardines and larger crustaceans such as crabs and lobsters, as well as whales, which filter feed, and basking sharks.
Tertiary consumers acquire energy by eating other carnivores but may be preyed upon. Owls are an example of tertiary consumers; although they feed off mice and other herbivores, they also eat secondary consumers such as stoats. In turn, owls may be hunted by eagles and hawks, and are therefore not apex predators.
Apex predators are organisms at the top of the food chain, and which do not have any natural predators.
Eagles, wolves, large cats such as lions, jaguars, and cheetahs, and marine animals such as sharks, tuna, killer whales, and dolphins are all examples of apex predators, although there are many more.
Apex predators often have specific adaptions, which make them highly efficient hunters, such as sharp teeth and claws, speed and agility, and stealth; sometimes they work within groups, enhancing the success of their hunting abilities.
However, not all apex predators are vicious hunters. Whale sharks are large filter feeders, consuming only small fish and plankton, although because they have no natural predators, they are apex predators in their environment.
Apex predators play an extremely important role in an ecosystem; through predation, they control populations of the lower trophic levels.
If apex predators are removed from an ecosystem, organisms such as grazing herbivores can over-populate, therefore placing intense grazing and browsing pressure on the plants within a habitat.
If there are fewer available plant resources, other organisms that depend on the plants (although are not hunted by the apex predator), such as insects and small mammals, will suffer population declines, and in turn, can affect all trophic levels within an ecosystem. This disturbance is called a top-down trophic cascade and can lead to ecosystem collapse.
Other trophic levels
The organism that feeds on a secondary consumer is called a tertiary consumer and the one that eats on a tertiary consumer is referred to as a quaternary consumer. The tertiary consumers and the quaternary consumers occupy trophic levels 4 and 5, respectively.
The last of the trophic level is occupied by decomposers, such as detritivores. They feed on dead plants and animal matter. Detritivores are decomposers that specifically fragment to consume their food. Examples of detritivores are worms, millipedes, dung flies, woodlice, and slugs.
Other decomposers include fungi and bacteria. They consume nutrients at the molecular level as opposed to other consumers who eat their food and digest them.
These decomposers rely on readily available nutrients in the simplest form, e.g. materials that have been digested or substrates derived from deceased or rotting organisms. Parasites that feed on available organic materials but do not necessarily kill the host may also be included in this group.
Decomposers occupy the last trophic level or the top of the ecological pyramid. The most common decomposers are fungi. They are the first instigators of decomposition. They have the enzymes and other compounds to break down the biomolecules of the deceased organism.
Bacteria also have enzymes that break down organic compounds into simpler forms. After the decomposition process, the detritivores then act on the remains, scavenging for detritus or decomposing organic matter.
The role of decomposers in the ecosystem is vital as they are the ones that break down the organic matter of dead organisms where a part of them returns to earth as a geochemical component.
Planktons are microscopic organisms that live in aquatic habitats. Some of them are photosynthetic and others are heterotrophs. Thus, they may be distributed to different trophic levels in the food chain or ecological pyramid.
Those that are capable of photosynthesis, such as phytoplankton, are considered producers. Heterotrophic plankton, such as zooplankton may be consumers as they feed on other plankton.
Biomass Transfer Efficiency
Biomass in an ecological pyramid is lost progressively from the bottom up. The greatest biomass amount is found at the base trophic level which includes the producers.
Since the primary consumers rely on producers for sustenance, the biomass amount of the producers would, therefore, be a limiting factor to the biomass of the primary consumers.
Likewise, the secondary consumers rely on primary consumers, and consequently, their biomass would also be affected by the available biomass amount of primary consumers.
Thus, in an ecosystem, it is usual to find a biomass pyramid wherein the first trophic level is the widest while the topmost trophic level is the narrowest.
The distribution of biomass in an ecosystem implicates ecosystem stability. If in the case of an inverted pyramid, the ecosystem could fail when there are more consumers than primary producers.
Monitoring the trophic levels is essential to gain an understanding of the interrelationships between organisms as well as the ecological processes that occur within an ecosystem. In that way, the magnitude of herbivory, predation, and decomposition processes could be used to know the status and stability of an ecosystem.