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Theme VI
Interaction and
Interdependence
An Introduction
to Ecology
24
After you have finished reading this chapter, you should be able to:
Define
ecology and ecosystems.
Identify
some biotic and abiotic factors and explain their effects on
organisms.
Discuss
the ways in which limiting factors affect organisms.
If the Earth were only a few feet in diameter, floating a few feet above a field
somewhere, people would come from everywhere to marvel at it.
Anonymous
Introduction
An aquarium is a complete small world in itself. Plants live together with
animals in one place. By looking closely, you can see a snail crawling
along the leaf of an underwater plant. Elsewhere on the plant, hidden
between some leaves, are masses of snail eggs surrounded by small globs
of clear jelly. Within the eggs are the individual dark spots that are the
developing snail embryos. A goldfish swims slowly by. The mouth of the
fish opens and closes again and again. Water enters its mouth, bringing
the oxygen the fish needs. The fish takes a bite out of a plant leaf for food,
perhaps eating some snail eggs, too. (See Figure 24-1 on page 512.)
The aquarium is near a window. Light pours into the tank, and the
plants grow toward the sun. The plants are anchored by their roots in a
mixture of crushed stones and gravel. In the corner of this small aquatic
world is a plastic tube that releases bubbles. The fish swim in and out of
the bubbles that form a curtain of air.
Why do some people spend long periods of time gazing quietly into
an aquarium? Some answers become obvious upon reflection.
An aquarium is a self-contained miniature world of life. The basis of
511
Figure 24-1 The
aquarium fulfills
the life-support
needs of all its
inhabitants.
life on Earth can be found in an aquarium. Like the living things in an
aquarium, every organism on Earth lives in its surroundings, its environment. No living thing exists alone. All living things interact, or affect
other living things and their environment. All living things depend on
each other and their environment—they are interdependent. These relationships of interaction and interdependence between living organisms
and their environment are studied in the branch of biology known as
ecology.
■■ ECOLOGY AND ECOSYSTEMS
Every plant and animal lives somewhere. The environment in which an
organism lives is molded by many different factors. These include availability of food and water, amount of sunlight, temperature, and type of
soil in the area. If these conditions involve other living organisms, they
are known as biotic factors. For a bird, biotic factors include, but are not
limited to, trees, other birds, insects, and worms. Other conditions involve
nonliving or abiotic factors. For example, some abiotic factors for a bird
are water, wind, temperature, and light.
Earth is a living planet. It is far too complex to try to understand all
at once. However, the interaction and interdependence of organisms and
their environment can be understood by examining specific areas. Ecologists call a specific place an ecosystem when all the living organisms
and nonliving factors in that one place are considered together. A pond
is an ecosystem. A forest is an ecosystem. So, too, is a rocky seashore. Our
Figure 24-2
Earth is the
ultimate
ecosystem—
it fulfills the
needs of all its
inhabitants.
aquarium, as small as it is, is also an ecosystem and can therefore be used
to study ecology. (See Figure 24-2.)
■■ ADAPTATIONS AND EVOLUTION
It is truly amazing how so many organisms seem to fit so well with their
environment. Camels have extremely wide two-toed feet with skin that
stretches between the toes. With these feet, camels can avoid sinking into
the desert sand. (See Figure 24-3 on page 514.) In fast-moving tropical
streams, some species of catfish have suckers on the bottom of their bodies. The suckers hold onto a rock and thus prevent the catfish from being
swept downstream. The larval stage of the tiger swallowtail butterfly
resembles a bird’s dropping. With this disguise, it is not the first choice of
a hungry bird searching for a meal!
In a tropical rain forest, the fertile soil is extremely shallow. Most of the
nutrients available for plants are located in this thin layer. Many rain forest trees have shallow root systems. To support themselves upright, these
trees have wide supports called buttresses. The tree’s buttresses, located at
514 Interaction and Interdependence
Figure 24-3 The wide, two-toed feet of camels enable them to walk in sand
without sinking.
the base of their trunks, support the tree in much the same way as the
stone buttresses of a Gothic cathedral support its walls. (See Figure 24-4.)
The giant saguaro cactus is able to take in enormous amounts of water
during the rare desert rainstorms. The stem of this cactus, once it is
swollen with water, can keep the plant alive throughout the long dry
months ahead. (See Figure 24-5 on page 516.) The tiny pond skater insect
has a waxy substance on its six feet. The wax keeps the insect from breaking through the top layer of water molecules, and it can easily walk across
the surface of a pond. Wherever we look on this living planet, we see
more examples of adaptations that enable organisms to survive Earth’s
vastly different living conditions. How did all this happen?
First, we must be clear about what did not happen. No individual
organism intentionally changed to survive in a particular environment.
No individual rosebush purposely developed brighter flowers to attract
more insects. No individual arctic hare intentionally became whiter to
hide in the snow.
What did happen is that individuals in a population have always been
somewhat different from one another. Sometimes the differences are obvious. Two dogs from the same litter, for example, might be very different
sizes or colors. Sometimes the differences are not obvious to an observer.
A slight difference in the biological makeup of an organism, for example,
the ability to make a particular enzyme, might provide that organism
Chapter 24 / An Introduction to Ecology 515
Figure 24-4 Some rain-forest
trees with shallow roots have
woody buttresses for support.
with a slight survival advantage over other organisms. In the examples
given, not all individuals in the hare population or in the rose population
are identical. These variations in individuals are due to inherited genetic
differences. Because of the differences, some individuals are better suited
to certain environmental factors. In an ancestral population, some hares
had somewhat lighter-colored fur and could hide better. Those hares
would have been more likely to be missed by a predator and survive to
pass on their genetic traits to future generations. It is through this process
of natural selection that a species, not individual organisms, evolves. This
is the basis of evolution. Over time, a species’ traits make a remarkable fit
with its environment. If they do not, the species will probably not survive
in that particular environment. The characteristics of an organism that
give it this fit are called adaptations. Adaptations can be in physical traits,
such as the size, shape, or color of an organism. They can also be in behavioral traits, such as the building of nests by birds or the yearly release of
seeds by plants. Humans, too, have many physical and behavioral
adaptations.
Every organism is therefore adapted through evolution to live in a
particular place. Each species of organism is adapted to a specific set of
516 Interaction and Interdependence
Figure 24-5 The giant
saguaro cactus, which
lives in the desert, stores
water in its stem.
conditions. The place where an organism lives is its habitat. The habitat
of a bullfrog is a pond. The habitat of a giant anteater is open grassland.
An organism’s habitat is its “address.”
To understand an organism’s relationship to its environment, we must
know more than its address, or habitat. We must know what it does, that
is, its “occupation.” The occupation of an organism is called its niche.
The niche of an organism includes how it gets its food, reproduces, and
avoids predators, among other things. The behavioral adaptations of an
organism make up its niche. These adaptations are the result of evolution
just as are the organism’s physical adaptations. For example, woodpeckers that were best able to find insects in the bark of trees survived. Their
offspring inherited a whole series of structural and behavioral adaptations. The niche of an organism determines its habitat. In other words,
the ways that an organism has evolved to survive will also determine
where it can live. A woodpecker cannot live in the grasslands. Its niche
involves finding its insect food in the trunks of trees. Woodpeckers need
trees, and the insects that live in them, to survive.
Chapter 24 / An Introduction to Ecology 517
Just how important niches are has been made clearer with new
research on fossils reported in 2002 by scientists at the University of California–Berkeley. Their investigations on mass extinctions of long ago
showed that it took much longer than had been thought for the diversity
of species to return to Earth. The report stated, “One possible explanation for why diversification takes so long after an extinction is that extinction eliminates not merely species or groups of species, but takes away
ecological niches. It eliminates both organisms and the roles those organisms played in the ecosystem. Recovery thus becomes more complicated.”
■■ ENVIRONMENTAL FACTORS
Evolution occurs by natural selection. Natural selection is the process by
which the organisms that are best adapted survive. Now we can ask
another question: Adapted to what? Every organism is adapted to the conditions present in its environment. To understand why a plant grows the
way it does, or why an animal behaves the way it does, we must look at
the factors present in the environment of each organism.
If you have a garden, you already know that certain plants grow best
under particular conditions. (See Figure 24-6.) One type of plant grows
best in full sun, while other plants grow best in shade. Some plants
grow best under dry conditions, while others grow best in high humidity. Animals also thrive under certain conditions. Organisms often have
Figure 24-6 The conditions present in a garden determine which plants will thrive
there. Some plants prefer shade; others prefer more direct sunlight.
518 Interaction and Interdependence
preferences and thrive best in environments that approximate these conditions. Environmental factors can therefore limit where organisms live.
Each species is adapted to a specific set of conditions. These factors are
called limiting factors.
For example, the availability of sunlight is an important limiting factor for plants. Tall trees living in a tropical rain forest near the equator
receive a great deal of sunlight. Areas far to the north and south of the
equator receive less intense sunlight. The amount of sunlight in the
oceans and lakes also varies a great deal. Light penetrates water only to a
depth of about 200 meters. Below that depth, it is too dark for plants to
grow.
Temperature, of course, is another major limiting factor. Most organisms live in the range that is above 0°C and below 42°C. A few species of
bacteria can survive in conditions well outside of this temperature range.
Some live in hot springs, for example, where the water temperature is
nearly at the boiling point. Each type of plant or animal or microorganism has a fairly narrow temperature range that it prefers. In other words,
the organism has a tolerance only for temperatures within this range. Fish
show this tolerance range very clearly. If you measure water temperatures
at different depths in a lake during the summer, you will find that the
temperature drops as the depth increases. Each type of fish in the lake
has a different tolerance range for temperature. A species that prefers
warmer water lives in the relatively shallow regions or near the water’s
surface. A fish species that prefers colder water lives at greater depths. This
can also help explain why fish generally move to deeper water during hot
summer months. The water near the lake’s surface becomes too warm for
many fish to survive in, since warmer water contains less dissolved oxygen. (See Figure 24-7.)
5
10
Figure 24-7 A lake’s
temperature decreases with
increasing depth.
Lake depth (m)
0
15
5
10 15
0
Water temperature (°C)
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 24-7 s/s
Chapter 24 / An Introduction to Ecology 519
Figure 24-8 The maple is a deciduous tree.
All life depends on water. However, organisms have evolved adaptations that enable them to survive
in very specific ranges of available moisLIVING ENVIRONMENT BIOLOGY, 2e/fig. 24-8 s/s (rev.10/22/03)
ture. Deciduous trees, such as maples, have large, flat leaves. Because
water is constantly moving out of the openings on the leaf surfaces, trees
with large, flat leaves live in areas with abundant rainfall. Such trees can
live only where rainfall exceeds 100 centimeters per year. (See Figure
24-8.) Other species of trees have evolved ways to save water. Coniferous
trees, such as pines, have narrow leaves called needles. Little moisture is
lost from this type of leaf; thus, coniferous trees can survive in areas that
have less water available. (See Figure 24-9.)
Other environmental factors are much less obvious, but they are still
very important for a species’ survival. For example, chemical nutrients in
the environment may determine which organisms survive in a particular
place. Two such nutrients are the elements nitrogen and phosphorus. Not
only do these substances have to be present in adequate amounts for many
White pine
Cone on branch of
white pine tree
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 24-9 s/s
Figure 24-9 The pine is
a coniferous tree.
520 Interaction and Interdependence
plants, they also have to be present in the right forms. Farmers have long
been aware of this. In many cases, soil contains less than the minimum
amount of nutrients needed by plants to grow well. Many crops cannot be
grown unless fertilizers are added to the soil. Fertilizers increase the
amounts of nitrogen and phosphorus to levels needed by the crops.
Almost every gardener has experienced another important limitation.
While too little of a limiting factor may prevent the growth of an organism, too much may also limit the organism’s growth. Some gardeners
think that the more fertilizer they give their plants, the more flowers and
vegetables they will get. To their surprise, they find that their plants often
die when greater amounts of fertilizer are added to the soil. Organisms
have a tolerance range, sometimes a very narrow one. The amount of an
environmental factor should be neither too low nor too high. The tolerance range determines the best conditions for a specific type of organism
in a specific location.
Check Your Understanding
Discuss two ways biotic factors and two ways abiotic factors can
affect the life of a tree.
■■ SALTWATER ECOSYSTEMS
Because we live on land, we tend to think of Earth as a land planet. However, 75 percent of Earth’s surface is covered by water. One of the main
reasons that life exists on Earth, and probably not on other planets in our
solar system, is that only the temperatures present on Earth allow for the
existence of liquid water. On Earth, life evolved first in water, not on land.
Today, the climates of all areas, including land areas, depend heavily on
the parts of Earth’s surface covered by water. Because water is so important to life, we will begin our study of ecosystems by looking at those in
the water.
A map of the world shows individual oceans. A closer look, however,
shows that all the world’s oceans are connected. There is really one world
ocean. Some ecologists consider this ocean to be one tremendously large
ecosystem. Scientists call a very large area in which a common group of
organisms lives a biome. Because the oceans share many of the same types
of organisms, we will call this environment the ocean, or marine, biome.
Two of the main limiting factors in the ocean biome are the saltiness,
or salinity, and the temperature of the water. We drink freshwater—water
that contains little or no salt. However, water in the ocean contains about
Chapter 24 / An Introduction to Ecology 521
3.5 percent salt. Anyone who swims in the ocean knows the taste of salt
water and probably also knows that it is not a good idea to drink it.
The actual amount of salt in ocean water varies. As the amount of salt
varies, the density of the water also changes. The more salt, the denser the
water. This is why a person can float easily in very salty water, while it is
difficult for many people to float at all in freshwater.
The temperature of ocean water also differs from place to place. Temperature affects water density, just as the level of salt does. Cold water is
more dense than warm water. Because of density differences, water in the
ocean is constantly moving. Denser water sinks, while less dense water
rises. These water movements, along with winds on the surface, cause
currents to occur in the ocean. Some ocean currents are so large that they
actually form “rivers” in the ocean. The Gulf Stream is an example of a
major ocean current. It originates in the Gulf of Mexico, travels north
along the east coast of the United States, and moves across the Atlantic
Ocean to bring warm water toward Great Britain. In fact, some of the
islands off the coast of Britain are so warm that semitropical plants can
survive there. (See Figure 24-10.)
Light is another important limiting factor in oceans. Different parts of
the ocean have different sets of conditions. Because of this, the ocean is
considered to be made up of different zones. The top zone in the ocean
down to about 200 meters is where sunlight penetrates. Below this depth,
it is always completely dark. Because plants need light to make food
St
G
ul
rea
m
f
Figure 24-10 The Gulf Stream is one of Earth’s ocean currents.
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 24-10 s/s
522 Interaction and Interdependence
Intertidal
zone
Neritic zone
Figure 24-11
Sunlight
penetrates
about 200
meters below
the ocean’s
surface.
Continental
shelf
Pelagic zone
Oceanic zone
Photic
zone
200 m
Aphotic
zone
Benthic zone
through photosynthesis, all plant life in the ocean lives only in the top
zone. (See Figure 24-11.)LIVING ENVIRONMENT BIOLOGY, 2e/fig. 24-11 s/s
Another zone occurs at the very edge of the ocean. This zone experiences very unusual environmental conditions—it varies, at different times
of the day, between being either wet or dry. Why do alternating wet and
dry conditions occur here?
The force of gravity from the moon and sun pulls on Earth. The land
does not move, but the water does. The water in the world’s oceans therefore rises and falls twice each day. These movements of the ocean water
are called tides. At high tide, the shoreline of the ocean is covered by
water. At low tide, the same place is left exposed to the air. The area that
is alternately covered with water and left exposed to the air is called the
intertidal zone. Only a very special group of plants and animals can live
under the unique environmental conditions present in the intertidal
zone. (See Figure 24-12.)
Moving into the ocean away from the shore, the water is quite shallow. Because light penetrates the water in this near-shore zone, a great
many organisms live here. Much of the abundant sea life caught for food
by humans inhabits the near-shore zone. Unfortunately, this important
part of the ocean is often mismanaged by humans. The near-shore zone
is where some of the greatest harm is being done to the ocean and the life
it supports. Pollution, mostly from human causes, and overfishing are
two major problems in this part of the ocean.
Finally, the great open areas of the ocean—where the water is tremendously deep—make up another zone. This zone has its own conditions.
Because the bottom is far beyond the reach of sunlight, photosynthesis
occurs only in organisms that float near the surface of deep water. These
tiny photosynthetic floating microorganisms are called phytoplankton.
Chapter 24 / An Introduction to Ecology 523
Blue-green
bacteria
and algae
Upper
intertidal
zone
Snails
Limpets
Barnacles
Dog whelk
Rockweed
Midintertidal
zone
Mussels
Sea star
Lower
intertidal
zone
Fish
Sea urchin
Seaweeds
Sand dollar
Figure 24-12 The intertidal zone is home to a special group of plants and
animals.
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 24-12 s/s
Phytoplankton are an extremely important source of food for much of
the life in the ocean. (See Figure 24-13 on page 524.)
■■ FRESHWATER ECOSYSTEMS
Less than three percent of Earth’s water is freshwater. Of this, two-thirds
is frozen near the North and South Poles and at the tops of mountains.
Of the liquid freshwater, most of it is stored in the ground. We must take
524 Interaction and Interdependence
Figure 24-13 Phytoplankton are photosynthetic microorganisms that float near the surface of the ocean.
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 24-13 s/s
great care to protect this groundwater. A very serious environmental
problem is the rapid use of too much groundwater and the polluting of
what is left.
There are two main types of freshwater ecosystems on the surface of
Earth. Lakes and ponds, bodies of water that are usually still, make up
one type. In these, temperature and light are the main limiting factors.
The winter temperatures in cold climates may cause the water to freeze.
This has a particular effect on the life of a lake or pond ecosystem. The second type of freshwater ecosystem consists of rivers and streams. In these,
the water always moves. In a running-water ecosystem such as a river, the
temperature and the amount of light are fairly constant at any given
point. However, a river or stream is different at various places along its
length. Near its beginning high in the mountains, the river is young; it is
cold, flowing swiftly, and usually clear. Near its end at the ocean, the river
is old; the water has become warmer, moves more slowly, and is often
muddy. Trout are powerful swimmers and are well adapted to the swiftmoving water of young rivers and streams. Freshwater bass, on the other
hand, live well among the thick growth of plants in an older, slower river.
■■ LAND ECOSYSTEMS
For an interesting reason, life on land is much more diverse than life in
water. Conditions in water are fairly constant over wide areas. Conditions
in water also change little over time. When conditions do change in
water, they change slowly.
Chapter 24 / An Introduction to Ecology 525
Conditions on land are very different. From season to season, and
from one part of the day to another, temperatures on land can vary
widely. The temperature in a desert can be above 50°C during the day and
drop to well below freezing at night. Conditions on land can change rapidly over relatively short distances, particularly when changes in altitude
are involved. Life in a valley differs from life on either side of the valley
as you move away from the valley floor.
Variations in temperature, amount of moisture, soil type, length of
days and nights, and seasonal variations, along with altitude, operate
together to produce different land ecosystems. Sometimes humans have
found out the hard way just how very diverse land ecosystems can be.
For example, people who have moved to new lands have tried to grow
crops in the same way in their new home as they did in the place they
came from. The failure of the crops to grow well made it very clear that
too little was understood about the new environment. The study of ecology has many practical applications.
Ecologists use the idea of biomes to describe and study life in various
land ecosystems. A land biome is a large area in which one main type of
Temperature (°C)
32.2
Desert Grassland
Tropical forest
Temperate forest
15.6
Coniferous
forest
–1.1
Arctic and
alpine tundra
0
102
406
305
203
Mean annual precipitation (cm)
Figure 24-14 The
temperature and
moisture in a biome
determine which plants
will grow there.
plant lives. It is mainly a biome’s climate—that is, its temperature and
moisture—that determines which plants will grow. (See Figure 24-14.)
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 24-14 s/s (rev.10/22/03)
Therefore, a map of land biomes looks approximately the same as a map
of the main climate zones on Earth. Because animals depend on plants as
a food source, a biome over one part of the world usually has the same or
similar types of animals as that biome in another part of the world. Let’s
tour the main land biomes of Earth. (See Figure 24-15 on page 526.)
Trees are obviously an important type of plant on land. In fact, there
526 Interaction and Interdependence
Desert
Grassland
Tropical rain forest
Deciduous forest
Taiga
Tundra
Figure 24-15 Land biomes of the world.
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 24-15 s/s
are three main forest biomes on Earth, each with characteristic types of
trees: tropical rain forests, deciduous forests, and coniferous forests.
Tropical rain forests exist in a wide band between the Tropic of Cancer and the Tropic of Capricorn, north and south of Earth’s equator. In the
tropical area, there are more than 250 centimeters of rainfall each year.
The warm temperatures and the hours of daylight remain roughly equal
throughout the year. Abundant life of all kinds exists in the tropical rain
forest biome. A small area in a tropical rain forest may contain more than
100 tree species, 300 orchid species, and thousands of species of animals,
most of them insects. Monkeys, bats, squirrels, parrots, snakes, frogs, and
lizards are just a few of the kinds of animals that inhabit this biome. Conditions in a tropical rain forest vary. At ground level, it is dark because
little light penetrates the leaves of trees that grow overhead. Plants that
live on the floor of a tropical rain forest are adapted to life under conditions of low light. Many houseplants, also adapted to low-light conditions, are originally from the tropical rain-forest floor. It is also moist on
the floor. Higher up in the trees, it is cooler, windier, and brighter. (See
Figure 24-16.)
Dead plants and animals decay very quickly in the warm, moist conditions on the rain-forest floor. However, rain quickly washes away most
soil nutrients. The nutrients that remain are confined to the few top cen-
Chapter 24 / An Introduction to Ecology 527
Bright
light
Emergent
Upper
canopy
Filtered
light
Lower
canopy
Understory
Shade
Thin layer of
leaf litter and topsoil
Figure 24-16 Conditions on the rain-forest floor are warm, moist, and dark; higher
up in the trees, it is cooler, windier, and brighter.
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 24-16 s/s
timeters of soil. As a result, tropical rain-forest soils are poor soils. Farmland created by destroying tropical rain forests quickly becomes useless as
the nutrients are used up. As a result, more tropical rain forests are being
destroyed to make more farmland. This ever-increasing need for farmland to grow crops is the main reason tropical rain forests—along with the
remarkable diversity of life they contain—are destroyed.
Farther north and south of the equator, climate patterns change. Definite seasons, from hot wet summers to cold drier winters, occur. The seasonal nature of these areas creates the deciduous forest biome. Here, fewer
tree species are found, although there are many individual members of
the same tree species. More light reaches the forest floor where a wide
variety of plants live. Deciduous trees, such as maple, beech, hickory, and
oak, lose their leaves each autumn. This seasonal dropping of leaves is
528 Interaction and Interdependence
El Niño and La Niña
Will the risk of a major hurricane in Florida be higher than normal this
summer? Will Buffalo, New York, one of the snowiest cities in America, have
another winter with no snowfall? Will intense Pacific storms cause
destructive mudslides that bring soil and rocks crashing down hillsides
along the California coast?
Questions like these have been asked in the United States and in other
countries for years. The answers to these questions focus on the effects
produced by two atmospheric events: the “little boy” El Niño and the “little
girl” La Niña. Both El Niño and La Niña events occur regularly in the Pacific
Ocean.
El Niño is a periodic warming of the water in the eastern Pacific Ocean near
the equator. La Niña—sometimes called the cooler sister—is marked by
cool waters near the equator in the Pacific. Both of these events change the
atmosphere above the ocean. A change in the wind causes a change in the
movement of the ocean’s surface waters. Changes in water temperature
change the amount of moisture that leaves the ocean’s surface and enters
the atmosphere.
El Niño events tend to transfer heat from the ocean to the atmosphere. This
increases the strength and speed of the jet stream—the high winds that
move from west to east across the United States—and contributes to
relatively predictable climatic patterns. During La Niña, the colder Pacific
water near the equator weakens the jet stream that helps to keep our
weather constant. As a result, La Niña events cause unusual weather
patterns in the United States. El Niño events occur every three to seven
years. Because the effects of El Niño and La Niña are opposite each other,
we are always living in one event and heading toward the other. Without
a doubt, we will continue to learn more about how changes in water
temperatures in the Pacific Ocean are related to changing weather patterns
that occur in distant parts of the world and right at home.
typical of trees in these areas. Dropping their leaves prevents trees from
losing water during cold weather. Deciduous forest animals include
wolves, gray foxes, deer, raccoons, squirrels, birds, and insects. The soil in
temperate deciduous forests is much richer than the soil in a tropical rain
forest.
Farther north of the equator are the great northern coniferous forests
of North America, Asia, and Europe. The main plants in these forests are
the needle-leaved evergreen trees, such as spruce, pine, fir, hemlock, and
cedar. These are called conifers, or coniferous trees, because they produce
their seeds in cones. In a coniferous forest, the winter is long; the warmer
Chapter 24 / An Introduction to Ecology 529
growing season is short. Because of cold temperatures over much of the
year, little decay occurs. These forest floors develop a thick layer of dead
needles.
Farthest from the equator, the temperatures are too low, and the
amount of precipitation too small, for trees to grow. Only one or two
months of the year are warm enough to support plant growth. The biome
here is the tundra. Only the upper layer of the soil warms during the summer. Below this layer, the soil remains permanently frozen all year. A thick
layer of plants, such as mosses, lichens, grasses, and low shrubs, covers the
surface. Although life in the tundra is harsh, more animals live there than
you might expect. These include caribou, arctic foxes, snowy owls, great
numbers of insects, and migrating geese and ducks during the summer.
So far, in our tour of land biomes, we have been following decreasing
temperatures as we moved away from the equator. If we explored climates
where precipitation—but not temperature—decreases, we would first find
ourselves in grasslands. Grasslands are usually found in the interiors of
continents, where limited moisture prevents the growth of trees. With
less than 100 centimeters of rain per year, only grasses can grow. In different places in the world, grasslands have different names. Grasslands
are called prairies or plains in North America, steppes in Russia, savanna
in parts of Africa, veldt in South Africa, and pampas in South America.
The rich soil that has slowly developed in grasslands has created some of
the best agricultural land on Earth. Grasslands produce most of the
world’s grain crops. Because of this, grasslands are often called the
“world’s breadbasket.” (See Figure 24-17.)
Figure 24-17
Wildebeest
migrate across
the grasslands, or
savanna, of Africa
in search of food.
530 Interaction and Interdependence
Large animals that eat grasses are native to grasslands. These include
bison and antelope. In many areas, domestic cattle and sheep have
replaced the native grazing animals. Grasshoppers, prairie dogs and other
small rodents, hawks, and snakes are also typical grassland animals.
Finally, our tour brings us to the driest places on Earth, which produce
the desert biomes. In a typical desert, temperatures may be very high during the day. However, because of limited water vapor in the air, the heat
accumulated during the day is quickly lost at night. As a result, temperatures fall dramatically at night. Plants include water-storing cactuses,
shrubs with roots that grow deep underground to reach water, and wildflowers and grasses that grow and mature in the short period of time after
the rare rain. Animals also show adaptations in this area of limited water
availability. The kangaroo rat lives underground for much of the day. It
gets its water from the tissues of green plants and by breaking down the
fat in dry seeds that it eats. Predators such as coyotes and foxes are often
active at night to avoid and survive the harsh desert conditions.
The table below lists the six major land biomes and their main
features.
THE LAND BIOMES OF EARTH
Biome
Features/Conditions
Tropical rain forests
Found just north and south of equator; have more than 250 cm rain
per year; warm temperatures and stable day length all year; abundant and diverse forms of plant of animal life.
Deciduous (temperate)
forests
Farther north and south of equator; definite seasons, from hot wet
summers to cold drier winters; compared to tropical rain forests,
have fewer tree and animal species, but richer soil.
Coniferous forests
Farther north than deciduous forest; mostly needle-leaved evergreen
trees; winters are longer; growing season shorter; very little decay of
leaf litter (due to cold).
Arctic & alpine tundra
Farthest north; temperatures and rainfall too low for trees; brief
growing season for mosses, lichens, grasses, shrubs; have resident
and migratory bird and mammal species.
Grasslands
In interior of continents; warm temperatures, but less than 100 cm
rain per year; have rich soil; support large grazing animals (antelope); produce most of world’s grain crops.
Deserts
Driest places (lowest rainfall); temperatures high in day, low at
night; have plants that store water (cactus), and animals that stay
underground during day and are active at night.
EXTENDED LABORATORY INVESTIGATION 24
What Can We Learn by “Adopting” a Tree?
INTRODUCTION
Observations of a single organism in its environment over an extended
time period can provide important information. You can learn about the
relationship of an organism to the physical conditions in its surroundings. Common organisms in our environment are trees. Trees have an
amazing variety of ways to carry out such life functions as producing food
and producing more trees. In many parts of the United States, the best
time to observe trees is during the spring. At this time, deciduous trees
come out of their dormant winter state and begin active growth and
reproduction. In this investigation, you will “adopt” a deciduous tree and
make observations during the spring growing season. You can write your
observations in your science journal. (No laboratory materials required.)
PROCEDURE
1. During February or March, select a deciduous tree that you will be able
to observe daily. No leaves should be present at this time. Your tree
should have at least one branch close to the ground so that you can
observe the ends of the branches.
2. Begin a journal for your periodic observations. Describe the location
of your tree, including specific geographic references so that another
person reading your journal can locate your tree.
3. Describe the appearance of your tree as fully as possible. You can add
drawings or photographs to your description. In your written description include:
◆ overall appearance and shape
◆ the approximate size of your tree: height, spread of branches
◆ bark color and texture
◆ trunk diameter: make this measurement a standard distance from
the ground; 1.5 meters is a good height at which to measure the
diameter
◆ kind of branching
◆ appearance of buds and twigs
Chapter 24 / An Introduction to Ecology 531
4. Begin close-up observations of the buds and twigs on your tree on a
regular basis. Keep daily journal entries when you first observe changes
in the buds and twigs.
5. Throughout your observation period, keep records of the daily high
and low temperatures as well as the sunrise and sunset times. Graph
these data.
6. Write a complete description of your tree as it appears at the end of
May.
INTERPRETIVE QUESTIONS
1. In the library, study the sexual reproduction of trees. Explain how your
observations relate to this process.
2. Research the species of trees that commonly grow in your area. What
features are usually used to identify a species? To what genus and
species does your tree belong?
3. Relate the changes you observed on your tree to the changes in temperature and daylight.
4. What have you learned about the process of observation by keeping
your tree journal?
532 Interaction and Interdependence
Chapter 24 / An Introduction to Ecology 533
■■ CHAPTER 24 REVIEW
Answer these questions on a separate sheet of paper.
VOCABULARY
The following list contains all of the boldfaced terms in this chapter. Define
each of these terms in your own words.
abiotic factors, adaptations, biome, biotic factors, coniferous,
deciduous, ecosystem, groundwater, habitat, limiting factors, niche,
phytoplankton, tides
PART A—MULTIPLE CHOICE
Choose the response that best completes the sentence or answers the question.
1. The interactions among living things and between living things
and their nonliving environment are the focus of the area of
biology known as a. system dynamics b. cryptozoology
c. herpetology d. ecology.
2. An example of an abiotic factor in a pond ecosystem is
a. floating duckweed b. a dragonfly c. mud at the bottom
d. a school of fish.
3. An adaptation for life in the desert might be a. large, flat leaves
with many stomates b. a fleshy, water-storing stem c. long,
sharp thorns on branches d. shedding leaves in winter.
4. Together, the living and nonliving factors in a particular place,
such as a pond or field, are called a. a biome b. abiotic factors
c. an ecosystem d. a population.
5. The land biome that is characterized by a warm, wet climate and
the greatest number of species of plants and animals is the
a. tropical rain forest b. deciduous forest c. coniferous forest
d. tundra.
6. The insects called walking sticks look exactly as their name
suggests. Factors in the walking stick’s environment might include
a. insect eaters that use sight to find their prey b. cactuses that
have a single large, barrel-shaped stem c. brightly colored flowers
d. plants that are poisonous to plant eaters.
7. An organism’s “address,” or where it lives, is best described as its
a. biome b. habitat c. ecosystem d. niche.
8. An organism’s “occupation,” or what it does, is best described as its
a. habitat b. ecosystem c. niche d. biome.
534 Interaction and Interdependence
9. In a desert, the limiting factor for most plants is probably
a. temperature b. sunlight c. nutrients in the soil d. water.
10. Trees that shed their leaves in winter are a. coniferous
b. crepuscular c. benthic d. deciduous.
11. Approximately how much of Earth’s surface is covered by water?
a. 25 percent b. 50 percent c. 75 percent d. 99 percent
12. A very large area characterized by a particular climate and set of
plants and animals is a. an ecosystem b. a niche c. a habitat
d. a biome.
13. In the marine biome, photosynthesis takes place primarily in
a. deciduous plants b. phytoplankton c. coniferous trees
d. grasses.
14. Most of the world’s grain crops are produced in
a. deciduous forests b. grasslands c. coniferous forests
d. tropical rain forests.
15. The biome closest to Earth’s poles is a. deciduous forest
b. grassland c. tundra d. coniferous forest.
PART B—CONSTRUCTED RESPONSE
Use the information in the chapter to respond to these items.
Increasing Latitude
Terrestrial Biomes
16. For each of the following
descriptions, identify the area of
1
the map that most closely fits that
2
description.
a. “Moose, goose, spruce”
4
b. Fields of wheat where bison
3
5
once roamed.
c. Snowy owls hunt mice among
the lichens on frozen subsoil.
d. Monkeys and toucans move in
a warm, humid canopy of leaves.
6
17. Identify the biomes in areas 3, 4,
and 5. Explain why these areas are so different from one another.
18. Distinguish between the two types of freshwater ecosystems.
Explain what effect damming rivers has on these ecosystems.
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 24-Q16 s/s (rev.10/22/0
19. Describe the three major areas of the marine biome. Which area is
most strongly affected by tides?
20. Select an organism that lives in your environment. Make a list of at
least five biotic and five abiotic factors that affect the organism.
Chapter 24 / An Introduction to Ecology 535
PART C—READING COMPREHENSION
Base your answers to questions 21 through 23 on the information below and
on your knowledge of biology. Source: Science News (October 19, 2002): vol.
162, p. 253.
Tests Revise Image of Kangaroo Rats
The textbook case of how to survive in a desert may have important
details wrong, according to new studies of kangaroo rats.
Species in the genus Dipodomys, nocturnal rodents that scurry
through North America’s deserts, have epitomized toughness in punishing climates, says Randall Tracy of the University of Connecticut in
Storrs. Earlier researchers, he says, marveled at how the creatures apparently got water by metabolizing seeds and avoided overheating by staying in cool burrows until late at night.
In an upcoming issue of Oecologia, Tracy and Glenn E. Walsberg from
Arizona State University in Tempe challenge those views. They made
their observations near Yuma, Ariz., in the Sonoran desert.
The animals’ burrows get hotter than expected, the researchers
found. For more than 100 days of the year, soil temperatures rose to
over 30°C at depths of 2 meters. Yet during most of the summer, the
kangaroo rats remained less than a meter deep, where it’s about 35°C.
Nor did the animals emerge only in the cool part of night; they ventured above ground right after sundown. Also, forget the seeds-only
menu. The rodents ate a considerable amount of green plant tissue, presumably a substantial water source during tough times.
21. Explain the two methods that scientists had first thought kangaroo
rats used to survive in the desert.
22. State the research findings that challenge the traditional
explanation of how kangaroo rats stay cool.
23. Explain why scientists now doubt that kangaroo rats get their water
from the metabolism of seeds.