Feeding relationships are often shown as simple food chains – in reality, these relationships are much more complex, and the term ‘food web’ more accurately shows the links between producers, consumers and decomposers
A food web diagram illustrates ‘what eats what’ in a particular habitat1. Pictures represent the organisms that make up the food web, and their feeding relationships are typically shown with arrows. The arrows represent the transfer of energy and always point from the organism2 being eaten to the one that is doing the eating.
Trophic levels
Organisms in food webs are commonly divided into trophic levels. These levels can be illustrated in a trophic pyramid where organisms are grouped by the role they play in the food web. For example, the 1st level forms the base of the pyramid and is made up of producers. The 2nd level is made up of herbivorous consumers and so on. On average, only 10% of the energy from an organism is transferred to its consumer3. The rest is lost as waste, movement energy, heat energy4 and so on. As a result, each trophic level5 supports a smaller number of organisms – in other words, it has less biomass6. This means that a top-level consumer, such as a shark, is supported by millions of primary producers from the base of the food web or trophic pyramid.
Marine trophic pyramid
Food webs throughout the world all have the same basic trophic levels. However, the number and type of species that make up each level varies greatly between different areas and different ecosystems.
Download a PDF of this diagram.
Food webs throughout the world all have the same basic trophic levels. However, the number and type of species7 that make up each level varies greatly between different areas and different ecosystems8.
Producers
Producers are described as autotrophic9, which means they are able to make their own food. Just like producers on land, producers in the marine environment convert energy from the sun into food energy through photosynthesis10. Phytoplankton are the most abundant and widespread producers in the marine environment. Other producers include seaweeds (a type of macroalgae) and seagrasses (the only flowering plant11 found in marine environments). New Zealand has only 1 species of seagrass but many species of seaweed.
Consumers
Consumers are described as heterotrophic12, which means they are unable to make their own food and rely on consuming other organisms or absorbing dissolved organic13 material in the water column14.
Consumers are divided into herbivores15 and carnivores16 and are typically further divided into 1st, 2nd or 3rd level consumers. For example, many zooplankton in the marine environment are herbivorous consumers. They form the 2nd level of the trophic pyramid and consume phytoplankton17. Zooplankton18 are eaten by the 1st level carnivorous consumers, which includes juvenile stages of larger animals like fish and jellyfish as well as small fish and crustaceans19. 2nd and 3rd level carnivorous consumers include larger fish and some species of squid and octopus. Predators at the top level of the trophic pyramid include animals like sharks and dolphins. However, not all top marine predators live in the sea. The albatross is an important predator20 at the top of the marine food web in Otago. Humans are also top-level consumers in the marine food web.
Decomposers
Decomposers exist on every trophic level. They are mainly bacteria21 that break down dead organisms. This process releases nutrients22 to support the producers as well as the consumers that feed through absorbing organic material in the water column. This process is very important and means that even top-level consumers are contributing to the food web as the decomposers break down their waste or dead tissue.
Changes to food webs
The effect of removing or reducing a species in a food web varies considerably depending on the particular species and the particular food web. In general, food webs with low biodiversity23 are more vulnerable to changes than food webs with high biodiversity. In some food webs, the removal of a plant species can negatively affect the entire food web, but the loss of one plant species that makes up only part of the diet of a herbivorous consumer may have little or no effect.
Some species in a food web are described as ‘keystone’ species. A keystone species24 is one that has a greater impact on a food web than you would expect in relation to their abundance. The removal of a keystone species characteristically results in a major change, in the same way that removing a keystone from an arch or bridge could cause the structure to collapse.
In Fiordland, the New Zealand sea star is a keystone species that controls the numbers of the species it feeds on, for example, mussels. If the sea star is removed, this can cause a large increase in the numbers of mussels, and this has flow-on effects throughout the food web.
Many scientists investigate food webs in order to better understand how they may be affected by human impacts such as fishing, pollution and tourism.
Useful links
Listen to this Radio New Zealand programme Sea Lions As Food Web Ambassadors. Lucy Jack is hoping that her research will give insights into marine food webs and how they’ve changed over time.
Learn about Trophic level: definition, categories, structure, examples and importance on Biology Online.
- habitat: The natural environment in which an organism lives.
- organism: A living thing.
- consumer: 1. An organism that feeds on other organisms to obtain energy for life processes. These organisms are also called heterotrophs. 2. A person who purchases goods and services for personal use.
- heat energy (heat): Heat energy: the transfer of energy in materials from the random movement of the particles in that material. The greater the random movement of particles the more heat energy the material has. Temperature is a measure of the heat energy of a material.
Heat: the flow of energy from a warm object to a cooler object. - trophic level: The feeding level in food webs. For example, producers are in the first trophic level.
- biomass: 1. Organic matter, such as trees, plants, reject fruit, straw, algae, dairy effluent or tallow (waste fat), which can be turned into biofuel. 2. The mass of living biological organisms in a given area or ecosystem at a given time.
- species: (Abbreviation sp. or spp.) A division used in the Linnean system of classification or taxonomy. A group of living organisms that can interbreed to produce viable offspring.
- ecosystem: An interacting system including the biological, physical, and chemical relationships between a community of organisms and the environment they live in.
- autotroph: An organism that is able to make its own food.
- photosynthesis: A process that uses the energy from sunlight to convert carbon dioxide and water into carbohydrates, releasing oxygen as a byproduct. Photosynthesis occurs in the green parts of plants, in algae and in some microorganisms.
- flowering plant: A plant with a life cycle that includes the formation of seeds inside flowers. The scientific name for a flowering plant is angiosperm.
- heterotroph: An animal that is unable to make its own food and relies on consuming nutrients from other organisms.
- organic: 1. Molecules that contain carbon and that have a biological origin. 2. Grown using natural processes with nutrients from natural sources.
- water column: The vertical section of water between the freshwater or ocean floor and the surface.
- herbivores: An animal that only eats plants, compared to carnivores, which only eat meat, or omnivores, which eat plants and meat.
- carnivores: Animals that eat other animals.
- phytoplankton: Very small plant organisms that drift with water currents and, like land plants, use carbon dioxide, release oxygen and convert minerals to a form animals can use.
- zooplankton: Tiny shrimp-like animals that drift with water currents.
- crustaceans: A large group of arthropods, which includes animals such as crabs, lobsters, crayfish, shrimp, krill and barnacles.
- predator: An animal that kills and eats other animals, called its prey.
- bacteria: (Singular: bacterium) Single-celled microorganisms that have no nucleus.
- nutrient: A substance that provides nourishment for growth or metabolism.
- biodiversity: The range of species found in a particular region. The more species that exist (the higher the biodiversity), the more likely it is that an ecosystem will survive episodes of change.
- keystone species: A species that has a greater impact on the community of organisms in an ecosystem than you would expect in relation to its abundance. The removal of a keystone species often has a dramatic effect on the ecosystem.
