Sea otters don’t eat algae. And yet, their diet influences the abundance of seaweed. How? Indirectly. Sea otters eating sea urchins (spiky animals in the same class as sea stars) eating kelp has become a textbook example of a trophic cascade, and Serendipity is a first-hand account by ecologist James A. Estes of how this happened. A trophic cascade refers to the indirect effects that ripple through a food web as a result of, for example, a predator consuming its prey. Simultaneously, the book is a searingly open account of how science is done, how ideas change, and how fortuitous events can suddenly send your research programme off in a whole new direction.
Estes’s story begins in 1970. Narrowly avoiding being drafted into the Vietnam war as a soldier, he ended up in the Aleutian archipelago, the chain of islands that are strung between Alaska and Russia, to research the population status of sea otters in the area. Little did he know he would return here time and again for the next 45 years.
At the time, when ecologists wrote and thought about food chains and food webs (the plants and animals connected to each other because they are each other’s food), they focused mostly on bottom-up forcing. This is the idea that the lower levels in such a chain, for example the abundance and productivity of plants, influence higher levels, such as the abundance of their grazers and in turn the predators that eat those grazers. The notion that the arrow of causality could also point the other way, top-down, with predators impacting herbivores impacting plants, was not much considered, if at all.
As described here, much of Estes’s career has revolved around showing the presence and importance of top-down forcing in food webs. It started with him noticing differences between islands with and without sea otter populations. Where sea otters were present, they preyed on sea urchins, which prevented kelp from being grazed, resulting in flourishing kelp “forests”. Without sea otters, the urchins thrived, grew large and munched their way through the kelp, leaving a barren underwater landscape.
“When ecologists wrote and thought about food chains and food webs [in the 1970s] the notion of top-down forcing was not much considered, if at all.”
From those initial observations, Estes chronologically explains how he expanded on those findings, showing this occurred around islands throughout the Aleutians, impacted other animals in this food web (fish, sea stars, even eagles), and affected the coevolution of chemical defences of kelp in response to predation by sea urchins. When the sea otter populations sharply declined in the 1990s, observations pointed to orcas having switched diet. In yet another example of a trophic cascade, Estes links this to a hangover from intensive whaling during the 1960s and ’70s. Bereft of their prey, orcas switched to smaller marine mammals, causing populations of one after the other species to crash. This, in turn, had knock-on effects elsewhere, showing how human impact can ripple through an ecosystem (see Whales, Whaling, and Ocean Ecosystems for more on this).
And the net is cast wider still, as further research showed that trophic cascades can cross from marine into terrestrial ecosystems. A final noteworthy achievement in this field was to bring together ecologists studying all of the Earth’s major ecosystems to exchange experiences and views, revealing that top-down forcing occurs in food webs throughout ecosystems, not just marine ones (that meeting resulted in the book Trophic Cascades: Predators, Prey, and the Changing Dynamics of Nature).
Doing observations and measurements on animals in the wild is – I can tell you from experience – a sometimes nerve-wracking undertaking. Estes lively details the many questions, worries, and unknowns as he embarked on new research projects. How do you measure sea otter abundance when there are no established protocols? How do you turn a hunch into a dataset that withstands academic scrutiny? How do you deal with logistical limitations? Without going into the nitty-gritty, he nevertheless provides plenty of details on hypotheses formulated, measuring protocols used, choices made, problems encountered, statistical analyses employed, and so forth. For general readers, some of this might be a bit much, but for biologists and ecologists (especially those just starting out) this will be a very interesting aspect of the book, and not a little bit reassuring. From the polished publications of senior researchers, you might almost forget that they had their own struggles back then.
“[…] for an author to write what we should stop focusing on, and where further research would be a waste of effort […] is something you do not see often.”
What really struck me is the almost searing honesty and openness with which Estes narrates how some of his ideas were wrong, how some of them were initially just suspicions that required more work to pass the gauntlet of peer review, and what the limitations of some of his conclusions are. I found his last chapter, in which he looks to the future of the field, particularly interesting. This kind of epilogue is not uncommon in academic books. But for an author to not just write what research should be done, but to also explicitly write what we should stop focusing on, and where further research would be a waste of effort, now that is something you do not see often.
Some of that soul-searching might have resulted from the unexpected controversy that erupted when Estes suggested that orcas were responsible for the sudden decline in sea otters as a result of earlier whaling. Dubbed the “megafaunal collapse hypothesis” in the literature, a segment of the marine science world did not take kindly to it at all (much to my surprise, to be honest). Estes here spends several chapters explaining what he based these ideas on and how he dealt with the subsequent fall-out. He does so with the care and circumspection of someone who has been scarred by the experience.
Serendipity is full of colourful anecdotes, as 45 years of fieldwork provides plenty of hair-raising fodder, but the book always has its eye firmly on the science. The way Estes describes it, his battle is far from over. The idea of top-down forcing in food chains is still not widely accepted in some quarters and plenty of scepticism remains. In my opinion, however, he convinces that trophic cascades are an important part of how ecosystems function. On top of that, Serendipity is both a fascinating memoir and an intimate and insightful look at how ecological research is done in practice. Readers interested in ecology and marine biology, in particular, will find much here to enjoy.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own however.
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