A food web describes the links between species in an ecosystem. The web is a complex network of food chains. Food chains are often based on plants that provide food for other animals.
Some animals only eat plants and are called herbivores. Others eat plants and animals and are called omnivores. Animals that only eat other animals are called carnivores.
Food webs are finely balanced ecosystems. The loss of even one species can break the delicate threads of the web. This can have a serious impact on the environment.
It starts with krill
In Antarctica and the Southern Ocean, the food web begins with microscopic plants called phytoplankton. These ‘feed’ on the energy of the sun. Phytoplankton are food for Antarctica’s most important species – Antarctic krill.
Krill look like smaller versions of familiar crustaceans such as prawns or shrimp. They are mostly transparent, although their shells have a bright red tinge from small pigment spots. Their digestive system is usually visible and coloured a vivid green from the microscopic plants they eat. They have large black eyes. Adult Antarctic krill are approximately 6 cm in length and weigh over 1 gm.
Who’s eating who?
Nearly everything in Antarctica has krill for dinner. This includes scale fish, squid, and seabirds such as albatross and petrels.
They’re also meals for iconic marine predators including penguins, some seals as well as humpback and blue whales.
Predators can also be prey, with leopard seals eating penguins and other seals. As well as killer whales eating seals and penguins. It’s a tough world!
Squid are carnivorous marine cephalopods. Their size varies from a few inches to over 30 feet. A squid's position in the food chain is determined by its size. The smaller the squid the lower its position; in turn, the larger the squid, the higher it becomes in the food chain.
Squids are predators. They eat fish, crustaceans (like shrimp), crabs and even other squids. They are secondary and tertiary consumers, meaning they eat herbivores and other carnivores. They are able to catch prey with their two feeding tentacles, then hold the prey with their eight arms and bite it into small pieces using a parrot-like beak.
Although small squid are predators, they are also prey for many other creatures, including birds, fish, sharks, whales, humans and larger squid. Small squid have the ability to move rapidly through the water. Their skin is covered in chromatophores, pigmented cells arranged in layers that allow squids to change color to match their surroundings. Small squids' high speed and ability to change colors not only help them evade predators, but allow them to more easily pounce on prey.
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Giant and colossal squid are two of Earth's largest animals, their length exceeded only by the sperm whale. Reaching 46 feet long, colossal squid are the largest known invertebrate. Colossal and giant squid are close to the top of the food chain, able to eat any sea animal they can catch. Whales have been found with scars from the suckers of the giant squid and the hooks of the colossal squid. Colossal squid consume little food relative to other large sea creatures, and subsist mainly on large fish, chiefly the Antarctic toothfish. The full-sized adults are only preyed on by sleeper sharks and the sperm whale.
There is still much to learn about squid, as they tend to live closer to the bottom of the ocean. The colossal squid has the largest known eyes in the animal world. These eyes help them to escape from predators. They are slower than other squid. To see their prey in the deep ocean they use bioluminescence, the ability to emit light from their bodies. Some of these prey are Chilean sea bass, Antarctic toothfish and chaetognatha, which they catch using their hooks.
Food webs describe who eats whom in an ecological community. Made of interconnected food chains, food webs help us understand how changes to ecosystems — say, removing a top predator or adding nutrients — affect many different species, both directly and indirectly.
Phytoplankton and algae form the bases of aquatic food webs. They are eaten by primary consumers like zooplankton, small fish, and crustaceans. Primary consumers are in turn eaten by fish, small sharks, corals, and baleen whales. Top ocean predators include large sharks, billfish, dolphins, toothed whales, and large seals. Humans consume aquatic life from every section of this food web.
Whales are the largest animals ever to have lived, larger even than the largest dinosaurs. There are two reasons that they have managed to attain such enormous size - well over a hundred tonnes for the largest blue whales and nearing this amount for some other whale species.
1/ They live in the oceans and so the buoyancy of the water can support their great bulk rather than having to be propped up on land by legs and muscles. As with most other mammals, the density of a whale is very close to that of water.
2/ Whales tap the food chain low down - close to the producers, there are few steps and so little energy is lost meaning more is available to the whales, so they are able to grow to enormous sizes. The higher up a food chain you get, the lower the biomass of animals (that is number of animals multiplied by their weight) because there are more steps and so more energy is lost. The more steps in a chain means less food available at the top.
The Antarctic Food Web is relatively simple compared to ecosystems in other parts of the world. There are fewer different species, but greater numbers of individuals of each. The second most numerous large mammal in the world (after man) is the crab-eater seal, an archetypal Antarctic animal.
A key part of the Antarctic food web are krill small shrimp-like crustaceans that the great majority of Antarctic animals, seal, whales, penguins and other birds, fish etc. feed upon.
What are the Producers in Antarctic Food Chains?
There are few land plants in Antarctica, all the large animals including the ones that come onto land like seals, penguins and other birds take their food from the sea. The producers in Antarctic food chains are tiny single celled plants known collectively as phytoplankton that float in the upper layer of the sea though they can grow at depths down to about 100m.
There are many different species and types of phytoplankton, the two largest groups are Diatoms and Dinoflagellates, they are from 2-2000 micrometers in size, by comparison the width of a human hair is in the region of 100 micrometers.
Diatoms are made of two (di) external silica shells that fit together like a small box. Dinoflagellates have a flagellum a whip-like "tail" that they can use to move about.
Food chain: A sequence of organisms starting with a producer (usually a plant), followed by the animal that eats the plant, then an animal that eats that animal and so on to the "top carnivore" an animal that eats others but is not killed and eaten by any other. Though everything eventually gets "eaten" by decomposers when it dies by some other cause. The arrows always point to the animal that does the eating and from the organism that gets eaten, more specifically they indicate the flow of energy.
Food web: A complex combination of a number of animals and plants in an ecosystem or habitat that shows what eats what and what gets eaten by what. A food web shows a more accurate picture as in reality it is rare for anything to just eat one kind of food.
Plankton: organisms that live in the top layers of a body of water, whether sea, lake, pond, river etc. Plankton are at the mercy of the currents and movement of the water. Some plankton dwellers can move about within the water column, up and down, but drift where the tides and currents take them.
Phytoplankton: Phyto- plant, Plankton - see above. These are the tiny plants that capture the energy of the sun and turn it into food, they are the Producers of the Antarctic food web. As they are so tiny, they can divide and grow very quickly in response to the more intense and longer lasting light of the summer months.
Zooplankton; Zoo - animal, Plankton - see above. These are the tiny (and not so tiny) animals that feed directly on the phytoplankton, In Antarctica they are often krill which provide the food for most of all the larger animals.
Producer: An organism that produces food. Usually a green plant, anything from microscopic algae (as in phytoplankton) to a tree. The raw materials are sunlight for energy, with carbon dioxide and water providing the main raw materials for growth. Producers drive all food webs and chains. At each step along the chain energy is lost, only 10% or often much less is passed on between steps.
Consumer: An organism that eats food in the form of other organisms, plants, animals or a mixture of the two. If the consumer eats plants it is called a primary consumer as it is the first consumer in the food chain. If it eats an animal that itself is a consumer it is a secondary consumer as the second consumer in the food chain. In the Antarctic food chain krill are primary consumers and baleen whales, penguins, seals and many kinds of fish and other birds are secondary consumers when feeding on krill. Many animals are a mixture of primary, secondary, tertiary (3rd) and quaternary (4th) consumers as they eat a variety of prey.
Big floes have little floes all around about 'em And all the yellow diatoms couldn't do without 'em. Forty million shrimplets feed upon the latter,And they make the penguin and the seals and whales...Much fatter.
Thomas Griffith Taylor - geologist on Scott's 1909-11 Antarctic expedition
Big fleas have little fleas,Upon their backs to bite 'em,And little fleas have lesser fleas,
and so, ad infinitum.
The original nursery rhyme "Fleas".