Editor's Note: Lakes and streams receive a disproportionate share of the disruption that follows the spread of human influences over the globe. The disruption reaches from the direct effects of dams and modifications of water courses to the disposal of sewage and the introduction of toxins including agricultural poisons. There is in addition the series of problems associated both with the depletion of the fish populations through harvest and with the introduction of exotics. And, more recently, we have recognized the seriousness of the problems associated with air pollution, including the acidification of rain and the cumulative effects on lakes. Now we see the further possibility that genetic engineers will be adding to the burdens already well known from the introduction of exotics by producing novel combinations of genes that may escape into nature to produce totally unforeseen effects.

In many ways, among all of science, the limnologists have led in defining the transitions under way in nature. The process of eutrophication was recognized first in aquatic systems and has since been recognized as a general phenomenon, the enrichment with nutrients mobilized by human activities, a first step in the continuum of change that is gradualy recognized as pollution, and, in a twist peculiar to nature, impoverishment.

The full array of effects and patterns laid forth so explicitly by Schindler is almost overwhelming. But the other message, only slightly less powerfully articulated, is that the changes are a continuum away from the status quo toward progressive impoverishment.

Editor's Note: Glacial advances and retreats over several hundreds of thousands of years have sorted and resorted the plant populations of the middle and high latitudes of eastern North America and have winnowed from those populations a flora of hardy survivors. The survivors among trees are almost uniquely singular species, unencumbered by dependence on other species, by the existence of a community, by a narrow dependence on edaphic perfection, or on special mechanisms for pollination or dispersal of seeds. The species of fir, spruce, birch, oak, poplar, hemlock, and pine can occur in almost any mixture in closed forests or in open woodlands, even on occasion in savannas, and on a range of habitats. To be sure, the species are not interchangeable, have different distributions and requirements and roles in succession, but, as wind-pollinated species (in contrast to tree species of the tropics that may depend on insects, birds, or mammals for pollination), they have few direct dependencies on other species that restrict them to any special community. We might go so far as to suggest that they are preadapted to climatic change as a result of selection by some of the most rapid climatic changes that we know of beyond the catastrophe of the Cretaceous/Tertiary boundary.

Margaret B. Davis, a distinguished scholar who has specialized in retrospective studies of vegetation, has addressed the challenge of estimating the rate of migration of one of these glacial survivors, Tsuga canadensis, under the climatic changes anticipated for the next decades.

Editor's Note: Lichens, many of them, are surface dwellers. They have evolved to occupy, among other places, the surface of rocks, the surface of the soil, the bark of trees. These surfaces offer some of the most difficult habitats for life, habitats marked by extremes. The surface of a rock, for example, may bake in the sun during the day and rise in temperature to 70° Celsius or more, dropping at night to freezing or below. Similar extremes in water availability accompany such changes. Yet such habitats may support communities of lichens that garner water and nutrients directly through the surfaces of their thalli. Despite the apparent hardiness of lichens, their populations have long been considered sensitive indicators of air pollution: certain lichens appear to be particularly vulnerable to air pollution and their vigor appears to offer potential as an early index of impoverishment.

The difficulty is that under disturbance the populations are ephemeral: they often disappear quickly and the index becomes neither morbidity nor other active change, but mere absence. Professor Hawksworth warns, to add to the difficulty, that it is not always appropriate to transfer experience with lichen communities from one place to another. Nonetheless, he provides compelling documentation of the sensitivity of lichen populations to airborne pollutants. The effects are unquestionably impoverishment, a reduction in diversity that is virtually universal and that applies in particular to fruticose and foliose, less to crustose, lichens.

I recall speculating years ago with the distinguished Russian scholar, Victor Kovda, at dinner in his Moscow apartment, about the productivity of the currently impoverished vegetation surrounding the modern city of Tblisi in Soviet Georgia. I had seen in a museum in Tblisi an ancient oaken cart with wheels fashioned of single disks of oak cut from the end of a log fully 4 feet in diameter. Large oak trees or forests of any description do not exist in the region now. Kovda guessed that the current productivity of the region, measured as ecologists would measure it as “primary production”, is 1% of what it once was. The cause was deforestation followed by the grazing of goats, sheep, cattle, and, later, over much of the land, by tillage, all of which contributed to the erosion of a thin soil.

Tblisi, a landscape that has sustained human occupation from time immemorial, is but one example of the cumulative effects of long-term human activities on the capacity of the landscape for sustaining life. The conference that was the basis for this book emerged from many years of puzzlement over how ecologists address the human-caused transitions that now dominate plant and animal communities globally. For many ecologists, perhaps most, the answer seems to be to focus on some of the more fascinating aspects of the evolution and extinction of species. But human effects and disturbance reach much farther than to extinction alone.

Editor's Note: Islands have provided some of the most fundamental of insights into evolution and ecology while bearing some of the greatest burdens of human depredation. What biologist can remain long innocent of the saga of Darwin's finches as spun out by Darwin himself and David Lack and others who have been puzzled by the diversity of life of the Galapagos? Whose curiosity is not piqued by the biotic anomalies of New Zealand, Australia, Madagascar, Easter Island, Surtsey? And yet, while scholars have found extraordinary insights in the biota of islands, the biota was often devastated early in the period of human expansion through the introduction of goats or other ungulates thought to benefit seamen on future visits.

Both the puzzles and the depredations grow more complicated as human influences spread. Dieter Mueller-Dombois addresses the current transitions in the forests of Pacific Islands. He describes the rain forests of the Hawaiian Islands as “originally impoverished and secondarily enriched.” They were impoverished by comparison with other large islands of the Pacific that are closer to continental areas of higher diversity; they have been enriched by high endemism, a product of their insularity and period of isolation. Mueller-Dombois introduces a concept new in this treatment, progressive development through a peak period of a thousand years or more followed by regression as soils deteriorate.

Editor's Note: Why are ecologists concerned about the imminent destruction of the last of the tropical forests?

The answers are legion, so many as to seem diffuse, so powerful as to seem exaggerated and shrill, so fundamental as to seem obvious, and so demanding as to seem preemptive.

Forests are the great biotic flywheel that keeps the biosphere functioning more or less predictably. They are the major biotic component of the global carbon cycle, contain about three times as much carbon as the atmosphere, and their destruction contributes directly to the warming of the earth. Their presence determines the reflectivity of the earth over large areas, energy balance, water balance, nutrient fluxes, and air and water flows. They are, moreover, the major reservoir of biotic diversity on land: there is no habitat richer in species, none more promising as a source of succor for a swelling, scrambling, grasping human population uncertain as to where its great hopes lie. And yet, no habitat is being addressed more rapaciously than the tropical forests of Brazil.

Philip Fearnside is an ecologist with many years’ experience in research, writing, and teaching in Brazil. He writes here about one of the world's greatest tragedies and touches on the transitions in plant communities that accompany the process around the world. The shift from forest to grassland or lesser communities is common here as elsewhere under chronic disruption.

Editor's Note: Tests of theory in ecology are often difficult, sometimes impossible. John Cairns has brought a lifetime of experience to bear on the practical questions of pollution using simple aquatic systems to shed light on hypotheses in ecology that are otherwise difficult to examine. He recognizes two types of impoverishment, losses of diversity in a landscape as human effects spread, and the in situ transitions that accumulate under chronic disturbance, usually, but not always, reducing the diversity of a community and making it more vulnerable both to invasion and other disruption. The insights are refreshing, if complex and disturbing. For us they confirm the patterns now familiar: chronic disturbance in aquatic systems favors the hardy cosmopolitan species that reproduce rapidly and usually do not form the basis of complex food webs. The patterns are systematic and predictable and increasingly common. One of the greatest dangers is that the disturbances will become so close to universal that sources of species for reintroduction will be lost, despite the general hardiness of the protists with which he works.

Introduction

Anthropogenic stress commonly leads to changes in ecosystems that are regressive and best described as “impoverishment.” Most of these disturbances are chronic. Catastrophic disturbances, such as nuclear war or rapid global climatic change, will cause immediate and severe effects that we can only poorly estimate. For other disturbances, such as acid rain and various toxins, experience is abundant and data are available.

Editor's Note: The world is changing rapidly under human influences and the changes are the enemy of life. The seriousness of the problem has remained buried until recently in the litter of aspirations for economic growth at any cost and in the convenient and comforting assumption that the biosphere is resilient, capable of absorbing any insult. Recognition of biotic impoverishment as pervasive, even global, and as a threat to economic and political security as serious as war itself is new, not yet a part of the body politic. Yet, for the environment as for any machine, once parts start breaking, damage spreads rapidly.

Michael Oppenheimer offers a physical scientist's review of the physical and chemical changes under way in our world. Although it is common to acknowledge that “everything is connected to everything else”, we rarely recognize the concatenation of physical, chemical, and biotic changes in environment that make a term such as “acid rain” a reality. The complexities acknowledged by Bormann in his assertion that the decline of forests in eastern North America is not simply and certainly ascribed to air pollution become real in Oppenheimer's discussion of the mechanisms.

The effects of changes in environment appear in plants, not as a simple response to drought or heat or toxins such as ozone, but as an increase in insect or fungal damage in trees, a mysterious chlorosis, or unexplained mortality.

Repairing a troubled world will require fundamental alterations in many aspects of human behavior.

Editor's Note: The Arctic with its limited flora and fauna, its extremes of climate, its indigenous people, its ties to montane habitats of lower latitudes, and its uncertain history has always captured the interest of ecologists. The landscape, despite the vicissitudes of climate and the long months of darkness and frost, is a living landscape, a tightly integrated community, not open to easy invasion by exotics, its living systems strangely immune to disease. It is vulnerable, but disruption, when it comes, can be severe: “thermal karst erosion” is the phrase the specialists use to describe the destruction that occurs as the surface albedo drops and the sun melts the ground that is usually frozen. A trickle of water becomes an erosive force, transforming the tundra into a slurry of mud and eroding peat.

Efforts at stabilizing landscapes disturbed by human activities, building plant successions where none exists, have met with limited success. Massive efforts at introducing exotics have, fortunately, failed. The arctic ecosystems are closed corporations, not open to outsiders. Bliss shows that plant succession as recognized in lower-latitude communities is weakly developed, in some circumstances non-existent. But the communities are surprisingly resilient … to the point where the erosion starts. Once it starts, there is no cure in time of interest to this generation … or the next.

Bliss, who has devoted a career to the Arctic, has written comprehensively and sensitively about arctic ecosystems. He sees no mitigating circumstance in a rapid warming and no effective countermeasure.

Editor's Note: Much of the New England landscape of today would be foreign to the New Englander of the nineteenth century, immersed as it was in 1850 in an intensive subsistence agriculture that reached to virtually every corner of the land. The nineteenth-century farm preserved access to springs, divided pastures and hayland among heirs to assure survival, and saw wildlife principally as a threat, competitors and predators in a lifelong struggle to make the farm support an expanding family. The land was heavily used, too often abused, impoverished, eroded. Juniperus communis var. depressa spread over pastures, devouring space that might otherwise have supported herdsgrass and marking at once the impoverishment of pasture, farm, and family. Swamps were drained, ploughed, and planted. Species were lost. In the 1980s much of this land, once farmed, all pastured, has returned to forest. The forest and the land are threatened now with a different set of insults: division into houselots, highways, paved commercial sites, small holdings of various types, none of them dependent on energy from the land. The land is now considered mere place, space, access to people, water, view. And its biotic resources, already impoverished through 200 years of intensive use of the land, are threatened again by an equally pervasive, less obvious assault: air and water pollution from afar.

F. H. Bormann has summarized his perspective of this latter problem with special emphasis on air pollutants common today over large areas of the globe.

Editor's Note: Bogs and peat and acid waters are commonly thought to be of the higher, cooler latitudes, not tropical. But peatlands and acid waters occur around the world. The traveler in the Amazon Basin, for instance, until recently restricted to its rivers, found a new world in the transition from the silt-laden water of the main stem of the Amazon or the Solimoes to the black, acid water of the Rio Negro. The traveler is blessed there with an abrupt relief from insect pests: I have slept comfortably, without screens or mosquito netting, 60 miles above Manaus in the magnificent riverine bog-swamp known by the lyrically liquid name Anavilhanas. The black water is the drainage from bog soils, extensive in that part of the basin, and supports an extraordinary fauna of herbivorous and seed-eating fishes that graze in the rich varzea forests, flooded annually to a depth of 30–50 feet in many places.

The fact is that peatlands are widespread around the world, apparently the product of a biotically caused acidification of moist habitats that are low in nutrients. Their plant and animal communities are at once impoverished by comparison with other sites that are rich in nutrients and less acid, and yet the bogs are rich in fascinating endemics such as the seed-eating fishes of the Rio Negro. The habitat is ancient, common, widespread, important, still evolving, sometimes thought to be the product of a regressive series of changes that reduces tree growth and leads otherwise to impoverishment.

Editor's Note: The challenge of proving the effects of air pollution on vegetation has been awkward and frustrating at best, the more awkward as venal interests have pressed ever more insistently for proof of specific causes and evidence that the damage is worth correction.

Walter Westman has addressed this challenge in the coastal region of southern California where the effects of one of the world's most insidious problems with air pollution have been accumulating for decades. He has used an extraordinary combination of techniques including field studies along well-defined gradients of pollution and chamber studies under controlled conditions. The field studies were supplemented with remotely sensed imagery.

The conclusions are classical, powerful, persuasive…and about as specific and definitive as they come: a trend toward impoverishment involves systematic reduction in the vigor of indigenous plants, an increase in the abundance of exotic annuals, an increase in the frequency of fire, and, on slopes, increased erosion including landslides. In parallel with the changes in the structure and successional patterns of the vegetation, Westman shows a series of biochemical changes in plants that include increases in the concentration of nitrogen in tissues after ozone exposure, an increase in the ash content with increasing pollution, and shifts in the chlorophyll content. Here, in an apparently hardy, drought-resistant vegetation, the patterns of impoverishment become conspicuous when sought systematically and follow patterns similar to those found in other vegetations around the world.

Editor's Note: The eucalypt forests of Australia have evolved to occupy a narrow habitat of restricted nutrient and water availability. They have proven vulnerable to fire, the introduction of exotics, and to the eutrophication that accompanies human activities. They are also vulnerable to a host of aggressive, introduced annuals that are more responsive to the nutrients and that carry fires through the forest at unusual times and in novel ways. The effect is the replacement of the forest by grasslands made up of exotics, a pattern of impoverishment now recognized as common around the world. But the special sensitivity of these forests and those of New Zealand sets them apart as a lesson in both the details of evolution and in the importance of knowledge of those details in management of a potentially rich, productive, and enduring resource that is now rapidly being lost. The loss is through a classical series of stages of impoverishment and results in a conspicuous loss in the capacity of the land for support of people.

R. L. Specht is a distinguished ecologist, long a student of the vegetation of Australia. He writes here about the transitions he has observed in forests in response to cumulative human disturbance.

Introduction

Eucalypt Forests/Woodlands in Australia

Only a quarter of the continent of Australia has the subhumid to perhumid climate favorable to eucalypt-dominated open-forests and woodlands (Table 10.1 and Figure 10.1).

Editor's Note: The landscape around Sudbury, Ontario, has been devastated over several decades by the gaseous emissions of smelters. The extent of the devastation is difficult to define, but forests have been destroyed by air pollution and prevented from recovering over an area of more than a hundred square miles. The cause is oxides of sulphur and heavy metals emitted over decades from a series of smelting operations. Lakes and streams have been affected as well.

Yan and Welbourn address the problem of defining the patterns of changes in plant and animal communities of lakes and ponds affected by air-borne pollutants 13 km from the smelter. Surprisingly enough, the effects, although cumulative, are reversible, at least in part. They are faithful to patterns reported elsewhere: the small-bodied, rapid reproducers are more resistant; the large-bodied are commonly more vulnerable. Food webs are abridged, fish populations lost, arid certain populations explode in response to removal of competitors. So the effects present an interesting mixture of direct effects of toxins, including aluminum washed off the acidified land, and indirect effects due to changes in predator or consumer pressure.

The transitions are all in the direction of steepening the dominance/diversity curves by concentrating production in one or a few species. The impoverishment leads to increased vulnerability to further disturbance. The example is unusual and compelling.

These extraordinary circumstances offer powerful insights into the patterns of nature, which are much closer to universal than we might have expected.

Editor's Note: Ecologists normally think of evolution as independent of human influences. But people have been around for at least 2 million years, long enough to have influenced not only the landscape but evolution itself.

Naveh, a landscape ecologist, shows how long habitation of the Mediterranean Basin has affected both species and the structure of the communities of this region. The vegetation of the basin is both unique in having evolved over many thousands of years with dense human populations and, strangely, common to the point of illustrating the central principle of impoverishment.

The story is fascinating but not elevating: the human role has been persistent, unrelieved, continuous pressure toward impoverishment. The evolutionary response has been, not surprisingly, adaptation in the pattern now so familiar in these pages. Where we started on this path is obscure, buried in geological history hundreds of thousands of years back and interpretable in more recent times only from such fragmentary documents as the records of the rain of pollen left in special places such as bogs.

Naveh brings a lifetime of research and intimate knowlege of the region to bear on the history and development of the Mediterranean vegetation.

Editor's Note: Economic gradients usually dominate in determining details of the management of resources, including land and forests. One of the most important examples is the influence that inexpensive oil and cheap transportation have had on the use and value of land and the size and shape of cities in the United States and eleswhere. Cheap energy has enabled the spread of dwellings across the landscape; has raised the value of rural land for housing beyond its value for farm, pasture, or forest; and has favored massive agricultural enterprises far from markets. The management of forests, too, has yielded again and again to economic gradients that place a hundred years’ experience with research in forestry secondary to the expense of harvesting. Monster clipping machines now wander across the landscape, mowing forests in acre increments, building clearcuts on clearcuts no matter what science and common sense might dictate.

Robert Repetto, an economist on the staff of the World Resources Institute, examines in lucid detail the economic forces that are currently leading to deforestation around the world. Most such gradients are artificial, sometimes flagrantly corrupt, often established or condoned by governments for the advantage of some favored few at public expense. One of the most flagrant examples is the practice of the U.S Forest Service of supporting “logging on over a hundred million acres of national forests that are economically unfit for sustained timber production. production... The timber is sold at prices that do not cover governmental expenses, with a cost to the taxpayers of a hundred million dollars a year."