What Is a Food Web? Definition, Types, and Examples

A food web is a detailed interconnecting diagram that shows the overall food relationships between organisms in a particular environment. It can be described as a "who eats whom" diagram that shows the complex feeding relationships for a particular ecosystem.

The study of food webs is important, as such webs can show how energy flows through an ecosystem. It also helps us understand how toxins and pollutants become concentrated within a particular ecosystem. Examples include mercury bioaccumulation in the Florida Everglades and mercury accumulation in the San Francisco Bay.

Food webs can also help us study and explain how the diversity of species is related to how they fit within the overall food dynamic. They may also reveal critical information about the relationships between invasive species and those native to a particular ecosystem.

Food Web Definition

The concept of a food web, previously known as a food cycle, is typically credited to Charles Elton, who first introduced it in his book Animal Ecology, published in 1927. He is considered one of the founders of modern ecology and his book is a seminal work. He also introduced other important ecological concepts like niche and succession in this book.

In a food web, organisms are arranged according to their trophic level. The trophic level for an organism refers to how it fits within the overall food web and is based on how an organism feeds. 

Broadly speaking, there are two main designations: autotrophs and heterotrophs. Autotrophs make their own food while heterotrophs do not. Within this broad designation, there are five main trophic levels: primary producers, primary consumers, secondary consumers, tertiary consumers, and apex predators

A food web shows us how these different trophic levels within various food chains interconnect with one another as well as the flow of energy through the trophic levels within an ecosystem.

Trophic Levels in a Food Web

Primary producers make their own food via photosynthesis. Photosynthesis uses the sun's energy to make food by converting its light energy into chemical energy. Primary producer examples include plants and algae. These organisms are also known as autotrophs.

Primary consumers are those animals that eat the primary producers. They are called primary as they are the first organisms to eat the primary producers who make their own food. These animals are also known as herbivores. Examples of animals in this designation are rabbits, beavers, elephants, and moose.

Secondary consumers consist of organisms that eat primary consumers. Since they eat the animals that eat the plants, these animals are carnivorous or omnivorous. Carnivores eat animals while omnivores consume both other animals as well as plants. Bears are an example of a secondary consumer.

Similar to secondary consumers, tertiary consumers can be carnivorous or omnivorous. The difference is that secondary consumers eat other carnivores. An example is an eagle.

Lastly, the final level is composed of apex predators. Apex predators are at the top because they do not have natural predators. Lions are an example.

Additionally, organisms known as decomposers consume dead plants and animals and break them down. Fungi are examples of decomposers. Other organisms known as detritivores consume dead organic material. An example of a detrivore is a vulture.

Energy Movement

Energy flows through the different trophic levels. It begins with the energy from the sun that autotrophs use to produce food. This energy is transferred up the levels as the different organisms are consumed by members of the levels that are above them.

Approximately 10% of the energy that is transferred from one trophic level to the next is converted to biomass—the overall mass of an organism or the mass of all the organisms that exist in a given trophic level.

Since organisms expend energy to move around and go about their daily activities, only a part of the energy consumed is stored as biomass.

Food Web vs. Food Chain

While a food web contains all constituent food chains in an ecosystem, food chains are a different construct. A food web can be composed of multiple food chains, some that can be very short, while others may be much longer. Food chains follow the flow of energy as it moves through the food chain. The starting point is the energy from the sun and this energy is traced as it moves through the food chain. This movement is typically linear, from one organism to another.

For example, a short food chain may consist of plants that use the sun's energy to produce their own food through photosynthesis along with the herbivore that consumes these plants. This herbivore may be eaten by two different carnivores which are a part of this food chain. When these carnivores are killed or die, the decomposers in the chain break down the carnivores, returning nutrients to the soil that can be used by plants.

This brief chain is one of many parts of the overall food web that exists in an ecosystem. Other food chains in the food web for this particular ecosystem may be very similar to this example or may be much different. 

Since it is composed of all of the food chains in an ecosystem, the food web will show how the organisms in an ecosystem interconnect with one another.

Types of Food Webs

There are a number of different types of food webs, which differ in how they are constructed and what they show or emphasize in relation to the organisms within the particular ecosystem depicted.

Scientists can use connectance and interaction food webs along with energy flow, fossil, and functional food webs to depict different aspects of the relationships within an ecosystem. Scientists can also further classify the types of food webs based on what ecosystem is being depicted in the web.

Connectance Food Webs

In a connectance food web, scientists use arrows to show one species being consumed by another species. All of the arrows are equally weighted. The degree of strength of the consumption of one species by another is not depicted.

Interaction Food Webs

Similar to connectance food webs, scientists also use arrows in interaction food webs to show one species being consumed by another species. However, the arrows used are weighted to show the degree or strength of consumption of one species by another.

The arrows depicted in such arrangements can be wider, bolder, or darker to denote the strength of consumption if one species typically consumes another. If the interaction between species is very weak, the arrow can be very narrow or not present.

Energy Flow Food Webs

Energy flow food webs depict the relationships between organisms in an ecosystem by quantifying and showing the energy flux between organisms.

Fossil Food Webs

Food webs can be dynamic and the food relationships within an ecosystem change over time. In a fossil food web, scientists attempt to reconstruct the relationships between species based on available evidence from the fossil record.

Functional Food Webs

Functional food webs depict the relationships between organisms in an ecosystem by depicting how different populations influence the growth rate of other populations within the environment.

Food Webs and Type of Ecosystems

Scientists can also subdivide the above types of food webs based on the type of ecosystem. For example, an energy flow aquatic food web would depict the energy flux relationships in an aquatic environment, while an energy flow terrestrial food web would show such relationships on land.

Importance of the Study of Food Webs

Food webs show us how energy moves through an ecosystem from the sun to producers to consumers. This interconnectedness of how organisms are involved in this energy transfer within an ecosystem is a vital element to understanding food webs and how they apply to real-world science.

Just as energy can move through an ecosystem, other substances can move through as well. When toxic substances or poisons are introduced into an ecosystem, there can be devastating effects.

Bioaccumulation and biomagnification are important concepts. Bioaccumulation is the accumulation of a substance, like poison or a contaminant, in an animal. Biomagnification refers to the buildup and increase in the concentration of said substance as it is passed from trophic level to trophic level in a food web.

This increase in toxic substances can have a profound impact on species within an ecosystem. For example, man-made synthetic chemicals often do not break down easily or quickly and can build up in an animal's fatty tissues over time. These substances are known as persistent organic pollutants (POPs).

Marine environments are common examples of how these toxic substances can move from phytoplankton to zooplankton, then to fish that eat the zooplankton, then to other fish (like salmon) who eat those fish, and all the way up to orca who eat salmon. Orcas have a high blubber content so the POPs can be found at very high levels. These levels can cause a number of issues like reproductive problems, developmental issues with their young as well as immune system issues.

By analyzing and understanding food webs, scientists are able to study and predict how substances may move through the ecosystem. They are then better able to help prevent the bioaccumulation and biomagnification of these toxic substances in the environment through intervention.

Sources

• “Food Webs and Networks: the Architecture of Biodiversity.” Life Sciences at the University of Illinois at Urbana-Champaign, Biology Department.
• “11.4: Food Chains and Food Webs.” Geosciences LibreTexts, Libretexts.
• “Terrestrial Food Webs.” Smithsonian Environmental Research Center.
• “Bioaccumulation and Biomagnification: Increasingly Concentrated Problems!” CIMI School.