【正文】 e top unless some older trees die, creating a hole in the canopy. When this happens, all of the seedlings on the ground level pete intensely to reach the sunlight. Many plant species reach the top of the forest by climbing the tall trees. It is much easier to ascend this way, because the plant doesn39。t have to form its own supporting structure. Some plant species, called epiphytes, grow directly on the surface of the giant trees. These plants, which include a variety of orchids and ferns, make up much of the understory, the layer of the rainforest right below the canopy. Epiphytes are close enough to the top to receive adequate light, and the runoff from the canopy layer provides all the water and nutrients (養(yǎng)分 ) they need, which is important since they don39。t have access to the nutrients in the ground. Stranglers and Buttresses Some epiphytes eventually develop into stranglers. They grow long, thick roots that extend down the tree trunk into the ground. As they continue to grow, the roots form a sort of web structure all around the tree. At the same time, the strangler plant39。s branches extend upward, spreading out into the canopy, Eventually, the strangler may block so much light from above, and absorb such a high percentage of nutrients from the ground below, that the host tree dies. Competition over nutrients is almost as intense as petition for light. The excessive rainfall rapidly dissolves nutrients in the soil, making it relatively infertile except at the top layers. For this reason, rainforest tree roots grow outward to cover a wider area, rather than downward to lower levels. This makes rainforest trees somewhat unstable, since they don39。t have very strong anchors in the ground Some trees pensate for this by growing natural buttresses. These buttresses are basically tree trunks that extend out from the side of the tree and down to the ground, giving the tree additional support. Rainforest trees are dependent on bacteria that are continually producing nutrients in the ground Rainforest bacteria and trees have a very close, symbiotic (共生的 ) relationship. The trees provide the bacteria with food, in the form of fallen leaves and other material, and the bacteria break this material down into the nutrients that the trees need to survive. One of the most remarkable things about rainforest plant life is its diversity. The temperate rainforests of the Pacific Northwest are mainly posed of a dozen or so tree species. A tropical rainforest , on the other hand, might have 300 distinct tree species. 。 All Creatures, Great and Small Rainforests are home to the majority of animal species in the world. And a great number of species who now live in other environments, including humans, originally inhabited the rainforests. Researchers estimate that in a large rainforest area, there may be more than 10 million different animal species. Most of these species have adapted for life in the upper levels of the rainforest, where food is most plentiful. Insects, which can easily climb or fly from tree to tree, make up the largest group (ants are the most abundant animal in the rainforest). Insect species have a highly symbiotic relationship with the plan! life in a rainforest. The insects move from plant to plant, enjoying the wealth of food provided there. As they travel, the insects may pick up the plants39。 seeds, dropping them some distance away. This helps to disperse the population of the plant species over a larger area. The numerous birds of the rainforest also play a major part in seed dispersal. When they eat fruit from。 a plant, the seeds pass through their digestive system. By the time they excrete (排泄 ) the seeds, the bird39。 may have flown many miles away from the fruitbearing tree. There are also a large number of reptiles and mammals in the rainforest. Since the weather is so hoi and humid during the day, most rainforest mammals are active only at night, dusk or dawn. The many rainforest bat species are especially well adapted for this lifestyle. Using their sonar, bats navigate easily through the mass of trees in the rainforest, feeding on insects and fruit. While most rainforest species spend their lives in the trees, there is also a lot of life on the forest floor. Great apes, wild pigs, big cats and even elephants can all be found in rainforests. There are a number of people who live in the rainforests, as well. These tribes— which, up until recently, numbered in the thousands— are being forced out of the rainforests at an alarming rate because of deforestation. Deforestation In the past hundred years, humans have begun destroying rainforests at an alarming rate. Today, roughly acres of rainforest are destroyed every second. People are cutting down the rainforests in pursuit of three major resources: ? Land for crops ? Lumber for paper and other wood products ? Land for livestock pastures In the current economy, people obviously have a need for all of these resources. But almost all experts agree that, overtime, we will suffer much more from the destruction of the rainforests than we will benefit. The world39。s rainforests are an extremely valuable natural resource, to be sure, but not for their lumber or their land. They are the main cradle of life on Earth, and they hold millions of unique life forms that we have yet to discover. Destroying the rainfore