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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 plant'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 competition 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 don't have very strong anchors in the ground. Some trees compensate for this by growing natural buttresses. These buttresses are basically tree trunks that extend out from the side of the tree and clown 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 composed 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 plant 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 plants' 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 birds may have flown many miles away from the fruit-bearing tree。
There are also a large number of reptiles and mammals in the rainforest. Since the weather is so hot 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 1.5 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, over time, we will suffer much more from the destruction of the rainforests than we will benefit。
The world'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 rainforests is comparable to destroying an unknown planet we have no idea what we're losing. If deforestation continues at its current rate, the world's tropical rainforests will be wiped out within 40 years。
1. Virtually all plant and animal species on Earth can be found in tropical rainforests。
2. There is not much change in the weather in the tropical rainforests all the year round。
3. The largest number of rainforests in the world are located on the African continent。
4. Below the canopy level of a tropical rainforest grows an overabundance of plants。
5. New tree seedlings will not survive to reach the canopy level unless ______。
6. Epiphytes, which form much of the understory of the rainforest, get all their water and nutrients from ______。
7. Stranglers are so called because they ______ by blocking the sunlight and competing for the nutrients。
8. Since rainforest bacteria and trees depend on each other for life, the relationship they form is termed ______。
9. Plant species are dispersed over a large area with the help of ______。
10. As we are still ignorant of millions of unique life forms in the rainforest, deforestation can be compared to the destruction of ______。
