As the Worms Churn: Burrowing Animals Mix Soil and Sediments, Shaping the Environment and Scientists' Understanding of It

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To celebrate the 200th anniversary of Charles Darwin's birth, hordes of readers are reveling in On the Origin of Species, which sets forth the case for evolution via natural selection. Others are poring over The Voyage of the Beagle, the chronicle of Darwin's five-year, round-the-world expedition.

It's probably safe to say, however, that only die-hard Darwinistas are cracking the spine on his last book, The Formation of Vegetable Mould, Through the Action of Worms, with Observations on Their Habits. In this work, which Darwin himself described as "a curious little book," he discusses the role that earthworms play in the formation and erosion of soil. "The subject may appear an insignificant one," he modestly noted, "but we shall see that it possesses some interest." Indeed, for a short while after this book was first published in 1881, it sold more quickly than On the Origin of Species had.

But only much later did scientists begin to appreciate the widespread effects of bioturbation, the displacement and mixing of sediments by animal and plant life. Today, scientists recognize that the process has implications not just for geology, but also for archaeology, ocean chemistry, evolutionary biology and resource management. And, basically, for anyone who works in or on the ground.

For researchers who study subtle layering of sediments to understand a site's history, bioturbation makes work complicated and renders results uncertain. Fossilized remnants of burrows can also make rock much more porous than expected, affecting fluid flow through aquifers and oil fields, for example. And new studies suggest that when it first came on the scene, bioturbation may have accelerated an evolutionary arms race among creatures. Some researchers make an even stronger claim based on other newwork: Bioturbation, they argue, substantially changed ocean chemistry, rendering the seas more hospitable to life at the base of the food chain and therefore more biologically productive.

Slowly stirred

Like Darwin's other studies of natural history, his observations of worms and the results of their burrowing were numerous, varied and took place over a considerable period of time.

In 1837, Darwin dug a hole in a pasture in Staffordshire where lime had been spread in 1827 and cinders had been dumped several years after that. Well beneath the thickly matted roots of the overlying grass, Darwin noted the layers that formed as a result of the dumping--one of black cinders and, two inches below, one of lime. In holes dug in the same pasture nearly five years after the original hole, each layer sat about an inch deeper than it had before--a result of worms' nocturnal aboveground excursions, Darwin concluded. The worms eat soil, carry it to the surface and excrete the material in their fecal matter.

Some scientists scoffed at Darwin's notion that something as small and insignificant as a worm could substantially impact the terrain, but Darwin countered that large numbers of worms could indeed roil the soil: In the course of a year, he estimated, the 25,000 or more worms living beneath each acre of land in his area would bring between 14 and 18 tons of material to the surface.

Most of Earth's surface is home to burrowing animals of all sorts and sizes, from ants to aardvarks. In parts of some streams, spawning fish stir up more sediment than spring floods do. Even the floor of the deep sea, a milieu once thought lifeless, is substantially altered by burrowing creatures. They tunnel along, breaking up and exposing sediments. Previously buried material comes to the surface, and the fresh, nutrient-rich stuff gets sent downward.

"It's like turning over a compost pile," says Robert C. Aller, a marine biogeochemist at Stony Brook University in New York. The boost in microbial action triggered by bioturbation speeds nutrient cycling, he notes. Plants also get involved in the cycle, pulling food and water from soft. …