Which of the following is the correct order of trophic levels, from the most biomass to the least biomass?

Producers → Primary Consumers → Secondary Consumers → Tertiary Consumers or Apex Predators

Food Chains and Food Webs: Difference

Organic materials and the energy to build must come from somewhere. Therefore, organisms must either eat other organisms to sustain themselves or make their own energy, as plants do. The flow of this energy from organism to organism is represented by diagrams known as food chains and food webs, the subject of this article!

Energy: sustaining life on earth

Energy- or the ability of an organism to do work—must come from somewhere. For life on earth, most life is sustained via the energy produced by the sun. Sunlight can be used by organisms via photosynthesis to transfer light energy into chemical energy, which is the energy stored in the chemical bonds between molecules.

Energy stored in these bonds can then be used by organisms to carry out life processes.

What is primary productivity?

Primary production is the process by which plants or other autotrophs convert the sun’s energy into energy that can be used for food by the plants themselves or by other organisms.

The rate at which autotrophs do this is called primary productivity.

Scientists measure different types of primary productivity to help understand the flow of energy through ecosystems. The gross primary productivity (GPP) of an ecosystem is the total rate at which light energy is converted into chemical energy through photosynthesis by autotrophs or primary producers.

Net primary productivity is the total rate of primary production minus what the producers use to sustain themselves. Thus, the net primary productivity represents the available energy that may be used by the other organisms in an ecosystem.

Vast like the ocean

You wouldn’t think the open ocean would do much in the way of primary productivity, since it is dark and blue instead of green and fertile like tropical forests. However, floating in the upper layers of the ocean are phytoplankton, which can carry out photosynthesis. Because the open ocean is so large and occupied with phytoplankton, it is actually responsible for over half of the total net primary productivity on earth every year! Although, the actual rate compared to the size of the ocean is lower than in areas like tropical rainforests or coastal marshes.¹

But not all organisms can use the sun’s energy directly. Some organisms must consume each other to receive energy. How organisms get their energy is the basis for learning about food chains and food webs!

Autotrophs versus heterotrophs

Before we continue to learn about energy flow through ecosystems, we should cover some common terms that scientists use to describe how an organism gets its energy.

Autotrophs are organisms that can make their own food and nutrients for survival, these include chemoautotrophs and photoautotrophs.
- Chemoautotrophs produce their own food by synthesizing chemical energy from their environment.
  - One example is the bacteria that live on the floor of the ocean at hydrothermal vents.
    They can synthesize energy from the hydrogen sulfide found at these vents.
- Photoautotrophs can make their own food through the energy in sunlight via photosynthesis (making sugars from light and carbon dioxide).
  - Photoautotrophic organisms include plants and cyanobacteria.

Because autotrophs produce their own food, we call them producers as well.

Heterotrophs on the other hand get their energy by consuming other organisms.
- Examples of heterotrophs are some bacteria, fungi, insects, and animals.

As a result, we call this group of organisms “consumers”.

Food chains in ecosystems

Food chains in ecosystems are simple representations of how energy is transferred between organisms. Food chains demonstrate what eats what in sequential order from producers to different types of consumers.

For example, grass might be the start of the food chain followed by rabbits, which are eaten by coyotes.

Food chains, especially in ecosystems, often move in order from plants to herbivorous animals to omnivorous or carnivorous animals.

A food chain shows the flow of energy through organisms in an ecosystem in a simple, sequential order from producers to different levels of consumers (i.e., herbivores, carnivores, etc.).

What are trophic levels?

Each group of organisms at similar points in the food chain make up what ecologists call a trophic level. Organisms at the same trophic level get their energy from similar sources.

The different trophic levels include:

Producers (autotrophs): make their own food; examples include terrestrial plants, phytoplankton, and cyanobacteria.
Primary consumers (heterotrophs): consume the producers; examples include herbivorous mammals such as deer or rabbits and plant-eating insects.
Secondary consumers (heterotrophs): consume the primary consumers and, if omnivorous, the producers as well; examples include
Tertiary consumers (heterotrophs): consume the primary and secondary consumers; examples include apex predators such as lions or sharks.

But wait, there are more ways in which heterotrophs feed! When organisms die or excrete waste, it is recycled by other heterotrophs known as decomposers and detritivores.

Decomposers break down nonliving biological material,
- these include organisms like fungi, worms, and bacteria.
Whereas detritivores feed off the broken-down material,
- these include animals that are scavengers, such as vultures.

Types of food chains

Ecologists have determined that two types of food chains are the basis for most ecosystems:

Detrital food chain: In this food chain the decomposers make up the base, which then breaks down materials in the soil and detritivores feed off of this decaying material. Eventually, detritivores themselves become food for carnivores (secondary consumers and beyond).
Grazing food chain: This is the food chain that is easy to imagine. In this food chain, producers make up the base from which primary consumers “graze” or eat. Then energy moves onto secondary consumers (carnivores, omnivores), tertiary consumers, and so on and so forth.

Energy pyramids and energy flow through ecosystems

Between each organism in a food chain, only about 10% of the energy from the previous trophic level makes it to the next trophic level. In other words, only 10% of the energy is used to make other organic matter (also known as biomass) at the next trophic level. The rest of the energy is lost through heat or other inefficiencies in digestion, respiration, and similar life processes.

Because so much energy is lost between trophic levels, food chains are often only four or five trophic levels long!

Food chains help to visualize energy flow between trophic levels. Born from a similar idea, energy pyramids show the decreasing energy transfer between trophic levels.

The biomass of the different trophic levels decreases as you move from producer to tertiary consumer. This is reflective of what occurs in ecosystems, as energy is lost, the total biomass of organisms that make up higher trophic levels decreases. Primary producers then always make up the largest collective biomass of any trophic level, and tertiary consumers (or apex predators) make up the smallest.

Food webs in ecosystems

Food chains provide great visualization tools to understand what is happening in an ecosystem. Food webs are, however, quite simplified. In reality, an ecosystem is much more complex.

For example, a grasshopper wouldn’t be the only organism making a meal out of the grass, there might also be rabbits, deer, or other insects using that grass as a food source. Small birds may eat the grasshoppers as well, and be consumed by the snakes or hawks in turn. Additionally, you may have omnivores in an ecosystem, which can consume both plants and other animals. Once you think of all the possibilities, who eats who becomes a tangled web!

A food web is a compilation of food chains showing the multiple feeding relationships between organisms in an ecosystem. It would be almost impossible to track every interaction between organisms, but food webs help to visualize how energy is transferred through a community at a more complex level than food chains.

A food web shows complex feeding relationships between organisms in an ecosystem. It is a compilation of multiple food chains.

Differences between food chains and food webs

Food chains and food webs are both modeling systems used by ecologists to quantify and understand an ecosystem. We know that what actually happens in an ecosystem is oftentimes more complex than we can model, but it is still important to demonstrate the flow of energy. Below, Table 1 offers the main differences between food chains and food webs.

Table 1: The differences between food chains and food webs.

Type of model	Food chains	Food webs
What do they show?	Food chains show transfer of energy from single feeding relationships from producer to consumer in sequential order of trophic level.	Food webs show the transfer of energy among organisms in an ecosystem with many feeding relationships.
Can trophic levels of organisms be easily determined?	Yes, food chains show the tropic levels increasing sequentially.	Not always, it may be easy to tell what a producer is, but an organism may exist at multiple trophic levels in a food web.
Can organisms exist at multiple trophic levels in the model?	No, food chains only show organisms occupying a single trophic level.	Yes, food webs show that omnivores may be both primary and secondary consumers.
Encompasses the energy of the whole ecosystem?	No, food chains only show a single thread within an ecosystem.	Yes, food webs attempt to show the intricate relationships among organisms.

Food Chains and Food Webs - Key takeaways

Primary productivity is the rate at which autotrophs convert the sun's energy into energy (food) that is usable for themselves or for other organisms. Most of the life on earth is sustained by energy from the sun.
Autotrophs produce their own food; heterotrophs must consume autotrophs or other autotrophs for food.
A food chain shows the flow of energy through trophic levels of increasing hierarchy (producer -> primary consumer -> secondary consumer -> tertiary consumer).
A food web brings multiple food chains together to make a more complete, complex model of who eats who in an ecosystem.
Energy is lost through heat between trophic levels, which means only about 10% of energy gets transferred between trophic levels. Biomass of trophic levels decreases as you move from producer to tertiary consumer.

References

Primary Productivity by the Editors of Encyclopaedia Britannica. Last updated: January 24th, 2022. Britannica.com.

Frequently Asked Questions about Food Chains and Food Webs

What is the difference between food chains and food webs?

A few differences exist between food chains and food webs, the biggest is that food webs are actually a compilation of many food chains, meaning they are more complex than food chains.

More specific differences include:

Food chains show distinct trophic levels for each organism. In a food web, because multiple relationships between organisms are highlighted, an organism may belong to more than one trophic level.
Because organisms can only belong to one trophic level in a food chain, it does not account for organisms that feed from multiple trophic levels, such as omnivores. These organisms are able to be seen in a food web.
A food chain shows a single thread of energy transfer in a food web, which shows multiple threads.

What is the importance of food chains and food webs?

Food chains and food webs help ecologists visualize what is happening within an ecosystem. A food chain helps realize a single thread within a food web.

A food chain also shows where the most biomass in an ecosystem exists, as the start of the food chain, or the base organism, usually has the most biomass (producers, decomposers) since approximately 10% of the biomass from each level is converted to biomass at the next trophic level.

A food web reflects the multiple feeding relationships among organisms in the larger ecosystem. Organisms (consumers) may feed from more than one tropic level and on different types of organisms, which a food web demonstrates.

What is a food chain?

A food chain is a representation of energy flow in an ecosystem from one organism to the next. A food chain moves from producer to primary consumer to secondary consumer and so forth, lasting only about 4 or 5 trophic levels long. Trophic levels are tiers within a food chain that group together organisms with similar feeding relationships (organisms that have similar sources of energy).

A food chain that starts with a producer and moves to primary consumers and then secondary and tertiary consumers, is known as a grazing food chain. A food chain that starts with a decomposer (breaks down non-living organic matter), moves to a detritivore (feeds off of non-living organic matter), and then a carnivore (secondary/ tertiary consumer), is known as a detrital food chain.

How does a food web work?

A food web is made up of multiple food chains. A food web is made by identifying what each organism eats or where it gets its energy from. Then, all the organisms in that specific ecosystem (i.e., temperate forest) you are observing can be connected by arrows, indicating what organism becomes food for another organism. All the arrows within a food web represent a transfer of energy from one organism to the next.

Since a food web combines food chains, it may show the transfer of energy between food chains. For example, a grazing food chain, which is producer-based, may move from producer to primary consumer to secondary consumer. Then, the secondary consumer may die and provide energy for the decomposers, which starts the beginning of another food chain from decomposers to detritivores to back to secondary and tertiary consumers.

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