Life Everlasting
Page 12
TREES, ALIVE AS well as dead, are also life-promoting for fish. Trees growing along stream banks shade the water and keep it cool, which helps trout breathe. They need lots of oxygen, and warm water holds little of it. But beyond that, the brook trout, Salvelinus fontinalis, a beautiful green-marbled char, with red dots surrounded by blue haloes, red-edged fins, and a pink or red belly, needs places to hide and to rest—as all organisms do. The roots of trees along the banks of a brook play a pivotal role in holding the soil, while the rushing water undercuts the banks, creating cavities where the trout lie in wait to snatch insects drifting by. Once the trees die, they are recycled into the stream.
Along the dirt road where I live in Vermont is a drainage where, were it not for beavers, the water would be seasonal. Thanks to the beavers, who harvest trees for food and for dam construction, the water is now there year-round. The beavers have built a series of dams (fifteen at my last count) that impound the water in steps down the valley incline. The dams range from about twenty to several hundred feet in length. The largest of the beaver-made ponds holds three species of fish and is the breeding place of six species of frogs, one toad species, and at least two types of salamanders. These ponds are too shallow and warm in the summer to be suitable for brook trout, but at higher, cooler elevations, some beaver works are prime trout habitat.
Although beavers haul a lot of wood into the water to make their lodges and dams, streams are also dammed up, creating fish habitat, when trees fall over into the water on their own. Over the years, spring freshets may move a tree downstream, where it lodges against the bank, some boulders, or other trees to make a logjam. Water swirls over, under, or around the jam, gouging holes and making pools where the trout hide and stay cool on hot summer days when water levels are low. These rare bottlenecks make all the difference to trout and salmon survival.
SIMILARLY IN A FOREST, the standing dead trees eventually fall and create an ecosystem of quite another set of organisms. But the tree as ecosystem keeps changing, as the work of the fungi and bacteria progresses. Moisture near the ground, plentiful oxygen, and warmth all help fungi soften the wood. Moss grows over the decaying trunk, holding rain water that would otherwise run off. Seedlings of some trees, such as yellow birch in my forest, which are seldom able to punch through the heavy annual leaf fall on level ground, can get a start on moss-covered fallen logs. From reading the accounts of the Swedish naturalist Peter Kalm, written during his travels in North America in the mid-1750s, I suspect that these moss-coated “nurse trees” are especially important for maintaining some tree species in mature old-growth deciduous forests. While Kalm was in Pennsylvania, he marveled at the large size of the trees and the sparseness of the forest understory. Hordes of squirrels abounded, and people let their pigs range in the woods to forage on nuts. Some hint of why many nut-bearing trees were favored in these forests can be gleaned from his comments on November 13: “The leaves have at present fallen from all the trees, both from the oaks and from all those which have deciduous leaves, and they cover the ground in the woods six inches deep.” Only seedlings from large seeds such as nuts can punch up through such a layer. Seedlings of small-seeded species use fallen trees as a competition-free launch pad for their race to reach the light of the canopy.
A yellow birch tree growing on an old pine log. Rotting logs serve as nurse trees by providing a space above the leaves covering the ground where tree seeds with minute food stores can get a root hold.
As a log starts to disintegrate, it begins to admit centipedes and millipedes, and in the fall wasps, beetles, and other insects burrow into it to hibernate. In subsequent years, as it sinks further into the ground and is covered with leaves in the fall, the wood returns to the soil as humus.
ABOUT TWENTY YEARS ago, scientists from the College of Forestry at Oregon State University started a two-hundred-year study of 530 rotting logs in the Cascade Mountains. The study has a long way to go, because huge logs take a long time to decay. But already, as Mark E. Harmon, a professor of forest science at OSU, has said, “Much of what we’ve found has run contrary to conventional wisdom.”
Their main finding so far is that rotting logs feed the nutrient cycle and are far more important to forest health than previously supposed (by some). The availability of nitrogen is a key limiting factor in a forest’s growth, and the decay of rotting logs releases nitrogen for reuse. But even more important, the decay process extracts gaseous nitrogen from the air and makes it available for organisms to turn into protein. Another point is that the brown-rot fungi—a group that cannot break down the lignin component of wood—leave behind structural material that helps build soil. The white-rot fungi degrade all parts of the wood, but they act only in some tree species and at varying rates, depending on the species. The species composition of a forest thus has long-term implications for the soil and for regeneration. I suspect it will turn out that tree diversity ultimately enhances tree growth through its effects on the soil and that soil is created not only from the trees’ remains at death but also from the shedding of dead leaves throughout their lives.
Every fall the manicured pet grass of suburban lawns receives a blanket of leaves from the surrounding birch, ash, and maple trees. Many people conscientiously rake up the leaves (or, worse, remove them with noisy, gas-powered leaf blowers) as though they were some kind of trash. They stuff the leaves into black plastic bags, which they leave at the curb for the garbage truck to haul off. I leave leaves where they fall, and rain and snow flatten them onto the ground. In early spring, after the first soaking rain and before the grass starts to grow, the leaf undertakers (earthworms) come up out of the ground at night to begin their work. They stretch up out of their burrows, grab the limp, wet leaves in their mouths, and pull them toward and into their tunnels. In the morning you may see leaf tufts standing upright all over the lawn where worms were halfway done with a leaf at daylight. Most worms pause their work at first light and retreat underground; those who don’t risk the robin. The more leaves the worms find, the more they multiply, to fertilize and aerate the lawn and make the grass grow.
Soil making in forests is similar. The Maine woods that I am familiar with are wild and wonderful despite logging, and what makes them appealing and keeps them wild is their hugeness, which largely discourages “cleanliness” and encourages litter and slash. There is time for decay and regeneration of the trees back into humus and soil.
Forest soil is a complex, species-rich ecosystem that in some ways acts like an organism itself. Edward O. Wilson described (in Biophilia) a handful of soil thus: “This unprepossessing lump contains more order and richness of structure, and particularity of history, than the entire surface of all the other [lifeless] planets combined. It is a miniature wilderness that can take almost forever to explore.” Here I will explore it only briefly. Bacteria in forest soil take the ammonium that results from the decay of protein and turn it into usable nitrate. Other bacteria fix gaseous nitrogen from the atmosphere and put it back into the soil. The kinds of bacteria that thrive in and depend on the soil in turn determine the kinds of plants that can grow there. In oxygen-poor environments, other bacteria act as denitrifiers: they return nitrogen to the atmosphere. Another group, the actinobacteria, decompose organic matter to form humus, as do a huge variety of fungi. Still other fungi live in a symbiotic relationship, known as a mycorrhiza, with the roots of trees and other plants. Mycorrhizae are necessary for the tree to absorb nutrients from the soil.
By decomposing dead plant and animal matter, soil microbes release organically bound nitrogen and phosphorus in forms that the plants can use for growth. Thus, over the long run, the forest soil needs dead trees, or the slash and “waste” from logging, to feed it. Aside from the complex chemistry, however, soil with organic matter folded into it has a texture that binds water, making it continuously available for the trees’ growth. The carbon, nitrogen, and water cycles all meet in the soil, intersecting on dead trees, which give the forest life.
We may think we are God’s gift to earth as the greatest undertakers when we dispose of old or dead trees and plant young, “healthy” ones to catch excess carbon out of the atmosphere. We think we do nature one better by planting preferentially “superior,” genetically engineered trees that are supposedly more “green” than those that nature has made over the course of four billion years of selection, in which only one of a tree’s tens of thousands of offspring may survive to reproduce. However, insects, molds, bacteria, and beavers have engineered the most amazingly foolproof, the most effective and intricate, cooperative system-solution to the death-into-life cycle of trees. The system has been tested in real-life conditions over eons, and it can hardly be improved on by our tinkering with details designed for our perceived immediate benefits.
Dung Eaters
What can’t be used is trash; what can, a prize Begotten from the moment as it flies.
—Johann Wolfgang von Goethe, Faust
IN THE MID-1970S I TRAVELED TO KENYA’S TSAVO NATIONAL Park with George A. Bartholomew from the University of California Los Angeles. It was Bart, who had been my PhD adviser, who inspired me to become a physiological ecologist. We went to Kenya to study the physiology, behavior, and ecology of dung beetles, with an initial specific interest in the elephant dung beetle Heliocopris dilloni. This sparrow-sized beetle is built like a tank, flies like a hawk, and tunnels through hard-packed soil like a bulldozer. Each monogamous pair rears one offspring on a diet of fresh elephant dung, which the parents carry down into their underground nest and then make into a ball.
I had seen my first African elephant dung beetles a decade earlier, during my undergraduate studies at the University of Maine, when I took a year off to accompany my parents on a bird-collecting expedition to Tanganyika (now Tanzania) for the Peabody Museum at Yale. One of my activities during that yearlong expedition was to set up nets in the forest around Mount Meru to catch birds. When I checked these nets early in the dawn, I found many dung beetles in them whenever elephants were near. While flying at night toward fresh dung, the beetles had become entangled in the nets. Four years later at UCLA, I studied the physiology of insect flight and its relation to body size and temperature. Large insects were hot, and small ones were not. Studying elephant dung beetles, which were larger than any I had ever seen, seemed a near “must” to round out my studies. I could not get any of them to fly in the lab, at least for long enough to stabilize their body temperature. But it would be easy to measure the temperature of flying dung beetles by intercepting them where I knew they would arrive: at fresh elephant dung. Many species of beetles, in a variety of body sizes, would come at the same time, and these would be the much-needed controls. It was not hard to convince Bart, a world authority on birds and mammals, that this would be an interesting project, and he graciously financed the trip and came along himself.
As many as 150 species of dung scarabs may be found in any one locality in Africa; 780 species live in southern Africa alone. At Tsavo it was the beginning of the rainy season, peak time for dung beetle activity. We came upon a herd of a hundred or so red dust–coated elephants slowly ambling along, grazing on the green grass that had just sprung up. Chalky white butterflies fluttered on scattered low bushes with white flowers. Metallic green cetoniine flower scarabs were humming around acacia trees ablaze in yellow bloom. Elephants were pulling up tufts of grass with their trunks and deftly stuffing them into their mouths. Each elephant daily processes hundreds of pounds of grass and twigs and at regular intervals leaves droppings the size of basketballs and the shape of yeast rolls.
As the elephant herd travels along, consuming a swath of vegetation as they go, they leave a trail of dung piles behind. A casual daytime tourist might be impressed by the number of dung beetles at each pile, but the scale of their activity during the day is as nothing compared to the astonishing spectacle after dark. Bart and I would have our shot of whiskey in the evening along with others at the Tsavo Lodge, but we were here for beetles at elephant dung, not elephants, and we had to be out at night when most of the beetles become active.
A pair of dung-ball-rolling scarab beetles (right), with the male pushing the ball and the female riding on it passively. The ball, once buried, provides food for their larva, which then pupates inside the ball, as shown in cross-section (left).
If you go out after dark where a herd of elephants has recently come through, or if you bring a bucket of fresh dung collected in the daytime and leave it on the ground, you will hear, shortly before the deep coughing roars of the lions begin, a low humming noise. It’s the sound of dung beetles barreling in—hundreds and then thousands of them—all making a beeline to your dung pile. They range in size from slightly larger than a rice grain up to that of the sparrow-sized Heliocopris dilloni. We once collected and counted 3,800 beetles coming in to a half-liter sample of fresh elephant dung in fifteen minutes! The total beetle weight exceeded that of the dung we had put out. Every night the dung was eaten on the spot, pulled underground into tunnels, or made into balls and rolled away to be buried elsewhere. After an hour or two, all that remained of the elephant dropping would be a two-meter-diameter pancake of loose, almost dry, fibrous material from which almost all of the juicy goodness had been extracted. But hundreds of rice-grain-sized beetles remained embedded in it a bit longer, trying to extract the last tiny nuggets of nourishment.
THE BEETLES THAT soon interested us the most were the numerous dung-ball rollers of one large-bodied species, later identified as Scarabaeus laevistriatus. These beetles came precisely at dusk, just as we were getting started on our night’s work. They approached fresh dung in a gingerly walk, palpated it with their antennae, then began to cut into it with the rakelike prongs on both front legs as well as a shovel-like extension on the front of the head. Each beetle pried off some dung, patted it with its front legs, and then pulled additional material from the pile. In this way it proceeded to sculpt an almost perfect sphere the size of a golf ball or as large as a baseball. The process could take from ten minutes to half an hour. When the ball maker was finished, it placed its long, slender hind feet on the ball, almost doing a handstand, with its front feet on the ground. Facing away from where it was going, it started walking backward on its front legs, while its hind feet kicked the ball to roll it along. Before it got a good rolling start, however, hordes of other beetles flew in, planning to steal the ball maker’s labor of and for love; some balls made by males are nuptial offerings or sexual displays, and they are prized by competing males.
Newly arr
iving S. laevistriatus beetles often inspected the whole dung pile but showed no immediate interest in ball construction. Instead, they approached a ball maker and jumped onto its almost finished ball. If it was a female and the ball maker a male, the female would flatten herself against the dung ball and stop moving. (In some species the female is the active partner who rolls the ball, and in others both partners roll it.) The male would then accept her and roll his ball away, paying no more attention to the hitchhiker, a mere bump on his ball. But very often two beetles, presumably both males, faced off on the ball and started a sparring match in which each one tried to flick the other off; the ball maker was unwilling to relinquish his creation to another male.
The single roller or couple proceeds in a probably random but consistent direction, possibly by maintaining a constant angle to some landmark or sky-mark. After traveling a suitable distance or arriving at a patch of soft ground, the beetles bury their ball, presumably in much the same way Nicrophorus beetles bury a mouse carcass. If it is a couple, the pair excavate a nest chamber and then mate, after which the male leaves. The female lays her one egg and stays on to care for the brood ball, which serves as both crib and larder for the developing larva. (The food may have the same volume as a carcass used by a burying beetle, but it has a lot less protein; hence the single offspring rather than the dozen or so of a burying beetle.) The dung-ball rollers have a life span of up to two years and thus may nest several times in their lives, each time with a different mate.