Ecology: The sea-otter whisperer

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Jane Lubchenco applauds James Estes's chronicle of his 45 years studying the complexities of an apex predator.

Serendipity: An Ecologist's Quest to Understand Nature

James A. Estes University of California Press: 2016. ISBN: 9780520285033

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Francois Gohier/VWPics/Alamy

Sea otters are crucial to kelp-forest ecosystems.

The sea otter (Enhydra lutris) has had a turbulent history. A cultural icon for tribes living around the coast of the North Pacific Ocean, it became an irresistible target for fur traders who almost drove it to extinction, triggering an international hunting ban in 1911. Then, as the species began to recover in parts of its former range, it became a magnet for tourists, a symbol of hope for marine conservation and the equivalent of Darwin's finches for one scientist: ecologist James Estes. Sea otters are now in decline again in most areas. Estes has relentlessly unpacked insights into the species' history and ecological complexities for more than four decades, in one of the most remote places on Earth.

“This relatively small marine mammal has an ability to control the very presence or absence of entire kelp forests.”

In Serendipity, Estes chronicles that research — in Alaska's Aleutian Islands and beyond — on this relatively small marine mammal, its voracious appetite for crabs, clams, fish and especially sea urchins, and its consequent ability to control the very presence or absence of entire kelp forests and their inhabitants.

In his refreshingly honest and humble narrative, Estes shares the intriguing insights that he gained through careful observation, intentional perturbations of environmental conditions, long-term monitoring, rigorous analysis, willingness to collaborate and openness to new ideas. He acknowledges the often circuitous pathways and serendipitous events that yielded new ideas to test. For example, he relates a pivotal conversation with ecologist Robert Paine, who coined the concept of keystone species, which have a disproportionately large effect on their ecosystems (see Nature 493, 286289; 2013). That led Estes to pose what would become the defining question of his career — 'What impact do sea otters have on kelp forests?' — rather than his initial query, 'What impact do kelp forests have on sea otters?'

The study of top-down control of an ecosystem by an apex predator and the use of perturbations to test ecological hypotheses were in their infancy in the 1970s, when Estes began his Alaska studies. The Aleutian otters' ongoing recovery from near-extinction provided the perfect mosaic of conditions: in some places, there were many otters; in others, there were few; and in yet others, otters were still absent. Estes' early work demonstrated conclusively that otters in Alaska did control much of the rest of the near-shore community by eating sea urchins, the primary consumers of kelps, and controlling their abundance. Where otters are present, so too are kelp forests and the plethora of other species (fish, crabs, starfish, mussels, snails, seals, seaweeds, birds and more) that depend on the plants for habitat or food. Where otters are absent, urchins thrive, gobble up kelp and keep the seascape barren.

Estes and his colleagues went on to demonstrate multiple aspects of 'trophic cascades', in which an apex predator's influence ripples through the food web through complex, indirect interactions. Thanks to the team's work, we know that the diets of gulls and bald eagles, the abundance of fish, the growth of mussels and the size and abundance of starfish are all strongly influenced by the presence or absence of otters in Aleutian kelp forests.

Some of the most intriguing ideas that emerged from Estes's studies focus on the surprising and precipitous decline of Aleutian sea otters that began in the early 1990s. Observations of increasing numbers of killer whales, or orcas (Orcinus orca), and of killer whales preying on otters, coupled with calculations about otter demography and killer-whale energetics, led to a startling suggestion by Estes and his colleagues: predation by killer whales was driving down otter populations. The team's subsequent hypothesis was even more surprising.

Realizing that something must have changed in the open sea to drive killer whales from their normal oceanic habitats to coastal ones, Estes and his colleagues began to consider broader-scale events in space and time. They knew that after the Second World War, intense industrial whaling in the North Pacific Ocean had reduced whale biomass 'sevenfold to eightfold', up until implementation of the international ban on commercial whaling in the late 1960s and early 1970s. They reasoned that if killer whales had previously fed on great whales, the demise of the latter might have forced killer whales to forage more widely and switch to alternate prey.

Piecing together data from the seas up to 370 kilometres around the Aleutian archipelago, they presented a compelling hypothesis of sequential collapses that spanned four decades. First, great-whale populations plummeted because of whaling. Then, killer whales switched to and decimated first one species, and then another, causing the collapse of populations of harbour seals (Phoca vitulina), then Steller sea lions (Eumetopias jubatus) and finally sea otters. This 'megafaunal collapse hypothesis', articulated by Estes, Alan Springer and their colleagues, was not received warmly by some scientists who had been studying individual species, but evidence for it continues to mount. Moreover, these ideas raise more intriguing questions: as at least some great whales recover from decimation by whaling, what will killer whales prey on? And what will then happen to seals, sea lions, otters and kelp-forest communities? Let us hope that Estes will continue to provide insights.

Many of the findings in the book, especially the early ones, are classics of ecology. For non-specialists, Estes's account is a fascinating introduction to key ecological concepts. For the cognoscenti and for students seeking understanding of how science actually works, Serendipity is an honest, behind-the-scenes peek at what really happened in the Aleutians and the Pacific more broadly, and what a now-esteemed scientist was thinking and feeling along the way. A rare and delightful insight into timely science.

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