Rain forests
Tropical rainforests are the most diverse ecosystem (生态系统) on Earth, and
also the oldest. Today, tropical rainforests cover only 6 percent of the Earth’s
ground surface, but they are home to over half of the planet’s plant and animal
species. 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. But
most of the time when people talk about rainforests, they mean the tropical
rainforests located near the equator. These forests receive
between 160 and 400 inches of rain per year. The total annual rainfall is spread
pretty evenly throughout the year, and the temperature rarely dips below 60
degrees Fahrenheit. This steady climate is due to the position
of rainforests on the globe. Because of the orientation of the Earth’s axis, the
Northern and Southern hemispheres each spend part of the year tilted away from
the sun. Since rainforests are at the middle of the globe, located near the
equator, they arc not especially affected by this change. They receive nearly
the same amount of sunlight, and therefore heat, all year. Consequently, the
weather in these regions remains fairly constant. The
consistently wet, warm weather and ample sunlight give plant life everything it
needs to thrive. Trees have the resources to grow to tremendous heights, and
they live for hundreds, even thousands, of years. These giants, which reach 60
to 150 ft in the air, form the basic structure of the rainforest. Their top
branches spread wide in order to capture maximum sunlight. This creates a thick
canopy (树冠) level at the top of the forest, with thinner greenery levels
underneath. Some large trees grow so tall that they even tower over the canopy
layer. As you go lower, down into the rainforest, you find less
and less greenery. The forest floor is made up of moss, fungi, and decaying
plant matter that has fallen from the upper layers. The reason for this decrease
in greenery is very simple the overabundance of plants gathering sunlight at the
top of the forest blocks most sunlight from reaching the bottom of the forest,
making it difficult for robust plants to thrive. The Forest for the
Trees The ample sunlight and extremely wet climate of many
tropical areas encourage the growth of towering trees with wide canopies. 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.
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. 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(养分)they need, which is important since they don’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 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. As we are still ignorant of millions of unique life forms in the rainforest, deforestation can be compared to the destruction of ______.