the atmosphere during photosynthesis. In this process, the producers use carbon from the carbon dioxide to produce other
carbon-containing molecules. These molecules include sugars
and starches. To obtain energy from these molecules, consumers
break them down into simpler molecules. Consumers release
water and carbon dioxide as waste products.
At the same time, oxygen is also cycling through the ecosystem. Producers release oxygen as a result of photosynthesis.
Other organisms take in oxygen from the atmosphere and use it
in their life processes.
wf e~
How is oxygen returned to the environment?
The Nitrogen Cycle
Figure 10 In the nitrogen cycle,
nitrogen moves from the air to the
soil, into living things, and back
into the air.
Interpreting Diagrams How do
consumers obtain nitrogen?
Like carbon, nitrogen is a necessary building block in the
matter that makes up living things. Since the air around you is
about 78 percent nitrogen gas, you might think that it would be
easy for living things to obtain nitrogen. However, most organisms cannot use the nitrogen gas in the air. Nitrogen gas is
called "free" nitrogen, meaning it is not combined with other
kinds of atoms. Most organisms can use nitrogen only once it
has been "fixed," or combined with other elements to form
nitrogen-containing compounds. You can follow this process in
Figure 10 below.
~
Free nitrogen in air
Bacteria release
some free nitrogen
back to air
Bacteria in root nodules
fix free nitrogen
into compounds
54. E
Nitrogen Fixation The process of changing free nitrogen gas
into a usable form of nitrogen is called nitrogen fixation. Most
nitrogen fixation is performed by certain kinds of bacteria. Some
of these bacteria live in bumps called nodules (NAHJ oolz) on the
roots of certain plants. These plants, known as legumes, include
clover, beans, peas, alfalfa, and peanuts.
The relationship between the bacteria and the legumes is an
example of mutualism. As you recall from Chapter 1, a symbiotic
relationship in which both species benefit is called mutualism.
Both the bacteria and the plant benefit from this relationship:
The bacteria feed on the plant's sugars, and the plant is supplied
with nitrogen in a usable form .
..Jt:'!.g~.~1!!.~~ Many farmers make use of the nitrogen_ ~}
TECHNOLOGY
fixing bacteria in legumes to enrich their
fields. Every few years, a farmer may plant a legume such as
alfalfa in a field. The bacteria in the alfalfa roots build up a new
supply of nitrogen compounds in the soil. The following year,
the new crops planted in the field benefit from the improved soil.
Once the nitrogen has been fixed into chemical compounds, it can be used by
organisms to build proteins and other complex substances.
Decomposers break down these complex compounds in animal
wastes and in the bodies of dead organisms. This returns simple
nitrogen compounds to the soil. Nitrogen can cycle from the soil
to producers and consumers many times. At some point, however, bacteria break down the nitrogen compounds completely.
These bacteria release free nitrogen back into the air. Then the
cycle starts again.
, Return of Nitrogen to the Environment
:
Figure 11 Lumpy nodules are
clearly visible on the roots of this
clover plant. Bacteria inside the
nodules carry out nitrogen fixation.
f Check Your Progress
1. Name and define the three major processes that
occur during the water cycle.
2. Explain the role of plants in the carbon cycle.
3. How is nitrogen fixation a necessary part of the
nitrogen cycle?
4. Where do nitrogen-fixing bacteria live?
5. Thinking Critically Comparing and
Contrasting Explain how the movement
of
matter through an ecosystem is different than
the movement of energy through an ecosystem.
~~~fil~/.l
.
:
:
:
Once your teacher has
approved your plan, place the
waste into your compost chambers.
Record your hypothesis about the effect
f of the variable you are investigating.
: Observe the two containers daily. (Hint: If
: there are no signs of decomposition
after
: several days, you may wish to stir the
contents of each chamber. Stirring allows
more oxygen to enter the mixture.)
Chapter 2
E. 55
~C.TI~,.,
"~ Earth's Biomes
DISCOVER
...........................................
~~.
How Mu(h Rain Is That?
The table shows the average amount of precipitation that falls each year in four different
regions. With your classmates, you will create a
full-size bar graph on a wall to help you visualize
these amounts of rain.
Blome
I
I
I
Rainfall (em)
Mojave Desert
15
Illinois prairie
70
Smoky Mountains
180
Costa Rican rain forest
350
1. Using a meter stick, measure a strip of addingmachine paper 15 centimeters long. Label this
piece of paper "Mojave Desert."
• What determines the type
of biome found in an area?
• Where can photosynthesis
occur in water biomes?
III
Reading Tip As you read,
make a list of the biomes
described in this section.
Under each biome name,
take notes on the characteristics of that biome.
2. Repeat Step 1 for the other three locations. If
necessary, tape strips of paper together to
make the correct length. Label each strip.
3. Now find a place where you can display the
four strips vertically. Ifthe wall of your
classroom is not tall enough, you may need
to use another wall in your school building.
Follow your teacher's instructions to hang
your precipitation strips.
Think It Over
Which ecosystem
receives the most precipitation? Which receives
the least? What effect do you think the amount
of rainfall might have on the types of species that
live in these ecosystems?
Developing Hypotheses
ongratulations! You and your classmates have been selected
as the student members of an around-the-world
scientific
expedition. Your mission is to study the major types of
ecosystems on Earth. You will be collecting data on the climate
conditions and typical organisms found in each of these ecosystems. The result of this expedition will be a database of information on the biomes you visit. A biome is a group of ecosystems
with similar climates and organisms.
Classifying ecosystems into biomes helps ecologists describe
the world. As you might expect, not all ecologists agree on the
exact number and kinds of biomes. The scientists guiding your
expedition have chosen to focus on six major land biomes and two
major water biomes.
Be sure to pack a variety of clothing for your journey.
During your trip, you will visit places ranging from
frozen, windy Arctic plains to-steamy tropical jungles.
In fact, it is mostly the climate conditions-temperature and rainfall-in
an area that determine its
biome. This is because climate limits the distribution
of plants in the area. In turn, the types of plants
determine the kinds of animals that live there.
C
Rain Forest Biomes
Tropical rain forest
Temperate rain forest
Rain Forest Biomes
The first stop on your expedition is a
tropical rain forest close to the equator.
The rain forest is warm and humidin fact, it's pouring rain! Fortunately,
you remembered to pack a poncho.
After just a short shower, the sun reappears. But even though the sun is
shining, very little light penetrates the
thick vegetation.
Plants are everywhere in the rain
forest.Some, such as the ferns, orchids,
and vines you observe hanging from
tree limbs, even grow on other plants.
Among the plants are many species of
birds as bright as the numerous flowers
all around you.
Tropical Rain Forests Tropical rain
forests are found in warm regions
close to the equator. Tropical rain
forests typically receive a lot of rain.
The warm temperatures do not vary
much throughout the year, and the
sunlight is fairly constant all year.
Tropical rain forests contain an astounding variety of
species. For example, scientists studying a 100-square-meter
area of one rain forest identified 300 different kinds of trees!
These trees form several distinct layers. The tall trees form a
leafy roof called the canopy. A few giant trees poke out above
the canopy. Below the canopy, a second layer of shorter trees
and vines form an understory. Understory plants grow well in
the shade formed by the canopy. Finally, some plants thrive in
the near-darkness of the forest floor.
Figure 16 Tropical rain
forests contain an amazing
variety of plants and other
organisms. In the large photo,
a river winds through the lush
Indonesian rain forest. The top
closeup shows a young
orangutan swinging from tree
limbs. In the bottom closeup,
a tarantula climbs over a
brightly colored bracket
fungus on the forest floor.
Chapter 2
E. 63