Equation 7.11 lets us do some simple calculations about the consequences of living in soils with various water contents, and I have tabulated some of these in Table 7.2. It is a very complicated table, but its contents are important, so please bear with me as I go through it with you.
At the heart of the table is an imposed variation of soil matric potential, listed in the leftmost column, each entry being double the one above it. These values span the range of soil matric potentials in which one most frequently finds earthworms, from a matric potential of zero, as would occur in a soil saturated with water, to a very dry soil with matric potential of about - 5 MPa. In the second column is the difference in water potential between the soil and worm, calculated with equation 7.11. As you can see, in very wet soils, the water potential difference is positive, dominated by the worm's osmotic potential, which draws water into the worm. At or around a soil matric potential of - 500 kPa, the worm and soil will be in water balance: net water potential is null, and there is no net force moving water one way or the other. In very dry soils, of course, the water potential difference points the other way, drawing water out of the worm and into the soil, because the soil's matric potential is so strong. The last three lines in Table 7.2 represent desiccating environments.
The really interesting numbers are the estimates of the energetic costs of living in these environments. We have already seen how such a calculation is done (equation 7.7): if the net water flux can be estimated, the work required to oppose it is the product of the water flux and the water potential.8 Calculating the
8. One has to make a few assumptions about the biology of the worm, of course, to make this model realistic. For example, we know that an earthworm's urine production varies from a maximum of about 60 percent of the body weight per day to a minimum of about 10 percent of the body weight per day (when net water flux requires estimates of water loss in urine and the movement of water across the worm's skin. Again, net water flux (tabulated in column 5) is inward at low matric potentials and becomes outward once the soil matric potential exceeds about -500 kPa.
In fairly damp soils, then, matric potentials in the soil are not sufficiently powerful to overcome the net potential difference drawing water into the worm. Even in fairly dry soil environments, matric water is sufficiently available to offset the voluminous losses of water through the worm's high-filtration nephridia. Over this range, there are finite metabolic costs, mostly associated with the cost of transporting water out, but these are small—the worm need invest at most about 5 percent of its resting energy consumption to stay in water balance.
Once soil matric potentials fall below -500 kPa, however, a very different picture emerges. The costs of transporting water at sufficiently high rates dramatically increase. A doubling of matric potential from 1.28 MPa to 2.56 MPa, for example, increases metabolic costs of water balance about eight-fold (from 38.1 mW to 310.0 mW). Worms can survive in soils with matric potentials as strong as - 5 MPa, but the energy costs are steep: roughly eight times the resting metabolic rate (expressed as 828 percent of metabolic cost). Earthworms are capable of mobilizing energy at this rate—most aerobic animals are capable of about a tenfold increase in energy consumption above their resting values—but the costs cut deeply into other uses, like locomotion or most important, reproduction. How deeply they cut becomes clear when you realize that an earthworm can only divert about 10 percent of its total energy production rate to growth and repro-duction—when more than 90 percent of its energy is going to maintaining water balance, as it will in soils the worm is in a drying environment). We also know that a worm will let its concentration of solutes in the body vary: in very wet environments, it is about 150 milliosmols, but in a drying environment it can rise to about 300 milliosmols.
with matric potentials stronger than about - 5 MPa, the worm's capacity to grow and reproduce will inevitably decline.
It seems, therefore, that the most effective water balance strategy of the worm will be to stay in soils with matric potentials of a few hundred kilopascals. This will at least give the worm a fighting chance against the torrents of water being sucked out of it by the matric forces in drier soils.
Where does a worm go to find the best soils? To some degree, locating a suitable soil environment is simply a matter of how deep the worm goes. Soils are commonly divided into horizontal layers, called horizons, that have different features at different depths. The trick for the worm is finding the right horizon.
Soil horizons reflect the processes that go into making a soil. The organization of different types of horizons are a soil's profile, essentially a record of the history of that soil. For example, a soil's lower horizons might be made up mostly of material derived from the bedrock below. Over this, flowing water might deposit sediments eroded from distant bedrock. Drying or waterlogging of the soil also might change the distribution of minerals in the horizon, and the activities of microbes might modify it further, as they do in marine muds (Chapter 6). Water filtering downward through a soil and the settling action of gravity may sort soil particles by grain size, with the smallest particles falling to the deepest levels. Organic material, like leaf litter, may accumulate at the top. The end result will be a characteristic pattern of horizons that differentiates between soil types and tells something about how they formed.
The diversity of soils and soil horizons is bewilder-ingly complex, and soil scientists have developed numerous classification schemes to make sense of them. One convenient way is to divide a soil column into three basic horizons that reflect the degrees to which water or air occupies the porous spaces of the soil.
Starting from the top and working down, these are the aerial, edaphic, and aquatic horizons.
The aerial horizon, as the name implies, is fairly dry, and most of the soil's void space is filled with air. The pore spaces tend to be large, but the water that does exist there is located mostly in the horizon's smallest pores. Consequently, matric potentials in the aerial horizon are very strong, well into the megapascal range. Air also diffuses easily through the large air-filled pores. Consequently, oxygen concentrations are typically greater than 19 percent, not much less than the 21 percent oxygen in the atmosphere. So, while oxygen is readily available in the aerial zone, water isn't. An earthworm living in the aerial horizon would face serious desiccation problems.
A worm would have no desiccation problems in the aquatic horizon. Gravity will draw water downward through the horizons above, and void spaces are filled nearly completely with water. Although pore spaces in the aquatic layer are usually very small, matric potentials are also very weak, almost always weaker than - 30 kPa or so. The only trouble with the aquatic horizon is its limited availability of oxygen. Oxygen can diffuse down from the air and through the horizons above, but because it must travel a long way it is slow getting there. Furthermore, biological activities in the upper horizons consume oxygen as it diffuses down, exacting the kind of "oxygen tax" described in Chapters 4 and 6. Consequently, oxygen concentrations in the aquatic horizon are commonly low, ranging from 16 percent down to nearly zero. So, while water is readily available, oxygen isn't.
A worm in soil seems to have a dilemma. If an earthworm descends to a soil horizon that is wet enough for it to stay in water balance cheaply, it will suffocate. Ascending to the horizon where oxygen is abundant puts the worm in jeopardy of drying out or incurring large energy costs for water balance.
Sandwiched in between the aquatic and aerial horizons, however, is an intermediate zone, the edaphic horizon, which is not too dry, as in the aerial horizon, and not too stuffy, as in the aquatic horizon. As I like to tell my students, it meets the "Goldilocks criterion": it is ju-u-u-u-ust right. In fact, earthworms most commonly inhabit the edaphic horizon, and if they happen to be displaced from it, into either aerial or aquatic horizons, they will migrate back to the soil environment they favor. So the earthworms' secret—how they are able to live in a terrestrial environment even though their water balance system is physiologically suited to aquatic life—is to live in the best of both worlds, the edaphic horizon of soils.
Up to this point, the water balance of earthworms seems fairly mundane. Clearly, earthworms need to seek out environments that suit their physiology, and these environments clearly exist in soils. What is interesting about this?
Many people would view the specialized habitat requirements of an earthworm as a limitation: sure, they've found their little niche, but their limited physiology traps them there. Owing to the fairly limited set of environmental circumstances to which their physiology suits them, they really are at the mercy of what the environment presents to them. If a soil offers no edaphic horizon, or if the edaphic horizon is thin, then opportunities for earthworms to live there will be commensurably limited. This viewpoint, unfortunately, reflects the attitude I described in Chapter 1, that animals always adapt to their environments, and never the other way round.
Few people who are not familiar with the history of biology know this about Charles Darwin, but his last great work (and the work that he claimed to have derived the most satisfaction from) was on earthworms and what they do to soils. Entitled The Formation ofVeg-etable Mould through the Action of Worms, it was published just a year before his death in 1882. On the face of it, it seems to be one of those curious anomalies so characteristic of the British eccentric. In fact, nearly all Darwin's work had a common theme, of which his brilliant synthesis of natural selection was but one dimension: his work on earthworms was another. Put simply, Charles Darwin was not so much an evolutionist as he was a uniformitarian, meaning that he believed that small, seemingly insignificant processes could be enormously powerful agents for change if they were allowed to operate over a sufficiently long time. Just as small changes in body shape from generation to generation in a line of evolutionary descent could, given enough time, generate a new species, and just as small accretions of calcite by corals could produce mighty atolls and reefs, so too could the seemingly insignificant activities of earthworms have enormous effects on the structure and functioning of soils. Indeed, soil scientists can claim Charles Darwin as the founder of their discipline as confidently as evolutionary biologists do.
Just as Darwin's thoughts on evolution aroused controversy, so too, remarkably, did his work on earthworms. Before Darwin, earthworms, if they came to mind at all, were considered agricultural pests, because they were thought to eat the roots of crop plants. One Mr. Fish, for example, wrote an article to the Gardener's Chronicle of 1869 criticizing Darwin's claim that earthworms were major forces in the origins and modifications of soils. "Considering their weakness and size, the work [earthworms] are represented to have accomplished is stupendous," he sniffed.
Well, we all remember Mr. Darwin and not Mr. Fish. In fact, earthworms are major agents for change in soil ecosystems, and they have three basic modus operandi. First, earthworms open up large persistent tunnels in the soil. As they tunnel, the worms press sideways on the soil, compacting and slightly stabilizing the walls of the burrow. Earthworms also leave behind a mucus coating on the burrow wall, which further stabilizes it. Second, a worm ingests the soil as it burrows and passes it through the gut, digesting the organic matter and bacteria and finally extruding the remnants as fecal pellets. The fecal pellets are permeated with mucus and other secretions from the gut. When these dry, they form large, surprisingly durable casts. Finally, earthworms come to the soil surface, gather large bits of decomposing organic matter, usually rotting leaves or grass, and drag them back into their tunnels. These items are either ingested immediately or are cached in a storage tunnel to be eaten later.
The end result of this incessant mixing and churning is to build soil. When multiplied by the numbers of earthworms in a typical field, the maneuvers described above produce enormous results. Darwin himself estimated the rate of soil-building by measuring how quickly large, immobile objects, like large stones or foundations, appeared to "sink" into the soil—what he was really measuring, of course, was how fast soil newly created by earthworms built up around the object. In the Kentish countryside where Darwin lived, for example, new soil accumulated at a rate of about 5 mm per year, which corresponds to an annual addition of new soil of roughly 18 tons per acre (40 metric tons per hectare). Earthworms clearly are doing a lot of work out there.
Soils, Earthworms, and the Second Law Soils are dynamic structures, and like all dynamic structures, they exist by resisting the inexorable slide toward disorder demanded by the Second Law. When earthworms build soils, they are working to oppose, even reverse, this slide toward disorder.
One important source of disorderliness in soils is weathering. Soils start as particles of rock: silica or some other mineral. A newly formed soil will consist predominantly of larger particles, a millimeter or so in diameter. During the course of weathering, water flowing through soil will jostle particles against one another, gravity will push them against one another, and so forth. The relatively large particles of newly weathered rock are in this way broken into smaller particles, and the process continues as long as weathering continues. If a soil's inexorable slouch toward entropy is not opposed, its particle size will decline and it will follow a progression from sand to silt and loam, ending finally at the lower limit of clay.
As you would expect from what I have said about the matric potential, the course of weathering in a soil will affect the interactions between soils and water. As the average particle size of a weathered soil declines, the soil's pore size likewise will decline. Because matric potential depends upon pore size, the soil will gradually become more and more clay-like in its interactions with water: it will hold lots of water, but will hold it with stronger and stronger matric forces (Fig. 7.6).
Opposing this tendency are processes that prevent or even reverse this decline in pore size. One obvious strategy is the aggregation of small soil particles into larger pieces. Aggregation can be a simple chemical process: sand grains sometimes become glued together into larger composites when minerals like calcite precipitate from soil water. Often, though, aggregation is a biological process. For example, numerous microorganisms in soil produce long-chain carbohydrate polymers (like the dextrans often used as thickening agents in processed foods). These literally glue soil particles together, making the average particle size larger, and slow the rate of weathering because the particles in the aggregate are coated and protected.
Earthworms aggregate soils in several ways. First, as they open a tunnel, they secrete mucus from the surface of their bodies, which infiltrates the soil around the tunnel. This aggregates the soil particles, keeps the tunnel open, and, because mucus is a great absorber of water, helps retain water around the tunnel. Second, as soil particles pass soil through the worms' guts, they also pick up some mucus. The fecal casts so formed obviously glue many small soil particles together into larger pieces. Third, one of the peculiar features of annelid biochemistry is their ability to eliminate bicarbonate from the body as crystalline calcium carbonate or calcite. This comes out in the worm's feces, and the combination of digested soil particles, mucus, and cal-cite makes the cast very resistant to erosion. Throw into the mix the incorporation of large pieces of leaf litter from the surface into feces and tunnels, and you can see that earthworms are powerful forces for the aggregation of soils, working against the disordering processes of weathering. Of course, by keeping soil particles large, they also keep matric potentials weak, which, as we have seen, makes it easier for worms to draw water into their bodies.
Earthworms change the soils they inhabit in another way. I have already spoken at length about the behavior of water in the soil's micropores, defined as spaces smaller than about a millimeter or so. There exist in soils, however, another class of pores, larger ones designated macropores, which, as the name implies, are larger than a millimeter. Macropores can arise from physical processes—cracks in blocks of bedrock or erosion features, for example—but most frequently, macropores arise biologically. One of their common origins, for example, is the penetration of roots into soil. While the root is alive, it occupies a space, but once the root dies and decays, it leaves behind a tunnel that can range from about a millimeter in diameter (for finely divided roots) to several centimeters in diameter (for main roots). Obviously, earthworms form macropores when they burrow through soils.
The macropore-micropore dichotomy is important because the movement of water through each type of pore is dominated by different physical forces. Movement of water in micropores tends to be dominated by the matric forces discussed extensively in this chapter. Water movement through macropores, on the other hand, is dominated by gravitational forces pulling water down these relatively large "drain pipes" into the soil. Thus, the degree to which water infiltrates into soil, as it would after a rain, is determined by the relative abundance of micropores and macropores. If rain falls on a dry clay soil, where micropores dominate, the surface layers of soil absorb a large quantity of water but will hold it there tightly, which means that the water will not percolate downward, nor be available to plants or animals that might have use for it. In the language of the water potential, the clay's matric potential is strong enough to overcome the water's gravity potential, and the water does not sink, at least not until enough of the pore spaces in the surface layer are filled and the local matric potential declines. After a rain, therefore, clay soils tend to hold water only in a thin layer at the surface. Because it is these surface layers that are warmed the most by the sun, this water will most likely evaporate right back to the atmosphere rather than sink deeper into the soil. Although clays are most susceptible to this phenomenon, even sands have strong enough matric potentials to keep rainwater confined to the top few centimeters of soil.
In a soil with abundant macropores, water flows rapidly downward, rather than being trapped by strong matric potentials in the upper layers of the soil. As it sinks down the macropore drain pipes, it interacts with soil along the way, "spreading the wealth" so to speak, deep into the soil. Technically, the sinking rate is measured by two important soil properties: the infiltration rate (how fast water poured onto the surface sinks into the soil) and the hydraulic capacity (how much water a particular volume of soil can hold).
Earthworms, in building their tunnels, obviously increase the abundance of macropores in the soil, and by doing so they commensurably alter the properties of the soils they occupy. Infiltration rates and hydraulic capacities of fields occupied by earthworms are several hundred percent higher than the rates in fields that do not have earthworms in them. Fields in which earthworms have been killed or their populations reduced substantially undergo a marked degradation in their abilities to absorb rainwater or to hold it weakly enough so that plants can use it. This is one of the reasons why no-till and low-till agricultural methods are so effective—deep tilling kills worms, and when worms are killed, more energy and money must be spent in delivering water to soils in quantities sufficient to support plant growth. The greater infiltration rates of "wormy" soils also make for warmer surface temperatures, and hence faster growth rates of roots and the plants that are supported by them.
The end "goal" of all this squishing and chewing and crawling and tunneling is to develop, maintain and expand a soil horizon where infiltration rates of water
_^ aggregation work macropore work
Figure 7.7 The physiological "choices" made by an earthworm.
are high (but not too high), where water is held capaciously (but not too tenaciously), and where there is sufficient air space so that oxygen is abundantly available (but humidities are high). In short, the building activities of earthworms expand the range in the soil horizon where the Goldilocks criterion is met. It is, in other words, the soil environment adapting to the earthworm, not the other way around.
The adaptation of the environment is occurring because earthworms are making a physiological "choice" (Fig. 7.7). They can use ATP energy to do physiological work against the physical forces in the environment drying them up. Or they can use ATP energy to do burrowing work, to synthesize secretion of chemicals— mucus and calcite—to stabilize their tunnels, and otherwise to modify their environment to expand the range of soil habitats in which a physiologically aquatic animal is able to live.
Something seems to have driven earthworms to the latter strategy, and it is interesting to speculate why. As I outlined at the beginning of this chapter, doing physiological work involves having an infrastructure—an engine, if you will, that can use ATP as a fuel to do the work expected of it. These engines we call organs. Moving from one environment to another in which the physiological demands might change necessarily involves a "retooling" process, converting the physiological engines to meet the new demands placed on them.
Clearly, this is something that animals have done repeatedly throughout their evolutionary history, so there is no intrinsic reason why earthworms could not have done the same when their ancestors moved onto land. Remember, however, that unbreakable connection between embryological development and body plans: retooling of physiological organs involves radical modifications of the body plan and the developmental programs that realize it. This means that retooling internal physiology is a laborious and time-consuming process. For example, the conversion of a gill-breathing fish to a lung-breathing mammal involved a massive plumbing renovation to modify the heart, major blood vessels, intestine, gills, and lungs, a process that took roughly 200 million years to complete. Vertebrates undertook the task and succeeded, obviously, but this success is no reason to suppose our ancestors followed the best, or even a particularly good, strategy for adapting to a new environment.
Earthworms, when they came onto land, seem to have opted out of the retooling option and pursued a strategy of using ATP energy to work against soil weathering. Along the way, they essentially co-opted the soil as an accessory organ of water balance. The advantage of adopting this strategy is clear: it is accomplished much more rapidly than a retooling of internal physiology. One can follow the "evolution" of the changing physiology of the soil by monitoring changes in the soils' physical properties in fields newly inoculated with earthworms. Prior to inoculation, soils are compact, they don't retain water well, and variations of surface temperature are extreme. Following the introduction of earthworms, one can see, within about a decade, marked changes in the infiltration rate, water holding capacity, and temperatures, all of which make the soil environment more equable for its annelid inhabitants. Even given the fairly short generation time of earthworms, you have to agree that a decade for remaking the soil environment is clearly preferable to the megadecades needed for a retooling of internal physiology by evolutionary adaptation.
"If I can fool a bug," thought Charlotte, "I can surely fool a man. People are not as smart as bugs." —e. b. white, charlotte's web (1952)
You may know the Greek legend of Arachne, the young Lydian weaver who was so skilled at her craft that she threatened Athena's stature as the source of all greatness in such arts. To punish Arachne both for her superior skill and her hubris, Athena turned her into a spider. As they say, it's not nice to fool Mother Nature.
We humans like to consider ourselves the master weavers of the planet, and indeed, we have a strong claim to that title. Just look around you and reflect on all the versatile things we weave: textiles, automobile tires, high-pressure hoses, duct tape, to name a few. Arachne's skill has also spread widely among animals, though—consider the fantastic array of woven structures to be found among birds' nests—and animal weavers are at least as versatile in the products they weave. Weaving seems particularly widespread among the insects and spiders, which produce their own fiber, silk, from glands in the body. The woven structures produced by these terrestrial arthropods include: nests, cocoons, egg cases, and other types of housings; nets, bolos, snares, booby traps (some devilishly clever), and other devices for capturing prey; parachutes and drag lines to aid in flight; signaling devices and accessory sense organs. In this chapter, I wish to look at woven structures as external organs of respiratory gas exchange. It takes up the theme begun in Chapter 7, in which we explored how the physiologically aquatic earthworms live in a terrestrial environment where they really have no business being. In this chapter, I turn the tables and examine how certain essentially terrestrial arthropods, mostly beetles, but including some spiders, manage to live underwater. The physiological problem faced by these animals is that they are irrevocably committed to breathing air, and this, of course, makes them completely unsuited physiologically for life underwater (Box 8A). Yet they live and prosper there: in some instances, woven structures are key to their ability to do so.
Most commonly, the woven structures built by underwater creatures are used to contain bubbles of air. A well-known example is the "diving bell" woven by spiders of the genus Argyroneta (Fig. 8.1). Diving bell spiders weave a dome-shaped web underwater, which contains a bubble held in place by the web's finely woven sheet of threads. The spider captures air at the surface and carries it down to its web as a bubble held to the body by a dense pile of water-repellent hairs, called hydrofuge hairs. Once it has filled its web with air, there the spider sits in its little diving bell, venturing out from time to time to hunt; like a spearfisherman, it returns to the bell after each foray to breathe. Occasionally, the spider ventures to the surface, where it gathers a fresh bubble and carries it down to replenish the air in its web.
Many aquatic insects, most commonly beetles, share this habit of carrying bubbles around with them. Usually, the bubbles are visible as a silvery coating on some portion of the animal's surface and are held in place by patches of hydrofuge hairs. Sometimes the bubbles are tucked away out of sight, under the wings or wing covers. Like diving bell spiders, many of these beetles come to the surface periodically to replenish their bubbles. Some bubble-carrying beetles do not surface, however; even if confined underwater indefinitely, they swim happily about, seemingly oblivious to the fact that they are supposed to be air-breathing animals. And herein lies a mystery. The ability of aquatic insects and spiders to breathe underwater is not a straightforward matter of carrying a bubble of air around and breathing from it. In fact, some mar-velously subtle physics is at work, which enables these animals to use bubbles as accessory gills. We shall spend some time exploring the physics of bubbles and bubble gills, in part because it is intrinsically fascinating, and in part because understanding the physics at
work pays a bonus: it gives us the tools to look for other animal-built structures that serve the same function but less obviously.
Simplicity and Complexity: Occam's Razor vs. Goldberg's Lever
The bubble-carrying beetles posed a mystery to biologists in the early part of the twentieth century in part because of a human failing: we are prone to confuse simplicity with truthfulness or authenticity. One
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