The Natural History of the Present, Chapter 3

Chapter 3: The Natural World
All forms of life influence their surroundings. Photosynthesizing bacteria and blue-green algae are thought to have created our oxygen-rich atmosphere about 2.5 billion years ago. Before that the atmosphere was a reducing one, carbon-rich and oxygen-removing. The creation of an atmosphere dominated by oxygen was a disaster from the point of view of most organisms; some retreated far underground to rock formations from whose chemical elements they could derive energy, some to the anoxic muds of wetlands, ponds and the deep ocean. The current global concentration of carbon dioxide is low thanks partly to the burial of so much carbon in fossil fuels 200 to 300 million years ago, and thus its effective removal from planetary chemistry. Large amounts of carbon are also stored in frozen northern bogs, in ocean sediments and ocean water, in soils, and in standing vegetation. Much of this carbon was sequestered by living things, some by simple chemical processes, and much of it can be released to the atmosphere under the right conditions. We, for instance, free the carbon in fossil fuels by burning them. Eventually that carbon would have been put back in the atmosphere as the continental plates in which it is buried are drawn down into the earth’s mantle and the carbon vaporized and carried back into the atmosphere through the vents of volcanoes. The oxygen content of the atmosphere is now maintained, somewhat mysteriously in the face of constantly exposed, oxydizing minerals (exposed by geological activity and digging humans); fires; and of oxygen-respiring living things, by bacteria and blue-green algae similar to those of 2.5 billion years ago, which live in soils or as photosynthesizing plankton on the ocean’s surface; and by rooted green plants.

It is now known that bacteria live in rock formations up to 3.5 kilometers below the surface of the ground and several miles deep under the ocean. They can tolerate temperatures up to 113º C., 13º C. above boiling. The bacteria in deep ocean muds derive their energy by reacting hydrogen with nitrate to produce ammonia. Some of the organisms that live in the deep, hot biosphere underground also depend on hydrogen fuel: these form the so-called subsurface lithoautotrophic microbial ecosystems; or, SLIMES, the rockeaters. SLIMES were first discovered in drill holes 1000 meters down in Columbia River basalts. It is thought these SLIMES use hydrogen generated by the reaction of water with the ferrous silicates in the basalt for an energy supply. As with the case of the sulfur-reducing bacteria about hot-water deep-sea vents, other organisms show up to eat the first ones; or to use their wastes. The mass of such depth-dwelling organisms, because of the immense size of their habitat, may be greater than that of life on the surface of the earth, much of which is in the first, well-oxygenated foot of soil, the first hundred feet above the soil, and the first half-inch of ocean surface. Whether the underworld life has much influence on the surface one isn’t known. Bacterial life in deep ocean muds has some effect on life on the surface, because what goes on in the muds influences the chemistry of the oceans, and ocean water circulates at greater speeds than crustal rocks (thousands of years rather than tens of millions: the carbon from fossil fuels wouldn’t have turned up in the atmosphere any time soon). At any rate, the living influences on the planet’s surface and atmosphere are set among more general planetary and gravitational ones (such as the output of the sun, the speed of the earth’s spin, its aspect to and distance from the sun); and those of plate tectonics. (Rocks circulate too, but slowly: the burial and rebirth of plates not only help recycle planetary carbon and also reposition continents and oceans; the position of the continents on the globe influences heat distribution, rainfall and other aspects of global climate.) It has become more and more clear that nothing—no climate regime, no ocean shoreline, no level of solar output, no stand of trees—is permanent. Things like the structure of a particular forest are only partly replicable. Ecosystems have histories: different histories explain differences among otherwise similar landscapes—landscapes similar in microclimate, soil, aspect, hydrology, but different in vegetation. Their different histories from, say, fire, grazing or windthrow, nudge them in different directions. Why the atmospheric concentration of oxygen, unlike that of carbon dioxide, remains constant in the face of fossil fuel combustion and the oxidation of exposed soils and rocks has to do with the large volume of atmospheric oxygen, compared to that of carbon and carbon dioxide (so large that burning all the trees on the planet would reduce the concentration of oxygen by only 300 parts per million out of 200,000 parts per million); and with the ability of bacterial populations to respond to small changes in the concentrations of planetary gases.

From a more local perspective, trees create conditions that favor forests: they transpire water from their leaves, cooling the atmosphere and creating mist and rain; their roots penetrate the soil, letting water and oxygen in; the yearly turnover of root hairs and rootlets (approximately one third of the total root mass) adds carbon and other nutrients to the soil. These nutrients are recycled by soil organisms and the trees. The damp, shaded, more temperate forest soils (not so hot in summer, not so cold in winter) become a home for insects, rodents, soil arthropods, bacteria, fungi. Fungi are important in all forests but especially so in evergreen forests, which are noted for their survival in poor soils, and whose needle-fall tends to resist recycling by the arthropods and bacteria, thus locking up nutrients for long periods of time. In some Pacific Northwest forests, soil fungi constitute half the soil biomass. Some evergreens won’t survive without specific fungi in the soil. Clear-cutting causes a dramatic decline in soil arthropods and fungi, which helps explain why some clear cut western sites, especially dry, south or west facing sites, are so difficult to reforest. The fungal hyphae provide water and other nutrients to the tree and increase its effective root area by 1000 to 10,000 times. They secrete enzymes that extract nitrogen and phosphorus directly from humus, eliminating the bacterial food chain. Hyphae secrete chemicals that break down the surfaces of stones and transport trace metals to the tree. They help protect tree roots from acid soils. (So firs will grow on piles of mine spoil.) They secrete growth hormones to increase tree root growth and antibiotics to protect tree roots from bacterial infection. Hyphae connect trees of the same and different species, and by providing nutrients from overstory to understory trees, help make forest succession possible. They turn the forest into a single organism. Such services are not free; approximately 40% of the photosynthesate produced by the leaves seeps out of the roots to the mycorrhizal and bacterial ecosystem that lies within a few millimeters of the root hairs (the so-called rhizosphere); the photosynthesate includes nutrients (sugars) and vitamins. Processes like this are the sort that commercial foresters (or farmers, in another situation) try to short-circuit, by using herbicides and commercial fertilisers to alter ecosystem function, and thus allocate more of the products of photosynthesis to plant growth. This approach ignores the place of the tree in the whole soil and forest community. By providing trees with water, fungi help them better withstand droughts. Polysaccharides secreted by the bacteria and fungi of the rhizosphere also change the soil’s structure, making it better aerated and better adapted to the movement and absorption of water (thus better adapted to tree growth). In test plots in a tropical forest in Guinea, trees responded better to a mulch of wood chips than to chemical fertilizers. In general, trees with mycorrhizal connections grow more vigorously than those without them.

The rest of the soil community is also important. The arthropod communities of Pacific Northwest forests include the grinders of successively smaller size that break down litterfall (five tons per acre per year in old growth forests), until the material, successive levels of bug poop, finally reaches the scale of the plant cell. Bacteria scavenge the available nutrients, die, or are eaten by other organisms, which excrete them, their nutrients becoming available to the trees; the nutrients are also transferred by fungal hyphae to the trees directly. Logs are broken down in a similar fashion. Total incorporation of a Douglas fir log into the soil can take 400 years. Because of the log’s inhabitants, while 5% of a tree consists of living cells, 20% of a rotting log is living tissue. Such soil arthropods can be used to define a site. From the structure of the arthropod community one can tell the time of year, the altitude of the collecting site, its aspect (north or south slope), the understory plants, the forest’s successional stage, whether it is old or second growth, its mix of trees. The arthropod community has a turnover time measured in weeks (compared to days for the soil bacteria and centuries for the trees) and so is a much better indicator of subtle changes than the trees, whose long lifetimes put them at a distinct disadvantage in terms of short-term changes. (Along the same lines, Douglas fir needles have fungi that live in them and manufacture chemicals that suppress the growth of needle-grazing insects. The fungi, unlike the tree, can evolve at a rate similar to the insects. As with the soil fungi, these fungi live on nutrients from the needles as well as on nutrients scavenged from killed insects.)

Forests intercept and soften rainfall. Rainwater splatters into a mist as it hits the canopy, drips off leaves and branches and runs down trunks, enters the soil along channels left by rotted roots, dead fungal hyphae, rodent burrows, the smaller burrows of soil invertebrates, cracks left by drying or ice. Rainwater is absorbed into the space that occupies 50-60% of a healthy forest soil. A considerable percent of rainwater evaporates from the trees. In grasslands, where much the same thing happens but on a smaller vertical scale, more water evaporates and runs off into streams, and less enters the ground, than in forests. In older forests, those over say, 150 years in the Pacific Northwest, which is about average for the beginning of maturity in temperate forests, the changing microclimate in the forest begins to favor the growth of lichens on the branches and trunks of the trees. In time, the lichens and other epiphytes on an old-growth tree may weigh four times its foliage. A tree may have 1000 species of affiliated fungi. The lichens gather their nutrients from the air. Rainwater leaches nutrients from them down to the forest floor, where they become available to the tree (pieces also break off and fall to the floor). Lobaria lichens are the largest single source of nitrogen in old-growth forests of the Pacific Northwest and fix several times the nitrogen they need for their growth and maintenance. Roughly speaking, the excess is what is available for the rest of the forest. Lobaria appear in Douglas fir forests at 100 years (20 years after the last of the planted trees in a plantation is cut) and become abundant at 200 years. A full range of all species develops at 400 years. Thus old-growth forests begin a new nutrient cycle, harvesting nutrients from the air to supplement what is available from the soil, much of which by then has been locked up in the trees. The lichens are a food source for arboreal rodents, and the rodents food for predatory birds, creating a new loop on the new nutrient cycle. More growth means more carbon stored in forest soils. Flying squirrels and red-backed voles, among whose main foods are the fruiting heads of the fungi of the forest floor, those valuable edible mushrooms, are the prey of Spotted Owls, a bird whose precipitous decline has led to the controversy over cutting the remaining old-growth forests of the Pacific Northwest.

The spongy forest soil, clumpy with polysaccharides secreted by tree roots and soil organisms, draws in the water it receives. Rainwater is naturally slightly acidic because of carbon dioxide in the air. In the soil it is enriched with more carbon dioxide exuded by respiring tree roots and soil micro-organisms. This increased acidity accelerates the process of subsoil erosion (or weathering), releasing nutrient minerals from rocks and rock particles under the tree. Water also carries rock-dissolving fungal enzymes. Most of the nutrients are taken up immediately by the forest community. Some of the soil water, stripped of its nutrients, sinks through the vadose zone to ground water, some is pulled up by the tree leaves and transpired. The 50-65% that evaporates or is transpired from the canopy to the atmosphere will fall on the landscape nearby in the convective rainfalls of spring and summer. Trees grow tall not so as to produce straight sawlogs, but because gravity tells them to and also to compete for sunlight, their source of energy. Another function is to intercept water. Precipitation under coastal redwoods in California is two to three times the yearly rainfall measured above the trees, thanks to the redwoods’ fog-catching abilities. Trees high on mountainsides may get half their moisture from cloud water condensing on their leaves and branches. Some of the moisture in both cases runs into the ground and feeds perennial streams. (The process supports forests on oceanic islands whose main source of water is fogs.) The great mass of the large trees of the American West Coast may be related to water storage, in a region of summer drought. For the most part, the dead heartwood tissue of trees is thought to be useless, and hazardous as the tree gets larger and its great weight, relative to its root area, increases its liability to fall. Some biologists have theorized that heart-rot fungi, which help the heartwood decay without harming the living outer shell of the tree, are adaptive mechanisms on the part of the tree to reduce its weight and convert the nutrients stored in the heartwood to something the living parts of the tree can use; thus the tree continues to produce seeds, which is the tree’s goal—surely a horrible notion to foresters. (Some old broad-leaved trees send adventitious roots from their branches down into their hollow cavities or into the soil that accumulates along their branches.)

Forests are part of the larger hydrologic and biogeochemical cycles of earth, and of the earth’s energy relationships with the sun. Through photosynthesis, evapotranspiration, and the absorption, reflection, and re-radiation of sunlight, forests affect the flow of solar energy through them and determine the forms in which this energy is dispersed. They change the form, amount and chemistry of the water that flows through them, the chemistry and speed of the wind-driven air, and help determine the amount and kinds of dust and sediment released to these fluids. They govern in some degree the behavior of the interconnected streams, rivers, lakes, and ocean estuaries. They affect local, regional and global climate. In temperate regions, forests, through the evapotranspiration of water from their canopies, have a cooling effect on the summer climate. In snowy regions, certainly in winter, and probably overall, forests have a warming effect on the climate, because they absorb more heat than bare or snow-covered ground. (Locally, this warming exceeds the global cooling effect caused by their absorption of carbon dioxide. So the movement of the taiga forests north as the climate warms will likely reinforce the warming.) But cutting the European forest seems to have raised midwinter temperatures in Europe by 2-3° C., while cutting the forest on Vancouver Island raised summer temperatures there by 2° C.

The storage of carbon in the tissues and soils of forests connects them to the global carbon cycle. One scheme to lower global carbon dioxide levels has been to plant millions of square kilometers of new forests, which while young and growing absorb much carbon dioxide. Existing older forests would be cut down to do this. Cutting down an existing forest releases large amounts of carbon dioxide from the logs, the soil and from rotting vegetation. In temperate forests, it takes 10 to 40 years before more carbon dioxide has been taken up by the growing trees than was released by decomposition. (Tropical forests take much longer.) Once the forest has grown to the point of maturity (that is, to the point where its increase in mass has slowed, and thus the storage of carbon in trunks and branches slowed), the problem is how to log the forest without releasing a high percentage of the carbon. Most of the carbon stored in the wood of a logged forest will return to the atmosphere within a decade. Much of it goes into paper or other short-lived products, and maybe a third is waste, which is burned or decomposes. Since it now appears that forest soils continue to absorb considerable carbon dioxide even as the trees themselves increase in mass more slowly, the likely answer is never to log the forest, or to log it very carefully, so that the soils themselves continue to absorb, or at least limit, their release of carbon. In this case, the choice of lands to reforest becomes important. We should choose more environmentally vulnerable or valuable ones, such as river floodplains, very steep slopes, watersheds used for water supply, and aquifer recharge areas. We should reforest connected areas of sufficient size for the birds and animals characteristic of a region. New forests on deforested or destroyed lands (such as worn-out agricultural lands or strip-mined lands) constitute an advantage from the start; establishing them may be difficult but involves no initial loss of carbon. Tropical forests cool the local climate and continue to store carbon over their lifetimes. They have a large influence on local rainfall and should not be cut, or be cut and reforested very carefully, without much disturbing the canopy, and thus kept from releasing their carbon. (Restored degraded grasslands, such as the American Plains or African savannahs, are other good candidates for carbon storage.)

Forests are connected to the larger world by the water and air that move through them, by the animals that graze on them, and by the plants and animals that otherwise take advantage of the habitat. Insect depredation is usually kept below 20% of leaf mass, partly by insect diseases, partly by chemicals secreted by the trees (24 hours after a caterpillar bites into an aspen leaf, the leaf has five times the amount of poisonous phenols), partly by predatory insects, and partly by insect-eating birds. While losses to insects are a serious matter, much of the energy in the grazed leaves returns to the trees through the insects’ dung and bodies. Summer bird densities in the mesic forests of the eastern United States can be startingly large. Eastern deciduous forests average 863 pairs of songbirds per square kilometer (in effect, about one pair per 1200 square meters, a square 30 by 40 meters on a side). Of these, 587 pairs migrate here from the tropics. Eastern coniferous forest averages 644 pairs, 412 of which are neotropical migrants. Riparian woodland in California averages 1596 pairs per square kilometer, 399 of them neotropical migrants. (California has a more moderate year-round climate and more year-round resident birds.) The neotropical migrants come to take advantage of the summer burst of insect life in the temperate and arctic regions. Birds of one species divide up the suitable areas of the forest into separate territories. A given plot of forest is however shared by many different species; the different species divide it into distinct feeding zones, which are gone over daily and intensively. Intensively, since the birds are feeding not only themselves but several growing young. Birds specialize by focussing on different parts of the forest (the forest floor, the mid-level, the canopy), in how they search those areas for food (scratching in the leaf litter, searching under fallen leaves, hawking for flying insects), in what they choose to eat. (Among wood warblers that forage in the conifers of the northeastern forests, for instance, one species forages in the outer surface, one near the trunk, one along a vertical axis, one horizontally, one chooses the upper third of the tree, one the lower third; some are specialists in specific insects, such as spruce budworm.) The winter territories of these birds are thousands of miles away. Between are the lands that are traversed during migration, slowly in the spring, where, in Central America, the birds’ northward movement corresponds with the ripening of some species of tree fruits, whose seeds the birds disperse. Along the East Coast of North America, spring migration follows the hatch of geometrid moth larvae, at bud break, up the Appalachian Mountains. The leisurely spring migration, during which the birds feed on abundant, protein-rich, nontoxic leafeating caterpillars, is quite different from the birds’ abrupt autumn departure from the Atlantic capes for flights over the ocean to the Caribbean islands or the northern coast of South America. The demanding autumn flights require abundant food supplies concentrated in small areas, that is, easy-to-find fruits, insects, or arthropods, as well as shelter.

To be effective in their job of insect control, the birds must be numerous; ideally they must flood their breeding territories, so that their summer densities are limited primarily by competition for food. This means their “winter” territories must also be optimal, as well as those lands used during migration. All these areas (summer territories, winter territories, migratory corridors) have been considerably degraded in the last 50 years. The Atlantic capes have been developed, reducing their supplies of food and shelter. Breeding areas suffer from fragmentation. Most neotropical migrants do best in large areas of unbroken forest. Smaller areas reduce the amount of their insect prey, as the drying effect of forest edges reaches into the forest and reduces its invertebrate life. Smaller woodlands also make neotropical migrants, most of which are small, brightly colored, and build rather visible cup nests, more vulnerable to nest predators and parasites. Cowbirds, the most important nest parasite in North America, lay their eggs in other birds’ nests and let those birds raise their young. As they grow, the large young cowbirds push the natural offspring out of the nest. Cowbirds, once a bird of the prairie, have increased tremendously and spread throughout the eastern United States with the fragmentation of the forest. (Other prairie birds, such as the clay-colored sparrow, are following them east.) Most neotropical migrants are too small to deter nest predators like crows, raccoons, blue jays, oppossums, house cats and skunks, all of which are abundant in forest edges. More and more winter range is being lost in southern Mexico, Central America and the West Indies to agriculture, logging, and development for tourism. Species that winter in tropical second growth, that is, cut-over woodland (these include northern parula warblers, indigo buntings, rose-breasted grosbeaks), are increasing in number, while those that winter in tropical primary forest (such as wood thrushes, blackburnian warblers, chestnut-sided warblers, hooded warblers) are declining. The quality of winter habitat makes a difference. American redstarts that winter in tropical forest in Jamaica and Honduras gain weight in winter and return early in spring, while those that winter in cutover dry scrub lose weight and return later. Lighthouses and lighted towers (TV towers, cellphone towers) kill 100 to 300 million migrating birds in North America each year, lighted windows 100 to 1000 million (dimming the lights in downtown skyscrapers during migration reduces deaths by 80% and saves millions of kilowatt-hours of electricity); cats 500 to 5000 million (a modest 5 to 10 per cat); ingested lead shot 300,000; hunters 120 million; cars perhaps a million a week. (There are perhaps 20 billion birds in North America in late summer.) The night sky through which songbirds fly south is no longer an empty ether, lit by moon and stars, but probed by beacons, crisscrossed by radio waves, swept by thundering engines, lit by the urban glow from below. Development removes roosting places, meadows and shrublands rich in food, forests from migratory corridors. Habitat degradation, along with pollution of the Northern Hemisphere by heavy metals and hydrocarbons (such as mercury, oil, benzene, DDT), and the depletion of calcium from forest soils by acid rain (which reduces the abundance and nutritional quality of their insect life), helps explain the falling numbers of neotropical migrants in North America. One count puts numbers of migrating songbirds at 5% of pre-Columbian populations, with a fall of perhaps 50% since the 1950s.

Migrating birds extend a forest’s reach into a larger realm, as does the water transpired from the trees (which enters the hydrological cycle, both local and global); or the chemicals exuded by the trees into the atmosphere. Some of these chemicals are part of the trees’ cooling mechanism, some function as warning signals of the presence of grazing insects to other trees (which also begin to secrete them, spreading the warning). Predatory insects are attracted by these deterent-and-warning chemicals, and eat the grazers or lay their eggs on them. (Some species of bean plants secrete chemicals attractant to predatory insects when the eggs of foliage-eating insects are laid on them.) Such chemicals are secreted in sufficient concentration to effect local atmospheric chemistry; in the presence of automobile exhaust and ultra-violet light, they form low level ozone and add to photochemical smogs. At least 120 chemicals are found in the air of California’s high Sierras. Among the most abundant are monoterpenes, which have a role in preventing and curing cancer. Such chemicals enter the bloodstream through the lungs or the limbic system through the olfactory nerves. So a breath of mountain air is a good thing.
A forest’s watery connections lead to the sea, whose nutrients may feed it. After Pacific salmon spawn, they die, and the nutrients in their bodies leach out into headwater streams and lakes. Up to 30% of the nitrogen and carbon in algae and aquatic invertebrates in the steep, nutrient-poor streams where Coho salmon spawn come from dying Coho. The Coho also contribute up to 18% of the nitrogen in streamside vegetation (up to 50% of the nitrogen in some old growth trees. Trees grow 3 times faster along salmon streams. Spawned-out fish contribute 25-40% of the nitrogen and carbon in juvenile Coho, a direct parental contribution to growth. Most of these nutrients come from the sea, so the Coho are part of an accumulated pool of nutrients shared among the sea, streamside forests, and the fish, and to a lesser extent with the bears, martins, otters, beetles, fly larvae, white-tailed deer, red squirrels and eagles that eat the salmon carcasses, and which spread the bounty from the sea further into the landscape through their own bodies and dung. In the salmon-fed rain forests of the Pacific coast, ancient murrelets nest among the roots of the trees and marbled murrelets in the tops: both are seabirds which contribute the nutrients in their droppings to the forest. East Coast streams also had abundant runs of anadromous fish (herring, sea-run trout, striped bass, shad, Atlantic salmon, eels) and so took part in a similar exchange of nutrients; 90% of Atlantic salmon also die after spawning. Fish that didn’t die contributed their voided wastes (and their milt and eggs), which correlated in amount with their body masses, much of which derived from the nutrients of the sea.

Forests are also influenced by the presence of large predatory animals. Ungulates such as deer and elk are capable of rapid increases in population. If unchecked by weather or predation, the animals overshoot their food supplies, their populations crash, and then cycle up and down at a much lower density, in a degraded habitat. Deer at densities greater than 10 per square mile inhibit regeneration in favored species such as hemlock, yew, and white cedar. Browsing that eliminates the understory reduces the value of the forest as habitat for many animals (including nesting birds). White-tailed deer densities in the eastern United States range from 5 per square mile in mature deciduous forest to 20 to 30 per square mile after logging; with protection, populations can reach 70 per square mile. High densities prevent forest reproduction and result in a permanent reduction in food supply for deer. In Yellowstone Park, before the wolf was re-introduced, browsing by elk had prevented regrowth in stands of cottonwoods along streams. Most of the cottonwoods were 60 years old or older and dated from the time wolves were eliminated from Yellowstone. Reintroducing wolves cut the elk population from 5% up to 30%. (Elk numbers are more influenced by food supply and weather than predation. Elk continue to increase during a period of mild winters even as the wolves increase.) But the presence of wolves changed the elks’ behavior. Elk now spent much less time in stream bottoms, where they were more vulnerable to ambush by wolves. This reduced their browsing there and let young willows and cottonwoods regrow. The regrowing forest improved habitat for nesting songbirds and also for beaver (another food animal of wolves). The trees stabilized stream banks and shaded the water. Trout became bigger and more abundant. The ponds the beaver built on small streams provided more habitat for birds and amphibians, slowed water flow, raised ground water tables, and let stream beds build up (aggrade) rather than downcut. Wolf predation also halved the coyote population, so numbers of smaller predators like red foxes and raptors that prey on rodents rose. Wolves control deer and elk by taking animals too sick, too hungry, too old or too young (or whose mothers are less powerful or aware) to defend themselves. (This is the usual story. In prairie dogs predation also falls on males obsessed with sex and on pregnant females, who are probably slightly slower. Predation on young deer or elk can become a problem if the prey population is reduced or otherwise stressed.) Thus animals like wolves exert different evolutionary pressures from those of human hunters on their prey. Human hunters take healthy animals of breeding age. To stabilize deer populations human hunters must take 35-45% of the females annually, not a small number. The difference in predatory behavior is thought to be one reason why prey populations tend to fall in body size under human predation but remain the same or rise under predation by wolves. Healthy wolf populations depend on migration of wolves from other regions, and thus on extensive connected areas of plains or forest.

The plants, animals, fungi and the invisible life of a landscape’s soil are influences on the landscape as important as its topography, climate, and mineral composition (all of which, on a local and a global scale, are modified by life). Natural landscapes are connected largely by the movement of water and its associated nutrients, but also by the flow of the atmosphere and the movements of nutrients and energy through seeds, fish, and other migratory animals. Through such connections, changes in one habitat (such as logging a part of a forest) can affect the whole landscape’s cycling of water, soil, or nutrients, that is, affect the rate of flow of these materials through the larger landscape. When the landscape was edible and its major purpose was to provide food, people were in most cases a less obtrusive part of this landscape. They altered it less, or in less disturbing ways than now, partly by influencing the abundance of large grazing animals, partly by small-scale horticulture, mostly through the use of fire. People were another part of the biota, comparable to an efficient predator like a wolf or a cougar, or to a transformer of soils like a prairie dog, an important but not an overwhelming influence on the landscape, not like now, a cometary impact.