Steller's sea cows weren't really cows. Apart from living in the Bering Sea, they were much, much larger — perhaps 10 meters long and 11,000 kilograms, roughly the size and weight of two mid-size pick-up trucks, though it's impossible to say for sure. They went extinct in the mid-18th century, possibly because of human hunting — hunting sea otters, that is.

A new study linking sea otter declines to the demise of an entire species is part of a special feature on megafauna — literally, large animals — published recently in Proceedings of the National Academy of Sciences. Apart from new insights into sea cows' extinction, researchers also report:

The sea cow-sea otter connection, however, highlights something often lost in the popular discussion of extinction and ecology — namely, the indirect yet enormous impact of human activity on our environment. In particular, ecologists are looking more and more at the role humans play in food webs, the complex networks that describe what eats what in an ecosystem, and the damage that losing one species can do throughout an ecosystem.

In the latest PNAS study, ecologists James Estes, Alexander Binder, and Daniel Doak focused on a relatively old and somewhat controversial question: How did the Steller's sea cow go extinct? According to one hypothesis, Russian sailors exploring the Bering Sea's Commander Islands simply hunted them to extinction. Considering the rates at which explorers killed the animals, that theory is entirely plausible.

But there's an alternative: Even if hunters never hunted sea cows, they still would have gone extinct because of sea otter hunting. Here's how it works: Sea otters eat sea urchins, which compete for resources with kelp — sea cows' main food source, the researchers argue. Therefore, killing sea otters indirectly culls kelp populations, ultimately starving sea cows. Estes and others recently tested part of that logic, finding that "kelp forest collapse after the loss of sea otters is a virtual certainty," the researchers write in their latest paper.

Incorporating historical data on sea otter populations and previously published results on how dugongs, sea cows' closest living relatives, respond to kelp losses, the team ran computer simulations to estimate how many of the roughly 1,500 sea cows alive when explorers arrived in the Commander Islands in 1741 would survive to 1768, the year they're believed to have gone extinct. The answer: At most six (more likely none at all), even if humans never touched a hair on a sea cow's head.

While that only shows that sea otter hunting could have killed off sea cows, similar, indirect interactions "can be powerful enough to cause significant changes in the abundance of other species, including extinctions" — something others should take into account in future research, the team writes.

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