PAUL R. EHRLICH
Professor of Biological Sciences, Stanford University, Stanford, California
Discussions of the current extinction crisis all too often focus on the fates of prominent endangered species, and in many cases on deliberate overexploitation by human beings as the cause of the endangerment. Thus black rhinos are disappearing from Africa, because their horns are in demand for the manufacture of ceremonial daggers for Middle Eastern puberty rites; elephants are threatened by the great economic value of ivory; spotted cats are at risk because their hides are in demand by furriers; and whales are rare because, among other things, they can be converted into pet food.
Concern about such direct endangerment is valid and has been politically important, because public sympathy seems more easily aroused over the plight of furry, cuddly, or spectacular animals. The time has come, however, to focus public attention on a number of more obscure and (to most people) unpleasant truths, such as the following:
The primary cause of the decay of organic diversity is not direct human exploitation or malevolence, but the habitat destruction that inevitably results from the expansion of human populations and human activities.
Many of the less cuddly, less spectacular organisms that Homo sapiens is wiping out are more important to the human future than are most of the publicized endangered species. People need plants and insects more than they need leopards and whales (which is not to denigrate the value of the latter two).
Other organisms have provided humanity with the very basis of civilization in the form of crops, domestic animals, a wide variety of industrial products, and many important medicines. Nonetheless, the most important anthropocentric reason for preserving diversity is the role that microorganisms, plants, and animals play in providing free ecosystem services, without which society in its present form could not persist (Ehrlich and Ehrlich, 1981; Holdren and Ehrlich, 1974).
The loss of genetically distinct populations within species is, at the moment, at least as important a problem as the loss of entire species. Once a species is reduced to a remnant, its ability to benefit humanity ordinarily declines greatly, and its total extinction in the relatively near future becomes much more likely. By the time an organism is recognized as endangered, it is often too late to save it.
Extrapolation of current trends in the reduction of diversity implies a denouement for civilization within the next 100 years comparable to a nuclear winter.
Arresting the loss of diversity will be extremely difficult. The traditional "just set aside a preserve" approach is almost certain to be inadequate because of factors such as runaway human population growth, acid rains, and climate change induced by human beings. A quasi-religious transformation leading to the appreciation of diversity for its own sake, apart from the obvious direct benefits to humanity, may be required to save other organisms and ourselves.
Let us examine some of these propositions more closely. While a mere handful of species is now being subjected to purposeful overexploitation, thousands are formally recognized in one way or another as threatened or endangered. The vast majority of these are on the road to extinction, because humanity is destroying habitats: paving them over, plowing them under, logging, overgrazing, flooding, draining, or transporting exotic organisms into them while subjecting them to an assault by a great variety of toxins and changing their climate.
As anyone who has raised tropical fishes knows, all organisms require appropriate habitats if they are to survive. Just as people cannot exist in an atmosphere with too little oxygen, so neon tetras (Paracheirodon innesi) cannot survive in water that is 40F (4.4C) or breed in highly alkaline water. Trout, on the other hand, cannot breed in water that is too warm or too acid. And the bacteria that produce the tetanus toxin cannot reproduce in the presence of oxygen. In order to persist, Bay checkerspot butterflies (Euphydryas editha bayensis) must have areas of serpentine grassland (to support the growth of plants that serve as food for their caterpillars and supply nectar to the adults). Whip-poor-wills, red-eyed vireos, Blackburnian warblers, scarlet tanagers, and dozens of other North American birds must have mature tropical forest in which to overwinter (see Terborgh, 1980, for example). Black-footed ferrets (Mustela nigripes) require prairie that still support the prairie dogs on which the ferrets dine.
This utter dependence of organisms on appropriate environments (Ehrlich, 1986) is what makes ecologists so certain that today's trends of habitat destruction and modification—especially in the high-diversity tropical forest (where at least one-half of all species are believed to dwell)—are an infallible recipe for biological impoverishment. Those politicians and social scientists who have questioned the extent of current extinctions are simply displaying their deep ignorance of ecology; habitat modification and destruction and the extinction of populations and species go hand in hand.
The extent to which humanity has already wreaked havoc on Earth's environments is shown indirectly by a recent study of human appropriation of the products of photosynthesis (Vitousek et al., 1986). The food resource of the animals in all major ecosystems is the energy that green plants bind into organic molecules in the process of photosynthesis, minus the energy those plants use for their own life processes—growth, maintenance, and reproduction. In the jargon of ecologists, that quantity is known as the net primary production (NPP). Globally, this amounts to a production of about 225 billion metric tons of organic matter annually, nearly 60% of it on land.
Humanity is now using directly (e.g., by eating, feeding to livestock, using lumber and firewood) more than 3% of global NPP, and about 4% of that on land. This is a minimum estimate of human impact on terrestrial systems. Since Homo sapiens is one of (conservatively) 5 million species, this may seem an excessive share of the food resource. But considering that human beings are perhaps a million times the weight of the average animal (since the overwhelming majority of animals are small insects and mites) and need on the order of a million times the energy per individual, this share might not be too unreasonable.
Yet human beings can be thought of as co-opting NPP not only by direct use but also by indirect use. Thus if we chalk up to the human account not only the NPP directly consumed, but such other categories as the amount of biomass consumed in fires used to clear land, the parts of crop plants not consumed, the NPP of pastureland (converted from natural habitat) not consumed by livestock, and so on, the human share of terrestrial NPP climbs to a staggering 30%. And if we add to that the NPP foregone when people convert more productive natural systems to less productive ones (such as forest to farm or pasture, grassland to desert, marsh to parking lot), the total potential NPP on land is reduced by 13%, and the human share of the unreduced potential NPP reaches almost 40%. There is no way that the co-option by one species of almost two-fifths of Earth's annual terrestrial food production could be considered reasonable, in the sense of maintaining the stability of life on this planet.
These estimates alone both explain the basic causes and consequences of habitat destruction and alteration, and give reason for great concern about future trends. Most demographers project that Homo sapiens will double its population within the next century or so. This implies a belief that our species can safely commandeer upwards of 80% of terrestrial NPP, a preposterous notion to ecologists who already see the deadly impacts of today's level of human activities. Optimists who suppose that the human population can double its size again need to contemplate where the basic food resource will be obtained.
A standard fool's answer to that question is that indefinite expansion of the human population will be supported by the immeasurable riches of the sea. Unhappily for that notion, the riches of the sea have been quite carefully measured and found wanting. People now use about 2% of the NPP of the sea, and the prospects even for doubling that yield are dim. The basic reason is that efficient harvesting of the sea requires the exploitation of concentrated pools of resources—schools of fishes and larger invertebrates. People cannot efficiently harvest much of the NPP that resides in tiny phytoplankton (the green plants of the sea) or in the zooplankton (animals too small to swim against the currents). Humanity appears to be already utilizing about as much of oceanic NPP as it can on a sustainable basis.
This discrepancy in the ability of Homo sapiens to exploit terrestrial and oceanic NPP is reflected in the general lack of an extinction crisis in the seas. Except for such organisms as some whales and fishes that are threatened by direct exploitation, animals that spend their entire lives in the open sea are relatively secure. Aside from some limited environments, such as certain coral reefs, the effects of habitat destruction are relatively small away from shorelines and estuaries. This situation could, of course, change rapidly if marine pollution increases—a distinct possibility.
The extirpation of populations and species of organisms exerts its primary impact on society through the impairment of ecosystem services. All plants, animals, and microorganisms exchange gases with their environments and are thus directly or indirectly involved in maintaining the mix of gases in the atmosphere. Changes in that mix (such as increases in carbon dioxide, nitrogen oxides, and methane) can lead to rapid climate change and, in turn, agricultural disaster. As physicist John Holdren put it, a carbon dioxide-induced climatic change could lead to the deaths by famine of as many as a billion people before 2020. Destroying forests deprives humanity not only of timber but also of dependable freshwater supplies and furthermore increases the danger of floods. Destruction of insects can lead to the failure of crops that depend upon insect pollination. Extermination of the enemies of insect pests (a usual result of ad lib pesticide spraying) can terminate the pest control services of an ecosystem and often leads to severe pest outbreaks. The extinction of subterranean organisms can destroy the fertility of the soil. Natural ecosystems maintain a vast genetic library that has already provided people with countless benefits and has the potential for providing many, many more.
These examples can be multiplied many fold—the basic point is that organisms, most of which are obscure to nonbiologists, play roles in ecological systems that are essential to civilization. When a population playing a certain role is wiped out, ecosystem services suffer, even if many other populations of the same organism are still extant. If the population of Engelmann spruce trees (Picea engelmanni ) in the watershed above your Colorado home is chopped down, you could be killed in a resulting flood, even though the species of spruce is not endangered. Equally, if that were the last population and it were reduced to just a dozen trees (so that, technically, the species still existed), you would not be spared the flood, and chance events would likely finish off the Engelmann spruce eventually anyway.
In most cases, numerous genetically diverse populations are necessary to ensure the persistence of a species in the face of inevitable environmental changes that occur naturally. The existence of many populations spreads the risk so that unfavorable conditions in one or a few habitats do not threaten the entire species. And the presence of abundant genetic variation within a species (virtually assured if its populations are living in different geographic areas) increases its potential for successfully evolving in response to long-term environmental changes. Today, this genetic diversity within species is declining precipitously over much of Earth's land surface—an unheralded loss of one of humanity's most vital resources. That resource is largely irreplaceable. Along with fossil fuels, rich soils, ancient groundwater, and mineral deposits, genetic diversity is part of the inheritance of capital that Homo sapiens is rapidly squandering.
What then will happen if the current decimation of organic diversity continues? Crop yields will be more difficult to maintain in the face of climatic change, soil erosion, loss of dependable water supplies, decline of pollinators, and ever more serious assaults by pests. Conversion of productive land to wasteland will accelerate; deserts will continue their seemingly inexorable expansion. Air pollution will increase, and local climates will become harsher. Humanity will have to forego many of the direct economic benefits it might have withdrawn from Earth's once well-stocked genetic library. It might, for example, miss out on a cure for cancer; but that will make little difference. As ecosystem services falter, mortality from respiratory and epidemic disease, natural disasters, and especially famine will lower life expectancies to the point where cancer (largely a disease of the elderly) will be unimportant. Humanity will bring upon itself consequences depressingly similar to those expected from a nuclear winter (Ehrlich, 1984). Barring a nuclear conflict, it appears that civilization will disappear some time before the end of the next century—not with a bang but a whimper.
Preventing such a denouement will prove extremely difficult at the very least; it may well prove to be impossible. Earth's habitats are being nickeled and dimed to death, and human beings have great difficulty perceiving and reacting to changes that occur on a scale of decades. Our nervous systems evolved to respond to short-term crises—the potential loss of a mate to a rival, the sudden appearance of a bear in the mouth of the cave. For most of human evolutionary history there was no reason for natural selection to tune us to recognize easily more gradual trends, since there was little or nothing one could do about them. The human lineage evolved in response to changes in the ecosystems in which our ancestors lived, but individuals could not react adaptively to those changes, which usually took place slowly. The depletion of organic diversity and the potential destruction of civilization may, ironically, be an inevitable result of our evolutionary heritage.
If humanity is to avoid becoming once again a species consisting of scattered groups practicing subsistence agriculture, dramatic steps will be necessary. They can only be briefly outlined here. Simply setting aside preserves in the remaining relatively undisturbed ecosystems will no longer suffice. In most part of the planet such areas are too scarce, and rapid climatic changes may make those preserves impossible to maintain (Peters and Darling, 1985). Areas already greatly modified by human activities must be made more hospitable for other organisms; for example, the spewing of toxins into the environment (leading to intractable problems like acid deposition) must be abated.
Above all, the growth of the human population must be halted, since it is obvious that if the scale of human activities continues to increase for even a few more decades, the extinction of much of Earth's biota cannot be avoided. Indeed, since Homo sapiens is now living largely on its inherited capital and in the future will have to depend increasingly on its income (NPP), one can argue persuasively that the size of the human population and the scale of human activities should be gradually reduced below present levels. Reducing that scale will be an especially difficult task, since it means that the environmental impacts of the rich must be enormously curtailed to permit the poor a chance for reasonable development.
Although improvements in the technologies used to support human life and affluence can of course help to ameliorate the extinction crisis, and to a limited extent technologies can substitute for lost ecosystem services, it would be a dangerous miscalculation to look to technology for the answer (see, for example, Ehrlich and Mooney, 1983). In my opinion, only an intensive effort to make those improvements and substitutions, combined with a revolution in attitudes toward other people, population growth, the purpose of human life, and the intrinsic values of organic diversity, is likely to prevent the worst catastrophe ever to befall the human lineage. Curiously, scientific analysis points toward the need for a quasi-religious transformation of contemporary cultures. Whether such a transformation can be achieved in time is problematic, to say the least.
We must begin this formidable effort by increasing public awareness of the urgent need for action. People everywhere should understand the importance of the loss of diversity not only in tropical forests, coastal zones, and other climatically defined regions of the world but also in demographically delineated regions such as areas of urbanization. The geological record can tell us much about catastrophic mass extinctions of the past. That, and more intensive studies of the living biota, can provide hints about what we might expect in the future. At the present time, data on the rates and direction of biodiversity loss remain sparse and often uncertain. As a result, estimates of the rate of loss, including the number and variety of species that are disappearing, vary greatly—in some cases, as pointed out by E. O. Wilson in Chapter 1, by as much as an order of magnitude. Moreover, scientists have also differed in their predictions of the eventual impact that will result from the diminishing biodiversity. Some aspects of these challenges are explored in the following five chapters comprising this section and are reflected throughout this volume.
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Terborgh, J. W. 1980. The conservation status of neotropical migrants: Present and future. Pp. 21–30 in A. Keast, editor; and E. S. Morton, editor. , eds. Migrant Birds in the Neotropics: Ecology, Behavior, Distribution, and Conservation. A symposium held at the Conservation and Research Center, National Zoological Park, Smithsonian Institution. Smithsonian Institution Press, Washington, D.C.
Vitousek, P. M., P. R. Ehrlich, A. H. Ehrlich, and P. M. Matson. 1986. Human appropriation of the products of photosynthesis. BioScience 36(6):368–373.
The major causes of biodiversity decline are land use changes, pollution, changes in atmospheric CO2 concentrations, changes in the nitrogen cycle and acid rain, climate alterations, and the introduction of exotic species, all coincident to human population growth. For rainforests, the primary factor is land conversion. Climate will probably change least in tropical regions, and nitrogen problems are not as important because growth in rainforests is usually limited more by low phosphorus levels than by nitrogen insufficiency. The introduction of exotic species is also less of a problem than in temperate areas because there is so much diversity in tropical forests that newcomers have difficulty becoming established (Sala, et al., 2000).
a. Human population growth: The geometric rise in human population levels during the twentieth century is the fundamental cause of the loss of biodiversity. It exacerbates every other factor having an impact on rainforests (not to mention other ecosystems). It has led to an unceasing search for more arable land for food production and livestock grazing, and for wood for fuel, construction, and energy. Previously undisturbed areas (which may or may not be suitable for the purposes to which they are constrained) are being transformed into agricultural or pasture land, stripped of wood, or mined for resources to support the energy needs of an ever-growing human population. Humans also tend to settle in areas of high biodiversity, which often have relatively rich soils and other attractions for human activities. This leads to great threats to biodiversity, especially since many of these areas have numerous endemic species. Balmford, et al., (2001) have demonstrated that human population size in a given tropical area correlates with the number of endangered species, and that this pattern holds for every taxonomic group. Most of the other effects mentioned below are either consequent to the human population expansion or related to it.
The human population was approximately 600,000 million in 1700, and one billion in 1800. Just now it exceeds six billion, and low estimates are that it may reach 10 billion by the mid-21st century and 12 billion by 2100. The question is whether many ecological aspects of biological systems can be sustained under the pressure of such numbers. Can birds continue to migrate, can larger organisms have space (habitat) to forage, can ecosystems survive in anything like their present form, or are they doomed to impoverishment and degradation?
b. Habitat destruction: Habitat destruction is the single most important cause of the loss of rainforest biodiversity and is directly related to human population growth. As rainforest land is converted to ranches, agricultural land (and then, frequently, to degraded woodlands, scrubland, or desert), urban areas (cf. Brasilia) and other human usages, habitat is lost for forest organisms. Many species are widely distributed and thus, initially, habitat destruction may only reduce local population numbers. Species which are local, endemic, or which have specialized habitats are much more vulnerable to extinction, since once their particular habitat is degraded or converted for human activity, they will disappear. Most of the habitats being destroyed are those which contain the highest levels of biodiversity, such as lowland tropical wet forests. In this case, habitat loss is caused by clearing, selective logging, and burning.
c. Pollution: Industrial, agricultural and waste-based pollutants can have catastrophic effects on many species. Those species which are more tolerant of pollution will survive; those requiring pristine environments (water, air, food) will not. Thus, pollution can act as a selective agent. Pollution of water in lakes and rivers has degraded waters so that many freshwater ecosystems are dying. Since almost 12% of animals species live in these ecosystems, and most others depend on them to some degree, this is a very serious matter. In developing countries approximately 90% of wastewater is discharged, untreated, directly into waterways.
d. Agriculture: The dramatic increase in the number of humans during the twentieth century has instigated a concomitant growth in agriculture, and has led to conversion of wildlands to croplands, massive diversions of water from lakes, rivers and underground aquifers, and, at the same time, has polluted water and land resources with pesticides, fertilizers, and animal wastes. The result has been the destruction, disturbance or disabling of terrestrial ecosystems, and polluted, oxygen-depleted and atrophied water resources. Formerly, agriculture in different regions of the world was relatively independent and local. Now, however, much of it has become part of the global exchange economy and has caused significant changes in social organization.
Earlier agricultural systems were integrated with and co-evolved with technologies, beliefs, myths and traditions as part of an integrated social system. Generally, people planted a variety of crops in different areas, in the hope of obtaining a reasonably stable food supply. These systems could only be maintained at low population levels, and were relatively nondestructive (but not always). More recently, agriculture has in many places lost its local character, and has become incorporated into the global economy. This has led to increased pressure on agricultural land for exchange commodities and export goods. More land is being diverted from local food production to “cash crops” for export and exchange; fewer types of crops are raised, and each crop is raised in much greater quantities than before. Thus, ever more land is converted from forest (and other natural systems) for agriculture for export, rather than using land for subsistence crops. The introduction of monocropping and the use of relatively few plants for food and other uses – at the expense of the wide variety of plants and animals utilized by earlier peoples and indigenous peoples – is responsible for a loss of diversity and genetic variability. The native plants and animals adapted to the local conditions are now being replaced with “foreign” (or “exotic”) species which require special inputs of food and nutrients, large quantities of water. Such exotic species frequently drive out native species. There is pressure to conform to crop selection and agricultural techniques – all is driven by global markets and technologies.
e. Global warming: There is recent evidence that climate changes are having effects on tropical forest ecology. Warming in general (as distinct from the effects of increasing concentrations of CO2 and other greenhouse gases) can increase primary productivity, yielding new plant biomass, increased organic litter, and increased food supplies for animals and soil flora (decomposers). Temperature changes can also alter the water cycle and the availability of nitrogen and other nutrients. Basically, the temperature variations which are now occurring affect all parts of forest ecosystems, some more than others. These interactions are unimaginably complex. While warming may at first increase net primary productivity (NPP), in the longer run, because plant biomass is increasing, more nitrogen is taken up from the soil and sequestered in the plant bodies. This leaves less nitrogen for the growth of additional plants, so the increase in NPP over time (due to a rise in temperature or CO2 levels) will be limited by nitrogen availability. The same is probably true of other mineral nutrients. The consequences of warming-induced shifts in the distribution of nutrients will not be seen rapidly, but perhaps only over many years. These events may effect changes in species distribution and other ecosystem processes in complex ways. We know little about the reactions of tropical forests, but they may differ from those of temperate forests.
In tropical forests, warming may be more important because of its effects on evapotranspiration and soil moisture levels than because of nutrient redistribution or NPP (which is already very high because tropical temperatures are close to the optimum range for photosynthesis and there is so much available light energy). And warming will obviously act in concert with other global or local changes – increases in atmospheric CO2 (which may modify plant chemistry and the water balance of the forest) and land clearing (which changes rainfall and local temperatures), for examples. (For an excellent discussion of these issues, see Shaver, et al., 2000.)
Root, et al.(2003) have determined that more than 80%of plant and animal species on which they gathered data had undergone temperature-related shifts in physiology. Highland forests in Costa Rica have suffered losses of amphibian and reptile populations which appear to be due to increased warming of montane forests. The golden toad Bufo periglenes of Costa Rica has become extinct, at least partly because of the decrease in mist frequency in its cloud forest habitat. The changes in mists appear to be a consequence of warming trends. Other suspected causes are alterations in juvenile growth or maturation rates or sex ratios due to temperature shifts. Parmesan and Yohe (2003), in a statistical analysis, determined that climate change had biological effects on the 279 species which they examined.
The migratory patterns of some birds which live in both tropical and temperate regions during the year seem to be shifting, which is dangerous for these species, as they may arrive at their breeding or wintering grounds at an inappropriate time. Or they may lose their essential interactions with plants which they pollinate or their insect or plant food supplies. Perhaps for these reasons, many migratory species are in decline, and their inability to coördinate migratory clues with climatic actualities may be partly to blame. The great tit, which still breeds at the same time as previously, now misses much of its food supply because its plant food develops at an earlier time of year, before the birds have arrived from their wintering grounds. Also, as temperatures rise, some bird populations have shifted, with lowland and foothill species moving into higher areas. The consequences for highland bird populations are not yet clear. And many other organisms, both plant and animal, are being affected by warming.
An increase in infectious diseases is another consequence of climate change, since the causative agents are affected by humidity, temperature change, and rainfall. Many species of frogs and lizards have declined or disappeared, perhaps because of the increase in parasites occasioned by higher temperatures. As warming continues, accelerating plant growth, pathogens may spread more quickly because of the increased availability of vegetation (a “density” effect) and because of increased humidity under heavier plant cover. As mentioned above, the fungus Phytophtora cinnamoni has demolished many Eucalyptus forests in Australia. In addition, the geographical range of pathogens can expand when the climate moderates, allowing pathogens to find new, nonresistant hosts. On the other hand, a number of instances of amphibian decline seem to be due to infections with chrytid fungi, which flourish at cooler temperatures. An excellent review of this complex issue may be found in Harvell, et al., (2002).
There may be a link between augmented carbon dioxide levels and marked increase in the density of lianas in Amazonian forests. This relationship is suggested by the fact that growth rates of lianas are highly sensitive to CO2 levels. As lianas become more dense, tree mortality rises, but mortality is not equal among species because lianas preferentially grow on certain species. Because of this biodiversity may be reduced by increased mortality in some species but not others (Phillips, et al., 2002).
f. Forest fragmentation: The fragmentation of forests is a general consequence of the haphazard logging and agricultural land conversion which is occurring everywhere, but especially in tropical forests. When forests are cut into smaller and smaller pieces, there are many consequences, some of which may be unanticipated.
i) Fragmentation decreases habitat simply through loss of land area, reducing the probability of maintaining effective reproductive units of plant and animal populations. Most tropical trees are pollinated by animals, and therefore the maintenance of adequate pollinator population levels is essential for forest health. When a forest becomes fragmented, trees of many species are isolated because their pollinators cannot cross the unforested areas. Under these conditions, the trees in the fragments will then become inbred and lose genetic variability and vigor. Other species, which have more wide-ranging pollinators, may suffer less from fragmentation. For instance, the pollen of several species of strangler figs (the fruit of which is an essential element in the diets of many animals) is dispersed by wasps over distances as great as 14.2 km (Nason, Herre, & Hamrick, 1998). Thus “breeding units” of these figs are extremely large, comprising hundreds of plants located in huge areas of forest. Isolated fig populations seem to survive and help to maintain frugivore numbers (if not diversity), so long as the number of trees within the range of the wasps does not fall below a critical minimum.
Most species are not so tolerant, however. Animals, particularly large ones, cannot maintain themselves in small fragmented forests. Many large mammals have huge ranges and require extensive areas of intact forest to obtain sufficient food, or to find suitable nesting sites. Additionally, their migrations may be interrupted by fragmentation. These animals are also much more susceptible to hunting in forest fragments, which accounts for much of the decline in animal populations in rainforests. Species extinctions occur more rapidly in fragments, for these reasons, and also because species depend upon each other. The dissection of forests into fragments in certain parts of the Amazon has led to extreme hunting pressures on peccaries, for instance, and in some places where they are locally extinct, three species of frogs have also disappeared, since they depended upon peccary wallows for breeding ponds. The absence of large predator species leads to imbalances in prey populations, and, since many of the prey species are seed-eaters, to declines in the population levels of many plant species. The prey, now at high population levels, consume most available seeds, leaving few to germinate. On small islands created after dam construction on the Chagres River in Panama, even large seed predators could not survive, and after 70 years, the former mixed tropical forest has become a forest of large-seeded plants only (Terborgh, 1992b). As Terborgh states, and we should attend to this lesson, “Distortions in any link of the interaction chain will induce changes in the remaining links.” (p. 289)
ii) When forests are cut down or burned, the resulting gaps are too large to be filled in by the normal regeneration processes. This permits the ascendancy of rapid-growing, light-tolerant species and grasses. Large gaps may then be converted to scrub or grassland.
iii) The “edge” effect: The cutting of forest into fragments creates many “edges” where previously there was deep forest. Many effects are consequent upon this. Edges are lighter, warmer and windier than the forest interior. These changes in microclimate alter plant reproduction, animal distribution, the biological structure and many other features of the forest. Tree mortality is much greater near edges, and climax species will be replaced by pioneer species. These effects can be seen as far as one kilometer into the forest. The drier and warmer conditions also make the fragment more flammable, with a concomitant increase in the frequency of fires. Without further stress, the forest may regenerate. However, if the fragment is surrounded by a human-dominated landscape, it may be inhibited from regeneration. This has occurred in certain areas of Brazil, where forest fragments are surrounded by sugar cane and Eucalyptus plantations. (For a discussion of edge effects, see Gascon, Williamson & da Fonseca, 2000). Thus, species requiring large areas of undisturbed primary forest are sacrificed to the benefit of those species which can exist on forest margins.
iv) Fire is particularly frequent in fragments. Recently, many forests have been subjected to deliberately-set and accidental fires, to which they have little resistance, and to which they are rarely naturally subjected. People often set fire to cut-over areas adjacent to forests to clear them of debris. These fires often get out of control and burn large areas, extend into the forest interior, and inhibit edge regeneration by killing pioneer forest vegetation. More than 90% of forest fires in certain eastern Amazon forest areas were associated with the edges of forest fragments (Wuethrich, 2000). If conditions remain severe, the forest will recede and be replaced by scrub.
v) The use of herbicides and the introduction of exotic species into areas surrounding forest fragments are detrimental to forest health. Herbicides blow from cleared agricultural areas into forests, and exotic species introduced by farmers and ranchers spread, often displacing native species. These exotic organisms interrupt the forest ecosystem and, since they have few or no natural enemies in their new environment, they are difficult to eradicate. According to Vitousek (1997), there are many islands where fewer than half of the species are native, and in many other terrestrial environments, more than 20% of species are foreign. These invasions drive the loss of indigenous species.
vi) For unknown reasons, fragmentation leads to the death of large canopy trees, even in the interior of fragments. Canopy trees dominate the forest structure, and they provide fruits and shelter for many animals. The mortality of trees in fragmented patches in Brazil has been found to be twice that of similar trees in the forest interior (Laurance, et al., 2000). Not only that, but tree mortality is confined disproportionately to large trees (an almost 40% increase in mortality). Large trees may be more vulnerable in fragmented forests because they are not as well buffered from wind and natural forces, because there are more tree parasites (lianas), and because they are more subject to dessication at forest edges. Loss of these largest trees has several corollary effects – the alteration of biogeochemical cycles (transpiration, carbon cycles), the reduction of species complexity, and the reduction of fecundity. As mentioned above, large trees are essential habitats and food sources for many other organisms, both plant and animal; they are the source of much of the primary productivity of the forest; and they are responsible for many effects on the water and nutrient cycles. They are irreplaceable in the forest ecosystem.
vii) The fragmentation of forests by logging and agricultural conversion also exaggerates the probability of major epidemics. Pathogens introduced through human activities by land use practices in areas surrounding the forest can be lethal to forest plants and animals.
viii) Rainforests are losing species, not only because of the disappearance of their habitat, but also because essential ecological processes are being interrupted by fragmentation. Fragments are much more easily accessible to human incursions than are intact forests. This leads to a variety of extractive activities within the forest interior. Intensive hunting, by depleting animal populations, inhibits plant reproduction, since many seeds can neither be dispersed, nor flowers be pollinated without them. Where these seed dispersers have been eliminated, are at low population densities, or cannot move between forest fragments, seed dispersal will be very limited, and as a result tree species dependent upon animal dispersers may become locally extinct. In the remnants of the Atlantic forest of Brazil, the seeds of 71% of tree species are dispersed by vertebrates (birds and mammals), and about 48% of these dispersers are birds which are deep-forest dwellers. As this forest becomes more and more fragmented, these birds are disappearing, so eventually the trees dependent upon them will be unable to replace themselves. In some fragments, all large vertebrates (including seed-eaters) have been hunted to extinction, and in some places the fragments are so distant from each other that these animals cannot pass from one to another. The Alagoas curassow, a large fruit-eating bird of this area, is now extinct; many others (toucans, aracaris, guans) are endangered by hunting pressures. Other species are sensitive to disturbance of their environment and they become locally extinct. The tiny bits of Atlantic forest remaining are becoming dominated by trees which are wind or water-pollinated or whose seeds are dispersed by animal species which can tolerate disturbed habitats or edges (“edge species”) (Cardoso da Silva and Tabarelli, 2000).
In addition, in fragmented forests, seeds will frequently land in deforested areas (where they are in the open, and exposed to heat, light and desiccation) in which they cannot germinate, and the seedlings cannot survive. In Brazil, three to seven times as many Heliconia acuminata seedlings planted in continuous areas of forest germinated as compared to those planted in fragmented areas (Bruna, 1999). Whatever the explanation for the lower rate of seedling germination in fragmented forests, whether due to inbreeding or other causes, fewer and fewer individuals in fragments grow to adulthood. Those which do will breed, but since populations are small, inbreeding occurs and the downward spiral continues until the population becomes locally extinct. This effect is seen frequently in forest fragments.
g. Hunting, fishing, and gathering: Many forests which appear intact are in fact “empty forests,” since most large animals have been hunted to unsustainable levels. These animals are mainly hunted for meat, but also for skins (jaguar, ocelot) or medicinal/chemical properties (poison-arrow frogs, collected to provide poisons for arrow tips, and the midwife toad, which in the Amazon is thought to have medicinal value). Turtles are heavily harvested for meat and their eggs are collected for food almost everywhere in the tropics and subtropics. Asian tropical freshwater turtles are in serious decline because they are extensively hunted for food or for use in traditional Chinese medicines. Thousands of tons of live turtles are caught or sent to China annually, a completely unsustainable level of collection. There are apparently no turtles left in the wild in Vietnam for this reason (Gibbons, et al., 2000). More than 80 species of Asian turtles are at such low population numbers that they will become extinct unless emergency measures – restrictions on international trade, increased habitat protection, captive breeding programs – are taken immediately.
Some of the hunting is done for subsistence purposes by villagers; some by farmers, miners and loggers, who live in the forest and use forest animals as a major food source; some by commercial hunters to supply urban markets. This is a major source of income in many rural tropical areas. In Gabon alone, a tiny country, 3,600 tons of bushmeat are consumed annually (Tuxill, 1998). The popularity of “bushmeat” in cities and towns located in or close to rainforests is rising. Surveys of bushmeat consumption in Bolivia and Honduras showed that people will eat more bushmeat as their income rises, but when they become more affluent, consumption declines. Consumption declines as well when bushmeat prices rise and the price of alternative sources of protein declines (Wilkie and Godoy, 2000). Part of the remedy for overexploitation of wildlife resources, then, lies with improving the income levels of local residents (so bushmeat becomes less attractive as a protein source), in increasing the costs of hunting, and in lowering the prices of alternative protein sources.
Many animals are trapped for the pet trade (tropical fish, birds, reptiles, monkeys) or for zoos or medical research. A 25-week survey of the Bangkok weekend market found specimens of 225 species of birds (most “protected” by government decree) for sale (Sponsel, Bailey and Headland, 1996). Other animals are trapped for their hides or furs, and some are killed because they live too close to human habitation and impinge on human activities. For instance, ocelots and other small carnivores may be shot when they attempt to prey on chickens or other domestic animals.
Many tropical animals are hunted mercilessly for their value in traditional Asian medicines. Tigers, bears, deer, snakes, and many other animals are near extinction in many places because of this trade. Tigers in India are almost gone and only 3-5000 tigers still exist in the wild anywhere (Tuxill, 1998). Many of these animals, or their parts, are smuggled illegally from Southeast Asian (and other) countries to China and other countries with large Chinese populations for these uses.
The effects of hunting are not just on the animals “taken.” Many animals which are human prey eat fruits and seeds, and are major seed dispersers in tropical forests (see above), and the seeds of certain species of trees must pass through the gut of an animal in order to germinate. In these ways many tropical plants and trees depend upon animals, for, without them, they will not be able to reproduce. For instance, the seeds of Inga ingoides, a South American tree, are dispersed widely by the spider monkey Ateles paniscus. Where this monkey is locally extinct (due to hunting pressures), the trees do not “outbreed”; the seeds fall to the forest floor and patches of seedlings of low genetic diversity surround the parent trees (Moore, 2001). This can be very detrimental to forests, which generally have high genetic diversity, because more homogeneous plants are generally less fit. The loss of elephants in African countries due to hunting has led to a loss of reproductive ability in many valuable tree species (Tuxill, 1998).
Fish and aquatic animals are killed indiscriminately by fishing techniques which employ insecticides and/or dynamite. These techniques not only catch the few desired specimens, but kill all of the other animals in the area. Commercial fishing operations are not sensitive to issues of sustainability. They catch as many marketable fish as possible, and intensify their efforts when fish populations drop (declines due in the first place to overfishing). Such unsustainable fishing operations have led and are leading to severe declines in fish in major river systems within tropical rainforests (see the case of the tambaqui, Part II, F3, b, ia).
Fragmentation may be more serious than previously imagined because the consequences of fragmentation are not static, but progressive. The edges of cut areas do not remain “in place” but gradually recede, further reducing the size of the fragments. Eventually fragments may disappear altogether or undergo ecological collapse.
Why do people heedlessly decimate the precious biodiversity of their planet? Some of them feel they have no economic alternative, while others are driven by the desire for short-term profit. Still others are uncomprehending. Unfortunately, so much of the depredation which is being inflicted upon areas of great biodiversity is, in the long run, and often in the short run, in vain. While tropical forests now occupy less than half of their former range, and much of what remains is damaged or fragmented, the net profit to humanity is slight. Clearing of tropical forests has provided only a relatively small percentage of total agricultural land, since much of the land converted for farms becomes rapidly degraded and is abandoned. Logging results in a one-time profit, mainly to large companies. Ranching is an activity which, on former rainforest land, is uneconomical, requires subsidizing, and is eventually abandoned. But the damage is permanent and the forest irreplaceable, so forest destruction has dire consequences. It degrades aquatic fisheries, causes floods and has many other consequences (see below) – so much harm for so little benefit.