The Physical Environment
Tropical rainforests are located in warm humid places around the world’s equator. Average daily temperature is about 25 degrees Celsius. Yearly rainfall is at least 1,500 mm, however some forests receive as much as 10,000 mm! Most of the rain falls during the monsoon, “the rainy season” between mid July and mid September.
On most days in the Tropics the rainforests receive 12 hours of sunshine providing plenty of energy for plant growth. But they grow in very poor soil as millions of years of rain has washed away the nutrients. Conveniently, the rainforest has adapted to these conditions and gets its nutrients by recycling the dead plants that fall to the forest floor. Micro-organisms rapidly break down this matter into usable nutrients like calcium, nitrogen and phosphorus.
There is a fantastic variety of rain forest plant life. A typical 10km square area contains 1,500 kinds of flowering plants and 750 tree species. These plants form a system of layers. The top layer consists of giant trees up to 75 metres tall that tower over the rest of the forest.
Canopy Trees, 20 to 30 metres tall, form the next layer. Shrubs and young trees make up the under layer, whilst the last layer is the forest floor itself. Ferns, herbs and seedlings that need little sunlight for growth are found in this bottom layer. Small plants called epiphytes that need more sunlight attach themselves to the trunks and branches of the canopy trees. They never touch the ground, but their aerial roots absorb water from the moist air. Vines that have roots in the ground climb trees of the top layer to obtain the sunlight they require.
Most tropical rain forest plants are exotic and very beautiful. Orchids and bromeliads for example are found throughout the canopy and under-story. The flowering Rafflesia arnoldii which grows on the forest floor has the largest flower in the world measuring up to 1 metre across. Unfortunately it smells like rotting meat! However, the odour attracts flies which carry out the necessary pollination.
The huge top layer trees are also quite strange. Many of them have huge base fins known as buttresses, which help support them in the poor soil, and prevent them being blown over by the high winds that can accompany the monsoon. Other trees send their roots down from their branches to provide extra support. Many trees have also evolved protection from leaf eating insects and animals, as they produce disagreeable chemicals in their leaves making them unpalatable. Others grow spines on their trunks and branches making it hard for animals to reach their leaves. Some have hollows in their branches for ants to nest in, and they return the favour by attacking those insects and vines that can harm the tree.
Many rainforest plants are very useful. Food such as pineapple, banana, grapefruit, avocado and coconut originated there, as did many spices like chocolate, vanilla, nutmeg, cinnamon, black pepper, ginger and paprika. Chicle, (chewing gum) as well as bamboo and balsa wood also come from the rainforest.
The rainforest is an ideal place for many types of animals to live. There is plenty of water, shelter and food, and it is warm all year. These conditions mostly benefit the insects which can grow and reproduce the year round, unlike the annual cycle in colder climates. Some insects grow very large. “Walking sticks” reach lengths of over 300mm. Beetles can be as large as your hand and some moths are the size of small birds. But the really amazing thing about them is their variety. One tree in the Amazon can house 200 different types of insects; not 200 insects but 200 different types! Scientists believe many insect types have yet to be named and catalogued.
Tropical birds are also very numerous. Over 500 species are found in one small reserve in Costa Rica which is almost as many as the entire number of bird species found in the United Kingdom. The Toucan is one of the best known tropical birds with its huge and colourful beak for picking and eating fruit. Other fruit eaters are parrots, macaws, lorikeets and hornbills. These birds are all large, colourful and noisy, and the rainforest echoes with their calls. Less obvious, but also very colourful are the small hummingbirds that buzz like bees through the forest gathering nectar.
Many reptiles and amphibians also live in the forest. The most spectacular is the anaconda which can measure 9 metres. Others include the colourful, but deadly poisonous coral snake, the python and the carpet snake (a delicacy according to the natives who eat it). The amphibians are best represented by the frogs many of which are brightly coloured, a warning that they are poisonous to eat. They are very vocal, and their songs permeate the forest. An unusual amphibian of the tropics is the blind salamander that has no legs and lives under logs and moss like a giant worm.
Many mammals thrive in the rainforest and bats are among the most common. However unlike their northern cousins (who eat insects) they feed mostly on fruit and nectar, although some prey on frogs. Rodents are almost as numerous as bats. The Capybaras is found near rivers and is the largest rodent in the world. It is almost as big as the pony-sized tapir. another resident of the rainforest which has a long snout-like nose. Both these animals are the natural prey of the jaguar the largest South American cat. Other rainforest cats include ocelot, margay and the largest of them all, the tiger.
The rainforest is home to monkeys and apes. Long tailed monkeys like the howler and spider live in Central and South America. They stay in the forest canopy and eat fruit and leaves. The howler has a very loud call that can be heard for miles.
Gorillas and chimpanzees are apes which are only found in Africa. The gorilla stays on the forest floor and its favourite food is fresh bamboo, whereas the chimpanzee is equally at home on the forest floor or in the canopy. It will eat just about anything, fruit, leaves, insects and small animals. The other great ape is the orang-utan which only lives on the Asian islands of Sumatra and Borneo and has an exclusive fruit diet. In contrast to gorillas and chimpanzees, which are gregarious, the orang-utan is a solitary beast.
It is important to understand that the rainforest animals play a vital role in maintaining their habitat. Because there is no wind in the lower layers to carry pollen from one flower to another, many plants depend on insects for pollination. The 900 varieties of fig tree is a good example of this process as there are 900 different kinds of wasps that pollinate them. Spreading plant seeds is necessary for their survival, and fruit eating birds, mammals and even fish help to accomplish this task.
People of the Rainforest
Native people have lived in the rainforests for thousands of years. In Brazil about 2,500 Kayapo natives live in an area about the size of Wales. They garden, hunt, fish and gather wild foods. They use over 600 species of plants for food, medicine, dyes, oils, soaps, fibres and insect repellent. Numbering about 20,000 the largest group of Indians that live in Brazil are the Yanomani.
The 125cm tall, nomadic Efe pygmies living in Central Africa have no gardening activity. Although one of the smallest races in the world the men sometimes hunt elephant and buffalo. More often they hunt monkeys, birds and rodents and collect fruit, honey, birds eggs and insects. The women gather nuts, roots and mushrooms. They have few possessions: baskets, bow and arrows, bark clothing, and some simple musical instruments.
Another group of indigenous rainforest people are the Gimi. About 10,000 of them live in the highlands of Papua New Guinea. The men hunt small mammals and birds, and the women tend gardens of sweet potatoes and other similar crops. Further west live the Penan of Borneo who are perhaps the most ancient of the forest dwellers and have lived there for nearly 40,000 years!
Over many generations tribal people have discovered many of the forests’ secrets. They have learned how to use the forest to fulfill their needs without harming it. Unfortunately many of these cultures are being wiped out as they come into contact with modern man
Deforestation in the tropics is as much a social problem as an ecological one. Inequitable land distribution in many developing countries forces the peasants to work marginal land and perpetuate a cycle of poverty and environmental destruction. If these people are given title to productive land, and access to such social programs as free education and family planning, the cycle can be broken.
What can be done to save the tropical rainforest?
Something that can be and is being done is the creation of reserves in the tropics. However ,this requires considerable capital which most developing countries can’t afford to spend for this purpose. Some money is being donated by western governments, but the balance is being raised by international conservation groups including Children’s Tropical Forests U.K.
Consumer action is another way to save the rainforests. For example, in 1988 the Rainforest Action Network Inc. and other groups in the USA threatened to boycott a major American forest products company if they proceeded with plans to clear 800,000 hectares for a eucalyptus plantation in New Guinea. The company cancelled its plans to do this action.
Many countries where tropical forests exist are burdened with tremendous foreign debt. Unless creditors and governments of western nations forgive large portions of these debts developing countries must increase the logging, mining and cultivation of forest land to produce exports that will pay off their debts. Debt-for-nature swaps are one good alternative to this. Through these swap programs countries are forgiven some of their debt in exchange for investing in conservation at home.
The consequences of rainforest destruction
The six hottest years of the century occurred in the last decade, and global temperatures are beginning to rise. This appears to be the result of the greenhouse effect caused by the millions of tons of carbon dioxide (CO2) our cars, factories and power plants discharge into the atmosphere. Tropical rainforests are especially good at trapping CO2. Yet trees that are cut down cannot act as carbon traps, and when they are burned more carbon is released into the air which of course magnifies the problem.
Rainforests cover only 5% of the earth’s surface yet over half of the world’s species live in them. 25% of all known medicines contain compounds from rainforest plants, yet only 1% of the plants have been tested for their medicinal qualities. A large proportion of the most important basic foods known to man have their origins in the rainforests.
Unfortunately these nurseries are being eradicated at an alarming rate. Unless conservation efforts in the tropics are intensified most rainforests will disappear early next century.
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