Animals as the Beating Heart of the Planet: On Joe Roman’s “Eat, Poop, Die”

IN EARLY SPRING, as temperatures rise and snow starts to melt, more than 5,000 bison migrate from low-elevation valleys to higher-elevation plateaus in Yellowstone National Park. Like many large herbivores, they initially follow green waves of fresh growth across the landscape in order to graze on resprouting grasses and other plants. By summer, however, the bison begin to retrace their steps, regrazing the same areas rather than seeking new ones. Why?

Through many years of dedicated research, wildlife biologist Chris Geremia and his colleagues discovered that the bison in Yellowstone do not simply follow spring’s tempo but modulate it. By intensively grazing and fertilizing the park’s grasslands, the bison encourage plants to continually produce tender and nutritious new growth, thereby maintaining areas of high-quality forage in their vicinity. As a consequence, they significantly prolong and intensify the growing season within Yellowstone, making the park greener and keeping it so for longer. In early spring in particular, bison collectively exert a stronger influence on plant growth than rainfall, temperature, or other environmental factors do. In essence, Yellowstone’s bison are as much cultivators of spring as its dependents.

Today, fewer than 8,000 wild bison move freely across public lands in North America. Before colonization, however, some 30 to 60 million bison roamed the plains. It is no exaggeration to say that bison once shaped the heart of an entire continent. What scientists observe today is but a trace of their former power.

For far too long, Western science has overlooked and undervalued the many ways in which animals shape their environments. This is partly because, by sheer mass, trees, grasses, forbs, ferns, mosses, seaweeds, phytoplankton, and other photosynthetic life forms dwarf the animal kingdom. Plants and other producers also form the foundation of most major ecosystems, and are thus more obvious subjects for the study of how life defines its surroundings. When it comes to animals, we have always been more interested in the drama of competition, reproduction, and survival: the dynamic roles that animals play within their habitats, rather than what they do to their environments, which can be harder to observe and quantify. And by hunting much of the world’s megafauna to extinction—and then continuing to endanger and winnow diverse animal populations around the world—we have further obscured Earth’s zoological imprints.

In recent decades, however, scientists have developed a much keener understanding of how animals transform our world. Conservation biologist Joe Roman lucidly and eloquently recounts this “radical shift” in his new book, Eat, Poop, Die: How Animals Make Our World (2023). The title might not have quite the same gravitas as Elizabeth Gilbert’s best-selling memoir Eat Pray Love: One Woman’s Search for Everything Across Italy, India, and Indonesia (2006), but don’t let that fool you: this is a compelling account of important scientific insights. “Landmark studies of seabirds, whales, sea otters, salmon, wildebeests, bison, spiders, grasshoppers, cicadas, and other animals,” Roman writes, “have shown that they can alter the landscapes and seascapes where they live, with major impacts on ecological function.” Animals rapidly cycle nutrients—the molecular building blocks of all life—through different regions of the planet. Without the daily bustle and long-distance migrations of animals—without their continual consumption, defecation, and decomposition—these nutrients would primarily be recycled by much slower geological processes, often remaining imprisoned in one part of the planet for ages, and thus severely restricting life’s domain. “A geological journey that would take thousands or millions of years […] can be reversed in a single dive, a short flight back to a barren rock, and a Pollock-like spatter,” Roman continues. “Animals pump nitrogen and phosphorus from deep-sea gorges up to mountain peaks and across hemispheres from the poles to the tropics.” Animals, he says, “are the beating heart of the planet.”

The radical shift Roman describes is part of an even larger movement within science: a much deeper appreciation for the many ways that living creatures of all kinds—not only animals, but microbes, plants, and fungi too—have profoundly altered the planet over the past several billion years. Collectively, life is responsible for many of Earth’s defining features: its breathable atmosphere, blue sky, rapid rain cycles, fertile soils, and bountiful seas. Life may even have played a significant role in the formation of the continents. Without life, Earth would not simply be bleak and barren, like an old abandoned house, but utterly unrecognizable: a jumble of raw materials that never attained the complexity, diversity, and beauty that distinguish our cosmic home. In many ways, life built the world as we know it.

Roman organizes his study of the planet’s zoological transformations around different species and ecosystems. In the more aquatic-focused chapters, we learn how whales help cycle carbon, nitrogen, phosphorus, and other vital elements through the ocean by sloshing water with their enormous bodies, releasing massive fecal plumes at the sea’s surface, and sinking to the seafloor when they die in open water. Some of the research that helped elucidate these ecological processes, it should be noted, was itself zoologically powered—namely by dogs trained to sniff out whale poop. Before the advent of whaling, dead whales falling to the seafloor sequestered more than six million pounds of carbon each year in the deep sea—a figure that has now been reduced by about two-thirds. The behavior of a much smaller creature, the parrotfish, is also consequential. Parrotfish scrape at rocks and coral with beak-like teeth to feed on algae, and then release crushed limestone from their gills and in their poop in the form of sand and silt. These expelled sediments comprise large portions of the sand on beaches in Hawaii and the Caribbean. A single greenhead parrotfish can poop almost 10,000 pounds of sand annually. The presence or absence of otters has similarly dramatic effects. When otters are hunted to near extinction, sea urchin populations explode and devour seaweeds, reducing once-lush kelp forests to slime-covered “urchin barrens,” as researchers call them. When conservationists return otters to their native habitats, they keep urchins in check, allowing kelp forests to regenerate.

In terrestrial sections of the book, Roman details how insects, birds, and mammals colonize newborn volcanic islands; how bears may help fertilize forests by dragging the carcasses of salmon inland; how the reintroduction of wolves and beavers restored Yellowstone’s riparian ecosystems; and how the periodic emergence of cicadas in eastern North America results in trillions of insects dying and decomposing all at once, which constitutes an “enormous but rare resource pulse.” We also learn about the “landscape of fear”—a fascinating and relatively new scientific concept describing how the stress-inducing presence of predators indirectly but dramatically alters the physiology and behavior of nearby herbivores, and thus the surrounding landscape. “The dynamics that shape our physical world—atmospheric chemistry, geothermal forces, oceanography, plate tectonics, and erosion through wind and rain—have been explored for decades, even centuries,” Roman writes. In contrast, it has only been in the past three decades that ideas like the landscape of fear, the whale pump, and zoogeochemistry—how animals influence the cycling of nutrients—have “made their way into the ecological literature.”

Books authored by academics, even those ostensibly written for a general readership, are sometimes dull, verbose, or inscrutable. Roman’s book is nothing of the sort. His writing is lean, lively, and engaging. Throughout, he interweaves scientific exposition with entertaining tales from field expeditions. Most of the time, these narratives dovetail nicely with the book’s central themes and give us a window onto the day-to-day lives of scientists. Occasionally, however, Roman devotes a bit too much text to recounting an adventure that may have great personal meaning for him but does not propel the book forward or improve the reader’s overall understanding. For a book entirely dedicated to a rather specific subject, there are also a few conspicuous gaps. The text would benefit from a more comprehensive treatment of Earth’s history and life’s evolutionary saga, as well as from a stronger organizing principle. One of the most consequential changes animals made to the planet occurred around 540 million years ago when ocean creatures with newly evolved shells and spines revolutionized the marine ecosystem by tearing up microbial mats and irrigating the seafloor. The animal-engineered Cambrian substrate revolution, as it’s known, created whole new layers of ocean habitat and spurred further evolutionary innovation, ultimately making Earth much more diverse and habitable. Yet Roman doesn’t mention it.

A brief cautionary note: As the title might suggest, this is not a book for the squeamish. Some sections contain vivid descriptions of defecation, urination, and other bodily functions. Roman has a unique appreciation for such processes, comparing the multiplicity of whale fecal plumes to the individual beauty of snowflakes; some plumes, he writes, “sparkle with scales, like the sun glinting on the water,” whereas others are “more subtle, a cloud of unknowing, like oversteeped green tea.” Elsewhere, his descriptions may be discomfiting for some readers—as in one passage concerning dolphin fecal plugs, or another detailing the acoustic properties of diarrhea and flatulence.

These caveats aside, Roman’s book is an engrossing, timely, and important contribution to ecological writing for the general public—and surely one of the first in a wave of new books exploring life’s profound influence on the planet. For Roman, these ecological insights offer more than a refined understanding of our world; they also provide a road map for how we can improve it. “The restoration of wildlife populations might be one of the best nature-based tools we have to face the climate crisis,” he writes. “Wild animals, through their movements and behaviors—their eating, pooping, and dying—can help rebuild ecosystems, recycle and redistribute nutrients, keep the planet a little cooler, and address the biodiversity crisis.” Animals and their behaviors are vital components of ecological cycles that sequester heat-trapping greenhouse gases like carbon dioxide and methane. Ecologist Oswald Schmitz and his colleagues have estimated that the protection and restoration of animals could yield an additional uptake of 6.4 billion tons of carbon dioxide each year, which is about a sixth of current emissions.

By the end of the book, it’s clear that Roman yearns for a new version of Earth in which wild animals, no longer dismissed as “bit players on the planet,” are revered. Getting there will require substantial changes to human civilization, in particular to our agricultural systems, which use close to half of the planet’s habitable land. “Look, there’s no way around it,” he tells us. “[W]e’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.” If we give animals more of the space they once had—and thereby return Earth to a more dynamic and diverse state—we may also rekindle a unique feeling of awe: a sentiment our species has not truly felt since the days we painted mammoths and bison on cave walls by firelight. “When animals are abundant,” Roman writes, “they can be like the weather, moving nutrients and seeds like clouds move water. […] I thought, This is how the world should be, with animals as powerful as a thunderstorm—all-encompassing, seasonal, and not always predictable.”