Digging In

 kw: book reviews, nonfiction, biology, paleontology, burrowing, trace fossils

Are we still cavemen, somewhere deep inside us? Some folks are. The people who live in a certain part of Cappadocia certainly are, if not cave dwellers, certainly burrow dwellers. The soft volcanic stone in the area is easily dug. Several thousand people live in underground, or within-rock, dwellings. Some of these unique burrow-houses, along with churches and other public places, were carved in the rock as long ago as 300 AD.

Does this make humans the largest burrowing animals? Actually, that distinction belongs to grizzly bears, as we read in The Evolution Underground: Burrows, Bunkers, and the Marvelous Subterranean World Beneath Our Feet, by Anthony J. Martin.

Dr. Martin is an ichnologist, a scientist who studies trace fossils: fossilized tracks, trails, and burrows made by animals. His book shows how knowledge of the ways animals have trod on, dug into, and tunneled underground have created the natural environment. For example, a motto in his field is, "Without animals that tunnel and poop, there would be no mud." Geological forces tend to mix clay and silt and organic sludge into larger-grained sediments. Animals that tunnel within those sediments are frequently like earthworms and marine worms, that ingest the "dirt", digest the organic part, and defecate pellets of the remaining mineral bits mixed with mucus, typically onto the surface (look for little piles of pellets near wormholes after a rainstorm). These pellets glomp together into "mud". (Yes, Virginia, dirt is mostly silt and clay mixed with animal poop and poop eaten and re-pooped. Now, don't you want to wear gloves when you garden?)

It is likely that ants plus termites make up more than half of the total mass of all animals. And they are nearly all inveterate tunnellers. How far back did "bioturbation" (the stirring of the sediment by animals) begin? The book has a tentative answer: Around 541 million years ago, or a little before, during the transition from the Ediacaran Period to the Cambrian Period.

The Ediacaran Period, from 635 to 541 million years ago (mya) is named for a region in Australia where these unusual soft-bodies fossils were first found. In this image the scale bars are either 1/2cm (black) or 1cm (white). The best analysis of the environment of these animals, or proto-animals, is of quiet seabeds with a surface composed of bacterial mats, which sometimes humped up into stromatolites, which originated about two billion years earlier. None of these critters had shells or teeth, and they seem to have fed on the waste products of the bacteria and perhaps a little bit on the bacteria themselves. It seems they did not feed on each other; there were no predators yet.

They apparently did not have the wherewithal to dig into or under the bacterial mats. At the very end of this period, transitional animals called the Small, Shelly Fauna (SSF) appeared, and they did begin to dig in. They also seem to have fed on the soft-bodies feast around them, because the "softies" soon vanished.

The SSF quickly gave way to the animals of the Cambrian Period, from 541 to about 485 mya, which were shelled creatures such as the beloved trilobites, but included all modern phyla plus a number of phyla that have gone extinct. Here we see a trilobite and a blastozoan (distant relative of sea stars).

The book has quite a chapter on the trackways left by trilobites, and the confusion that sometimes results when other many-legged creatures leave tracks that look similar at first glance.

Cambrian animals didn't just leave tracks in the bottom. Burrowing as a lifestyle seems to have begun among nearly all phyla during the Cambrian Period.

Why burrow? For some, food is found there (ask any mole or earthworm). Protection and privacy: it is easier to defend eggs and babies when they are in tunnels or bunkers or burrows. Making babies is safer in a burrow also; the blissful couple is less likely to be interrupted. There are actually birds that tunnel to protect their eggs and young.

The creatures that survived the "big five" extinctions were mostly burrowers. This is seen on a small scale in a description that begins Chapter 9, "Viva La Evolución: Change Comes from Within". Pocket gophers that happened to be in their tunnels during the Mount St. Helens eruption of May 18, 1980, found their tunnel mouths buried under loose ash, through which they had to tunnel upwards to attain the new surface of the ground. They did so, in large numbers, all over the area that was devastated and incinerated by the nuée ardent ("glowing cloud") of superheated gas and melted glass that roared off the mountain. 57 humans that were within around a 10 mile radius of the volcano died. Thousands of pocket gophers, including some very much closer to the mountain, were safe in their dens and emerged to repopulate the area with their own mini-population explosion.

Chapter 8, "Rulers of the Underworld", surveys the breadth of kinds of animals that live literally underfoot, from ants to armadillos, and some that are (or were) a bit too big to be literally underfoot, such as the giant ground sloths that left tunnels you can almost drive a car through in parts of South America.

A major theme of the book, found in most chapters, is that the diggers all around us are ecosystem engineers. The gopher tortoise is a superstar of ecosystem engineering. These middlin-sized tortoises tunnel industriously, making spaces not only for themselves, but for about 400 species of animals that get the opportunity to dwell in those spaces, or in side tunnels off of them. A foot-long tortoise makes a one-entrance tunnel 5-15 meters long, going as deep as 3 meters, to an enlarged den. If you were to excavate a well-used tunnel, however, you would find numerous side tunnels made by mice and toads, also by dung beetles and other insects. The tortoises move tons of earth about, and areas with many burrowing animals in general are well-aerated because they are to well-perforated! The constant digging and mixing means we live amidst an extensively re-worked landscape…or, at least, those of us who live outside cities.

Even in my suburban area, a typical shovelful of garden soil contains one or two dozen earthworms (multiply by the thousands of square feet in my yard). There are also the burrows and tunnels of mice, voles, camel crickets, and a dozen species of ant.

It's good to be reminded, or enlightened, regarding the many uses of the underground and the wildlife that inhabits and creates it. A thoroughly enjoyable book.

Pardon me for continuing with a criticism or two; you can stop reading here if you prefer. The points below don't diminish the value or enjoyment of the book.

More and more I find myself wishing authors and publishing houses would make more and better use of copy editors and proofreaders. A spell-checker is only 10% of the task. Some examples:

• On page 142 I found this in a description of the impact of the asteroid that wiped out the dinosaurs 65 million years ago: "The impact…instantly converted its potential energy into kinetic energy…". Hardly! The rock was moving about 30 km/s, and that's all kinetic energy. It was converted, first to thermal energy (melting and evaporating rock and ocean water), and then to more kinetic energy of the "splash stuff", molten rock lobbed halfway around the planet. The copy editor needs to know some physics.
• The word "had" was omitted from a phrase that should have read, "…cobbles of sandstone that had fallen off the slope…" Page 148.
• Faulty math: the statement that ants probably outweigh humans (true), is followed up by "one million ants per person". Hmm. I weigh just under 100 kg, or 100,000 grams. One millionth of my weight is 100 mg. I suspect a 100 mg ant would be a fearsome critter! Large (12mm) carpenter ants weigh 20-30 mg; maybe the colony's queen approaches 50 mg. The average worker ant of all species weighs about 2 mg, so it would take 50 million ants to balance me on the scales. Page 226.
• The author in one place states that a hectare is 100 square meters, but a hectare is actually 100 meters squared, or 10,000 square meters (107,639 sq ft). That is 2.471 acres. I found a few places (p. 235 is one), where the ratio is reversed, indicating that the author (or someone he quoted) calculated 2.5 hectares per acre. That's quite different from both 100 sq m and 10,000 sq m.

To be honest, these complaints total half a page; out of a 400-page book, that isn't bad. I like Dr. Martin's writing.