Why are rainforests so diverse?
By Rhett A. Butler
April 1, 2019
RAINFOREST DIVERSITY - ORIGINS AND IMPLICATIONS
Although they cover less than 2 percent of Earth's surface, rainforests house an estimated 50 percent of all life on the planet's land masses.
No one knows exactly how many species live in the world's tropical rainforests — estimates range from 3 to 50 million species — rainforests are the undisputed champions of biodiversity among the world's ecosystems, containing far higher numbers of species on a per-area basis relative to sub-tropical, temperate, and boreal ecosystems. For example, whereas temperate forests are often dominated by a half dozen tree species or fewer that make up 90 percent of the trees in the forest, a tropical rainforest may have more than 480 tree species in a single hectare (2.5 acres). A single bush in the Amazon may have more species of ants than the entire British Isles. This diversity of rainforests is not a haphazard event, but is the result of a series of unique circumstances.
What is biodiversity?
Biodiversity -- short for biological diversity -- is the the number and types of organisms in an habitat, ecosystem, region or environment. It can refer to genetic, species, or habitat variation at any scale.
CLIMATE AND BIODIVERSITY
The hot and humid climate plays an important role in rainforest variety. As a general rule, diversity and ecosystem productivity increase with the amount of solar energy available to the system. Sunlight is captured in the leaves of canopy plants via photosynthesis, converted into simple sugars, and transferred throughout the forest energy system as the leaves and fruit are eaten or decomposed by various organisms. The primary measure of ecosystem net primary production is the fixation of carbon by plants. Tropical rainforests have the highest mean net primary production of any terrestrial ecosystem, meaning an acre of rainforest stores more carbon than an acre of any other vegetation type. The humid climate adds another ingredient essential to rich diversity: water.
The stable tropical rainforest environment promotes diversity by allowing plants and animals to interact all year round without needing to develop protection against cold or frost. In addition, because the sun shines all year long providing plants with the energy to manufacture food via photosynthesis there is no seasonal food shortage in the ecosystem. The abundant food source for plants (sunlight) is passed up through the system to herbivores, which consume the plant leaves, seeds, and fruits, to carnivores which consume the herbivores. Over the course of millions of years, with abundant food, rainforest species have adapted to take full advantages of all the available niches.
Millions of years of battle between predator and prey have resulted in an extensive array of defenses, weapons, and specializations. Camouflage, mimicry, specialized breeding and feeding habits, symbiotic relationships with other species, and other complex adaptations have allowed species to out-compete rivals by making use of resources not available to generalists. Virtually no niche in the rainforest is unfilled and many different species can coexist in a relatively small area, without encroaching on their neighbors. The evolutionary process continues and species are pushed into narrower and narrower niches until they are unbelievably specialized to their particular way of life.
This evolutionary process ensures that no one well-adapted species (i.e. beetle) dominates the whole population of beetles because that one species cannot be possibly adapted to all the niches available in the forest. As a generalist, the species would be quickly out-competed by more specialized species. Generalists appear to thrive most under disturbed conditions, such as areas cleared for agriculture. Here these "weedy" species may be quite common. Furthermore, any species abundant in natural forest faces the threat that a predator would adapt to exploit its abundance. For example, the failure of rubber tree (Hevea brasiliensis) plantations in the Amazon is due to leaf blight. In the ordinary rainforest, rubber trees are widely dispersed so blight can never wipe out more than one individual tree at a time.
Tropical rainforests are markedly different from temperate forests. In temperate regions many plant and animal species have wide distributions, and a forest may consist of a half dozen or so tree species. In contrast, tropical species have evolved to fit narrow niches in a relatively constant environment, producing grandiose diversity. For example, more than 480 tree species have been identified in a single hectare of tropical rainforest.
Visitors to the rainforest are often disillusioned by what they see because they confuse the word "diversity" with "abundance." They visit the rainforest expecting to see ten jaguars, dozens of iguanas lying on the lodge patio, and large toucans waiting for them with breakfast. You will not encounter giant herds of wildebeest or zebra as on the African savanna. Nor will you find an eruption of flowers or even an abundance of colorful birds. Life in the rainforest is strikingly subtle.
Rainforests are diverse, in terms of numbers of species, but any one given species is not necessarily plentiful. Some rainforest species have populations that number in the millions, whereas others may consist of a handful of individuals. The biology of tropical rainforests is a biology of rare species. The reason for this occurrence is that the majority of rainforest species are scarce over the range of the forest and may be common in only a few small areas where they are particularly well adapted. A certain species may be quite common in one area but exceedingly rare only 500 yards away, where it is replaced by another similar, but distinct, species. There are a few common species found in scattered patches and a great number of rare species scattered throughout a forest. Some of these species are extremely rare and on the verge of extinction, especially where the forest has been disturbed. The reason for this pattern is that many species are highly specialized to fit a particular niche. Where that niche exists, that species may have a large population and constantly produce offspring that head off to colonize new areas. However, the colonizers almost always fail, because they cannot compete with the specialized species of other areas. Thus these colonizers are rare in the areas where they try to establish a foothold.
CANOPY STRUCTURE AND BIODIVERSITY IN THE RAINFOREST
The canopy system characteristic of tropical rainforests further increases diversity by creating new niches in the form of new sources of food, new shelters, new hiding places, and new areas for interaction with other species. In fact, it is estimated that 70-90 percent of life in the rainforest is found in the trees. One of the best examples of a canopy niche which multiplies diversity are the epiphytes, many of which form tiny ecosystems of their own. The tank bromeliads of New World forests can hold over eight liters (two gallons) of water in catchments formed in their stiff, upturned leaves. These pools of water serve as nurseries for frog tadpoles and insect larvae specifically adapted to life in this tiny obscure niche, and provide water for millions of other canopy dwellers. Over 28,000 epiphyte species are known to science, although many more have never been catalogued.
In addition to epiphytes, other plant species including lianas and creepers, create new means for ground-dwelling animals to access the resources of the canopy. Many of the ground-dwelling animals of the temperate zone, like porcupines, kangaroos, anteaters, earthworms, and crabs, have moved up into the canopy in tropical regions.
AREA AND BIODIVERSITY IN THE RAINFOREST
The size of a habitat is another factor in the great diversity of the rainforest. Area increases diversity because a larger plot is likely to have more habitats, hence niches, to support a greater variety of species. In addition, many species require a large range for adequate prey or seed forage. The basis for this idea was set forth by MacArthur and Wilson in The Theory of Island Biogeography (1967) using small islands in the Florida Keys. Soon after the work was published, research focused on whether island biogeography could be applied to fragments of habitat. Evidence for this concept was found in an experiment devised by Thomas Lovejoy in the late 1970s. The experiment was known as the Minimum Critical Size of Ecosystems Project and measured ecosystem decay in forest patches ranging in size from 2.5 acres (1 hectare) to 2,500 acres (1,000 hectares). During the late 1970s the Brazilian government was encouraging widespread clearing of rainforest by offering tax incentives to landowners. However, in an area known as the Manaus Free Zone, just north of the Amazonian city of Manaus, the government required that 50 percent of the forest on a developed area must be saved. Lovejoy used this stipulation for his experiment, convincing landowners to leave their required forest patches in neatly cut squares.
The experiment, today known as the Biological Dynamics of Forest Fragments Project, found that the most seriously degraded forest with the least diversity were the smallest, one- hectare reserves, while the reserves that retained the most diversity were the ones of the largest area. In the smaller reserves, drying winds reached the interior, affecting tree species and resulting in more tree falls. Gaps in the canopy allowed more sunlight to reach the forest floor, further altering the understory microclimate and causing changes in the makeup of resident species. Larger herbivores left the patches since the limited number of trees could not provide sustenance, soon followed by predators, which could not cope with the loss of prey. The loss of predators caused an imbalance in the food chain, and the populations of small herbivores and omnivores increased, adding pressure on forest seed banks and impairing the reproducing ability of forest trees. Troops of army ants could not be supported by meager forest patches and they too left, along with the bird, butterfly, and other insect species that depended on the troop. Shade- loving plants and animal species died off as more sunlight penetrated the diminished canopy, and "gap" species, like vines and certain bird and insect species, proliferated. These losses continued to set off a chain reaction that caused profound changes in the system, eventually resulting in its collapse.
Similar experiments carried out around the world have yielded similar results (although in some cases diversity among certain groups may actually increase). The colonization of forest patches by forest- edge species, light-gap specialists, and savanna species can counter the loss of species less tolerant of the changed forest and maintain the diversity of the patch. In some cases, forest fragment diversity may hold steady, but overall (global) diversity declines as some unique species lost from the forest patch are not replaced. Floor- dwelling species appear more affected by forest fragmentation than canopy species. Declining biodiversity in accordance with decreasing land area is an important trend to consider for conservation (see section 10).
In global studies, larger forest patches lost fewer of their species. Diversity declined but at a rate and to a degree inversely proportional to the size of the patch. In other words, the larger the patch, the more organisms survived and were successful in reproducing. Thus these experiments demonstrated that the area of an ecosystem directly affects biodiversity.
SOIL AND BIODIVERSITY IN THE RAINFOREST
The soils of a rainforest affect the diversity of the forest. Although nearly 70 percent of tropical rainforest exists on poor acidic soils, it retains its fertility in a large part thanks to nutrient recycling and other processes. However, in some areas, soils are so poor that only a limited number of tree species can grow (though these forests are still highly diverse by temperate standards). One example is the so-called "white-sand" or "blackwater" forests that grow on rocky, sandy soils. Some of these forests grow on nothing but rocks and the roots of other trees. Trees that grow under these conditions tend to be species with tannins in their leaves, which in turn, turn local rivers into "blackwater" rivers. The bitter tannins in their leaves limit insect populations, thus reducing the number of animals the forest can support (insects serve as a major food source for larger animals in most rainforests). These "blackwater" forests are self-perpetuating, since the "blackwater" rivers that result from the decay of their leaves only make the soils more acidic and prevent other tree species from growing on the already nutrient-lacking soils.
Forest tree diversity, and hence total diversity, may also be reduced in forests with soggy soils like those of the igapò or "swamp forest." The limited number of tree species like Cecropia and palms that can tolerate these wet soil conditions means that these few trees species tend to dominate these areas. Subsequently only the animals that feed on their fruits, leaves, and seeds are abundant in these areas.
High-diversity forests are often found on nutrient rich—sometimes volcanic— soils that are well-drained. These forests are frequently found in areas protected from major disturbances like strong wind and regular flooding.
SHORT-TERM VARIATION AND BIODIVERSITY IN THE RAINFOREST
Rainforests and their diversity do not exist in a constant state, but are the product of a series of impacts including fires, tree falls, small-scale human clearing, and even lava flows. These events can increase forest diversity by giving new species a chance to grow in the absence of the towering canopy trees. The growth of new tree species spells new opportunities for their symbiotic species (for example new pollinators or seed dispersers).
Forests that are regularly stressed, like those affected seasonally by strong winds and storms, tend to be dwarfed with a less developed canopy and reduced diversity. "Typical" tall rainforests are typically found where they are protected from strong winds, as in valleys and certain geographical areas.
Within a relatively small area there can be great variations in forest dynamics. For example, in the terra firme rainforests of the Central Amazon—where average canopy tree age can exceed 300 years and some trees can be more than one thousand years old— forest turnover rates can be extremely low. In contrast, nearby floodplain forests may have turnover rates of less than 70 years due to migrating river channels that periodically undercut river banks and trees.
Diversity is usually sharply reduced in forests degraded by activities such as logging, burning, and agricultural development. Generally, when forest is logged, the dense canopy structure is disturbed, allowing more sunlight to penetrate to the forest floor. The forest is more likely to dry out, and less water can be recycled through the system of evaporation and transpiration. Many rainforest species are unable to cope with the changes in the forest microclimate and either move on or gradually perish. In addition, the loss of certain valuable hardwood trees to logging has a major impact on species with which they have interdependent relationships. Studies suggest that logging in any form reduces tropical forest diversity—studies around the world show declines of certain species, especially primates, birds, and insects in degraded forests. While there may be a local increase in the abundance and diversity of certain species, there is an overall regional or global decline in biodiversity due to the loss of species specially adapted to the conditions of undisturbed forest. Degraded forest is also more prone to be developed or burned by humans, severely reducing diversity. Heavy logging in the forests of Indonesia and Brazil was partly responsible for creating the dry forest conditions that drove the widespread forest fires of 1997-1998.
ICE AGES AND ECOTONES AND BIODIVERSITY IN THE RAINFOREST
Recent studies suggest that ecotones, transition zones between habitats, play an important role in the biodiversity of rainforests. Ecotones bordering rainforests and savannas, secondary forests, plantations, and other forest types are evolutionary hotbeds where evolutionary competition may lead to the rise of new species. Scientists say that populations in ecotones may specialize to the niche and diverge significantly from populations of the interior of the forest. This new theory initially appears to challenge the popular view that the ice ages had a highly significant role in rainforest diversity. However, some scientists speculate that the receding forest and fragmentation of the ice ages would create a larger area of ecotones, contributing further to biodiversity. Therefore the combination of both conditions may have contributed to the well- known diversity of tropical rainforests.
The relative age of a tropical rainforest plays a role in its diversity, although the role is still largely debated. Tropical rainforests are probably the planet's oldest continuous ecosystems. Tropical rainforests began to take their form some 140 million years ago during the age of the dinosaurs, the late Cretaceous. It was during this period, when much of the world's climate was tropical or sub-tropical, that flowering plants originated and later spread across the globe.
Over their long history, species have come and gone, communities have been destroyed and reformed, and entire systems altered. Along with the changes, new relationships within the system form as new species emerge. Generally the changes are relatively slow, although there have been times of upheaval where drastic change occurred over a short period of time. These natural upheavals appear to foster an increase in biological diversity as evidenced by the effect of the ice ages, especially on the Malay Archipelago in Southeast Asia.
Today, many of the 20,000 or so islands of the Malay Archipelago are covered with tropical forest. Some of these rainforests have existed in some form or another for the past 100 millions years, although, as discussed in section one, the ancient forests had fewer large mammals and no flowering plants. When the ice ages came and ocean waters condensed or became locked up in polar ice, the floor of the shallow South China Sea was exposed, allowing the crossover of species from mainland Asia. Although this region was less affected by the temperature drop than other areas because of its proximity to the ocean and the equator, the climate cooled significantly enough to cause tropical rainforest to recede to scattered patches. The areas formerly forested with tropical rainforest gave way to savannas and montane forest ecosystems. Most of the region had a distinct short rainy season.
When the ice ages came to an end, a warmer climate returned and the ocean rose again to re-flood the shallow areas of the South China Sea. Many of the plants and animals that had crossed over from the mainland were trapped on the reformed island habitats. In addition, some of the montane and more temperate species adapted to the gradually warming climate and became tropical species. The small pockets of tropical rainforest that survived the ice ages served as biological reservoirs to repopulate the expanded tropical forest zone. Some of the tropical species that had been separated into different pockets had radiated enough during their isolation that when they did again cross paths, their habits and physiological features had changed enough (adapted to their niche within the tropical pocket) that they could no longer breed, and could be considered distinct species.
Diversity was again multiplied by subsequent ice ages which caused isolation and subsequent adaptive radiation into more distinct species. For example, take a hypothetical elephant species that began as a single species on mainland Asia. During the ice ages, it expanded its range to some of the islands of the Malay Archipelago, which, with the lower ocean levels, had become connected to the mainland. When the ice ages came to an end, elephants became stranded on the islands. On the smaller islands, those elephants with a smaller body size tended to survive and reproduce more successfully because their lower dietary requirements could be sustained by the smaller amount of food available on the island. The larger individuals tended to be less successful reproductively. Thus evolution favored the dwarfing of elephants on the islands over the course of several thousand years and when the next ice-age crossover came, the island elephants would not breed with the mainland elephants. Since the island elephants filled a different role on the Asian mainland during the crossover, some dwarfed island elephants remained on the mainland during the next drop in water levels. These elephants, now isolated from their island ancestors could diverge enough to be unable to breed with the island elephants during the next crossover. Thus over the course of two ice ages, one species of elephant became three, not considering the other forms that would develop on islands with different niches, like those with more mountainous terrain, swampy bogs, or different plant species on which to feed. And so the process of evolution through geographic isolation, continues, and more species are formed.
The Amazonian rainforest was affected in a different way than Southeast Asia by the ice ages because the change in sea level did not play the same role as in the existence of islands. Instead, the cooler temperatures may have lead to a great contraction of the tropical rainforest and resulted in its replacement with savanna. During the ice ages, carbon dioxide levels drop by as much as 50 percent, causing the majority of plants, which require high levels of carbon dioxide (known as C3 plants) to decline. Some plants, known as C4 plants, especially grasses, grow well under low carbon dioxide conditions. Thus (according to a leading theory), when carbon dioxide levels dropped during glacial periods, rainforests full of C3 plants retreated and savanna grasses (C4 plants) expanded their range. Rainforest was broken up into islands separated by savanna, while communities of species were divided in isolated pockets. Some communities diverged and when the forests expanded and the communities were rejoined, they were altered enough so they could or would not breed.
This "refugia" theory, though plausible and supported by some pollen evidence, is not universally accepted. Recent studies in a few limited locations suggest that the Amazon may have remained densely forested during the past ice age. In 1999 Hooghiemstra and van der Hammen suggested that pollen evidence supports both theories and both scenarios may have occurred in different parts of the Amazon basin and at different periods of time.
A theory proposed in 2005 argues that Amazon rainforest biodiversity has much less to do with climate change than it does with the biology of native species and the forest itself. Looking at the "DNA-clock" of butterfly species in the Amazon basin, scientists from University College London concluded that rainforest butterflies evolved at very different rates, a finding that suggests their evolution is largely independent of external factors like the ice ages. Lead author of the study, Jim Mallet, says that research "rules out geographic isolation caused by past climate change as the main cause of species evolution. Instead the evolution of species must largely be caused by intrinsic biological features of each group of species."
DIVERSITIES OF IMAGE
Because plants grow year round in the tropical rainforest, they must continuously defend themselves against an array of predators. Over the course of millions of years of evolution, plants have developed a variety of mechanical and biochemical defenses. Mechanical defenses like thorns, spines, and stinging hairs appear to be secondary protection to chemical compounds produced by plants, like alkaloids, tannins, and toxic amino acids.
In response, like biochemical warfare, herbivorous insects have adapted to these compounds and insects that eat these plants are able to detoxify the chemicals. The result is that any given insect species has adapted to feed on only a limited number of plants species, while leaving these individual plant species toxic to most other insects.
Through the rigorous process of natural selection, plant species have been perfecting various chemical defenses to ensure survival over millions of years of evolution, and are proving to be an increasingly valuable reservoir of compounds and extracts of substantial medicinal merit. These plants have synthesized compounds to protect against parasites, infections and herbivores, creating acutely powerful chemical templates with which pharmacologists can create new drugs.
Interesting associations have developed between plants and insects like that of the Heliconid butterflies and passion flower vines of the genus Passiflora. Passion flower vines contain cyanide-based compounds for protection against predators. However, Heliconid caterpillars have adapted to these compounds and are able to eat the vine's leaves. Therefore, Heliconid butterflies lay their eggs directly on the passion flower vine, so the larvae will have easy access to their food source. Passion flowers have counter-adapted to the behavior by developing mechanisms to discourage Heliconid butterflies from laying eggs on their leaves. Some Passiflora have evolved structures (actually nectaries) that create housing and produce excess nectar for ants. In return, the ants attack anything, including butterfly eggs, that intrudes on their host. One- upping their predators, some Passiflora have structures that mimic the eggs of Heliconid butterflies. Since a Heliconid butterfly will not lay its eggs on leaves that already have (or appear to have) these eggs, she will move on to another plant. In this manner, Passiflora deter Heliconid butterflies without devoting any resources to the production of nectar for a guard of ants, a technique of protection adopted by many other plants as their primary means of defense. Heliconid caterpillars that develop into butterflies retain the cyanide they consumed as larvae, making the adult butterflies highly unpalatable to predators. The distinct pattern and color of Heliconid butterflies acts as a sort of warning for predators of its toxic composition. When a predator eats one of these butterflies and experiences a foul taste and other ill effects, it learns to associate the colors and patterns of the prey with the bad experience. The next time the predator recognizes the color pattern, it is likely to avoid that potential prey.
This use of warning coloration to advertise bad taste or toxic composition is employed frequently in the rainforest by a variety of animals. The toxic chemicals almost never kill the predator, but cause some irritation to violent sickness. There would be no use if the poison killed the predator, since the next predator that came along would make the same mistake and eat the prey. By making the predator ill, the toxin causes the predator to recognize and avoid the unpalatable prey and similar-looking species known as mimics.
There are three forms of mimicry utilized by both predator and prey: Batesian mimicry, Muellerian mimicry, and self-mimicry. Mimicry refers to the similarities between animal species; camouflage refers to an animal species resembling an inanimate object.Batesian Mimicry
Batesian mimicry is named for Henry Walter Bates, a British scientist who studied mimicry in Amazonian butterflies during the mid- and late nineteenth century. Batesian mimicry refers to two or more species that are similar in appearance, but only one of which is armed with spines, stingers, or toxic chemistry, while its apparent double lacks these traits. The second species has no defense other than resembling the unpalatable species and is afforded protection from certain predators by its resemblance to the unpalatable species, which the predator associates with a certain appearance and a bad experience. Examples of Batesian mimicry are the several species of butterflies that mimic the toxic Heliconid butterflies. Another fascinating butterfly mimic is the non-toxic Papilio memmon of Indonesia. Each female butterfly (regardless of her coloration) can produce one or more different female forms which mimic any of five other species of foul-tasting butterflies. Batesian mimicry is also found in venomous coral snakes and the harmless milk and king snakes of the New World. Both snakes are marked with alternating yellow, red, and black bands causing possible predators to avoid both. The snakes can often be distinguished by using an old scout saying: "Red against yellow: kill a fellow. Red against black: friend to Jack." The deadly coral snake has bands in the order of red, yellow, black, while the innocuous species have the pattern of red, black, yellow (although the rule is not failsafe and there are exceptions).Muellerian Mimicry
Muellerian mimicry is named for Fritz Mueller, a German zoologist who worked in the Amazon three decades after Bates. This form of mimicry refers to two unpalatable species that are mimics of each other with conspicuous warning coloration (also known as aposematic coloration). Thus all mimics share the benefits of the coloration since the predator will recognize the coloration of an unpalatable group after a few bad experiences. Since several species have the same appearance to the predator, the loss of life will be spread out over several species, reducing the impact on each individual species. Poison arrow frogs of South America and Mantella frogs of Madagascar are examples with their conspicuous coloration of bright colors against black markings and toxic composition.Self Mimicry
Self-mimicry is a misleading term for animals that have one body part that mimics another to increase survival
during an attack or helps predators appear innocuous. For example, countless moth, butterfly, and freshwater fish
species have "eye-spots": large dark markings that when flashed may momentarily startle a predator and
allow the prey extra seconds to escape.
"Eye-spots" also help prey escape predators by giving predators a false target. A butterfly has a better chance of surviving an attack to the outer part of its wing than an attack to the head.
Less often predators utilize self-mimicry to aid in catching prey by appearing less threatening or fooling the prey as to the origin of the attack. For example, several turtle species and the Frogmouth Catfish (Chaca sp.) of Southeast Asia have tongue extensions that are used as a sort of lure to attract prey to a position where they become an easy catch. One of the most interesting examples of self-mimicry is the so-called "two-headed" snake of Central Africa which has a tail that resembles a head and a head that resembles a tail. The snake even moves its tail in the way most snakes move their heads. This adaptation functions to trick prey into believing the attack is originating from where it is not.
A completely different approach for deception is camouflage, whereby animals seek to look inanimate or inedible to avoid detection by predators and prey. There are many examples of rainforest species which are cryptically colored to match their surroundings. For example, the Uroplatus geckos of Madagascar are incredible masters of disguise and are practically unnoticeable to the passer-by. An even more amazing group is the katydids, a group of grasshopper-like insects found worldwide. Katydids are nocturnal insects which use their cryptic coloration to remain unnoticed during the day when they are inactive. They remain perfectly still, often in a position that makes them blend in even better. Katydids have evolved to the point where their body coloring and shape matches leaves--including half-eaten leaves, dying leaves, and leaves with bird droppings--sticks, twigs, and tree bark. Other well-known camouflage artists include beetles, mantids, caterpillars, moths, snakes, lizards, and frogs.
Some species appear to have conspicuous coloration when they are not in the proper surroundings. For example, among the brilliant butterflies of the forest, the magnificent electric blue Morpho, has iridescent blue upper wings and a seven-inch wingspan. However, because the underwings are dark, when the Morpho flies through the flickering light of the forest or even out in broad daylight, it seems to disappear. Other forest species, especially mammals, have spots or stripes to help break up the animal's outline. In the shade created by the canopy, large mammals like leopards, jaguars, ocelots, and okapi are surprisingly difficult to see with their disruptive coloration.
Review questions - Part I
- Most of the plant and animal species live in what level of the rainforest?
- What are epiphytes?
- What is an example of an epiphyte? (Hint: think of a popular kind of flower)
- What are lianas?
- What is a symbiotic relationship?
- What is a keystone species?
- Why are agoutis important in the rainforest ecosystem?
Review questions - Part II
- Why does biodiversity generally increase towards the tropics?
- Where does the rainforest ultimately get its energy?
- Why are few species relatively abundant in the rainforest?
Review questions: - Part III
- How does the canopy amplify rainforest biodiversity?
- How does area impact biodiversity?
- Does forest fragmentation reduce forest diversity?
- How do soils affect forest diversity?
Review questions: - Part IV
- How does area impact biodiversity?
- Does forest fragmentation reduce forest diversity?
Review questions: - Part VII
- How can climate change affect the distribution of species?
Review questions: - Part VIII
- Why are some rainforest animals (especially insects and frogs) brightly colored?
- How do plants protect themselves from predators?
Review questions: - Part IX
- What are three types of mimicry?
- Why is camouflage important?
Citations - Part I
- The opening quotation comes from The Song of the Dodo (New York: Scribner, 1996) by David Quammen.
- In his The Diversity of Life (Cambridge, Mass.: Belknap Press, 1992), E.O. Wilson eloquently depicts rainforest diversity using the example of the number of ants in a bush: a single bush in the bush in the Amazon may have more species of ants than the entire British Isles.
- The "Mean Net Primary Production by Ecosystem" table is derived from Holdgate, M. ("The Ecological Significance of Biological Diversity," Ambio Vol. 25, No. 6, Sept. 1996).
- E.O. Wilson demonstrates the Increase in Diversity Towards the Tropics using the number of bird species in locations of similar size (The Diversity of Life, Cambridge, Mass.: Belknap Press, 1992).
- The box, "Portraits of Rainforest Diversity" is derived from several sources: plant species (E.O. Wilson, The Diversity of Life, Cambridge, Mass.: Belknap Press, 1992); butterflies (Robbins, R.K. and Opler, P.A., "Butterfly Diversity and a Preliminary Comparison with Bird and Mammal Diversity," p 69-75 in Biodiversity II. Reaka-Kudla, Wilson, Wilson, eds., Joseph Henry Press, Washington D. C. 1997); and insects (Didham, R.K. and Stork, N.E., "Rise of the Supertramp Beetles," Natural History, Vol. 107, No. 6. July/August 1998).
Citations - Part II
- The section on stability - especially on competition and evolutionary processes - is heavily influenced by E.O. Wilson, The Diversity of Life, Cambridge, Mass.: Belknap Press, 1992.
Citations - Part IV
- MacArthur and Wilson presented the idea that habitat size is correlated with the diversity of species in The Theory of Island Biogeography, Princeton, N.J.: Princeton University Press, 1967.
The background for the Minimum Critical Size of Ecosystems Project (Biological Dynamics of Forest Fragments Project) is given in Lovejoy, T.E. et al., "Ecosystem Decay of Amazon Forest Remnants," in M.H. Nitecki, ed., Extinction, Chicago: University of Chicago Press, 1984; Lovejoy, T.E. et al., "Edges and other effects of isolation on Amazon Forest Fragments." in M.E. Soulè, ed., Conservation Biology: The Science of Scarcity and Diversity, Sunderland: Sinauer, 1986; Wilson, E.O., The Diversity of Life, Cambridge, Mass.: Belknap Press, 1992; Quammen, D., The Song of the Dodo, New York: Scribner, 1996; and Laurance, W.F. and R.O. Bierregaard, Jr, eds., Tropical Forest Remnants: Ecology, Management, and Conservation of Fragmented Communities, Chicago: University of Chicago Press, 1997.
- Smaller fragments suffered greater disturbance through tree falls and suffered losses of biomass according to Laurance, W.F. and R.O. Bierregaard, Jr, eds., Tropical Forest Remnants: Ecology, Management, and Conservation of Fragmented Communities, Chicago: University of Chicago Press, 1997; and Laurance, W.F., "Biomass Collapse in Amazonian Forest Fragments," Science Vol. 278 (1117-1118), Nov. 1997. The work edited by Laurance and Bierregaard further surveys fragmented sites around the world coming to the conclusion that fragmentation reduces global biodiversity. A similar result is reached in Bawa, K.S. and Seidler, R., "Natural Forest Management and Conservation of Biodiversity in Tropical Forests," Conservation Biology Vol. 12 No. 1 (46-55), Feb 1998.
- Island biogeography is discussed further in Williamson, M. (Island Populations, Oxford: Oxford University Press, 1981); Quammen, D. (The Song of the Dodo, New York: Scribner, 1996); Oosterzee, P. (Where Worlds Collide, New York: Cornell University Press, 1997); James H. Brown, J.H., and M.V. Lomolino (Biogeography (2nd edition), Sunderland: Sinauer Associates, 1998); and Whittaker, R.J. (Island Biogeography: Ecology, Evolution and Conservation, Oxford: Oxford University Press, 1999).
Citations - Part VI
- Eldredge, N. and Gould S. ("Punctuated equilibrium: an alternative to phyletic gradualism." in T. Schopf, Models in Paleobiology, New York: WH Freeman 1972) introduce the idea of punctuated equilibrium as a new theory for evolution.
- "Doomsday genes" which may enable species to undergo radical structural changes in mere generations in response to sudden environmental changes are discussed in Rutherford, S.L. and S. Lindquist, "HSP90 as a capacitor for morphological evolution," Nature 396: 336-342, 1998.
Citations - Part VII
- The merits of the "refugia" ice age theory are debated between Colinvaux, P.A., et al., "A long pollen record from lowland Amazonia: forest and cooling in glacial times," Science Vol. 274 (85-88), Oct.1996; Turcq, B. et al., "Amazonia rainforest fires: a lacustrine record of 7000 years," Ambio Vol. 27 No. 2 (139-142), March 1998; and Hooghiemstra, H. and van der Hammen, T., "Neogene and Quaternary development of the Neotropical rain forest: the refugia hypothesis, and a literature overview," Earth-Science Reviews, Vol. 44, issue 3-4 (147-183) Sept. 1998.
- Whitmore, T.C. (Biogeographical Evolution of the Malay Archipelago, Oxford: Clarendon Press, 1987) and Van Oosterzee, P. (Where Worlds Collide, New York: Cornell University Press, 1997) review the effect of the Ice Ages on Indonesia and New Guinea in their discussion of the Wallace Line. Both also briefly discusses some of the theories on the causes of global ice ages. More detail on ice ages is provided in J. Imbrie, (Ice Ages : Solving the Mystery, Harvard: Harvard University Press, 1986); Raup, D, (Extinction: Bad Genes or Bad Luck? New York: W.W. Norton, 1991); Lundqvist, J. ("Quaternary climatic fluctuations, global environment changes, and the impact of man," Nature and Resources, Vol. 32, No. 4, 1996); Van Oosterzee, P. (Where Worlds Collide, New York: Cornell University Press, 1997); and Bradley, R.S. (Paleoclimatology (International Geophysics Series vol 64), Academic Press Limited, 1999).
- The box on population diversity draws from Hughes, J.B., G.C. Daily, and P.R. Ehrlich, "Population diversity: Its extent and extinction," Science 278: 689, Oct. 24, 1997.