An area of the Amazon rainforest in Brazil. The tropical rainforests of South America contain the largest diversity of species on Earth.
Tropical rainforest climate zones (Af).
Tropical rainforests are rainforests that occur in areas of tropical rainforest climate in which there is no dry season – all months have an average precipitation of at least 60 mm – and may also be referred to as lowland equatorial evergreen rainforest. True rainforests are typically found between 10 degrees north and south of the equator (see map); they are a sub-set of the tropical forest biome that occurs roughly within the 28-degree latitudes (in the equatorial zone between the Tropic of Cancer and Tropic of Capricorn). Within the World Wildlife Fund's biome classification, tropical rainforests are a type of tropical moist broadleaf forest (or tropical wet forest) that also includes the more extensive seasonal tropical forests.
Overview
Amazon River rain forest in Peru
Tropical rainforests can be characterized in two words: hot and wet.
Mean monthly temperatures exceed 18 °C (64 °F) during all months of the
year.
Average annual rainfall is no less than 1,680 mm (66 in) and can exceed
10 m (390 in) although it typically lies between 1,750 mm (69 in) and
3,000 mm (120 in). This high level of precipitation often results in poor soils due to leaching of soluble nutrients in the ground.
Tropical rainforests exhibit high levels of biodiversity. Around 40% to 75% of all biotic species are indigenous to the rainforests. Rainforests are home to half of all the living animal and plant species on the planet. Two-thirds of all flowering plants can be found in rainforests.
A single hectare of rainforest may contain 42,000 different species of
insect, up to 807 trees of 313 species and 1,500 species of higher
plants. Tropical rainforests have been called the "world's largest pharmacy", because over one quarter of natural medicines have been discovered within them. It is likely that there may be many millions of species of plants, insects and microorganisms still undiscovered in tropical rainforests.
Tropical rainforests are among the most threatened ecosystems
globally due to large-scale fragmentation as a result of human activity.
Habitat fragmentation
caused by geological processes such as volcanism and climate change
occurred in the past, and have been identified as important drivers of
speciation.
However, fast human driven habitat destruction is suspected to be one
of the major causes of species extinction. Tropical rain forests have
been subjected to heavy logging and agricultural clearance throughout the 20th century, and the area covered by rainforests around the world is rapidly shrinking.
History
Tropical rainforests have existed on earth for hundreds of millions
of years. Most tropical rainforests today are on fragments of the Mesozoic era supercontinent of Gondwana.
The separation of the landmass resulted in a great loss of amphibian
diversity while at the same time the drier climate spurred the
diversification of reptiles.
The division left tropical rainforests located in five major regions of
the world: tropical America, Africa, Southeast Asia, Madagascar, and
New Guinea, with smaller outliers in Australia. However, the specifics of the origin of rainforests remain uncertain due to an incomplete fossil record.
Other types of tropical forest
Several biomes may appear similar-to, or merge via ecotones with, tropical rainforest:
- Moist seasonal tropical forest
Daintree "rainforest" in Queensland is actually a seasonal tropical forest.
Moist seasonal tropical forests
receive high overall rainfall with a warm summer wet season and a
cooler winter dry season. Some trees in these forests drop some or all
of their leaves during the winter dry season, thus they are sometimes
called "tropical mixed forest". They are found in parts of South
America, in Central America and around the Caribbean, in coastal West Africa, parts of the Indian subcontinent, and across much of Indochina.
- Montane rainforests
These are found in cooler-climate mountainous areas, becoming known as cloud forests
at higher elevations. Depending on latitude, the lower limit of montane
rainforests on large mountains is generally between 1500 and 2500 m
while the upper limit is usually from 2400 to 3300 m.
- Flooded rainforests
Tropical freshwater swamp forests, or "flooded forests", are found in Amazon basin (the Várzea) and elsewhere.
Forest structure
Rainforests are divided into different strata, or layers, with
vegetation organized into a vertical pattern from the top of the soil to
the canopy.
Each layer is a unique biotic community containing different plants and
animals adapted for life in that particular strata. Only the emergent
layer is unique to tropical rainforests, while the others are also found
in temperate rainforests.
Forest floor
Western lowland gorilla
The forest floor, the bottom-most layer, receives only 2% of the sunlight. Only plants adapted
to low light can grow in this region. Away from riverbanks, swamps and
clearings, where dense undergrowth is found, the forest floor is
relatively clear of vegetation because of the low sunlight penetration.
This more open quality permits the easy movement of larger animals such
as: ungulates like the okapi (Okapia johnstoni), tapir (Tapirus sp.), Sumatran rhinoceros (Dicerorhinus sumatrensis), and apes like the western lowland gorilla (Gorilla gorilla), as well as many species of reptiles, amphibians, and insects. The forest floor also contains decaying plant and animal matter, which disappears quickly, because the warm, humid conditions promote rapid decay. Many forms of fungi growing here help decay the animal and plant waste.
Understory layer
The understory layer lies between the canopy and the forest floor.
The understory is home to a number of birds, small mammals, insects,
reptiles, and predators. Examples include leopard (Panthera pardus), poison dart frogs (Dendrobates sp.), ring-tailed coati (Nasua nasua), boa constrictor (Boa constrictor), and many species of Coleoptera.
The vegetation at this layer generally consists of shade-tolerant
shrubs, herbs, small trees, and large woody vines which climb into the
trees to capture sunlight. Only about 5% of sunlight breaches the canopy
to arrive at the understory causing true understory plants to seldom
grow to 3 m (10 feet). As an adaptation to these low light levels,
understory plants have often evolved much larger leaves. Many seedlings
that will grow to the canopy level are in the understory.
The canopy at the Forest Research Institute Malaysia
Canopy layer
The canopy is the primary layer of the forest forming a roof over the
two remaining layers. It contains the majority of the largest trees,
typically 30–45 m in height. Tall, broad-leaved evergreen trees are the dominant plants. The densest areas of biodiversity are found in the forest canopy, as it often supports a rich flora of epiphytes,
including orchids, bromeliads, mosses and lichens. These epiphytic
plants attach to trunks and branches and obtain water and minerals from
rain and debris that collects on the supporting plants. The fauna is
similar to that found in the emergent layer, but more diverse. It is
suggested that the total arthropod species richness of the tropical
canopy might be as high as 20 million. Other species habituating this layer include many avian species such as the yellow-casqued wattled hornbill (Ceratogymna elata), collared sunbird (Anthreptes collaris), grey parrot (Psitacus erithacus), keel-billed toucan (Ramphastos sulfuratus), scarlet macaw (Ara macao) as well as other animals like the spider monkey (Ateles sp.), African giant swallowtail (Papilio antimachus), three-toed sloth (Bradypus tridactylus), kinkajou (Potos flavus), and tamandua (Tamandua tetradactyla).
Emergent layer
The emergent layer contains a small number of very large trees, called emergents, which grow above the general canopy, reaching heights of 45–55 m, although on occasion a few species will grow to 70–80 m tall. Some examples of emergents include: Balizia elegans, Dipteryx panamensis, Hieronyma alchorneoides, Hymenolobium mesoamericanum, Lecythis ampla and Terminalia oblonga.
These trees need to be able to withstand the hot temperatures and
strong winds that occur above the canopy in some areas. Several unique
faunal species inhabit this layer such as the crowned eagle (Stephanoaetus coronatus), the king colobus (Colobus polykomos), and the large flying fox (Pteropus vampyrus).
However, stratification
is not always clear. Rainforests are dynamic and many changes affect
the structure of the forest. Emergent or canopy trees collapse, for
example, causing gaps to form. Openings in the forest canopy are widely
recognized as important for the establishment and growth of rainforest
trees. It is estimated that perhaps 75% of the tree species at La Selva
Biological Station, Costa Rica are dependent on canopy opening for seed
germination or for growth beyond sapling size, for example.
Ecology
Climates
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Artificial tropical rainforest in Barcelona
Tropical rainforests are located around and near the equator,
therefore having what is called an equatorial climate characterized by
three major climatic parameters: temperature, rainfall, and dry season
intensity.
Other parameters that affect tropical rainforests are carbon dioxide
concentrations, solar radiation, and nitrogen availability. In general,
climatic patterns consist of warm temperatures and high annual rainfall.
However, the abundance of rainfall changes throughout the year creating
distinct moist and dry seasons. Tropical forests are classified
by the amount of rainfall received each year, which has allowed
ecologists to define differences in these forests that look so similar
in structure. According to Holdridge's classification
of tropical ecosystems, true tropical rainforests have an annual
rainfall greater than 2 m and annual temperature greater than 24 degrees
Celsius, with a potential evapotranspiration ratio (PET) value of <0 .25.="" a="" however="" href="https://en.wikipedia.org/wiki/Tropical_forest" lowland="" most="" title="Tropical forest">tropical forests
can be classified as tropical moist or wet forests, which differ in
regards to rainfall. Tropical forest ecology- dynamics, composition, and
function- are sensitive to changes in climate especially changes in
rainfall.Soils
Soil types
Soil types are highly variable in the tropics and are the result of a
combination of several variables such as climate, vegetation,
topographic position, parent material, and soil age. Most tropical soils are characterized by significant leaching
and poor nutrients, however there are some areas that contain fertile
soils. Soils throughout the tropical rainforests fall into two
classifications which include the ultisols and oxisols.
Ultisols are known as well weathered, acidic red clay soils, deficient
in major nutrients such as calcium and potassium. Similarly, oxisols are
acidic, old, typically reddish, highly weathered and leached, however
are well drained compared to ultisols. The clay content of ultisols is
high, making it difficult for water to penetrate and flow through. The
reddish color of both soils is the result of heavy heat and moisture
forming oxides of iron and aluminium, which are insoluble in water and
not taken up readily by plants.
Soil chemical and physical characteristics are strongly related
to above ground productivity and forest structure and dynamics. The
physical properties of soil control the tree turnover rates whereas
chemical properties such as available nitrogen and phosphorus control
forest growth rates.
The soils of the eastern and central Amazon as well as the Southeast
Asian Rainforest are old and mineral poor whereas the soils of the
western Amazon (Ecuador and Peru) and volcanic areas of Costa Rica are
young and mineral rich. Primary productivity or wood production is
highest in western Amazon and lowest in eastern Amazon which contains
heavily weathered soils classified as oxisols.
Additionally, Amazonian soils are greatly weathered, making them devoid
of minerals like phosphorus, potassium, calcium, and magnesium, which
come from rock sources. However, not all tropical rainforests occur on
nutrient poor soils, but on nutrient rich floodplains and volcanic soils
located in the Andean foothills, and volcanic areas of Southeast Asia,
Africa, and Central America.
Oxisols, infertile, deeply weathered and severely leached, have developed on the ancient Gondwanan shields. Rapid bacterial decay prevents the accumulation of humus. The concentration of iron and aluminium oxides by the laterization process gives the oxisols a bright red color and sometimes produces minable deposits (e.g., bauxite). On younger substrates, especially of volcanic origin, tropical soils may be quite fertile.
Nutrient recycling
This high rate of decomposition is the result of phosphorus levels in
the soils, precipitation, high temperatures and the extensive
microorganism communities. In addition to the bacteria and other microorganisms, there are an abundance of other decomposers
such as fungi and termites that aid in the process as well. Nutrient
recycling is important because below ground resource availability
controls the above ground biomass and community structure of tropical
rainforests. These soils are typically phosphorus limited, which
inhibits net primary productivity or the uptake of carbon.
The soil contains microbial organisms such as bacteria, which break
down leaf litter and other organic matter into inorganic forms of carbon
usable by plants through a process called decomposition. During the
decomposition process the microbial community is respiring, taking up
oxygen and releasing carbon dioxide. The decomposition rate can be
evaluated by measuring the uptake of oxygen.
High temperatures and precipitation increase decomposition rate, which
allows plant litter to rapidly decay in tropical regions, releasing
nutrients that are immediately taken up by plants through surface or
ground waters. The seasonal patterns in respiration are controlled by
leaf litter fall and precipitation, the driving force moving the
decomposable carbon from the litter to the soil. Respiration rates are
highest early in the wet season because the recent dry season results in
a large percentage of leaf litter and thus a higher percentage of
organic matter being leached into the soil.
Buttress roots
A common feature of many tropical rainforests is the distinct buttress roots
of trees. Instead of penetrating to deeper soil layers, buttress roots
create a widespread root network at the surface for more efficient
uptake of nutrients in a very nutrient poor and competitive environment.
Most of the nutrients within the soil of a tropical rainforest occur
near the surface because of the rapid turnover time and decomposition of organisms and leaves.
Because of this, the buttress roots occur at the surface so the trees
can maximize uptake and actively compete with the rapid uptake of other
trees. These roots also aid in water uptake and storage, increase
surface area for gas exchange, and collect leaf litter for added
nutrition.
Additionally, these roots reduce soil erosion and maximize nutrient
acquisition during heavy rains by diverting nutrient rich water flowing
down the trunk into several smaller flows while also acting as a barrier
to ground flow. Also, the large surface areas these roots create
provide support and stability to rainforests trees, which commonly grow
to significant heights. This added stability allows these trees to
withstand the impacts of severe storms, thus reducing the occurrence of
fallen trees.
Forest succession
Succession
is an ecological process that changes the biotic community structure
over time towards a more stable, diverse community structure after an
initial disturbance to the community. The initial disturbance is often a
natural phenomenon or human caused event. Natural disturbances include
hurricanes, volcanic eruptions, river movements or an event as small as
a fallen tree that creates gaps in the forest. In tropical
rainforests, these same natural disturbances have been well documented
in the fossil record, and are credited with encouraging speciation and
endemism.
Human land use practices have lead to large scale deforestation. In
many tropical countries such as Costa Rica these deforested lands have
been abandoned and forests have been allowed to regenerate through
ecological succession. These regenerating young successional forests are
called secondary forests or second-growth forests.
Biodiversity and speciation
Young orangutan at Bukit Lawang, Sumatra
Tropical rainforests exhibit a vast diversity in plant and animal
species. The root for this remarkable speciation has been a query of
scientists and ecologists for years. A number of theories have been developed for why and how the tropics can be so diverse.
Interspecific competition
Interspecific competition
results from a high density of species with similar niches in the
tropics and limited resources available. Species which "lose" the
competition may either become extinct or find a new niche. Direct
competition will often lead to one species dominating another by some
advantage, ultimately driving it to extinction. Niche partitioning is
the other option for a species. This is the separation and rationing of
necessary resources by utilizing different habitats, food sources, cover
or general behavioral differences. A species with similar food items
but different feeding times is an example of niche partitioning.
Pliestocene refugia
The theory of Pleistocene refugia was developed by Jürgen Haffer in 1969 with his article Speciation of Amazonian Forest Birds.
Haffer proposed the explanation for speciation was the product of
rainforest patches being separated by stretches of non-forest vegetation
during the last glacial period. He called these patches of rainforest
areas refuges and within these patches allopatric speciation occurred.
With the end of the glacial period and increase in atmospheric humidity,
rainforest began to expand and the refuges reconnected.
This theory has been the subject of debate. Scientists are still
skeptical of whether or not this theory is legitimate. Genetic evidence
suggests speciation had occurred in certain taxa 1–2 million years ago,
preceding the Pleistocene.
Human dimensions
Habitation
Tropical rainforests have harboured human life for many millennia,
with many Indian tribes in South- and Central America, who belong to the
Indigenous peoples of the Americas, the Congo Pygmies in Central Africa, and several tribes in South-East Asia, like the Dayak people and the Penan people in Borneo.
Food resources within the forest are extremely dispersed due to the
high biological diversity and what food does exist is largely restricted
to the canopy and requires considerable energy to obtain. Some groups
of hunter-gatherers have exploited rainforest on a seasonal basis but
dwelt primarily in adjacent savanna and open forest environments
where food is much more abundant. Other people described as rainforest
dwellers are hunter-gatherers who subsist in large part by trading high
value forest products such as hides, feathers, and honey with
agricultural people living outside the forest.
Indigenous peoples
A variety of indigenous people live within the rainforest as
hunter-gatherers, or subsist as part-time small scale farmers
supplemented in large part by trading high-value forest products such as
hides, feathers, and honey with agricultural people living outside the
forest.
Peoples have inhabited the rainforests for tens of thousands of years
and have remained so elusive that only recently have some tribes been
discovered.
These indigenous peoples are greatly threatened by loggers in search
for old-growth tropical hardwoods like Ipe, Cumaru and Wenge, and by
farmers who are looking to expand their land, for cattle(meat), and
soybeans, which are used to feed cattle in Europe and China. On 18 January 2007, FUNAI reported also that it had confirmed the presence of 67 different uncontacted tribes in Brazil, up from 40 in 2005. With this addition, Brazil has now overtaken the island of New Guinea as the country having the largest number of uncontacted tribes. The province of Irian Jaya or West Papua in the island of New Guinea is home to an estimated 44 uncontacted tribal groups.
Pygmy hunter-gatherers in the Congo Basin in 2014
The pygmy peoples are hunter-gatherer groups living in equatorial
rainforests characterized by their short height (below one and a half
meters, or 59 inches, on average). Amongst this group are the Efe, Aka, Twa, Baka, and Mbuti people of Central Africa. However, the term pygmy is considered pejorative so many tribes prefer not to be labeled as such.
Some notable indigenous peoples of the Americas, or Amerindians, include the Huaorani, Ya̧nomamö, and Kayapo people of the Amazon. The traditional agricultural system practiced by tribes in the Amazon is based on swidden cultivation (also known as slash-and-burn or shifting cultivation) and is considered a relatively benign disturbance.
In fact, when looking at the level of individual swidden plots a number
of traditional farming practices are considered beneficial. For
example, the use of shade trees and fallowing all help preserve soil organic matter, which is a critical factor in the maintenance of soil fertility in the deeply weathered and leached soils common in the Amazon.
There is a diversity of forest people in Asia, including the Lumad peoples of the Philippines and the Penan and Dayak people of Borneo. The Dayaks
are a particularly interesting group as they are noted for their
traditional headhunting culture. Fresh human heads were required to
perform certain rituals such as the Iban "kenyalang" and the Kenyah
"mamat". Pygmies who live in Southeast Asia are, amongst others, referred to as "Negrito".
Resources
Cultivated foods and spices
Yam, coffee, chocolate, banana, mango, papaya, macadamia, avocado, and sugarcane
all originally came from tropical rainforest and are still mostly grown
on plantations in regions that were formerly primary forest. In the
mid-1980s and 1990s, 40 million tons of bananas were consumed worldwide
each year, along with 13 million tons of mango. Central American coffee
exports were worth US$3 billion in 1970. Much of the genetic variation used in evading the damage caused by new pests is still derived from resistant wild stock. Tropical forests have supplied 250 cultivated kinds of fruit, compared to only 20 for temperate forests. Forests in New Guinea alone contain 251 tree species with edible fruits, of which only 43 had been established as cultivated crops by 1985.
Ecosystem services
In addition to extractive human uses, rain forests also have non-extractive uses that are frequently summarized as ecosystem services. Rain forests play an important role in maintaining biological diversity, sequestering and storing carbon, global climate regulation, disease control, and pollination.
Half of the rainfall in the Amazon area is produced by the forests. The
moisture from the forests is important to the rainfall in Brazil, Paraguay, Argentina Deforestation in the Amazon rainforest region was one of the main reason that cause the severe Drought of 2014-2015 in Brazil.
Tourism
Canopy walkway for seeing the diverse tropical forest in Costa Rica
Despite the negative effects of tourism in the tropical rainforests, there are also several important positive effects.
- In recent years ecotourism in the tropics has increased. While rainforests are becoming increasingly rare, people are travelling to nations that still have this diverse habitat. Locals are benefiting from the additional income brought in by visitors, as well areas deemed interesting for visitors are often conserved. Ecotourism can be an incentive for conservation, especially when it triggers positive economic change. Ecotourism can include a variety of activities including animal viewing, scenic jungle tours and even viewing cultural sights and native villages. If these practices are performed appropriately this can be beneficial for both locals and the present flora and fauna.
- An increase in tourism has increased economic support, allowing more revenue to go into the protection of the habitat. Tourism can contribute directly to the conservation of sensitive areas and habitat. Revenue from park-entrance fees and similar sources can be utilised specifically to pay for the protection and management of environmentally sensitive areas. Revenue from taxation and tourism provides an additional incentive for governments to contribute revenue to the protection of the forest.
- Tourism also has the potential to increase public appreciation of
the environment and to spread awareness of environmental problems when
it brings people into closer contact with the environment. Such
increased awareness can induce more environmentally conscious behavior.
Tourism has had a positive effect on wildlife preservation and
protection efforts, notably in Africa but also in South America, Asia,
Australia, and the South Pacific.
Conservation
Threats
Deforestation
Mining and drilling
The Ok Tedi Mine in southwestern Papua New Guinea
Deposits of precious metals (gold, silver, coltan) and fossil fuels (oil and natural gas)
occur underneath rainforests globally. These resources are important
to developing nations and their extraction is often given priority to
encourage economic growth. Mining and drilling can require large amounts of land development, directly causing deforestation. In Ghana,
a West African nation, deforestation from decades of mining activity
left about 12% of the country's original rainforest intact.
Conversion to agricultural land
With the invention of agriculture, humans were able to clear sections of rainforest to produce crops, converting it to open farmland. Such people, however, obtain their food primarily from farm plots cleared from the forest
and hunt and forage within the forest to supplement this. The issue
arising is between the independent farmer providing for his family and
the needs and wants of the globe as a whole. This issue has seen little
improvement because no plan has been established for all parties to be
aided.
Agriculture on formerly forested land is not without
difficulties. Rainforest soils are often thin and leached of many
minerals, and the heavy rainfall can quickly leach nutrients from area
cleared for cultivation. People such as the Yanomamo of the Amazon, utilize slash-and-burn
agriculture to overcome these limitations and enable them to push deep
into what were previously rainforest environments. However, these are
not rainforest dwellers, rather they are dwellers in cleared farmland that make forays into the rainforest. Up to 90% of the typical Yanamomo diet comes from farmed plants.
Some action has been taken by suggesting fallow periods of the land allowing secondary forest to grow and replenish the soil.
Beneficial practices like soil restoration and conservation can benefit
the small farmer and allow better production on smaller parcels of
land.
Climate change
The tropics take a major role in reducing atmospheric carbon dioxide. The tropics (most notably the Amazon rainforest) are called carbon sinks. As major carbon reducers and carbon and soil methane storages, their destruction contributes to increasing global energy trapping, atmospheric gases.
Climate change has been significantly contributed to by the destruction
of the rainforests. A simulation was performed in which all rainforest
in Africa were removed. The simulation showed an increase in atmospheric
temperature by 2.5 to 5 degrees Celsius.
Protection
Efforts to protect and conserve tropical rainforest habitats are diverse and widespread. Tropical rainforest conservation
ranges from strict preservation of habitat to finding sustainable
management techniques for people living in tropical rainforests.
International policy has also introduced a market incentive program
called Reducing Emissions from Deforestation and Forest Degradation
(REDD) for companies and governments to outset their carbon emissions
through financial investments into rainforest conservation.
