What if an ocean food web changed?

In the icy sea near Antarctica, tiny krill swim in huge groups. Penguins, seals and whales eat them.

What if all the krill disappeared? Who would be hungry?

A food web shows who eats whom. Arrows run from a food to the animal that eats it, so you can follow energy from tiny ocean plants all the way to the biggest hunters.

In the Southern Ocean around Antarctica, one small animal sits in the middle of the web: Antarctic krill. What would happen to the rest of the web if the krill were gone? In this lab you can take krill, or any other species, out of a model web and watch the effects spread.

Food webs map trophic links: who consumes whom. Removing a species can act bottom-up (its consumers lose food) and top-down (its prey lose a predator), and the effects can spread as a trophic cascade.

The Southern Ocean is a classic case of a web funneled through one species. This lab removes Antarctic krill from a simplified model web and asks which species are affected, and what the model cannot tell us about the real ocean.

Tiny floating plants grow in the sunny sea. Krill eat them.

Then penguins, seals and whales eat the krill. Blue whales eat almost nothing else!

Antarctic krill grow to about 6 centimeters long and weigh just over a gram, but there are an estimated 400 million tonnes of them in the Southern Ocean. Their swarms can be so thick that they color the sea reddish-brown.

Krill mainly eat phytoplankton, tiny floating plants that need sunlight to grow. In turn, Adélie penguins, seabirds, seals and whales eat krill. Crabeater seals, despite their name, eat krill, and blue whales feed almost only on krill; the biggest may eat up to 6 tons in a day.

The Australian Antarctic Division estimates about 400 million tonnes of Antarctic krill (Euphausia superba), with swarm densities up to 30,000 per cubic meter, and says most of the larger Antarctic animals (seals, whales, seabirds, fish and squid) depend on krill directly or indirectly. Krill feed mainly on phytoplankton and, in winter, on algae under the pack ice.

The model's web has eight species: phytoplankton; krill; silverfish (eats krill); Adélie penguin (krill, silverfish); crabeater seal and blue whale (krill); leopard seal (krill, penguins, silverfish); and orca (crabeater seals, penguins, silverfish).

In our pretend lab, Olly takes all the krill out of the food web.

In the model, the penguins, seals, whales and fish have nothing left to eat. Only the tiny plants have more room to grow.

Choose 'Antarctic krill' and run the model. The changes spread in waves. First, the silverfish, the crabeater seal and the blue whale, which eat only krill in the model, lose their food. Then the animals that eat them lose their food too. In the end, all six animals in the web are marked 'could disappear', because every food the model lists for them is gone.

One thing gains: the phytoplankton. With no krill eating them, the model marks them as possibly more common.

With removeOcean = 2 the model marks 7 species affected: 6 'could disappear (no food left)' (silverfish, Adélie penguin, crabeater seal, blue whale, leopard seal, orca) and 1 'could become more common' (phytoplankton, released from grazing). No species is merely 'finding less food', because the cascade empties every consumer's prey set.

Compare other removals: phytoplankton gone affects all 7 others; silverfish gone affects 5; the blue whale affects only 2 (krill could rise, phytoplankton could fall). Apart from the phytoplankton at the base, krill is the only species whose loss empties the web.

Krill are like the middle of a big net. Many lines run through them.

Cut the middle, and the whole net comes apart.

Energy in the Southern Ocean flows mostly through one doorway: plants make food from sunlight, krill eat the plants, and almost everything bigger eats krill or eats something that does. When so many links pass through one species, losing it cuts the path for nearly everyone.

In a web with many different middle animals, losing one leaves other paths open. That is why scientists study which species hold a web together.

The model applies rules repeatedly: a consumer with no remaining foods could disappear; prey that lost a predator could become more common; and their own foods could then become fewer. Because krill is the sole or main path from primary producers to every consumer here, the bottom-up rule propagates to all of them.

The model's limits, from its documentation: it is qualitative, with no population numbers or timescales; each link is simply present or absent; only one level of top-down effect is followed; and real predators such as orcas and leopard seals eat many prey not shown, so real losses would usually be smaller and slower than 'no food left' suggests.

Krill are not disappearing all at once. But scientists watch them closely.

Sea ice helps baby krill grow. People who fish for krill have rules to follow.

Krill are not about to vanish, but their world is changing. The Australian Antarctic Division says young krill's survival is affected by changes in climate, sea ice and ocean chemistry. Sea ice matters: tiny algae grow on it, and when it melts in spring the algae help feed plankton blooms that krill eat.

People also catch krill. An international commission, CCAMLR, sets the rules for krill fishing in the Southern Ocean and reviews the fishery every year.

The Australian Antarctic Division notes that early krill survival is affected by changes in climate, sea ice and ocean chemistry; NSIDC describes sea-ice algae seeding spring blooms that feed krill and, through them, Adélie penguins, seabirds, seals and whales.

The krill fishery is managed by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR), whose working group and scientific committee review it each year and whose conservation measures set the catch limits. What a real collapse of krill would do is an open question; the model shows the web's dependence, not a forecast.