What would happen if cetaceans disappeared?

The question may seem abstract, almost hypothetical. Yet several cetacean species are today Critically Endangered: the vaquita numbers only a few individuals, the Yangtze baiji is already considered functionally extinct, and many European killer whale populations could disappear within the coming decades. Imagining an ocean without cetaceans is no longer science fiction, it is a scenario that marine ecologists are studying seriously, because the consequences would run far deeper than one might think.

The collapse of the biological pump

Whales play an ecological role that is often invisible but essential, known as the “whale pump”. By diving to hunt in the depths and returning to the surface to breathe, these giants carry out a massive vertical transfer of nutrients. Their faeces, rich in nitrogen, iron and phosphorus, are released near the surface in the sunlit layers where phytoplankton proliferate, the microalgae that form the base of the whole marine food chain.

Phytoplankton produce about 50% of the atmospheric oxygen we breathe, and absorb nearly 40% of the carbon dioxide emitted by human activities. Without cetaceans, this flow of nutrients towards the surface dries up. Less phytoplankton means less oxygen produced and a critical reduction in the capacity of the oceans to sequester CO₂. The ocean, the first bulwark of the planetary climate, would lose a significant part of its effectiveness.

Regulation of the food webs

Cetaceans are not only predators: they are ecosystem regulators. Krill, for example, those tiny crustaceans that make up most of the diet of southern right whales, paradoxically sees its populations increase when whale numbers fall, with no control by their main predator one might have expected the opposite. This puzzle, known as the “krill paradox”, is explained precisely by the whale pump: fewer whales = less fertilisation = less phytoplankton = less krill. Whales consume krill, but above all they produce the conditions for it to proliferate.

At the top of the chain, killer whales play a role equivalent to that of the wolf in a forest: they regulate populations of seals, porpoises and small cetaceans. Their disappearance brings population explosions among their prey, which in turn unbalance the ecosystem, a phenomenon called a trophic cascade. The consequences can reach as far as the algae, the seagrass beds and the reefs.

Whales, allies of the climate even after their death

The climatic role of cetaceans does not stop with their lives. When a great whale dies naturally, its body sinks slowly towards the abyss, a phenomenon called a “whale fall”, in French chute de baleine. A single carcass can sequester several tens of tonnes of carbon on the ocean floor, where that carbon stays trapped for centuries or millennia. At the scale of pre-industrial cetacean populations, this sequestration is estimated to have represented a significant share of the oceanic carbon cycle.

A whale carcass on the bottom also creates an ephemeral but rich ecosystem, which can feed a specialised fauna for fifty to a hundred years: sleeper sharks, deep-sea octopuses, tube worms, chemosynthetic bacteria. Each whale fall is, on its own, an oasis of biodiversity in the abyssal deserts.

The impact on humanity

The consequences would also be direct for human societies. Fishing depends on the balance of the food chains that cetaceans help to maintain: a collapse would upend commercial fish populations. Whale watching tourism, which generates more than two billion dollars a year worldwide, would disappear. The coastal communities that live from watching, from measured fishing and from marine ecosystem services would be lastingly affected.

And beyond the figures, there would be a loss that is harder to quantify: that of an irreplaceable part of the planet’s living heritage, the extinction of animals endowed with an intelligence comparable to our own, with a transmissible culture, with an evolutionary history going back fifty million years. A loss that future generations would probably not forgive us.

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Further reading