What if orcas left the Southern Ocean?

Orcas are big black-and-white whales. They live in every ocean.

Near Antarctica, orcas catch seals, penguins and fish. What if all the orcas swam away?

Orcas, also called killer whales, live in every ocean on Earth. NOAA says they are most abundant in cold waters, such as the seas around Antarctica. They are apex predators: they eat at the top of the food web.

So what if every orca left the Southern Ocean around Antarctica? Would anything else notice? In this lab you can take the orca out of a model food web and follow what could happen next.

Killer whales occur in all oceans and are most abundant in cold waters such as those around Antarctica (NOAA). As apex predators, their effect on an ecosystem runs downward through the animals they eat. Researchers studying Antarctic killer whales note that their predation may be a major force structuring marine communities.

This lab asks what a food web model predicts if that top level is removed entirely. It is a qualitative experiment: the model shows which species could be affected and in which direction, not by how much.

In the cold sea, tiny plants called phytoplankton feed the krill.

Krill feed fish, penguins, seals and whales. Orcas eat seals, penguins and fish.

The Australian Antarctic Division says the Southern Ocean food web begins with tiny plant-like phytoplankton that use sunlight. They feed Antarctic krill, small shrimp-like animals, and nearly everything else eats krill: fish, penguins, some seals and great whales.

Our model web has eight members. Krill eat phytoplankton. Silverfish, crabeater seals and blue whales eat krill. Adélie penguins eat krill and fish. Leopard seals eat krill, penguins and fish. At the top, orcas eat crabeater seals, penguins and fish.

The model's Southern Ocean web has eight nodes in four rows: phytoplankton; Antarctic krill; silverfish, Adélie penguin, crabeater seal and blue whale; and the leopard seal and orca. Each link is present or absent, and all of a species' foods count equally.

This matches the outline the Australian Antarctic Division gives: phytoplankton feed krill, krill feed fish, squid, seabirds, penguins, some seals and baleen whales, and predators such as leopard seals and killer whales eat penguins and other seals. The real web has hundreds of species of plankton, birds and fish.

Now Olly takes the orcas out of the web.

The seals, penguins and fish have one less hunter. There could be more of them!

Take the orca out of the model and nothing is left without food, because orcas are at the top. Instead, the change flows downward. The silverfish, Adélie penguins and crabeater seals lose a hunter, so they could become more common.

All three eat krill, so with more of them eating, krill could become fewer. And then the blue whale and leopard seal, which also eat krill, could find less food. In all, six of the other seven species are affected.

With the orca removed, the model's bottom-up rule finds nothing starved: no species depended on orcas for food. The top-down rule marks the orca's three prey (silverfish, Adélie penguin, crabeater seal) as possibly more common. Their shared food, krill, is marked as possibly fewer, and the blue whale and leopard seal, whose foods include krill, as possibly finding less food.

Readouts: affected 6, more 3, fewer 1, less food 2, gone 0. Only phytoplankton is untouched, because the model follows top-down effects just one level.

Every animal in a food web is joined to others. Take one away, and others feel it.

When a hunter goes, the animals it ate can grow in number. Then they eat more of their own food.

A food web is a set of links between who eats whom. When a hunter disappears, the animals it ate are no longer being caught, so their numbers could grow. More of them means more mouths eating their food. This chain of knock-on effects is called a trophic cascade.

Our model is simple. It only says who could be affected, never how many animals or how fast. Real orcas and leopard seals eat many foods the model leaves out, and real animals can switch foods.

Top-down control: a predator limits its prey, which in turn limits what the prey eat. Removing the predator can release the prey and shift pressure to the next level, a trophic cascade. The model applies this as rules: release the removed species' prey, mark their foods as eaten more, and flag species whose foods are reduced.

Limits from its documentation: it is qualitative, gives no population numbers, follows top-down effects only one level, ignores competition, disease, weather and people, and real predators often switch prey. Note too that the model's orca does not eat leopard seals, although the Australian Antarctic Division names the killer whale as the leopard seal's only natural predator.

Some orcas from Antarctica really do take long trips. They swim to warm water and come back!

Scientists followed them with special tags.

Real Antarctic orcas do sometimes leave, for a while. NOAA scientists Durban and Pitman put satellite tags on 12 orcas hunting near the Antarctic Peninsula. Six of them swam fast and straight away from Antarctica, and five reached warm waters off Uruguay and Brazil.

One orca made a round trip of almost 9,400 kilometers in just 42 days. The scientists found no sign that the trips were for feeding or for having calves. They suggest the warm water might let the orcas renew their skin without losing too much body heat.

Durban and Pitman (NOAA Southwest Fisheries Science Center, Biology Letters, 2011) tagged 12 type B killer whales, predators of ice seals and penguins, near the Antarctic Peninsula. Six made fast (over 12 km/h), direct movements away from Antarctic waters; five reached subtropical waters at 30–37° S off Uruguay and Brazil, in water from −1.9 °C to 24.2 °C. One whale completed a non-stop round trip of almost 9,400 km in 42 days.

With no sign of calving or prolonged feeding, the authors suggest periodic maintenance migrations allowing skin regeneration without high heat loss: a hypothesis. A real, lasting departure of all orcas has never been observed, so its effects remain an idea.