The rainforests of the world are part of a 3.4 million square mile band of green that encircles the equator, and are home to over half of all living things. Some of the most incredible animal and plant life can be found in these regions and the children will be shown several video tapes that focus on the rainforests of South America as enrichment activities. The North American rainforests are much more limited in their numbers of species due to their island nature. Hawaii and Puerto Rico received their plant and animal inhabitants by chance, carried by wind or water from other regions over millions of years. The earliest immigrants found a new home with few competitors and were able to establish themselves and adapt to the special conditions of the islands. In Hawaii five million years of evolution has allowed a limited number of species to adapt to a wide variety of habitats and niches. An outstanding example is the variation among the Hawaiian honeycreepers which the children will study further in an activity.
To understand the incredible diversity of the rainforest in general we must go back in time about 30 million years when the climate of the world was much drier in the equatorial regions. Isolated wet regions were surrounded by dry barriers, this resulted in the evolution of species in individual pockets of habitat that were quite different from one another. As the climate changed and became more uniformly wet, these isolated pockets expanded and ran into each other creating a mix of life-forms that is the most diverse in the world.
The equatorial location of rainforests means more solar radiation, no winter, no reduced daylength, and therefore a relatively stable 70-80°F temperature year-round. The abundant rainfall is the defining characteristic of this biome, with the island of Kauai, the oldest of the Hawaiian chain receiving a staggering average of 486 inches per year making it perhaps the wettest place on earth! Most of the world’s rainforests receive between 70 and 110 inches of rain per year which is generally distributed evenly throughout the year. Of that about 25% evaporates from the canopy 40% trickles down limbs and bark being absorbed and evaporated, and only about 10% actually reaches the forest floor. This slow movement of moisture downward through the trees results in extremely high humidity, 70% in the canopy and 90-95 % at floor level. This constant cycling of water through rainfall and evaporation keeps several thousand gallons of water in the atmosphere per acre of rainforest each day.
The abundant rainfall has a significant impact on the formation of the soil also. The daily rains and warm temperatures cause the leaves that fall to break down very quickly. Humus does not have a chance to collect and organic nutrients would be washed away if it were not that the rainforest vegetation is well adapted to taking up those nutrients immediately. The plants have extensive shallow root systems and in the poorest soils the root mat may be only 15 to 40 cm. deep. In experiments done on these root systems there was 99.9% absorption of cations sprinkled there and of radioactive tracers on fallen leaves none leached through the root mat.
Unlike the forests of the drier temperate regions, in the rainforest most of the nutrients are tied up in the trees themselves. As much as 75% of the carbon in this system is in the wood and leaves. Decomposition occurs rapidly and whole trees may completely disappear in ten years or less, once they have fallen to the ground. Decomposition does not only take place on the ground however. As dead leaves fall through the canopy they often land on outstretched branches where hosts of bacteria, worms, and insects break the leaves down into rich humus soil. Some of the trees are able to send out a kind of aerial root to absorb nutrients that never even made it to the ground.
On the forest floor the remainder of the leaves break down and are quickly taken up by the vegetation and by various insects and worms. One study showed that 32% of the leaf litter was taken up by termites every week. The remaining soil is rich in aluminum and iron oxides giving it a distinctive red coloration and an acidic nature. Once the overlaying vegetation is removed the nutrient cycle comes to a screeching halt and the soil is virtually useless.
The plants of the rainforest are the key to its fertility. Every conceivable corner of the biome is utilized by plant growth. The canopy, or upper layer of tree growth, rises 150-180 feet up in the air from the forest floor. Most of these giant trees are about 100 feet tall but a few super giants tower above them at heights of up to 200 feet. These trees are very vulnerable to wind due to their shallow root systems and have developed a unique kind of buttress root that extends into the ground only a few feet but reaches out laterally many feet. These buttressing roots may emerge from as high as thirty feet up the side of the trunk providing the tree with great stability.
Many other plants have found a home high in the upper level of the canopy by taking advantage of the structure of the tall trees. Woody vines rooted in the ground climb up the trunks of the trees to reach the sun and wind around the high branches, sometimes reaching a length of 300 feet or more. Epiphytes rest on outstretched branches taking their nutrients from the rain and falling debris that collect on every surface. These include orchids, ferns, cacti, and bromeliads.
Beneath the canopy an understory of trees, saplings, bushes, and shrubs grow as high as 50 to 80 feet tall. Their growth is limited by the lack of sunlight until one of the old giants falls clearing an opening for new growth. On the forest floor itself the shade is very deep, less than 10% of the sunlight reaches this level. Very little low growth is possible in such sparse light so the floor is relatively bare and open.
In Hawaii, tree ferns are the most common type of forest vegetation. They are not true woody trees, but rather more like a rosette of ferns living atop their own dead stem. The epiphytic vegetation is mostly ferns, mosses, and liverworts. These particular plants are successful colonists because their spores are tiny and light; easily carried by the wind in large numbers. The flowering plants of the Hawaiian rainforest tend to have white or light green flowers attractive to the tiny pollinators looking for a light spot in the shade of the forest. Although pollinators like bees and hummingbirds seem to be attracted by red and orange showy flowers there are few of those kinds of pollinators on Hawaii.
The native animals of Hawaii were limited to those that could be caught in strong winds and carried to the islands. Insects, birds, and bats were the first arrivals. Snails also appeared perhaps carried on mats of reeds or brush. From the few successful travellers evolved a wide variety of forms to fill every niche. From some 250 insect arrivals evolved 3,722 separate species and varieties. 22 snails evolved into 1,064, and 15 bird species took on 70 variations from the originals.
The honeycreepers are a good example of how a single species of bird evolved into many variations based on adaptations to fill many niches. The beak shape has changed over time to permit feeding on insects; beetles, caterpillars, and larva, or nectar from a variety of flowers. Some are adapted to feed on seeds and others on fleshy fruits. Activity 3 provides an outline sheet of beak shapes and suggested food sources the children can choose from for each bird. A discussion of beak shape should take place before the sheets are handed out.
There are no native mammals in Hawaii except the forest-dwelling hoary bat. All other mammals now found there are considered introduced species brought by man. Pigs, dogs, cats, mongoose, rats, and goats have changed the environment in many serious ways. Rats strip away bark killing the trees, pigs root through the vegetation tearing up the soil, goats overgraze areas. Alien species of plants have been brought in for landscaping and have taken over native species habitat. Animal activity has accelerated the erosion process in many areas removing valuable topsoil, flushing the silt into streams and eventually the ocean.
Rainforest preservation is an important topic that should stimulate a good classroom discussion. The children probably have an understanding of the carbon dioxide-oxygen cycle played out there, and some knowledge about the cutting and burning of rainforests for agriculture and cattle. Concepts of biodiversity are more difficult but should be brought up, as should soil depletion caused by removal of trees. Economic and humanitarian issues can also be brought into the discussion, their significance made very clear by the recent Earth Summit held in Rio de Janeiro.