The ground layer of the rainforest

By Rhett A. Butler
April 1, 2019

Part I:

THE FOREST FLOOR

The forest floor of primary tropical rainforest is rarely the thick, tangled jungle of movies and adventure stories. It is actually rather the opposite: the floor is relatively clear of vegetation due to the deep darkness created by perhaps 100 feet (30 m) of canopy vegetation above. The canopy not only blocks out sunlight, but dampens wind and rain, so much so that a visitor to the rainforest may not immediately know it is raining because raindrops are deflected and collected by various canopy plants. Wind is also cut by overhead vegetation.

In undisturbed primary forests, a flashlight may be more useful than a machete since the subdued lighting limits ground growth. Instead of choking vegetation, a visitor will find large tree trunks, interspersed hanging vines and lianas, and countless seedlings and saplings and a relatively small number of ground plants.

The term "jungle" is frequently applied to forest areas having dense ground growth. Such "jungle" is characteristic of disturbed forest, usually near the forest edges, in recently opened light gaps, river banks, and areas where the forest is reclaiming previously cleared land. Vigorous ground growth is only possible where plenty of light is available—where there is a break or thinning in the light-absorbing canopy which screens out all but the 0.5-5 percent of light that reaches the floor in the primary forest.

CHARACTERISTICS

Despite its constant shade, the ground floor of the rainforest is the site for important interactions and complex relationships. The forest floor is one of the principal sites of decomposition, a process paramount for the continuance of the forest as a whole. It is also home to thousands of plants and animals, and provides support for trees responsible for the formation of the canopy. The ground level is the region of the forest which was first explored and has been the most intensively studied.

Part II:

SOILS and NUTRIENT CYCLING IN THE RAINFOREST

Understanding the basic composition of forest soils helps explain the concept of nutrient cycling in the rainforest; why there are problems with clearing rainforest lands for agriculture; and how soils are an important factor influencing forest complexity.

SOIL COMPOSITION

Over two-thirds of the world's rainforests, and three-fourths of the Amazonian rainforest can be considered "wet-deserts" in that they grow on red and yellow clay-like laterite soils which are acidic and low in nutrients. Many tropical forest soils are very old and impoverished, especially in regions—like the Amazon basin—where there has been no recent volcanic activity to bring up new nutrients. Amazonian soils are so weathered that they are largely devoid of minerals like phosphorus, potassium, calcium, and magnesium, which come from "rock" sources, but are rich with aluminum oxide and iron oxide, which give tropical soils their distinctive reddish or yellowish coloration and are toxic in high amounts. Under such conditions, one wonders how these poor soils can appear to support such vigorous growth.

Rainforests are tremendously vegetated. Early European settlers in the tropics were convinced (and even assured by scientists at the time) that the lushness of the "jungle" was due to the rich soils, so they cut down large patches of forest to create croplands. The cleared land supported vigorous agricultural growth, but only for one to four years, when mysteriously, plant growth declined to a point where copious amounts of fertilizer were required for any growth. Settlers wondered why their crops perished and how such poor soil could support the luxuriant growth of tropical rainforest. The answer lies in the rapid nutrient cycling in the rainforest.

NUTRIENT CYCLING

The colonial settlers did not realize that they were dealing with an entirely different ecosystem from their temperate forests where most of the nutrients exist in the soil. In the rainforest, most of the carbon and essential nutrients are locked up in the living vegetation, dead wood, and decaying leaves. As organic material decays, it is recycled so quickly that few nutrients ever reach the soil, leaving it nearly sterile.

Decaying matter (dead wood and leaf litter) is processed so efficiently because of the abundance of decomposers including bacteria, fungi, and termites. These organisms take up nutrients, which are released as wastes when organisms die. Virtually all organic matter is rapidly processed, even fecal matter and perspiration. It is only a matter of minutes, in many rainforests, before dung is discovered and utilized by various insects. Excrement can be covered with brightly colored butterflies, beetles, and flies, while dung beetles feverishly roll portions of the waste into balls for use later as larval food. Insects are not only attracted to dung for the energy value, but often for the presence of nutrients like calcium salts. Human sweat is a treasure for several species of butterflies, which gather on the necks and hats of tourists, and for annoying sweat bees, which can cover seemingly every inch of exposed skin in some forests.

As vegetation dies, the nutrients are rapidly broken down and almost immediately returned to the system as they are taken up by living plants. Uptake of nutrients by plant roots is facilitated by a unique relationship between the roots and a fungi, mycorrhizae. The mycorrhizae attach to plant roots and are specialized to increase the efficiency of nutrient uptake nutrient from the soil. In return, plants provide the fungi with sugars and shelter among their roots. Studies have also shown that mycorrhizae can help a tree resist drought and disease.

Part III:

RAINFOREST TREE ROOT SYSTEMS

Tropical rainforest trees are well-adapted to their environment and have mastered the problem of poor soils. Since the first six to eight inches (15-20 cm) of soil is a compost of decaying leaves, wood, and other organic matter, it is the richest source of nutrients on the ground. To tap this resource, canopy trees are shallow rooted, whereas most temperate tree roots extend more than 5 feet (1.5 m) deep. Many tropical species have roots that actually grow out of the ground to form a mat on the forest floor in order to more efficiently collect nutrients. These tiny roots form a network that, along with the mycorrhizae fungi, rapidly absorb nutrients.

The configuration of shallow roots and great height causes a great deal of instability for rainforest trees, especially with wet soils and strong winds of the upper canopy that can accompany tropical storms. To counter this, many tree species have extensive root systems that in some cases may run for over 325 feet (100 m). Other trees, especially tall emergent species, have evolved buttress roots—large, thin extensions of the trunk that begin some 20 feet from the ground. These structures are thought to also aid in water uptake and storage, increase surface area for gas exchange, and collect leaf litter for additional nutrition. Some trees, especially palms, have stilt roots for support.

Thus when colonists cut the forest and burned the vegetation, they were destroying the delicate rainforest system which allows vigorous growth on such poor soil. Burning the dead wood and vegetation released enough nutrients into the soil to allow crops to grow for several years, but without the mycorrhizae, and other soil organisms to fix nutrients, soils were rapidly leached by the harsh tropical sun or washed away by heavy rains. Essential minerals were not replaced by new decaying matter since there was no longer forest above to drop leaves and wood. Within a few years, the soil becomes nutrient deficient and can no longer support productive yields of conventional crops. Essentially, the colonists destroyed several links in the semi-closed nutrient system of the rainforest and had to abandon the site for another forested patch. While this seems similar to the "slash-and-burn" technique of native indigenous peoples, the difference is in the scale and form of the cleared sites. By clearing large areas, the colonists created a major break in the rainforest nutrient cycling system; something which takes generations to recover. In the smaller patches cleared by traditional forest dwellers, forest can quickly recolonize after agriculture is abandoned, especially if the patch is left surrounded by forest. Within 20 years, relatively well-developed secondary forest can reclaim such an agricultural plot.

Not all rainforest soils are so poor; some rainforests grow on nutrient-rich floodplain and volcanic soils. Some of the best soils are found on steep slopes because minerals are released when the exhausted topsoils erode. Such rich soils are found in the Amazonian floodplains, Andean foothills, and volcanic areas of Southeast Asia (Java), Africa, Central America, and the Caribbean. However, without proper management, these soils as well can be rapidly leached of nutrients by heavy rains and the sun. For example, a hectare of tropical rainforest rarely loses more than 1 ton of soil annually. However, when the forest is leveled and planted with various crops, the erosion increases drastically. If the forest is replaced with dense vegetation like a coffee plantation, the hectare loses between 20 and 160 tons, whereas if it is replaced with field crops, the patch can lose more than 1,000 tons annually.

Part IV:

REGROWTH AFTER RAINFOREST CLEARING

When Europeans cleared the rainforest for agriculture, they probably assumed that the forests would regrow relatively quickly like the temperate forests of Europe and North America. But rainforest will not readily return on lands with agricultural monocultures that have been devoid of forest for several years and have highly degraded soils. Tropical soils rapidly become inhospitable to growth due to swift leaching of nutrients caused by heavy rains and intense sunlight. In addition, many tropical hardwood trees are dependent on certain animal species for pollination, seed dispersal, and seed processing. The seeds of many tropical rainforest species are large (since they germinate in the shade of the canopy and must have enough food reserves to grow in the low light conditions of the forest floor) and require animal dispersers (wind or other mechanical means often are not sufficient for dispersing seeds of this size). The loss of these dispersal species when forest is leveled, means tree seeds are unlikely to be dispersed into cleared areas. Therefore these important forest tree species will not return.

The seeds and seedlings of those tree species not limited by animal dispersal and pollination agents are often specifically adapted to the light and climate conditions of the shaded rainforest floor. These seeds usually will not germinate in the hot, arid conditions of clearings, and those that do sprout rapidly succumb to the sun and poor soils. Tropical rainforest plants are accustomed to the stability of the rainforest, where they are robust. When they and their seeds are confronted with the entirely different set of conditions presented by forest clearing, they do not fare well.Their seeds have little or no capacity to remain dormant since they have no need under normal forest conditions.

The dry air of the forest clearing also desiccates the leaf litter causing the mycorrhizae to die. The elimination of the symbiotic mycorrhizae reduces the capacity of trees to take up nutrients from the soil. This fungi is especially difficult to replace since each species of tree may have its own symbiotic species of mycorrhizae. Regeneration is further stunted by the rapid encroachment of tough grasses and shrubs after the clearing of forest.

The situation is different when a cleared area is left surrounded by forest and the soils have not be decimated. Under these conditions, forest may rapidly reclaim the barren patch as fast-growing, weedy pioneer plants like forest grasses, bananas (Heliconia), gingers, and vines move into the clearing. Pioneer tree species—which require such conditions of bright sunlight and lower humidity for growth—like cecropia (Neotropics) and macaranga (Asia) quickly colonize forest gaps. After several years, the small number of pioneer plants and trees like kapok and cecropia are gradually replaced with diverse species more characteristic of older-growth primary forest. The necessary mycorrhizae fungi can recolonize from the surrounding forest and facilitate nutrient uptake. After 20 years the formerly cleared land may again support vigorous growth, although this forest is only a shadow of the original primary forest. The new secondary forest, is far less diverse, has a less developed canopy, fewer animals, and thicker ground vegetation. It is unknown how long it takes for secondary forest to return to the complexity of primary forest, but the estimates range from hundreds to thousands of years.

Part V:

SEEDS and FRUITS OF THE RAINFOREST

Many of the seeds and fruits produced by canopy trees fall to the ground where they provide food for seed gatherers (rodents, birds, fish, etc) and create a natural seed bank in the leaf litter. There are two growth strategies once seeds reach the ground. One strategy is to produce large seeds with food reserves enabling the seedling to survive in the low-light conditions of the understory. The second method characteristic of many pioneer species is to produce huge numbers of small seeds which only germinate under certain conditions (usually light gaps). When a light gap opens, these seeds sprout and the seedlings rapidly grow to once again plug the hole in the canopy.

Since seeds are the agents for future growth, many have defenses against predators to ensure some will be left untouched to germinate. Plants reduce herbivory and seed eating with structural defenses like thorns, stinging hairs, or spines and the production of toxic compounds which interfere with digestion or are unpalatable. Often the seed is surrounded by an edible fleshy material to make it palatable to birds and mammals who can disperse it, but the seed is toxic or indigestible so it will be passed out or discarded. For example, the cashew has a sweet and juicy stem but a toxic seed, so monkeys eat the fleshy stem and discard the seed.

Many rainforest trees, especially those of the forest interior, produce large cauliflorous fruits that grow directly out of the surface of the tree. This mechanism enables the tree to grow large, conspicuous fruits that attract larger animals. One of the largest cauliflorous fruits is the jackfruit of Southeast Asia which may be three feet long (1 m) and 110 pounds (50 kg). The jackfruit produces a strong odor that attracts nocturnal mammals like the flying fox. A famous cauliflorous fruit of Southeast Asia is the durian, a thorn covered fruit that is said to "taste like heaven but smell like hell." The odor of the durian is so offensive that its presence has been banned in some public places. Nevertheless, the durian is an important export for Malaysia, Indonesia, and Thailand, which exported nearly 1 million metric tons in 1992. Both cocoa and coffee beans grow as cauliflorous fruit.

Seeds and fruits provide an important food resource for forest-floor animals. To survive, a seed must elude these predators and many do so by being dispersed by specialized birds, mammals, and fish. These dispersers eat the fruit flesh without destroying the seed. Other seeds escape by being poisonous. Some of these toxins are useful to insects, which sequester them for their own purposes, and to humans. For example, many seed toxins have been chemically isolated and used to formulate drugs for humans, while umpteen insects naturally incorporate these compounds to render themselves toxic or use these toxins to produce scents to attract mates. For example, the male butterflies of the Danaiidae family convert alkaloids in the crotalaria plant into a scent to attract females.

Many large seeds have structured defenses against seed predators. In some cases seeds designed for gut passage by large mammals will have a sturdy seed coat, while bird-dispersed large-seeded fruit will often have seeds that are chemically defended. For example, the fruit of the nutmeg splits open to reveal a large seed covered with an orange-colored aril. The seed is highly aromatic, and the odor acts as a warning and toxic deterrent.

Part VI:

LIGHT GAPS IN THE RAINFOREST

The majority of the world's remaining forests are not the classical rainforest with towering trees, an open interior, and virtually no ground growth. Instead, most rainforests have been impacted in their recent history by storms, fires, logging, and landslides and subsequently have scattered areas in various stages of regrowth.

A common event in the tropical rainforest is the fall of an emergent tree, usually during a tropical thunderstorm. In fact, it is estimated that tree turnover rates in some rainforests are every 80-135 years. When one of these giants—laden with lianas connected to neighboring trees—falls, it takes out a sizable portion of the canopy. This hole in the canopy is known as a "light gap" because direct sunlight reaches the floor in contrast to the usual 1-5 percent under full canopy conditions. The opening of a light gap brings many changes to the section of rainforest.

The light gap is rapidly colonized by the same pioneer species that colonize clearings including trees like cecropia, balsa, macaranga, musanga, and bamboo, and shrubby plants like gingers, bananas, nightshades, climbing lianas, and rattan palms. These species are well-adapted for rapid growth, but not for long-term existence in the forest. Their often white wood and leaves with poor chemical protection are subject to infection and infestation by insects. Generally, these pioneers flower rapidly and produce numerous fruits, but are soon overtaken by the hardier, better adapted hardwood trees which fill in the gap in the canopy. Many forest tree species are dependent on light gaps to complete their life cycle.

As a result of the increased light and abundance of fruits produced by gap colonists, light gaps are areas of increased animal activity. Carnivorous animals follow the herbivorous animals that are attracted to the fruiting plants.

Part VII:

RAINFOREST SEEDLINGS

The fall of a tree gives new opportunity to the hundreds of hardwood seedlings that have been waiting years in a state of dormancy for just such an occasion of increased sunlight. The saplings are able to persist in such an extended state of dormancy thanks to several adaptations that enable them to cope with the poor light conditions: the 1-5 percent of sunlight that reaches the forest floor is not typical sunlight, but has high proportions of long-wavelength red and infrared light. Many species have colored pigments better adjusted to harvesting energy from the altered light, while others have red undersides thought to improve light exposure by reflecting light back through the leaf so the tissue gets a double dosage of light. Interestingly, the leaves of saplings on the forest floor differ substantially from those of the canopy, even within the same tree. Canopy leaves are rarely equipped with dark undersides and are usually small and waxy to conserve moisture and protect leaf tissues from the strong sun. Conversely, the leaves of saplings are often large, soft, and non-moisture conserving. In addition, saplings have low rates of growth to conserve the few precious sugars they are able to manufacture. Over the course of a decade, a sapling in the full shade of the canopy may show virtually no noticeable growth. For this reason, forest dwellers cut saplings to make semi-permanent trails since they will not need to cut them again for some time. These trails are suitable to the rainforest because they inflict little environmental harm and are undetectable to most non-forest peoples who may wander into the rainforest. New leaves of both floor and canopy height trees are often bright red to discourage predators by warning them of the leaves' bitter taste or toxic qualities.

As previously mentioned, there are hundreds of tree saplings waiting for the opportunity to take the place of the fallen tree. When the saplings are touched by direct sunlight, they are stimulated to grow rapidly. Each sapling seeks to become the first to reach the canopy, since those that do not slowly perish under the shade of the newly formed canopy.

Part VIII:

RAINFOREST SHRUBS

Like hardwood tree saplings, ground-dwelling rainforest plants and shrubs have adapted to life in the low light levels of the forest floor. They often have structures very similar to those of saplings: dark undersides, deeper colored leaves, and large leaves. Even so, because of the shaded conditions, a true shrub story is unusual in forests and only scattered vines and shrubs are present. Many of the herbaceous plants of the rainforest are epiphytes, including ferns, bromeliads, orchids, and arums, although there are also exclusively terrestrial plants like some of those of the Zingiberales Order including heliconia, gingers, bananas, and birds of paradise. These plants are known for their colorful bracts and are also found in light gaps and under secondary growth conditions.

The world's largest flower, from the rafflesia epiphyte, is found on the rainforest floor. The largest recorded specimen had a diameter of 45 inches (1.14 m), but the average diameter is around 30 inches (75 cm). The plant exists most of its life as vine root, and only becomes visible when it emerges as cabbage-like structure. From this ugly structure comes the maroon and yellow flower which blooms for 3-4 days before dying. The flower releases a putrid smell which attracts flies for pollination. Rafflesia seeds are thought to be dispersed when large mammals like pigs tread on them and carry the seeds away, stuck to their feet. The loss of large mammals may be hindering rafflesia populations.

Another noteworthy group of forest-floor plants from Southeast Asia are the monkey cups. These plants, like the pitcher plants of other parts of the world, have modified leaves for catching insects, which serve as a source of supplementary nutrients like nitrogen and carbon. Interestingly, pitcher plants raised with good soil conditions produce few, if any, pitchers.

Near light gaps and forest edges where sunlight is stronger, big-leaved plants like heliconia (bananas) are more common. Heliconia are famous in the New World for their large colorful flower-like bracts. These bright red, orange, and yellow structures house flowers that produce copious amounts of nectar which attract hummingbirds and insects. Hummingbirds like to visit more than one species of heliconia, creating a risk of hybridization, so different species of heliconia have flowering parts of different lengths allowing the heliconia to deposit its pollen on a specific part of the bird. When the hummingbird visits another heliconia of the same species, the flower has receptor parts that catch the pollen off the particular part of the bird.

Part IX:

MAMMALS of the RAINFOREST FLOOR

As a result of the lack of abundant ground growth, the tropical rainforest supports few large-bodied herbivores and consequently an even smaller population of large predators. The majority of ground-dwelling animals are small to medium-sized creatures that feed on fallen fruits and seeds, saplings, and small prey.

HERBIVORES

Because much of the vegetation is above the reach of forest animals, tropical rainforests support only a small biomass of large herbivores, especially in comparison with surrounding savanna areas. Much of their sustenance comes from the consumption of fallen fruits, seeds, and flowers.

FALLEN FRUIT AND SEED COLLECTORS

Whole niches have opened for species that feed exclusively on fallen matter. Convergent evolution has resulted in species with similar adaptations and appearance on each forested continent, even though none belong to the same family. This niche is filled in Asia by the mouse deer; in Africa by the duikers; and in South and Central America by the agouti. All three animal groups are similar in structure—about the size of a rabbit with thin legs and sharp claws on their hooves—and similar behavior—all three are nervous animals due to their highly sought and appreciated meat.

There are at least six species of duikers, a type of small antelope, which feed only on fallen fruit and seeds. The African rainforest is able to support this diversity in any one location because each species has adapted to feed on a unique type of seed. For example, one species is able to eat huge fruits by having a flexible jaw. Another species has special teeth which enable it to crack some of the hardest seeds in the rainforest.

Agoutis, acouchis, and pacas, all cat-sized, primarily nocturnal, rodents of the New World, are examples of the incredible rodent radiation in the New World. New World rodents are important seed dispersers in tropical rainforests, like the squirrels of temperate regions, in that they collect seeds and bury them in scattered stashes over a relatively wide range. A good portion of the seed caches go unclaimed and the seeds germinate, now safely distanced from the parent tree. Agoutis and pacas are active in the same forests, but avoid direct competition by feeding at different times: agoutis tend to be more diurnal (day-active), while Pacas tend to be nocturnal.

LARGE HERBIVORES

Mammal life in the tropical rainforest is as abundant and diverse as the other life forms there. However, small mammals are the rule and larger mammals are far less common than in temperate forests and the African savannas. The scarcity of large mammals is in part due to the lack of leaves at ground level on which to feed. Only a few large mammal species exist in tropical rainforests: the okapi, the elephant, the pygmy hippo, the bongo, and the gorilla of Africa; the tapir, the rhinoceros, the forest deer, and the elephant of Asia; and the tapir of South America.

Africa

The bizarre Okapi is a primitive giraffe, about the size of a horse, found in a small region of rainforest in Zaire bordered by the Ituri, Uele, and Zaire rivers. It has striped legs like a zebra to help it blend into the shadows of the forest and a long neck to reach leaves above the floor. The okapi is solitary or lives in pairs and feeds on foliage that is shunned by other forest creatures. The okapi was only discovered by Western science in 1899 and less than 10,000 individuals exist today. During the civil strife in the Democratic Republic of Congo (formerly Zaire) in the late 1990s and 2000s, the Okapi Faunal Reserve suffered much encroachment from fleeing refugees and militias, but the okapi population fared well. However in 2012, guerrillas attacked the headquarters of the reserve, killing rangers and more than a dozen okapi.

The gorilla is the world's largest primate, weighing up to 660 pounds (300 kg), having an armspan exceeding 6.6 feet (2 m), and standing six feet (1.8 m) on all fours and even taller when erect (females usually weigh less than 120 kg-265 pounds). Gorillas are represented by two subspecies: the western gorilla of lowland rainforests in Gabon, Cameroon, Nigeria, and Congo; and the eastern or mountain gorilla of the montane cloud forests in Uganda, Rwanda, and Congo (Zaire). Gorillas live in family groups consisting of one dominant silverback male, one to three sub-adult males, females, and juveniles. Gorillas build nests each night; males usually build theirs on the ground or low branches, while females build their quarters higher up in trees. Silverback males have virtually complete control over the group and dictate when to commence nest-building, when to forage, and how to accept intruders. Gorillas, despite a reputation for being brutal, are generally gentle, investigative creatures that feed on leaves, stems, and bamboo shoots [news and information on gorillas | photos].

African elephants are the largest land mammals of the world and are usually associated with the open savannas and plains of Africa. However, some African elephants (now considered a separate subspecies of elephant) do enter the Ituri rainforest of Zaire where they play an essential, although poorly understood, role as "architects of the rainforest." Elephants create clearings in the forest where they gather at mineral deposits and preferred drinking spots. After hundreds and even thousands of years of elephant trampling and digging, some of these clearings may reach several hundred yards across. The elephants keep these clearings open by trampling young colonizing plants and feeding on new leaves. These clearings attract other species which drink at the water hole and hunt prey exposed in the open area. Elephants, by feeding on young leaves in the forest, also allow more light to penetrate the canopy and reach the forest floor. Elephants not only clear areas, but also contribute to tree dispersal, in that several species of seed will only germinate after passing through an elephant's gut. [news and information on elephants | photos]

Asia

Like African elephants, Asian elephants play a similar role in the tropical rainforests of India and Southeast Asia. They have even been introduced into the world's third largest island, Borneo, when they were given as a gift to the sultan in the early 19th century.

Asia, like Africa, also possesses rhinoceroses, but those of Asia are more associated with the rainforest and are much smaller. Both the Sumatran and Java rhino are on the brink of extinction from habitat loss and hunting for their horn, which is ground up into a fine powder and used for medicinal purposes despite its composition: 100 percent keratin—the same structural material of fingernails. It is estimated that there are only 413-563 Sumatran rhinos left [news and information on rhinos | photos].

The Malay tapir is the largest of four species of tapir found worldwide. Related to horses and rhinos (Perissodactyla order), the tapir resembles a giant pig with a sawed-off elephant trunk, and has a short, thick body which may reach eight feet (2.4 m) in length. The tapir likes to be near water and is an excellent swimmer. It feeds on grass, leaves, and fallen fruits. Like the tapir of the New World, the Malay tapir lives alone or in pairs.

South America

There are three species of tapir in the Americas: two mountain tapirs of the Andes in Colombia and Ecuador, and the extremely rare Baird's Tapir ranging from Mexico to Ecuador. Tapirs are most closely related to horses and rhinos, despite their appearance, and have a mobile proboscis. Tapirs have recently become the focus of a new IUCN campaign because of their rapidly declining numbers.

Part XI:

PREDATORS of the RAINFOREST FLOOR

Due to the scarcity of large prey, larger predators are relatively rare in the rainforest. Many of these carnivores have adapted to cope with the shortage of large ground-dwelling prey by hunting in the canopy and supplementing their diet with smaller animals like fish, rodents, birds, and reptiles. The largest group of mammalian predators on the forest floor are the cats. Each forested region, except the Australasian realm has its own forest cat species.

CATS

The largest rainforest cat species is the tiger which once ranged from tropical India to Arctic Siberia to Southeast Asia. The tiger is threatened by habitat, hunting as vermin (farmers blame tigers for livestock losses), and killing for use in Chinese Traditional Medicine, where some believe tiger parts can cure illness. In the last century three subspecies of tiger have gone extinct: the Caspian, Javan, and Bali tigers. Of the six surviving subspecies, the Bengal or Indian Tiger is the most abundant [news and information on tigers | photos].

The second largest rainforest cat is the jaguar which once ranged from Arizona in the United States to Argentina, but is now mostly limited to forest areas and savannas in the Orinoco and Amazon basins. Ruthless hunting for its furs and as vermin along with forest clearing have dramatically driven this species to its endangered status throughout its range, and some of the eight sub-species are now extinct in the wild. Although it is protected under CITES (Convention on International on International Trade in Endangered Species), the jaguar is still hunted widely and vigorously. This aquatic cat's diet includes a broad range of animals: frogs, fish, rodents, turtles, deer, and caiman. The jaguar is an excellent swimmer and fisher and usually hunts at night. Jaguars are solitary cats that only pair up at mating times.

Leopards range from Asia to Africa and numerous sub-species and races exist. Some of these forms are extremely rare and some, like the Bali leopard, are extinct. The leopard forms of the Southeast Asian islands are smaller than mainland forms.

The puma, also known as the mountain lion, is a large New World cat which ranges from Canada to Patagonia. Besides the great cats, the rainforest also has several smaller species like the leopard cat (Asia), margay (New World), and ocelot (New World). These generally range from the size of a housecat to a dog. Most are nocturnal and hunt both on the forest floor and in the canopy.

OTHER CARNIVOROUS MAMMALS

The Civet family originated from Asia to Africa, but now has been introduced worldwide. The Civet family is made up of 16 genera including the well-known mongooses. One interesting member of the family is the fishing genet which has a unique feeding habit. Even though it feeds on fish, it is not a great swimmer and even tries to avoid getting wet whenever possible. The fishing genet fishes by tapping its paw against the water surface of small forest streams in order to attract fish. The fishing genet puts its long whiskers against the surface to detect vibrations caused by fish movements. When it senses movement, the genet takes the plunge.

The best-known members of the family are the mongooses which have been popularized by Kipling's Ricky-Ticky-Tavy story as eaters of snakes, though they also feed on insects, small mammals and birds, and eggs. Mongooses have an interesting technique for opening eggs in which they stand with their back to a rock, egg in hand, and violently thrust the egg against the rock.

The armadillo and giant anteater of the Edentata family are ground-dwelling carnivores of the New World, with 21 species of armadillo distributed from the southern U.S. to Patagonia. They are equipped with protective bony plates that render them virtually inedible to predators. Armadillos are excellent diggers who use their fine sense of smell to locate snakes, mice, lizards, and insects. Armadillos range in size from five inches (12 cm) and three ounces (90 g) to the rare giant armadillo which reaches 39 inches (1 m) and 120 pounds (55 kg), excluding the tail. The giant anteater inhabits savannas and rainforests from Guatemala to Argentina. It is a great swimmer and, like its arboreal relatives, a fine climber. It is well-adapted to feeding on ants and termites with its 39-inch (1 m) sticky tongue, strong sense of smell, and powerful, sharp claws. Sloth bears, weighing up to 300 pounds (135 kg), are distributed in the tropical rainforests of Sri Lanka and Southern India. They feed commonly on termites, forming their lips into a tubular shape and sucking them up like a vacuum sweeper. The related sun bear is found in the forests of Southeast Asia.

Part XII:

OMNIVORES of the RAINFOREST FLOOR

The most notable rainforest omnivores are the wild forest pigs which are found in both the New and Old World. These animals usually root in the forest soil for their food and in the process create pits that fill with water when it rains, providing a home to insect larvae, frog tadpoles, and even some fish species. Pigs are generally herd animals that move in groups ranging from 5-25 individuals. These herds are strongly territorial and some of the larger species will actually maim and kill larger animals (including humans) that attempt to pass through their territory.

New World pigs are represented by the peccaries which range from the Southern U.S. to Argentina. Peccaries have the uncanny ability to sense plant bulbs at depths up to 10 feet (3 m). They also feed on roots, insects, and small animals. Peccaries are active all day long, although mostly at dawn and dusk. The herd appears to lack a hierarchal structure and often seems to follow any adult member that makes a decision.

The Old World has a surprising diversity of forest pigs including the wild boar, warthog, and the bizarre babirusa. The babirusa is endemic to the Indonesian island of Celebes (Sulawesi) and is considered highly endangered. It has an arched back with long thin legs and strange tusks that rise for over one foot (30 cm) from the lower jaw. As they extend, they curve backward and downward to form partial semicircles. The babirusa is usually found in swamps and along streams.

Part VIII:

RAINFOREST GROUND BIRDS

The majority of ground-dwelling forest birds are insect eaters and elusive, although while quietly walking through the forest it is not unusual to startle some. The Asian forests are home to peafowl and jungle fowl (from which domestic chickens descended) in addition to the well-known common peacock of India and Sri Lanka. The common peacock lives much of the year in large flocks, but at the beginning of spring, the breeding period, a single male forms a harem with two to five females. Only the males have gaudy, elaborate plumage. The green peacock has a more extensive range across Southeast Asia and is larger, with predominantly green and metallic blue plumage.

Several Old World birds have peculiar nest-building habits including the mound builders (megapodes) of eastern Indonesia, New Guinea, and Australia and the bowerbirds of New Guinea and Australia. Megapodes are found east of the Wallace line where there are few ground-dwelling carnivorous mammals. The mound builders are fowl-sized birds named for the nests of huge compost heaps of leaf litter (these mounds can be in use for more than 40 years), which they build for their eggs. Pairs form life-long bonds and work feverishly to maintain the correct, almost exact, incubation temperature, by adding and removing leaves from the compost pile. Some species take over suburban compost heaps, while others occupy city dumps. Some species even utilize geothermal heat for incubating their eggs. Bowerbirds also have interesting nest-building techniques. Male bowerbirds construct display nests using grasses and leaves and decorate the walls with colored dyes of certain berries and pieces of charcoal held together with saliva. The nest and surrounding area are often adorned with iridescent beetles, flowers, feathers, fruit, and other colored and shiny objects to attract females.

The largest rainforest-dwelling ground bird that exists today is the cassowary which may reach 40 inches (1 m) in height. It has short, powerful legs and is well-adapted for running at high speeds (over 30 mph-50 km/h) through the forest. All three species of cassowary are odd-looking with a horny head growth like a helmet, dark body plumage, a turquoise head, a metallic blue neck, and a bright red throat.

In the New World one large group of birds, the antbirds, have adapted to feeding exclusively on the insects disturbed by army ants. These birds spend their life following the columns of army ants that move through the forest. There are numerous varieties of antbirds including antwrens, antshrikes, antthrushes, and antpittas.

Part VIII:

REPTILES OF THE RAINFOREST FLOOR

REPTILES

The most abundant vertebrate predators of the forest floor are reptiles, namely snakes and lizards. Best known of forest snakes are the giant constrictors, pythons of the Old World and Australasia, and boas of New World, but many of these are arboreal, aquatic, or relatively small. The majority of the snakes of the forest floor are small to medium sized, nocturnal, and mildly poisonous. They eat a range of amphibians, mammals, small birds, and insects. Very few of these species pose a threat to humans, although some are notorious for inflicting bites. The fer-de-lance viper of the New World, cobras of the Old World, and bushmaster of the New World are well-known examples.

There are several notable snakes of the rainforest with strange habits or characteristics. For example, the mole viper has long fangs that protrude beyond the corners of its mouth, while the African egg-eating snake, with its detachable jaw hinge, is specially adapted to feeding exclusively on bird eggs. This snake species is equipped with lower vertebrae that have an enamel-like coating and penetrate the esophagus to act as a saw-like structure. Although its head is about the size of a human finger, the African egg-eating snake can swallow eggs larger than that of a hen. One African snake species, sometimes labeled the "two-headed snake," has a tail that resembles its head, and a head that resembles a tail. In addition, to further confuse predators, the snake moves its tail in a manner that mimics the way most snakes move their heads. Thus if attacked, the tail (resembling the head) is the most likely target and the snake is much more likely to escape.

The Komodo dragon of folklore would seem to be one of the largest, more formidable beasts on earth. However, in reality, many of the "facts" about the Komodo dragon are exaggerated. First of all, the Komodo dragon is a lizard belonging to the group of 31 species of lizards known as monitors, found in the Old World. Secondly, the Komodo dragon does not reach 30 feet (9 m) as reported by many, but the largest recorded specimen was just over 10 feet (3 m) in length—still an impressive size. The Komodo dragon is found on a few scattered islands in Indonesia south of Celebes. With a belly full of deer, one may reach 500 pounds.

Chameleons, discussed in the canopy section are actually most active near the ground. There are some 135 species of chameleons found in Africa, half of which are endemic to Madagascar (Malagasy chameleon pictures). These solitary lizards are well adapted to their surroundings with their well- known ability to change color, along with their large eyes which can be moved independently, and their meter-long tongue. Interestingly, chameleons often do not change color to match their surroundings, but instead to convey emotions, defend territories, and communicate with mates. At the height of the breeding season, colors are spectacular, as males try to impress females. At night, in a relaxed state, chameleons turn pale, almost white.

Another group of lizards from Madagascar is the fringed, leaf-tailed, or Uroplatus geckos (pictures) which are incredible camouflage artists. Some species look exactly like bark, while others resemble moss when they flatten themselves against their surroundings. When discovered, Uroplatus geckos respond by throwing their mouths wide open, showing the bright orange-red interior, and erecting their tails.

Amazonian Reptiles - A Historical Account

Geckos are found worldwide and are quite famous for their abundance and loud call. Numerous species are kept widely as pets, while house geckos are found in virtually every house in the tropics, where they feed on house insects.

Part VIII:

RAINFOREST AMPHIBIANS

Amphibians are common on the forest floor, although not as abundant as those of the trees. Among the best known of rainforest amphibians are the tiny but brilliantly colored poison dart (arrow) frogs (members of the Dendrobatidae family). These striking frogs secrete powerful toxins from glands on their backs and use their color to advertise their toxic composition to potential predators. The potency of the toxin varies according to the species, and rainforest dwellers have been using these skin secretions for centuries to poison the tips of their blow-darts. The most toxic frog known is the yellow-gold Phyllobates terribilis, of Western Colombia, which is said to be potentially fatal if held in the hand. Indians need only rub and arrow tip across the frog's back and the arrow is good for a year. Other poison arrow frogs must be roasted to extract their poison. The skin secretions of poison arrow frogs have human healthcare applications as evidenced by the story of Epipedobates tricolor and ABT-594/epidatidine.

Dart frogs derive their toxicity from the ants and mites they consume. Frogs that are kept in captivity are generally not poisonous.

Not all rainforest frogs are so brilliantly colored. In fact more amphibians take the opposite approach to defense: camouflage. Several species throughout the world, including the horned toad and two unrelated frogs in Brazil, look like dead leaves and when disturbed stretch out their back legs and become totally still for 30 minutes.

Amphibians are on the decline worldwide—more than 150 are known to have gone extinct since the early 1980s. Several notable species including Costa Rica's Golden toad (Bufo periglenes) and the Gastric Brooding Frog (Rheobatrachus silus) of Queensland, Australia, have disappeared in recent decades. Habitat loss, introduced diseases like the chytrid fungus, over-harvesting, the effects of climate change, pollution, and invasive species are driving amphibian decline. More than two out of every five amphibians assessed by the IUCN are considered threatened.

Because amphibians have highly permeable skin and spend a portion of their lives in water and on land, they are sensitive to environmental change and can act as the proverbial canary in a coal mine, indicating the relative health of an ecosystem.

To keep up-to-date on the latest amphibian developments, be sure to check the Amphibian news feed.

Part VIII:

RAINFOREST INSECTS

Invertebrates are by far the most abundant and most diverse animals in the rainforest. They have invaded nearly every niche imaginable and many unimaginable, and each plays a unique, although still poorly understood, role in the ecosystem. For example, in the soil invertebrates are essential in the process of decomposition. These species feed on broken-down plant plant material and organic particles. Earthworms, termites, and others fragment larger particles into sizes more manageable for bacteria, fungi, and microorganisms.

Most invertebrates in both tropical and temperate regions are small and inconspicuous but rainforests house some of the world's largest. For example, Malagasy planaria and tropical American beetles may reach six inches (15-16 cm), and centipedes may reach eight inches (20 cm). Centipedes are often brightly colored, are carnivorous, and kill their prey with poison claws located under their first body segment. In some centipede species the female carefully guards the young. Millipedes feed on rotting logs. Jewel beetles, scarabs, termites, and earthworms are part of decomposition high in the canopy, in the soil-like debris found on epiphytes. Scorpions are much smaller than their arid and temperate counterparts but often pack a powerful sting.

Leeches are fascinating rainforest dwellers even though their feeding habits repel most people. Rainforest leeches of Southeast Asia, Africa, and Madagascar, unlike the leeches of the United States, do not live in water, but are able to live terrestrially by the humid conditions of the forest. Leeches are blood-suckers that are attracted to their prey by movement, temperature, and carbon dioxide. If one sits in the rainforest of Borneo for a few minutes, leeches, moving like inch worms, can be seen approaching from the forest floor and even dropping from the trees. The victim often does not feel the bite of the leech, which has razor-sharp teeth and releases an anticoagulant into the bite to allow the blood to flow freely. Leeches are amazingly persistent, and once attached they should be removed only by dousing them with salt or shampoo or burning them with a cigarette butt. Leeches are a nuisance, but relatively harmless, since they carry no known diseases. Leeches can ingest as much as fifteen times their body weight at a single feeding, enough to satisfy them for six to twelve months before their next meal.

ANTS

Army ants of the New World have long been depicted in fictional movies and books as a marauding force that threatens everything in its path including people and entire villages. This is hardly an accurate scenario. Some rainforest peoples actually welcome the periodic visitations by army ants to clear their hut of unwanted resident pests. In addition, forest peoples have been known to use large soldier ants (also soldier leaf-cutter ants) for medical purposes. The ant is picked up by its body with its powerful mandibles open, and placed over an open wound where it is allowed to clamp the wound closed. The native then twists the head off and the jaws remain as a temporary, natural suture. Although soldier army ants are formidable with their huge jaws, the majority of ants in a given column are medium-sized worker ants. The sheer numbers of these ants enable a column to overtake animals that normal-sized colonies could not. There are reports of tethered animals being devoured, but most of the column's prey consists of other invertebrates. The column scares up many insects that usually remain hidden or camouflaged during the day. Enough of these insects are scared up to support numerous species of birds that follow ant columns and feed exclusively on the insects. Dependent on the antbirds are ithomiine butterflies which feed on their nitrogen-rich droppings. Unlike other butterflies which are restricted to food reserves built up during their caterpillar stage, these ithomiine butterflies are able to live and reproduce for months thanks to the proteins gleaned from the bird droppings. The ithomiines are safe from predation by the antbirds, because as caterpillars they feed on leaves containing poisonous alkaloids and giving the adult butterflies an unpleasant taste. Other moths and butterflies mimic the warning coloration of ithomiine butterflies to afford themselves protection.

Other species benefit from the army ant columns. Trachinid flies wait in vegetation above the ant column for grasshoppers to appear. When one does, the fly lays an egg on its body. The egg hatches into a larva which burrows into the grasshopper and devours the insect from the inside. Certain insects including wasps, beetles, and millipedes are capable of chemically mimicking the odor of army ants so they are undetectable as they move through the column, since most ants have very poor vision and can only really distinguish between light and dark. These creatures are able to get free meals in the form of prey exposed or captured by the column.

The Old World equivalents of army ants are the driver ants, which are blind and move in massive armies (20 million) under the leaf litter.

Another well-known group of ants are the leafcutter ants of the genus Atta. Large columns of these ants are a common sight in tropical regions worldwide. Foragers, are the most apparent, carrying cut pieces of leaves, petals, and fruit from their place of origin back to their nest. Leafcutter ants exist in highly structured communities in which individual size determines the ants' specialized role in the community. The largest ant type is the soldier which may weigh 300 times more than the next largest ant type, the forager. At the bottom of the size scale is a tiny ant type whose function is to ride atop the leaves as they are carried by the foragers, and keep a watch out for species of flies that lay their eggs on the leafcutters' leaves. When the larvae of this fly hatch, they can decimate the entire colony.

Leaf Cutters in Perspective

The leaves that the foragers bring back to the colony are not eaten, at least not in the conventional sense. The leaves are taken into a chamber some 15-20 feet (4.5-6 m) underground where the leaves are cut into smaller pieces by smaller worker ants. The fragments are taken to another chamber where they are chewed up by smaller ants into a leaf paste. This leaf paste provides sustenance for a certain type of fungus, which grows and is farmed by even smaller ants. This fungus is fed upon by the ants. There is no central coordination of leafcutter activities: each ant simply carries out its job based on its size and age. Communication, as for most other ants, comes in the form of chemical pheromones which cause ants to react in a certain way (though audio signals are also important in ant communication). Even the harvested plant species benefit from the work of the leaf cutters; studies have found that growth rates increase for many plant species after "pruning" by leaf cutters.

REVIEW QUESTIONS

Review questions - Part I

• What is the difference between jungle and rainforest?
• Why is there generally little light on the forest floor?

Review questions - Part II

• Why does traditional agriculture often fail in rainforests?
• How are nutrients recycled in the rainforest environment?
• How are tropical forest soils different from soils in temperate forests?

Review questions: - Part V

• How do animals help the reproductive cycle of plants?
• Why are many seeds poisonous?

Review questions: - Part IV

Review questions: - Part VII

Review questions: - PREDATORS of the RAINFOREST FLOOR

• Why are large predators relatively rare in the rainforest?
• Why are tigers endangered?

Review questions: - OMNIVORES of the RAINFOREST FLOOR

• Why are large predators relatively rare in the rainforest?
• Why are tigers endangered?

Review questions: - RAINFOREST GROUND BIRDS

• What are bowerbirds known for?

Review questions: - REPTILES OF THE RAINFOREST FLOOR

Review questions: - RAINFOREST AMPHIBIANS

• Why are poison dart frogs colorful?
• Why are frogs endangered?

Review questions: - RAINFOREST INSECTS

CITATIONS

Citations - Part I

Citations - Part II

Citations - Part IV

Citations - Part VI

Citations - Part VII