Tropical rainforests are renowned for their biodiversity and are found in a broad belt across the Equatorial regions of the Earth. This is shown on the map below. They are found in a belt across central and South America, Western parts of central Africa (it is too dry in the east), in parts of Southeast Asia such as Indonesia and even in Northern Australia.
Tropical rainforest ecosystems have a range of distinctive characteristics, mainly physical, that promote their biodiversity. Within four square miles of tropical rainforest, you will find 1500 flowering plant species and 750 types of trees.
Factor 1 – Climate
This biome is typified by plentiful rainfall, as its name suggests. Typical tropical rainforests receive over 2000mm of rainfall per year, but the rainfall might not be EVENLY SPREAD. There will be rainfall in each month, but some months are wetter than others as shown on the climate graph for Manaus below.
Temperatures are very even, with most tropical forests averaging 27°C every day with very little variation. These 2 conditions allow for incredible plant growth, allowing a very lush amount of vegetation and of course consumers that feed on the vegetation or primary producers.
Above - Typical rainforest climate
Factor 2 – soils and nutrients
Tropical soils are very deep, some of the deepest in the world. This includes the Latosol, a typical tropical forest soil. The soils have been underneath tropical rain forests for millions of years and the high rainfall weathering the rock below and masses of vegetation allow deep soils to form. Tropical soils can be several metres thick BUT are often very nutrient poor as you go down through the soils. This is because the rainwater washes out or LEACHES the nutrients and minerals out of the soil. Soils are often red in colour as they are rich in iron.
This leaching means that the lower layers of the soils lack the nutrients and minerals needed by the lush vegetation. It is a huge system of NUTRIENT CYCLING that allows the vegetation to grow. This is a good example of the INTERDEPENDENT (where things rely upon each other) nature of the forest. As vegetation dies it is quickly decomposed by insects, bacteria and fungi. This releases nutrients into the surface of the soil which is taken up quickly by the plants.
Above - Tropical rainforest nutrient cycling
Factor 3 – water recycling
Water is also recycled within tropical forests. The roots of plants take up water from the ground and the rain is intercepted (caught and trapped) as it falls – the vast majority of it in the canopy. As the rainforest heats up during the day, the water evaporates into the atmosphere and forms clouds to make the next day's rain. This is convectional rainfall. The forests also protect the soil from being washed away or eroded by the heavy tropical rains.
Factor 4– Stratification (layers) of the forest and adaptations of plants and animals
The tropical rainforests are the most abundant and biologically diverse biomes on planet earth. The abundance of sunlight and rainfall allow huge amounts of plant growth via producers such as trees and forest ferns, which in turn allow an abundance of consumers.
The forest is packed full of plants and it is a genuine competition between plants for light and space. The rainforest is layered or stratified, as plants try to take advantage of what space and light there is. The tallest layer contains emergent trees, such as the Capoc tree, which grow tall to maximise the amount of sunlight that they can receive.
The structure of a Tropical Rainforest
Just below this layer is a near continuous layer of Canopy trees. Again these trees have grown very tall to enable them to access sunlight for photosynthesis. Most animals are found in the canopy where there is maximum light (monkeys are well adapted to living in trees) and where they can access food in the form of fruits and nuts, and be safer from predators on the forest floor. There are still predators in the forest canopy however such as pythons and eagles. Many of the trees have leaves with flexible bases that turn to face the sun. They also have huge Buttress roots to anchor these tall trees into the soil and prevent falling during high winds. Bark can often be smooth too, to allow water to flow easily down the tree. Some leaves have a tapered point, a drip tip, which stops water accumulating on the leave and damaging it.
Beneath this layer is the under canopy of smaller trees just waiting for an opportunity (such as a canopy tree dying) to grow up and occupy a place in the canopy. The lowest layer is the shrub and ground layer. Here the plants have to cope with low levels of sunlight as much is absorbed in the canopy above. Ferns grow here and have large leaf sizes to maximise the energy they can capture from the sunlight. Animals such as tapir and deer live on the forest floor eating seeds and berries. There are anteaters here too, plus an abundance of decomposers and insects.
Within the layers there are also epiphytes, which live on the branches and joints of trees and obtain their nutrients from the water and air rather than soil. These plants are therefore parasites. There are also lianas, these are wooden plants that take root in the ground and creep up trees to the canopy where they have their leaves and flowers. Rattan, a liana, is well known for its use in furniture and ropes. Rattan also produces large, edible fruits—a favourite of primates.
Factor 5 – competition and interdependence
Just like our pond ecosystem tropical rainforests have competition for resources and sunlight. Many of the animals and plants are in competition with one another and in many cases they are also reliant upon one another. Changes in one part of the ecosystem, either the living or non-living, could be very damaging for this ecosystem. The loss of some tree cover to deforestation or fire would affect both the water and nutrient cycles for example, and cause soil erosion, increased loss of nutrients from the soils via leaching and extra flooding. Similarly, if one of the elements of the tropical forest food web, shown opposite, were to change, there would be knock on impacts throughout that food web.
Factor 6 - people.
Some indigenous groups of people such as the Kayapo in Brazil have lived for generations in harmony with the forests. These groups are using the forests to meet their needs for food, water and shelter. However, increasing pressures and damage is being done to the forest because of human activity. Activities such as subsistence and commercial farming, logging, road building, mineral extraction, energy development, settlement, and population growth are all having a major impact as you will see in our case study of a tropical rainforest – the Amazon.