Passage Six
Rainforests
Tropical rainforests are the most diverse ecosystem on Earth, and also the oldest. Today, tropical rainforests cover only 6 percent of the Earth’s ground suce, but they are home to over half of the planet’s plant and animal species. In this completely unique world, there are thousands of species we have yet to discover.
In this article, we’ll travel into the tropical rainforest to see what makes it such a beautiful environment for plants and animals. We’ll also see why the rainforests are in er and look at some of the ways this affects us.
What Is a Rainforest
Generally speaking, a rainforest is an environment that receives high rainfall and is dominated by tall trees. A wide range of ecosystems fall into this category, of course, including the old-growth temperate forests of the Pacific Northwest. But most of the time when people talk about rainforests, they mean the tropical rainforests located near the equator.
These forests, concentrated in Africa, Australia, Asia, and Central and South America, receive between 160 and 400 inches (406.4 to 1016 cm) of rain per year. Unlike the rainforests farther to the north and south, tropical rainforests don’t really have a "dry season". In fact, they don’t have distinct seasons at all. The total annual rainfall is spread pretty evenly throughout the year, and the temperature rarely dips below 60 degrees Fahrenheit ( 16 degrees Celsius).
The Forest for the Trees
We saw in the last section that the ample sunlight and extremely wet climate of many tropical areas encourage the growth of towering trees with wide canopies (遮篷). This thick top layer of the rainforest dictates the lives of all other plants in the forest. New tree seedlings rarely survive to make it to the top unless some older trees die, creating a "hole" in the canopy. When this happens, all of the seedlings on the ground level compete intensely to reach the sunlight. Most other plants survive by taking advantage of the trees that form the canopy layer.
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 doesn’t have to form its own supporting structure. Lianas, long, woody plants that can grow more than 8 inches (20 cm) across, will often climb tall trees all the way up to the canopy layer. At the top of the forest, these climbers may spread from tree to tree, the canopy ceiling even thicker.
Some plant species, called epiphytes (附生植物), grow directly on the suce of the giant trees. These plants, which include a variety of orchids and ferns, make up much of the understory (下层矮生植被), 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 they need, which is important since they don’t have access to the nutrients in the ground.
Stranglers and Buttresses
Some epiphytes ually 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. ually, 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. When the host decomposes, the strangler’s lattice of roots remains, giving the plant the structure it needs to reach from the forest floor to the canopy.
Competition over nutrients is almost as intense as competition for light. The excessive rainfall rapidly dissolves nutrients in the soil, 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 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 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. Even with this amazing symbiotic cycle, nutrients are scarce. Some plant species gather additional nutrients by capturing bugs or catching plant material that falls from the canopy above.
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. This plant life is spread out over wide areas-in a square acre, an entire species might be represented by only a few individual plants. As we’ll see in the next section, rainforest animal life is similarly diverse.
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 l 0 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 (共生的) 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-underneath the canopy, the wind is not strong enough to carry seeds a significant distance, so plants depend entirely on animals for seed dispersal. Less harmful insects may also help a plant by fighting off more destructive insect species.
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 the seeds, the birds may have flown many miles away from the fruitbearing tree.
Passage SixRainforestsWhich of the following is NOT the result of the intense competition for nutrients