What a killer title. Rarely have I seen three snappy words so effectively capture the essence of a concept in biology. What concept is that? Zoogeochemistry. Many scientists have convincingly made the case that it is the small things that run the world. Though it is undeniably true that e.g. microbes and insects have shaped our planet, and continue to do so, it would be a mistake to think that larger animals are just along for the ride. I was stoked the moment the announcement for this book dropped and conservation biologist and marine ecologist Joe Roman did not disappoint. Eat, Poop, Die is fun and fascinating, while always keeping one eye firmly on the facts and complexities of ecology. Is it too soon to start earmarking titles for this year’s top 5? I think not.

Eat, Poop, Die: How Animals Make Our World, written by Joe Roman, published in Europe by Profile Books in April 2024 (hardback, 277 pages)

Given that this concept “is the beating heart of this book” (p. 214), what is zoogeochemistry? Physical forces shape the living world and living forces can shape the physical world right back—you may well have heard of biogeochemistry. Traditionally, research has focused on bottom-up interactions, revealing how e.g. microbes influence the climate and fungi and invertebrates help form soils. The phrase “ecosystem services” is often bandied about in this context, referring to how organisms keep our planet liveable by fertilizing soils, filtering water, or absorbing carbon dioxide from the atmosphere. Increasingly, however, it is becoming clear that larger animals similarly have the power to shape ecosystems, influencing the flow of chemical elements at large scales. Hence zoogeochemistry. How do animals do this? The title of the book says it all: rotting carcasses, deposited faeces, “and other unglamorous forces” (p. 52) can collectively move nutrients around ecosystems and influence the climate.

By visiting and interviewing fellow ecologists, and mixing in some of his own research, Roman introduces you to a variety of very interesting studies that are now uncovering these processes. He starts simply enough with the volcanic island Surtsey which appeared above the waves off the coast of Iceland in 1963. It has offered a unique opportunity for ecologists to study how a newly formed island gets colonized by plants and animals, with birds and seals enriching the initially scrappy plant communities through faeces, urine, carcasses, and eggshells. In the oceans, zooplankton move carbon and other elements from the surface to the deep , forming one component of the so-called biological pump. But whales, because of their sheer size, have the potential to counteract this. By bringing nutrients back from the deep, where they feed, to the surface, where they release nutrient-rich faecal plumes, they form what Roman calls a whale pump. (This happens to be his speciality, so expect a fair bit of lyrical waxing about whale poop.) In the Maasai Mara, wildebeest that die in river crossings provide an annual nutrient pulse, on top of the steady and intense input of hippos pooping in their pools. Coral-crunching parrotfish poop out the sand that covers the beaches of many tropical holiday destinations, and when periodical cicadas die in their trillions, they provide enormous but irregular nutrient pulses that have barely been studied. Roman appropriately concludes the book with James Estes’s work on sea otters in Alaska that has become the poster child for the concept of trophic cascades. In the process, he provides the interesting backstory of how the United States Atomic Energy Commission left a legacy of both atomic test sites and translocated otters that would repopulate the Pacific coast of the USA, a backstory Estes did not really detail in Serendipity. These and other examples form a delectable list of study systems, each more interesting and unbelievable than the last. No wonder Roman is often asked: “How did we overlook this for so long?”

“rotting carcasses, deposited faeces, “and other unglamorous forces” (p. 52) can collectively move nutrients around ecosystems and influence the climate.”

What I particularly appreciated is that for some of the better-studied systems, Roman gets the nuances right. The story of wolf reintroduction in Yellowstone National Park suppressing elk overgrazing and leading to a restored ecosystem has almost become a meme in ecology. The details, however, are a bit more complicated than the idea that you can just sprinkle wolves over a broken ecosystem like some sort of magical fairy dust. Similarly, the notion that bears indirectly fertilize trees by littering the forest with salmon carcasses, which Suzanne Simard approvingly mentioned in Finding the Mother Tree, has a few detractors. Roman spends time in the field with Daniel Schindler who thinks that poorly executed science and overinterpreted stable-isotope data have created a good story. One of his students adds that, though the cycling of nutrients between different animals in this system is not in question, the idea that it also fertilizes trees with nitrogen is considered more shaky by some. When it comes to the Pleistocene extinction of megafauna, Roman again hits the nail on the head by acknowledging the debate about the relative contributions of human hunting and a changing climate.

Eat, Poop, Die avoids becoming an endless showcase of stories about animals moving nutrients around ecosystems in said fashion. Chapter 5 (Chicken Planet) could have easily concluded the book but instead sits smack in the middle, punctuating the flow nicely. These sombre crescendos partially answer Roman’s earlier question of why we have overlooked the role of large animals for so long: because many have been driven to near-extinction by humans. The consequences of this have been studied in quite some detail for whaling, which resulted in impoverished marine ecosystems. But similar landscape changes resulted from the North American fur trade that saw the slaughter of bison, beavers, and otters. Humans have since become the prime mover of nutrients, with the mining of guano islands one sordid chapter in that history. The Haber–Bosch process might nowadays provide plentiful nitrogen (at a large environmental cost), but phosphorus still has to be mined. Future shortage is one of those creeping problems that is often overlooked in environmental discourse.

“[The book presents] a delectable list of study systems, each more interesting and unbelievable than the last. […] I particularly appreciated that for some of the better-studied systems, Roman gets the nuances right.”

Twice, Roman ruminates on whether rewilding our planet could help restore the nutrient flows that we have disrupted. It certainly has a better track record than geoengineering proposals. He is honest enough to highlight the downsides of large animals again roaming the land (from beavers flooding properties to otters hunting large clams that are commercially fished) and acknowledges that certain scenes from the past would cause social upheaval today. Achieving E.O. Wilson’s vision of setting aside half the planet for nature is “a long shot, especially if people are still eating meat on the bone and the population is increasing from eight billion to nine billion or ten” (p. 236). The most outspoken he gets is when he concludes: “Look, there’s no way around it. […] we’re going to have to consume less, consume better, if we’re going to rewild the world. A managed retreat is the only reasonable approach” (p. 238). I agree, and I cannot but marvel at how he is quietly radical here, almost sneaking in under the radar the three proverbial f-bombs (“veganism”, “degrowth”, and “overpopulation”) that often spark heated discussions. Though I could slate Roman for not developing this much further, I recognize that these are complex topics that could easily fill several books, and it would take us too far away from the main theme of this book.

Eat, Poop, Die is an absolute stonker that captivates from start to finish. The topic of zoogeochemistry is an incredibly fascinating development in ecological research and Roman recounts it here with flair.

Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.

Eat, Poop, Die

Other recommended books mentioned in this review:

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The reintroduction of wolves to Yellowstone National Park is one of the best-known examples of wildlife conservation. To celebrate its 25th anniversary and summarise the many lessons learned, Yellowstone Wolf Project leaders Douglas W. Smith and Daniel R. Stahler, together with wildlife ecologist Daniel R. MacNulty, bring together research from over 70 colleagues in this large, edited collection. The combination of academic content, excellent photography, guest essays, and an online bonus documentary with interviews make this the go-to reference work for anyone wanting to go beyond the headlines on this reintroduction project.

Yellowstone Wolves: Science and Discovery in the World’s First National Park, edited by Douglas W. Smith, Daniel R. Stahler, and Daniel R. MacNulty, published by the University of Chicago Press in December 2020 (hardback, 339 pages)

The first thing that struck me about Yellowstone Wolves is how well-organised the book is. Six parts contain nineteen chapters, none going beyond twenty pages, and most include a clearly signposted conclusion. To give equal airtime to so many different studies and opinions is remarkable—the six years the editors spent on this book have borne fruit. The second thing is how readable the book is. To do justice to the subtleties and complexities of real-world ecosystems means digging into scientific research. Thus, it discusses methodologies and research results and provides graphs galore, but without bogging the reader down with excessive jargon or complex statistics. Only occasionally, when the discussion turned to species interactions or ecosystem ecology, did I feel that I was reading an academic paper.

So, what have we learned from 25 years of having wolves back in Yellowstone? This book covers a wide range of topics, more than I can hope to discuss here. Sensibly, it opens with a short history of the park, wolf extermination, and the reintroduction—a dramatic story complete with last-minute lawsuits that almost scuppered the whole operation.

“The first thing that struck me about Yellowstone Wolves is how well-organised the book is. […] To give equal airtime to so many different studies and opinions is remarkable”

After some basic wolf biology, the bulk of the book discusses long-term research. This covers pathogens and parasites, the genetic studies that inform wolf pedigrees and explain why you see both black and grey wolves in the park, and, notably, the different aspects of wolf packs. How they form, how long they last, how they defend territories and compete with one another, and how they change over time. This introduces some of the legendary wolves from Rick McIntyre‘s Alpha Wolves of Yellowstone series, or the very popular female O-Six, but also reveals the value of older wolves to their pack, or the surprisingly high mortality due to intraspecific (i.e. wolf-wolf) conflict when wolves are not constantly hunted by humans.

My impression, however, is that the seven chapters across parts 4 and 5 will be the most relevant to many readers. Why? First, because the chapters on community ecology (particularly wolf-prey interactions) touch both on the concerns of the hunters and ranchers who opposed wolf reintroduction, and on the challenges faced by generations of park managers. Second, because the topic of ecosystem ecology (the effects of wolves on ecosystems) catapulted the park to internet fame.

We are going to need some history here.

Woven throughout this book is the story of how predator removal at the beginning of the 20th century saw elk populations boom, leading to concerns of too many elk overgrazing and trampling the park and surrounding farmland. From 1920 to 1968 park management and hunters culled and relocated tens of thousands of elk, leading to concerns of too few elk and, from 1969 onwards, new policies that let nature take its course. Predictably, without predators, there was a new elk boom. This is the context in which wolf reintroduction was finally put on the table.

“Of course, wolves eat elk, but the devil is in the details […] these chapters seek to correct misconceptions [as] wolves are not the ungulate killing machines some imagine”

Elk numbers have since declined again, causing—you cannot please everyone—renewed consternation. This time, though, wolves get the blame. Of course, wolves eat elk, but the devil is in the details: “what is in doubt is the size and timing of [their] contribution” (p. 187). So, these chapters seek to correct misconceptions. Though wolves are formidable pack hunters capable of taking down large prey, failure is frequent and the risk of injury high. Lacking the powerful bite and retractable claws of big cats or the muscular forelimbs of bears, wolves are not the ungulate killing machines some imagine, instead preying on young, old, and sick elk, or scavenging e.g. bison carcasses. Furthermore, elk decline started months before the wolves returned to Yellowstone in 1995. In subsequent years other predators such as cougars, bears, and coyotes also flourished, while hunters continued to shoot substantial numbers of elk just outside the park. Guest contributors weigh in here with lessons learned from other long-term wolf studies in national parks such as the island of Isle Royale, Banff, and Denali.

The other controversial topic tackled is ecosystem effects: the idea that the impact of predators on prey affects the prey’s food base, rippling down the food web and influencing a whole ecosystem. Now, such trophic cascades do occur in nature, but in Yellowstone’s case, the narrative has been hijacked by that one viral video clip, How Wolves Change Rivers. It presents a straightforward story of wolves killing elk, which reduced elk overgrazing of trees, in turn stabilising river banks and leading to the return of numerous animals. Broken ecosystem? Just add Wolves! Obviously, I am being facetious. In her contribution to Effective Conservation Science, Emma Marris examined the clip, the accusations of oversimplification, and the power of a good story. Ben Goldfarb, in his book Eager, noted that it downplays the effect of beaver reintroductions.

“the topic of ecosystem ecology […] catapulted the park to internet fame […] trophic cascades do occur in nature, but in Yellowstone’s case, the narrative has been hijacked by that one viral video clip”

Given this background, I was very curious to see how this book dealt with the matter. In one word: circumspect. The clip is only hinted at: “Some videos on the topic have garnered online audiences of millions. Although scientists have discredited some of these works as romantically simplistic […]” (p. 257). One chapter has a research group present the argument in favour of trophic cascades. They admit that indirect effects on vegetation have not been observed everywhere in the park where wolves now roam and add that players such as beavers, bison, wildfire, and disease complicate the picture. The next chapter has another research group consider more complex networks of interactions between wolves, other predators such as bears and cougars, scavengers, and herbivore prey. They open by writing that: “the preceding chapter considered […] processes in a single oversimplified food chain (i.e. wolves-elk-aspen/willow) in Yellowstone. Here, we discuss a broader set of food web relations that are too often ignored in the push to explain the links between wolves, elk, and vegetation” (p. 223). And so the discussion rumbles on.

The book ends with very relevant chapters on park management that explain the rationale behind visitor rules, celebrate dedicated wolf watchers, and hash out a framework for the perenially controversial topic of transboundary wolf management. That last one, in layman’s terms, gives recommendations for wildlife agencies on dealing with predators that cross the borders we draw on maps and come into conflict with stakeholders such as ranchers or hunters living near national parks.

As noted elsewhere, Yellowstone’s wildlife is subject to continuous change. By reintroducing wolves, scientists have had the unique opportunity to study the complexities of living, breathing ecosystems. Written by the very people who spent decades in the field doing the research, Yellowstone Wolves is a formidable achievement that presents a wide range of scientific topics in a well-organised, readable, and beautifully illustrated book.

Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.

Yellowstone Wolves

Other recommended books mentioned in this review:

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We tend to think of forests as static. Trees, after all, do not move. But that is a perspective foisted upon us by our limbed existence. Science reporter Zach St. George unmasks this illusion in plain terms: when trees die or new ones sprout, the forest has moved a bit. “The migration of a forest is just many trees sprouting in the same direction” (p. 2).

There is no shortage of books on trees, but this sounded like such an unusual take on the subject that I was utterly stoked when I learned of The Journeys of Trees. A journalist who delves into the palaeontological record to consider the slow-motion movement of forests over deep time? Get in here!

The Journeys of Trees: A Story about Forests, People, and the Future, written by Zach St. George, published by W.W. Norton & Co. in August 2020 (hardback, 244 pages)

The question of where in space and time organisms live where they do, and don’t where they do not, is the purview of the academic discipline of biogeography. St. George’s handling of ecological concepts here is excellent, especially when considering the many unknown factors that keep a tree from growing somewhere, or his clear breakdown of different niche concepts (his description of Hutchinson’s fundamental niche as a multidimensional hypervolume is fascinating). He traces the interest in questions of biogeography back to 18th-century naturalists such as Alexander von Humboldt, Asa Gray, and Darwin and Wallace. It was not just dinosaurs that fossil hunters unearthed, but also fossil trees, revealing that they had grown in places they now no longer did.

There are spine-tingling peeks into the deep past sprinkled throughout this book. Giant sequoias—today restricted to a few groves in North America—once towered over trees from Alaska to Europe while the quintessentially British oak spend the Pleistocene ice age in Spain and France. He mentions the ecological anachronisms of plants that have become marooned when their pollinators went extinct. Or, and this one blew my mind, the striking pattern seen in forest invasions. Time and again, tree species on one continent succumb to insects or fungi native to their sister species on another continent. This hints at the boreal forest that once ringed the globe at northern latitudes and connected these forests. As St. George explains, each continent now has its own oaks, maples, chestnuts, pines, etc. evolving separately, but their shared history is recent enough that they are still vulnerable to each other’s pathogens and pests.

“There are spine-tingling peeks into the deep past sprinkled throughout this book […] and I would have happily had St. George discuss nothing but palaeobiogeography.”

Through these examples, St. George cultivates a mindset that Marcia Bjornerud has called “timefulness“. For me, this is the rare, vertigo-inducing sensation where you find yourself peering down the yawning chasm of deep time and, for just a fleeting moment, connect with its incomprehensible vastness. Not many books manage to induce this and I would have happily had St. George discuss nothing but palaeobiogeography. However, there are two important topics linked to the present and the future that almost eclipse his writing on the ghosts of the past: the burden of invasive species and the question of forest conservation in the face of future climate change.

Plants do not just move themselves, we are increasingly doing the moving for them. While naturalists and geologists were still getting to grips with the age of the Earth and concepts such as biogeography, the Industrial Revolution was busy producing its legacy of Victorian plant hunters filling up botanical gardens and private collections in Europe, and entrepreneurs moving trees to improve colonies overseas. One such thread that St. George follows here is the story of how the Monterey pine growing in California and Mexico ended up blanketing New Zealand.

But each such translocation is an experiment. We take species out of their evolutionary context at our peril. Each journey that moves plants, wood, or even soil brings the risk of unwanted stowaways. Only a minority of translocated species become a problem, but with global trade and travel having increased exponentially, so have the risks posed by invasive species. A recent example recounted here that came to light only in 2001 is the decimation of North America’s ash trees when they fell victim to the emerald ash borer, a small beetle imported from Asia, possibly in pallet wood.

“it is exactly [the] legacy of species invasions that is now causing great tension […] when considering the future of the world’s forests, [particularly] the conservation initiatives taking matters in their own hands by planting trees in new areas.”

As St. George highlights, it is exactly this legacy of species invasions that is now causing great tension in the academic and conservation communities when considering the future of the world’s forests. As the deep-time perspective of The Journeys of Trees already reveals, forest migration is closely linked to climate change, and there is plenty of that in store, with St. George highlighting the risk of exacerbated droughts and fires in particular. One difference with past climate change is that in the geologically speaking short intervening time we have laid claim to vast swathes of the Earth’s surface, largely deforesting it.

Thus, the third and final important form of forest migration that St. George considers here are the conservation initiatives taking matters in their own hands by planting trees in new areas, often in the face of caution urged by ecologists. This varies from “assisted population migration” where trees are planted in their current range, to forward-looking initiatives better labelled “assisted range expansion” where they are planted outside of the current species’ range. St. George discusses Connie Barlow’s Torreya Guardians project that is planting Florida torreya all over North America, and the living seed bank of giant sequoias created by timber company Sierra Pacific Industries.

The flap text on the dust jacket mentions that the book focuses on five trees, but the story defies any rigid or chronological organisation, looping round in circles and delightfully intersecting its own narrative in numerous places. Much as I had hoped, The Journeys of Trees ended up being a fascinating sylvan road trip that sets itself apart from the many books written on trees, not least by its deep-time perspective.

Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.

The Journeys of Trees

Other recommended books mentioned in this review:

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“Serendipity: An Ecologist’s Quest to Understand Nature”, written by James E. Estes, published by the University of California Press in May 2016 (hardback, 275 pages)

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.

Serendipity paperback, hardback or ebook

Other recommended books mentioned in this review: