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.