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Ecosystems
and Ecosystem
Management .
Silver Lake on the Croton River, NY; a
typical view of the Eastern Deciduous Forest
of North America, home to the whitetailed
deer.
Learning Objectives
The Acorn Connection
As young children, most people learn that acorns grow
into oak trees. In fact, most acorns do not grow into trees
but become food for mice, chipmunks, squirrels, and deer.
In the woodlands of the northeastern United States,
where oak trees abound, large crops of acorns
(Figure 6.11) are produced every three to four years.
Acorns are rich in proteins and fats and are an excellent
source of nutrition. A steady supply of acorns would be
an excellent food base for the woodland animals.
The production of acorns is affected by the amount of
light and rain, the temperature patterns over the year,
and the quality of the soil. Scientists reason that if oaks
produced the same number of acorns each year, the populations of animals that feed on them would grow so
large that very few acorns would survive.
(a)
(b)
(d)
(e)
The tick that carries Lyme disease (a) feeds on both
the white-footed mouse (b) and the white-tailed deer (c). Oak leaves
(d) are an important food for the deer (c) and for gypsy moths (e),
Figure 6.1 •
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CHAPTER 6 •
In reality, the number of acorns produced varies from
year to year, with "mast" years-years of high production-occurring occasionally. In the years between
bumper crops of acorns, mice populations decline. With
the next bumper crop, there are more acorns than can be
eaten by the consumers of acorns, so many acorns survive to become oaks. Also, because of the abundance of
food, the mice populations increase.
White-footed mice (Figure 6.lb), feeders on acorns,
also carry tick larvae (Figure 6.la). When the ticks
feed on the blood of the mice, they inject the
microorganisms responsible for Lyme disease into the
mice. Mice populations are highest during the
summer following a bumper crop of acorns, and so
are tick larvae.
(c)
(j)
while oak acorns (j) are important food for the mouse. But the
mouse also eats the moths. The more mice, the fewer gypsy moths,
but the more ticks.
ECOSYSTEMS AND ECOSYSTEM MANAGEMENT
In later stages of their life cycle, ticks attach to other
animals, including deer (Figure 6.lc). As deer brush
against plants, ticks are deposited. The ticks can be
picked up by people brushing against the plants as they
walk past. If an infected tick bites a person, the person
may contract Lyme disease. As second-growth forest
area has increased and deer populations have soared,
Lyme disease has become the most common tick-borne
disease in the United States. 1 ' 2
Why has forest area increased? Beginning in colonial
times, the forests of the northeastern United States were
cleared to make space for farming and settlements, to
provide fuel, and to provide timber for commercial uses.
As coal, oil, and gas replaced wood as a primary fuel, and
as farming moved westward to the more fertile Great
Plains, fields that had been cleared were abandoned. In
many areas, the maximum clearing occurred around
1900. Since then, forests have grown back.
But let's return to the mice. In addition to feeding
on acorns (and other grains), mice feed on insects,
including larvae of the gypsy moth. Gypsy moth larvae
(Figure 6.le) feed on leaves of trees and are particularly
fond of oak leaves. Studies suggest that in years when
mice populations are low-the years between bumper
crops of acorns-gypsy moth populations can increase
dramatically. During these periodic outbreaks, gypsy
moth larvae can virtually denude an area, stripping the
leaves from the trees. Oaks that have lost most or all of
their leaves may not produce bumper crops of acorns.
Once the leaves are off the trees, more light reaches
the ground, and seedlings of many plants that could not
do well in deep forest shade begin to grow. As a result,
other species of trees may gain a foothold in the forest
and change its species profile. Of course, the next
generation of gypsy moth larvae find little to eat, and the
population of gypsy moths begins to decline again.
Abundant acorns draw deer into the woods, where
they browse on small plants and tree seedlings. Ticks
drop off the deer and lay eggs in the leaf litter. When
the eggs hatch, the larvae attach to mice, and the cycle
of Lyme disease continues. Deer do not eat ferns,
however, and in areas where deer populations are
dense, many ferns but few wildflowers and tree
seedlings are found.
Predators are also affected by the periodic nature of
acorn crops. For example, birds that feed on gypsy moth
larvae lose a food source when moth populations are
low. When motl1 populations are high, however, bird
nests are more exposed to predation because trees lose
so many leaves. 2
The acorn connection illustrates many of the basic characteristics of ecosystems and ecological
communities. First) all of the living parts of the oak forest community depend on the nonliving parts of the ecosystem for their survival: watery soil) airy and the light that provides energy
for photosynthesis. Second) the members of the ecological community affect the nonliving parts
of the ecosystem. When gypsy moths denude an area) for example) more sunlight can reach the
forest floor. Third) the living organisms in the ecosystem are connected in complex relationships that make it difficult to change one thing without changing many others. Fourth) the
relationships among the members of the ecological community are dynamic and constantly
changing. Many species are adapted to and benefit from a changing environment) as shown
by the advantages provided oaks by varying acorn production. Fifth) the implication for
human management of ecosystems is that any management practice involves trade-offs.
In this case) managing the forest to protect people against Lyme disease only results in more
potential for gypsy moth damage. 1
CASE STUDY
103
Figure 6.2 • The eastern deciduous
forest of the United States is a major
recreational resource, as illustrated
here by these hikers looking out
from a patch of this kind of forest to
the Hudson River, Croton Point,
NY. This makes the acorn
connection all the more of an
environmental problem.
6. 1
The Ecosystem:
Sustaining Life on Earth
We tend to associate life with individual organisms, for the
obvious reason that it is individuals that are alive. But sustaining life on Earth requires more than individuals or even
single populations or species. Life is sustained by the
interactions of many organisms functioning together, in
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CHAPTER 6 •
ecosystems, interacting through their physical and chemical environments. Sustained life on Earth, then, is a characteristic of ecosystems, not of inividual organisms or
populations. 3 To understand important environmental issues, such as conserving endangered species, sustaining renewable resources, and minimizing the effects of toxic
substances, we must understand certain basic principles
about ecosystems.
ECOSYSTEMS AND ECOSYSTEM MANAGEMENT
Basic Characteristics of Ecosystems
Ecological Communities and Food Chains
Ecosystems have several fundamental characteristics.
We have identified an ecological community as the set of
interacting species that makes up the living part of an
ecosystem. In practice, the term ecological community is
defined by ecologists in two ways. One method is to define the community as a set of interacting species found
in the same place and functioning together to make
possible the persistence of life. That is essentially the
definition we used earlier. A problem with this definition
is that it is often difficult in practice to know the entire
set of interacting species. Ecologists therefore may use a
pragmatic or operational definition, in which the community consists of all the species found in an area,
whether or not they are known to interact. Animals in
different cages in a zoo could be called a community according to this definition.
One way in which individuals in a community interact is by feeding on one another. Energy, chemical elements, and some compounds are transferred from
creature to creature along food chains, the linkage of
who feeds on whom . In more complex cases, these linkages are called food webs.
Ecologists group the organisms in a food web into
trophic levels. A trophic level consists of all those organisms in a food web that are the same number of feeding levels away from the original source of energy. The original
source of energy in most ecosystems is the sun. In other
cases, it is the energy in certain inorganic compounds.
Green plants, algae, and certain bacteria produce sugars through the process of photosynthesis, using only the
energy of the sun and carbon dioxide ( C0 2 ) from the air,
so they are grouped into the first trophic level. Organisms
in the first trophic level, which make their own food and
inorganic chemicals and a source of energy, are called
autotrophs. Herbivores, organisms that feed on plants, algae, or photosynthetic bacteria, are members of the second
trophic level. Carnivores, or meat-eaters, that feed directly
on herbivores make up the third trophic level. Carnivores
that feed on third -level carnivores are in the fourth trophic
level, and so on.
Food chains and food webs are often quite complicated
and thus difficult to analyze. A detailed look at one of the
simplest food chains is provided in A Closer Look 6.1. Next,
we look briefly at several more-complicated food chains.
• Structure. An ecosystem is made up of two major
parts: nonliving and living. The nonliving part is the
physical-chemical environment, including the local atmosphere, water, and mineral soil (on land) or other
substrate (in water). The living part, called the ecological community, is the set of species interacting
within the ecosystem.
• Processes. Two basic kinds of processes must occur in
an ecosystem: a cycling of chemical elements and a flow
of energy.
• Change. An ecosystem changes over time and can
undergo development through a process called succession, which is discussed in Chapter 9.
~
The processes that occur in an ecosystem are necessary for
the life of the ecological community, but no member of
that community can carry out these processes alone. That
is why we have said that sustained life on Earth, rather than
individuals or populations, is a characteristic of ecosystems.
We can see this by looking at cycling in an ecosystem.
As mentioned in Chapter 5, chemical cycling is complex.
Each chemical element required for growth and reproduction must be made available to each organism at the
right time, in the right amount, and in the right ratio relative to other elements. These chemical elements must also
be recycled-converted to a reusable form. Wastes are converted into food, which is converted into wastes, which
must be converted once again into food, with the cycling
going on indefinitely, if the ecosystem is to remain viable.
For complete recycling of chemical elements to take
place, several species must interact. In the presence of light,
green plants, algae, and photosynthetic bacteria produce
sugar from carbon dioxide and water. From sugar and
inorganic compounds, they make many other organic
compounds, including proteins and woody tissue. But no
green plant can decompose woody tissue back to its
original inorganic compounds. Other forms of lifeprimarily bacteria and fungi-can decompose organic
matter; but they cannot produce their own food. Instead,
they obtain energy and chemical nutrition from the dead
tissues on which they feed.
Theoretically, at its simplest, the ecological community
in an ecosystem consists of at least one species that produces its own food from inorganic compounds in its environment and another species that decomposes the wastes of
the first species, plus a fluid medium (air, water, or both).
We turn next to a more detailed discussion of ecological communities-in particular, food chains in ecological
communities.
A Terrestrial Food Chain
An example of terrestrial food chains and trophic levels
is shown in Figure 6.5. This north temperate woodland
food web existed in North America before European
settlement and includes human beings. The first trophic
level, autotrophs, includes grasses, herbs, and trees. The
second trophic level, herbivores, includes mice, an insect
6 . I THE ECOSYSTEM: SUSTAINING LIFE ON EARTH
105
Hot Spring Ecosystems in
Yellovvstone National Park
Perhaps the simplest natural ecosystem is
a hot spring such as those found in geyser
basins in Yellowstone National Park,
Wyoming. 4 Few organisms can live in
these hot springs because the environment is so severe. Water in parts of the
springs is close to the boiling point. In addition, some springs are very acidic and
others are very alkaline; either extreme
makes a harsh environment. Some of the
organisms that can live in hot springs are
brightly colored and give these pools the
striking appearance for which they are famous (Figure 6.3).
Typically, the springs have a wide range
of water temperatures, from almost boiling near the source to much cooler near
the edges, especially in the winter, when
there may be snow on the ground next to
a spring. In a typical alkaline hot spring,
the hottest waters, between 70° and 80°C
(158°-l76°F), are colored bright yellowgreen by photosyntl1etic blue-green bacteria, one of the few kinds of photosynthetic
organisms that can survive in hot springs.
In slightly cooler waters, soo to 60°C
(l22-l40°F), thick mats ofbacteria and algae accumulate, some becoming 5 em
thick. These mats are formed by long
strings of photosynthetic bacteria and algae. As the flowing springwater passes over
the mats, the long strings of cells trap and
hold single-cell algae.
First trophic level. Photosynthetic
bacteria and algae make up the spring's
first trophic level, which is composed of
autotrophs-organisms that make their
own food from inorganic chemicals and a
source of energy. In the hot springs, as in
most communities, the source of energy is
sunlight (Figure 6.4).
Second trophic level. Some flies, called
ephydrid flies, live in the cooler areas of the
springs. One species, Ephydra bruesi, lays
bright orange-pink egg masses on stones
and twigs that project above the mat. The
fly larvae feed on the bacteria and algae.
Since these flies eat only plants, they are
106
CHAPTER 6 •
Figure 6.3 • One of the many hot springs in Yellowstone National Park. The bright greenish color comes from photosynthetic bacteria, one of the few kinds of organisms that can survive in the hot temperatures and chemical conditions of the springs.
herbivores. These form the second trophic
level.
Third trophic level. Another fly, called
the dolichopodid fly, is carnivorous and
feeds on the eggs and larvae of the herbivorous flies. Dragonflies, wasps, spiders, tiger beetles, and one species of
bird, the killdeer, also feed on the herbivorous flies. The herbivorous ephydrid
flies also have a parasite, a red mite, that
feeds on fly eggs and travels by attaching
ECOSYSTEMS AND ECOSYSTEM MANAGEMENT
itself to the bodies of the adult flies.
Another parasite, a small wasp, lays its
eggs within the fly larvae. All these form
the third trophic level.
Fourth trophic level. Wastes and dead
organisms of all trophic levels are fed on
by decomposers, which in the hot springs
are primarily bacteria. These form the
fourth trophic level.
The entire hot springs community of
organisms-photosynthetic bacteria and
Spider mite
3rd
Trophic
Level
Dolichopodid
fly
Wasp
Killdeer
2nd
Trophic
Level
1st
Trophic
Level
4th
Trophic
Level
Figure 6.4 •
Food web of a Yellowstone National Park hot spring.
algae, herbivorous flies, carnivores, and
decomposers-is maintained by two factors: ( 1) sunlight, which provides an input of usable energy for the organisms;
and (2) a constant flow of hot water,
which provides a continual new supply of
chemical elements required for life and a
habitat in which the bacteria and algae
can persist.
Even though this is one of the simplest
ecological communities in terms of the
numbers of species, a fair number of
species are found. About 20 species in all
are important in this ecosystem. The eco-
logical community they form has been
sustained for long periods in these unusual habitats.
Another interesting aspect of the hot
springs ecosystem is species dominance .
Dominant species are those that are most
abundant or otherwise most important in
the community. (We discuss this in connection with diversity in Chapter 7.) As
noted earlier, in the hot springs community, the species of photosynthetic bacteria or algae that is dominant changes with
the temperature; one species dominates
the hotter springs and hottest regions
within a spring, and another species dominates cooler waters.
Because the algae are so brightly colored, this spatial patterning in dominance
is readily apparent to visitors. It was striking to one of the earliest explorers of
Yellowstone, a trapper named Osborne
Russell, who visited the springs in the
1830s and 1840s. He wrote that one boiling spring about 100m (328ft) across had
three distinct colors: "From the west side
for one-tllird of tl1e diameter it was white,
in the middle it was pale red, and the remaining tllird on the east light sky blue." 5
6. I THE ECOSYSTEM: SUSTAINING LIFE ON EARTH
1071
Figure 6.5 •
A typical terrestrial food web. Roman numerals identifY trophic levels.
called the pine borer, and other animals (such as deer) not
shown here. The third trophic level, carnivores, includes
foxes and wolves, hawks and other predatory birds, spiders,
and predatory insects. People are omnivores (eaters of both
plants and animals) and feed on several trophic levels. In
Figure 6.5, people would be included in the fourth trophic
level, the highest level in which they would take part.
Decomposers, such as bacteria and fungi, feed on wastes
and dead organisms of all trophic levels. Decomposers are
also shown here on the fourth level.
An Oceanic Food Chain
In oceans, food webs involve more species and tend to
have more trophic levels than they do in the hot springs
described in A Closer Look 6.1 or the terrestrial ecosystem
just considered. In a typical oceanic ecosystem (Figure 6.6),
microscopic single-cell planktonic algae and planktonic
photosynthetic bacteria are in the first trophic level. Small
invertebrates called zooplankton and some fish feed on the
algae and photosynthetic bacteria, forming the second
trophic level. Other fish and invertebrates feed on these
herbivores and form the third trophic level. The great
108
CHAPTER 6 •
baleen whales filter seawater for food, feeding primarily on
small herbivorous zooplankton (mostly crustaceans), and
thus the baleen whales are also in the third level. Some fish
and marine mammals, such as killer whales, feed on the
predatory fish and form higher trophic levels.
The Food Web of the Harp Seal
In the abstract, a diagram of a food web and its trophic
levels seems simple and neat; but in reality, food webs are
complex, because most creatures feed on several trophic
levels. For example, consider the food web of the harp seal
(Figure 6.7). The harp seal is shown at the fifth level. 6 It
feeds on flatfish (fourth level), which feed on sand lances
(third level), which feed on euphausiids (second level),
which feed on phytoplankton (levell). But the harp seal
actually feeds at several trophic levels, from the second
through the fourth, and it feeds on predators of some of
its prey and thus is a competitor with some of its own
food. 6 A species that feeds on several trophic levels typically
is classified as belonging to the trophic level above the
highest level from which it feeds. Thus, we place the harp
seal on the fifth trophic level.
ECOSYSTEMS AND ECOSYSTEM MANAGEMENT
Simple chemical
compounds
(phosphates,
nitrates, etc.)
Figure 6.6 •
6.2
An oceanic food web.
The Community Effect
Species can interact directly through food chains, as we
have just seen. They also interact directly through symbiosis and competition, discussed in the next chapter. But
a species can also affect other species indirectly, by affecting a third, a fourth, or many other species that, in
turn, affect the second species. In addition, a species can
affect the nonliving environment, which then affects a
group of species in the community. Changes in that
group affect another group. Such indirect and more
complicated interactions are referred to as communitylevel interactions.
Interactions at the community level are illustrated by
the sea otters of the Pacific Ocean. In fact, the community-level interactions of the sea otter are at the heart of
some arguments in favor of conservation of this species.
Sea otters feed on shellfish, including sea urchins and
abalone (Figure 6.8a). Sea otters originally occurred
throughout a large area of the Pacific Ocean coasts, from
northern Japan northeastward along the Russian and
Alaskan coasts, and southward along the coast of North
America to Morro Hermosa in Baja California, to
Mexico. 7 The otters were brought almost to extinction by
commercial hunting for their fur during the eighteenth
and nineteenth centuries; they have one of the finest furs
in the world. By the end of the nineteenth century, there
were too few otters left to sustain commercial exploitation,
and there was concern that the species would become
extinct.
A small population survived and has increased since then,
so that today there are approximately 111,500 sea otters.
According to the Marine Mammal Center, approximately
2,000 sea otters live along the coast of California, a few
hundred in Washington and British Columbia, and 100,000
along the Aleutian Islands of Alaska. The sea otter population in Russian waters is about 9,000. 8 Legal protection of
the sea otter by the U.S. government began in 1911 and
continues under the U.S. Marine Mammal Protection Act
of 1972 and the Endangered Species Act of 1973.
The sea otter has been a focus of controversy and research. On the one hand, fishermen argue that the sea ot6 .2 THE COMMUNITY EFFECT
109
5th Trophic
Level
Harp seals
4th Trophic
Level
Figure 6.7 • Food web of the harp seal showing how complex a real food web can be.
ter population has recovered-in fact, recovered too much.
In this view, there are too many sea otters today, and they
interfere with commercial fishing because they take large
amounts of abalone. 9 On the other hand, conservationists
argue that sea otters have an important community-level
role, necessary for the persistence of many oceanic species.
They say that there are still too few sea otters for this role
to be maintained at a satisfactory level.
What is this important role? It is made up of many effects on the community that result from sea otters' feeding on sea urchins. Sea urchins are a preferred food of sea
otters. Sea urchins, in turn, feed on kelp, large brown algae that form undersea "forests" and provide important
habitat for many species. Sea urchins graze along the bottoms of the beds, feeding on the base ofkelp, called holdfasts, which attach the kelp to the bottom. When holdfasts
are eaten through, the kelp floats free and dies.
Where sea otters are abundant, as on Amchitka Island
in the Aleutian Islands, kelp beds are abundant and there
are few sea urchins (Figure 6.8b). At nearby Shemya
Island, which lacks sea otters, sea urchins are abundant and
there is little kelp (Figure 6.8c) ? Experimental removal of
sea urchins has led to an increase in kelp. 10
11 Q
CHAPTER 6 •
Otters, then, affect the abundance of kelp, but the
influence is indirect. Sea otters neither feed on kelp nor
protect individual kelp plants from attack by sea urchins.
Sea otters reduce the number of sea urchins. With fewer
sea urchins, less kelp is destroyed. With more kelp, there
is more habitat for many other species; so sea otters indirectly increase the diversity of species. 9 •11 Thus, sea otters have a community-level effect . This example shows
that such effects can occur through food chains and can
alter the distribution and abundance of individual
species.
A species such as the sea otter that has a large effect
on its community or ecosystem is called a keyst one
species, or a key species. 12 Its removal or a change in its
role within the ecosystem changes the basic nature of
the community.
Community-level effects demonstrate the reality behind
the concept of an ecological community; they show us that
certain processes can take place only because of a set of
species interacting together. These effects also suggest that
an ecological community is more than the sum of its partsa perception called the holistic view. (Refer back to the discussion of environmental unity in Chapter 3.)
ECOSYSTEMS AND ECOSYSTEM MANAGEMENT
(a)
(b)
Figure 6.8 • The effect of sea otters on kelp.
(a) Sea otters feed on shellfish, including sea
urchins. The sea urchins feed on kelp. When
there are sea otters, there are few urchins (b)
but abundant kelp (c).
(c)
In spite of their importance, community-level interactions
are often difficult to recognize. O ne difficulty is knowing
when and how species interact. Community-level interactions
are not always as clear as those involving the sea otter. Even
there, considerable scientific research was required to under-
stand the interactions. Adding to the complexity, the set of
species that make up an ecological community is not fixed
completely but varies within the same kind of ecosystem from
time to time and place to place. This brings us to the question
of how to identifY ecosystems.
6.2 THE COMMUNITY EFFECT
111
Figure 6 .9 • Sometimes the transition from one
ecosystem to another is sharp and distinct, as in the
transition from lake to forest at Lake Moraine in Banff
National Park, Alberta, Canada.
6.3
How Do You Know When
You Have Found an Ecosystem?
An ecosystem is the minimal entity that has the properties
required to sustain life. This implies that an ecosystem is
real and important and therefore that we should be able to
find one easily. However, ecosystems vary greatly in structural complexity and in the clarity of their boundaries.
Ecosystems differ in size, from the smallest puddle of water to a large forest. Ecosystems and their communities differ in composition, from a few species in the small space of
a hot spring to many species interacting over a large area
of the ocean. Furthermore, ecosystems differ in the kinds
and relative proportions of their non biological constituents
and in their degree of variation in time and space.
Sometimes the borders of the ecosystem are well
defined, such as the border between a lake and the surrounding countryside (Figure 6.9). But sometimes the
transition from one ecosystem to another is gradual, as in
the transition from desert to forest on the slopes of the
San Francisco Mountains in Arizona and in the subtle gradations from grasslands to savannas in East Africa and from
boreal forest to tundra in the far north, where the trees
thin out gradually.
A commonly used practical delineation of the boundary
of an ecosystem on land is the watershed . Within a watershed, any drop of rain that reaches the ground flows out in the
same stream. Topography (the lay of the land) determines the
watershed. When a watershed is used to define the boundaries of an ecosystem, the ecosystem is unified in terms of
chemical cycling. Some classic experimental studies of ecosystems have been conducted on forested watersheds in U.S.
Forest Service experimental areas, including the Hubbard
Brook experimental forest in New Hampshire (Figure 6.10)
and the Andrews experimental forest in Oregon.
What all ecosystems have in common is not a particular physical size or shape but the processes we have
mentioned: the flow of energy and the cycling of chemical elements. Ecological communities change over time,
and it is the interactions among the species-a dynamic set
of processes-that are the key to the community concept.
6.4
Ecosystem Management
Ecosystems can be natural or artificial or a combination
of both. An artificial pond that is a part of a waste-treatment plant is an example of an artificial ecosystem.
The V-shaped logged
area in this picture is the famous
Hubbard Brook ecosystem study. Here,
a watershed defines the ecosystem, and
the V shape is an entire watershed cut as
part of the experiment.
Figure 6 .10 •
rl2
CHAPTER 6 •
ECOSYSTEMS AND ECOSYSTEM MANAGEMENT
Ho\N Are the Borders of an Ecosystem Defined?
The borders between ecosystems may be well defined or gradual.
Those considered well defined include freshwater streams. Such
ecosystems are often studied separately from surrounding ecosystems by researchers with different training and using different
methods. Research on streams in southeast Alaska in which
salmon spawn has raised questions about the practice of studying
aquatic and terrestrial ecosystems separately.
Salmon are anadromous fish-fish that come from the ocean to
spawn in freshwater streams. In southeast Alaska, enormous numbers of salmon spawn in over 5,000 streams. Although salmon are
born in freshwater, they migrate to the ocean, where most of their
growth occurs. After they return to their home streams, they spawn
and die. In one sense, therefore, salmon are a means of transporting resources from the ocean to freshwater. Because of their large
numbers, salmon have the potential to make significant contributions to organic and mineral content of streams.
Salmon have a high lipid content compared with many other
fish and are thus a good energy source for the animals that prey on
them. In addition, their decay adds nitrogen, phosphorus, carbon,
and other inorganic elements to freshwater. In one lake in western
Alaska, for example, 24 million fish add 170 tons of phosphorus to
the lake each year-an amount equal to or greater than recommended rates for applying fertilizers to trees. When the fish die,
their carcasses decay and provide nourishment for algae, fungi, and
bacteria. Invertebrates feed on these and on decaying bits of fish.
Other fish feed on the invertebrates. Finally, bears and other carnivores eat salmon, both live and dead, during their upstream migration. In that way, nutrients derived from salmon pass into the
soil and vegetation surrounding the streams.
Spawning fish have higher proportions of heavy isotopes of nitrogen and carbon esN and 13 C). These can be used to trace the
relative contributions of anadromous fish to the nitrogen and carbon content of organisms in the food web. One such study showed
that spawning salmon contributed 10.9% of the nitrogen found in
invertebrate predators and 17.5% in the foliage of riparian plants.
While it is not surprising to find aquatic invertebrates, which feed
on salmon eggs and juveniles, with large an1ounts of nitrogen derived from salmon, researchers were surprised at the high levels in
streamside vegetation. When terrestrial mammals and birds feed
on salmon, their feces and any uneaten salmon carcasses decay and
add nutrients to the soil, where they can be taken up through the
roots of plants. In southeast Alaska, over 40 species of mammals
and birds feed on salmon. Salmon migrations attract large numbers
of predators to streams and lakes. Salmon and other anadromous
fish thus appear to link the ocean, freshwater, and land to an extent
that is only beginning to be appreciated.
Ecosystems can also be managed, and the management
can include a large range of actions. Agriculture can be
thought of as partial management of certain kinds of
ecosystems (see Chapters 11 and 12), as can forests managed for timber production (see Chapter 13). Wildlife
preserves are examples of partially managed ecosystems
(see Chapters 13 and 14).
Sometimes, when we manage or domesticate individuals or populations, we separate them from their ecosystems.
We also do this to ourselves (see Chapter 3). When we do
this, we must replace the ecosystem functions of energy
flow and chemical cycling with our own actions. This is
what happens in a zoo, where we must provide food andremove the wastes for individuals separated from their natural
environments.
The ecosystem concept, then, lies at the heart of the
management of natural resources. When we try to conserve species or manage natural resources so that they are
sustainable, we must focus on their ecosystem and make
sure that it continues to function. If it doesn't, we must
Critical Thinking Questions
1. Given the intricate connections between the aquatic and terrestrial ecosystems along salmon streams, how would you define the
boundaries of the ecosystems?
2. When more adult salmon reach the spawning grounds than are
needed to maintain the population, some are considered excess.
How might the research described here affect that view?
3. Some biologists have called salmon a keystone species. Given
what you know about keystone species, how would you argue for or
against this designation?
4. In recent years, the numbers of anadromous fish along the
Pacific Coast of North America have declined precipitously because of overfishing and habitat destruction. What effects would
you predict this might have on the ecology of freshwater streams
and their adjoining land areas?
5. What types of management decisions about fish, wildlife, and
forests would follow from recognizing the connection between
aquatic and terrestrial ecosystems?
6 .4 ECOSYSTEM MANAGEMENT
1131
replace or supplement ecosystem functions with our own
actions. Ecosystem management, however, involves more
than compensating for changes we make in ecosystems.
It means managing and conserving life on Earth by considering chemical cycling, energy flow, community-level
interactions, and the natural changes that take place within
ecosystems.
Summary
• An ecosystem is the simplest entity that can sustain life.
At its most basic, an ecosystem consists of several species
and a fluid medium (air, water, or both). The ecosystem
must sustain two processes-the cycling of chemical elements and the flow of energy.
I
REEXAMINING
• The living part of an ecosystem is the ecological community, a set of species connected by food webs and
trophic levels. A food web or chain describes who feeds
on whom. A trophic level consists of all the organisms
that are the same number of feeding steps from the
initial source of energy.
• Community-level effects result from indirect interactions among species, such as those that occur when sea
otters influence the abundance of sea urchins.
• Ecosystems are real and important, but it is often
difficult to define the limits of a system or to pinpoint
all the interactions that take place.
• Ecosystem management is considered key to the
successful conservation of life on Earth.
THEMES
AND
ISSUES
Human Population
The human population depends on many ecosystems that are widely dispersed
around the globe. Our modern technology may appear to make us independent
of these natural systems. In fact, though, the more connections we establish
through modern transportation and communication, the more kinds of ecosystems we depend on. Therefore, the ecosystem concept is one of the most important we will learn about in this book.
Sustainability
The ecosystem concept is at the heart of managing for sustainability. When we
try to conserve species or manage living resources so that they are sustainable,
we must focus on their ecosystem and make sure that it continues to function.
Global Perspective
Our planet has sustained life for approximately 3.5 billion years. To understand
how Earth as a whole has sustained life for such a long time, we must understand the ecosystem concept, because the environment at a global level must
meet the same basic requirements as any local ecosystem.
Urban World
Cities are embedded in larger ecosystems. But like any life-supporting system, a
city must meet basic ecosystem needs. This is accomplished through connections between cities and surrounding environments. Together, these function as
ecosystems or sets of ecosystems. To understand how we can create pleasant and
sustainable cities, we must understand the ecosystem concept.
People and Nature
The feelings we get when we hike through a park or near a beautiful lake are as
much a response to an ecosystem as to individual species. This illustrates the
deep connection between people and ecosystems. Also, many effects we have on
nature are at the level of an ecosystem, not just on an individual species.
Science and Values
The introductory case study concerning acorns, mice, deer, and Lyme disease illustrates the interactions between values and scientific knowledge about ecosystems. Science can tell us how organisms like deer and mice interact. This
knowledge confronts us with choices. Do we want to have many deer and mice
and to live with Lyme disease? Do we want to invest in ecological and medical
resources to find a better way to control that disease? The choice we make
depends on our values.
rl4
CHAPTER 6 •
ECOSYSTEMS AND ECOSYSTEM MANAGEMENT
Key Terms==================================================================-autotrophs 105
community-level
interactions 109
decomposers 108
ecological community 105
food chains 105
food webs 105
keystone species 11 0
succession 105
trophic level 105
watershed 112
Study Questions===============================;;;;;;;;;;;;;;:
l. What is the difference between an ecosystem and an
ecological community?
2. In what ways would an increase in the number of sea
otters and a change in their geographic distribution
benefit fishermen? In what ways would these changes
be a problem for fishermen?
3. Based on the discussion in this chapter, would you expect a highly polluted ecosystem to have many species
or few species?
4. Is our species a keystone species? Explain.
5. Which of the following are ecosystems? Which are ecological communities? Which are neither?
a. Chicago
b. A 1,000-hectares farm in Illinois
c. A sewage-treatment plant
d. The Illinois River
e. Lake Michigan
FurtherReadinQ===========================================
Bormann, F. H. , and G. E. Likens. 1979. Pattern and Process in a
Forested Ecosystem. New York: Springer-Verlag. A synthetic view of
the northern hardwood ecosystem, including its structure, function,
development, and relationship to disturbance.
Odum, Eugene, G. W. Barrett 2004. Fundamentals of Ecology. Duxbury,
Brooks/Cole. Odum's original textbook was a classic, especially in providing one of the first serious introductions to ecosystem ecology. This
is the latest update of the late authority's work, done with his protege.
Molles, M. C. Ecology: Concepts and Applications. New York: McGrawHill. Currently, this is one of the most popular introductory ecology
textbooks.
Rockwood, L.L. 2006. Introduction to Population Ecology. Oxford, England:
Blackwell Publishing Professional. This is a new, up-to-date introduction
to a part of ecology crucial to management of natural resources.
FURTHER READING
115