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Ecological Restoration
Sunset in Yosemite, a nineteenth-century
painting by Albert Bierstadt, illustrates the
idea of the balance of nature. Nature is
portrayed as still (in a single state), beautiful,
and without people.
Learning Objectives
The Hands That Rock the Cradle
of Civilization: Demise and Possible
Restoration of the Tigris-Euphrates
Marshlands
The famous cradle of civilization, the land between the
Tigris and Euphrates rivers, is so called because the
waters from these rivers and the wetlands they formed
made possible one of the earliest sites of agriculture, and
from this the beginnings ofWestern civilization. This
well-watered land in the midst of a major desert was also
one of the most biologically productive areas in the
world, used by many species of wildlife, including millions of migratory birds.
Ironically, the huge and famous wetlands between
these two rivers, land tl1at today is within Iraq, have
been greatly diminished by the very civilization that they
helped create. "We can see from the satellite images that
by 2000, all of the marshes were pretty much drained,
except for 7 percent on the Iranian border," said Dr.
Curtis Richardson, director of the Duke University
Wetland Center. I
A number of events of the modern age led to the
marsh's destruction. Beginning in the 1960s, Turkey and
Syria began to build dams upriver in the Tigris and
Euphrates to provide irrigation and electricity, and now
these number more than 30. Then in the 1980s Saddam
Hussein had dikes and levees built to divert water from
the marshes so that oil fields under the marshes could be
drilled.
For at least 5,000 years, the Ma'adan people-the
Marsh Arabs-lived in these marshes. But the
Iran-Iraq War (1980-1988) killed many of them and
also added to the destruction of the wetlands (Figure
10.1a and b).
Today efforts are under way to restore the wetlands.
According to the United Nations Environment Program,
since the early 1970s the area of the wetlands has
increased by 58%.2 But some scientists believe that there
has been little improvement, and the question remains
here, as in many places around the world: Can ecosystems
be restored once people have seriously changed them?
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CHAPTER I 0 •
ECOLOGICAL RESTORATION
The purpose of this chapter is to provide an understanding of what is possible and what can be done to restore
damaged ecosystems.
In recent years, a new field called restoration ecology
has developed within the science of ecology. Its goal is to
return damaged ecosystems to some set of conditions
considered functional, sustainable, and "natural." Whether
restoration can always be successful is still an open question. For some ecosystems and species, success appears
achievable, but often success has required great effort.
10. 1 Restore to What?
The idea of ecological restoration raises a curious question: Restore to what?
The Balance of Nature
Until the second half of the twentieth century, the predominant belief in Western civilization was that any natural
area-a forest, a prairie, an intertidal zone-left undisturbed
by people achieved a single condition that would persist
indefinitely. This condition, as mentioned in Chapter 3, has
been known as the balance of nature. The major tenets of
a belief in the balance of nature are as follows:
l. Nature undisturbed achieves a permanency of form and
structure that persists indefinitely.
2. If it is disturbed and the disturbing force is removed,
nature returns to exactly the same permanent state.
3. In this permanent state of nature, there is a "great chain
of being" with a place for each creature (a habitat and a
niche) and each creature in its appropriate place.
These ideas had their roots in Greek and Roman philosophies about nature, but they have played an important role
miles
km
50
•
Permanent
marsh
•
Permanent
lake
Former
marshlands
- - International
boundary
(b)
(a)
Figure 10.1 • (a) A Marsh Arab village in the famous wetlands of
Iraq, said to be one of the places where Western civilization originated. The people in this picture are an1ong an estimated 100,000
Ma'adan people who now live in their traditional marsh villages,
many having returned recently. These marshes are an1ong the most
biologically productive areas on Earth. (b) Map of the Fertile
Crescent, where the Marsh Arabs live, and is called the Cradle of
Civilization. It is the land beween the Tigris (tl1e eastern river) and
Euphrates (the western river) in what is now Iraq. Famous cities of
history, such as Nineveh, developed here, made possible by the
abundant water and good soil. The gray area shows the known
original extent of the marshes, the bright green their present area.
in modern environmentalism as well. In the early twentieth
century, ecologists formalized the belief in the balance of
nature. They said that succession proceeded to a fixed, classic condition, which they called a climax state and defined
as a steady-state stage that would persist indefmitely and
have maximum organic matter, maximum storage of chemical elements, and maximum biological diversity. At that
time, people thought that wildfires were always detrimental
to wildlife, vegetation, and natural ecosystems. Bambi, a
Walt Disney movie of the 1930s, expressed this belief,
depicting a fire that brought death to friendly animals. In the
United States, Smokey Bear is a well-known symbol
employed for many decades by the U.S. Forest Service to
warn visitors to national forests to be careful with fire and
avoid setting wildfires. The message is that wildfires are
always harmful to wildlife and ecosystems.
All of this suggests a belief that the balance of nature
does in fact exist. But if that were true, the answer to the
question "Restore to what?" would be simple: Restore to
the original, natural, permanent condition. The method
of restoration would be simple, too: Get out of the way
and let nature take its course. Since the second half of the
twentieth century, though, ecologists have learned that
nature is not constant and that forests, prairies-all
ecosystems-undergo change. Moreover, since change has
been a part of natural ecological systems for millions of
years, many species have adapted to change. Indeed, many
require specific kinds of change in order to survive.
Dealing with change-natural and human-inducedposes questions of human values as well as science. This is
illustrated by wildfires in forests, grasslands, and shrublands, which can be extremely destructive to human life
and property. Scientific understanding tells us that fires are
natural and that some species require them. But whether
we choose to allow fires to burn, or light fires ourselves, is
a matter of values. In 1991 a wildfire that started in chaparral shrubland in Santa Barbara, California, burned just
83 minutes but did $500 million worth of damage. A few
years later, a wildfire in Oakland, California, did $1 billion
worth of damage. Restoration ecology depends on science
to discover what used to be, what is possible, and how different goals can be achieved. The selection of goals for
restoration is a matter of human values.
The Boundary Waters Canoe Area Wilderness:
An Example of the Naturalness of Change
One of the best-documented examples of natural disturbance
is the role of fire in the northern woods of North America.
The Boundary Waters Canoe Area-an area of more than
400,000 hectares (a million acres) in northern Minnesota
designated as wilderness under the U.S. Wilderness Actexemplifies nature relatively undisturbed by people. The area
is no longer open to logging or other direct disturbances by
people. In the early days of European exploration and settlement of North America, French voyagers traveled through
this region hunting and trading for furs. In some places, logging and farming were common in the nineteenth and early
twentieth centuries, but for the most part the land has been
relatively untouched. Despite the lack of human influence,
the forests show a persistent history of fire. Fires occur somewhere in this forest almost every year, and on average the
entire area burns once in a century. Fires cover areas large
enough to be visible by satellite remote sensing (Figure 10.2).
10 . 1 RESTORE TO WHAT?
1791
Figure 10.2 • Forest fires can be natural or caused by people. This
figure shows the change in a large area of the Superior National
Forest in Minnesota between 1973 and 1983, as observed by the
Landsat satellite. The black boundaries show a central corridor
where logging is permitted, surrounded by the Boundary Waters
Canoe Area at the top and bottom. This is an area that is protected
from all uses except certain kinds of recreation. The bright yellow
shows areas that were clear of trees in 1973 but had regenerated to
young forest by 1983. Most ofthis change is due to regrowth following a large fire that burned both inside and outside the wilderness. Red areas were forested in 1973 but cleared in 1983. Most of
these are outside the wilderness, and some of these are due to logging (red) and some to fire or storms. Greens show areas that were
forested both years. 3
When fires occur in the forests of the Boundary Waters
Canoe Area at natural rates and natural intensities, they have
some beneficial effects. For example, trees in unburned
forests appear more susceptible to insect outbreaks and
disease. Thus, recent ecological research suggests that
wilderness depends on change and that succession and disturbance are continual processes. The landscape is dynamic. 3
biological diversity that existed previously (Chapters 7 and 8).
According to this interpretation, ecological restoration
means restoring processes and a set of conditions that are
known to have existed for that ecosystem. From this point
of view, we examine populations that have declined and
ecosystems that have been damaged and try to learn what is
lacking. Then we go about trying to restore what is lacking.
But a wide variety of answers have been put forward to
answer the question: What is restoration? At the extreme
are those who argue that all human impacts on nature are
"unnatural" and therefore undesirable and that the only true
goal of restoration is to bring nature back to a condition
that existed prior to human influence. The anthropologist
PaulS. Martin takes this position. He proposes that the only
truly "natural" time is before any significant human
influence occurs. Specifically, he suggests that restoration to
conditions oflO,OOO B.c .-before farming-should be our
goal. He even suggests introducing the African elephant into
Nortl1 America to replace the mastodon, whose extinction,
he argues, was the result of hunting by Indians.4
But, in general, new ways of thinking about restoration
leave open choice, which is a matter of science and values.
Science tells us what nature has been and what it can be;
our values determine what we want nature to be. There is
no single perfect condition. However, for some of the
Goals of Restoration: What Is "Natural"?
With the examples of the wetlands in Iraq and forests of the
Boundary Waters Canoe Area, we can now return to the
question "Restore to what?" If an ecosystem passes naturally
through many different states, and all of them are "natural,"
and if change itself-including certain kinds of wildfire-is
natural, then what can it mean to "restore" nature? And how
can restoration that involves such things as wildfires occur
without undue damage to human life and property?
Can we restore an ecological system to any one of its past
states and claim that this is natural and successful restoration? A frequently accepted answer is that restoration means
restoring an ecosystem to its historical range of variation and
to an ability to sustain itself and its crucial functions, including the cycling of chemical elements (see Chapter 5 ),
the flow of energy (Chapter 9), and the maintenance of the
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CHAPTER I 0 •
ECOLOGICAL RESTORATION
Table 10.1 • Some Possible Restoration Goals
Goal
Approach
1. Pre-Industrial
2. Presettlement (e.g., of North America)
3. Preagriculture
4. Before any significant impact of human beings
5. Maximum production
6. Maximum diversity
7. Maximum biomass
8. Preserve a specific endangered species
9. Historical range of variation
Maintain ecosystems as they were in A.D. 1500
Maintain ecosystems as they were about A.D. 1492
Maintain ecosystems as they were about 5,000 B.C.
Maintain ecosystems as they were about 10,000 B.C.
Independent of a specific time
Independent of a specific time
Independent of old growth
Whatever stage it is adapted to
Create the future like the known past
goals in Table 10.1, specific conditions are especially desirable. It is possible to restore an ecosystem so that most
of the time it supports conditions that people desire for
one reason or another.
10.2
Everglades National Park is also the focus of a variety of
restoration efforts, and a large effort is under way to restore
bottomland-tl1at is, wetland-forests of the lower
Mississippi Valley (see discussion in Chapter 14).
What Needs to Be Restored?
Extent of Wetlands Losses
Ecosystems of all types have undergone degradation and
need restoration. However, certain kinds of ecosystems
have undergone especially widespread loss and degradation and are therefore a focus of current attention. In
addition to forests and wetlands, attention has focused on
grasslands, especially the North American prairie; streams
and rivers and the riparian zones alongside them; lakes;
and habitats of threatened and endangered species. Also
included are areas that people desire to restore for aesthetic
and moral reasons, showing once again that restoration
involves values. In this section, restoration of wetlands,
rivers, streams, and prairies is discussed briefly.
22.8%
1.1%
1.2%
3.5%
Missouri
Wetlands, Rivers, and Streams
In North America, large areas of both freshwater and
coastal wetlands have been greatly altered during the past
200 years (Figure 10.3). It is estimated that California, for
example, has lost more than 90% of its wetlands, both
freshwater and coastal, and that the total wetland loss for
the United States is about 50% (see Chapter 21). It is not
only the cradle of civilization that has suffered; wetlands
around the world are affected.
One of the largest and most expensive restoration projects in the United States is the restoration of the Kissimmee
River in Florida. This river was channelized, or straightened, by the U.S. Army Corps of Engineers to provide
ship passage through Florida. However, although the river
and its adjacent ecosystems were greatly altered, shipping
never developed, and now several hundred million dollars
must be spent to put the river back as it was before. The
task will include restoring the meandering flow of the river
channel and replacing the soil layers in the order in which
they had lain on the bottom of the river prior to channelization (see Chapter 21).
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I
North Dakota
=
1
10.9%
1.4%
South Dakota
10.9%
- - - - - - - - - - - 5.5%
-
5.5%
3.6%
Percentage of
wetlands 1780
27.3%
14.8%
II
Percentage of
wetlands 1990
Figure 10.3 • Loss ofwetlands in the midwestern United States.
These maps illustrate the extensiveness of wetland losses. In some
states, the losses are even greater. For example, it is estimated that
about 90% of the wetlands in California have been lost. [Source:
Based on Scientific Assessment and Strategy Team, Science for
Floodplain Management into the 21st Century (Faber, 1996), p. 84.]
I 0.2 WHAT NEEDS TO BE RESTORED?
181 1
(a)
(b)
Figure 10.4 •
Primary succession. (a) Forests developing on new lava flows in Hawaii and (b) at the edge of a retreating glacier.
10.3 When Nature Restores Itself:
Prairie Restoration
AB noted in Chapter 8, prairies once occupied more land
in the United States than any other kind of ecosystem.
Today, only a few small remnants of prairie remain.
Prairie restoration is of two kinds. In a few places,
original prairie exists that has never been plowed. Here,
the soil structure is intact, and restoration is simpler. One
of the best known of these areas is the Kanza Prairie near
Manhattan, Kansas. In other places, where the land
has been plowed, restoration is more complicated.
Nevertheless, the restoration of prairies has gained
considerable attention in recent decades, and restoration
of prairie on previously plowed and farmed land is
occurring in many midwestern states. The Allwine
Prairie, which is within the city limits of Omaha,
Nebraska, has been undergoing restoration from farm to
prairie for many years. Prairie restoration has also been
taking place near Chicago.
A peculiarity about the history of prairies is that although most prairie land was converted to agriculture, this
was not done along roads and railroads, so that long, narrow strips of unplowed native prairie remain on these
rights-of-way. In Iowa, for example, prairie once covered
more than 80% of the state-11 million hectares (28 million acres). More than 99.9% of the prairie land has been
converted to other uses, primarily agriculture, but along
roadsides there are 242,000 hectares ( 600,000 acres) of
prairie-more than in all of Iowa's county, state, and federal parks. These roadside and railway stretches of prairie
provide some of the last habitats for native plants, and
restoration of prairies elsewhere in Iowa is making use of
these habitats as seed sources. 5
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CHAPTER I 0 •
ECOLOGICAL RESTORATION
The Process of Ecological Succession
We have been discussing people's attempts to restore damaged ecosystems. Often, the damage has been caused by
people, but natural areas are subject to natural disturbances
as well. Storms and fires, for example, have always been a
part of the environment. 4 Recovery of disturbed ecosystems can also occur naturally, through a process called
ecological succession. This natural recovery can occur if
the damage is not too great. Sometimes, though, the
recovery takes longer than people would like.
We can classifY ecological succession as either primary or
secondary. P rimary succession is the initial establishment
and development of an ecosystem where one did not exist
previously. Secondary succession is reestablishment of an
ecosystem following disturbances. In secondary succession,
there are remnants of a previous biological community, including such things as organic matter and seeds. Forests that
develop on new lava flows (Figure 10.4a) and at the edges
of retreating glaciers (Figure 10.4b) are examples of primary
succession. Forests that develop on abandoned pastures or
following hurricanes, floods, or fires are examples of
secondary succession (see A Closer Look 10.1).
Succession is one of the most important ecological
processes, and the patterns of succession have many management implications . We see examples of succession all
around us. When a house lot is abandoned in a city,
weeds begin to grow. Mter a few years, shrubs and trees
can be found; secondary succession is taking place.
A farmer weeding a crop and a homeowner weeding a
lawn are both figh ting against the natural processes of
secondary succession.
An Example of Forest Secondary Succession
Within a few years after a fie ld is
abandoned, seeds of many kinds sprout,
some of short-lived weedy plants and
some of trees (Figure lO.Sa). After a few
years, certain species, generally referred
to as pioneer species, become established.
In the Poconos of Pennsylvania, red
cedar is such a species (Figure lO.Sb). In
New England, white pine, pin cherry,
white birch, and yellow birch are especially abundant. These trees are fastgrowing in bright light and have widely
distributed seeds . For example, seeds of
red cedar are eaten by birds and dispersed; birches have very light seeds that
are widely dispersed by the wind. Mter
several decades, forests of the pioneer
species are well established, forming a
dense stand of trees (Figure lO .Sc).
Once the initial forest is established,
other species begin to grow and become
important. Typical dominants in the
northeastern United States are sugar
maple and beech. These later-successional trees are slower-growing than the
ones that came into the forest first, but
they have other characteristics that make
them well adapted to the later stages of
succession . These species are what
foresters call shade-tolerant: They grow
relatively well in the deep shade of the
redeveloping forest.
Mter three or four decades, most of
the short-lived species have matured ,
borne fruit, and died. Since they cannot
grow in the shade of a forest that has
been reestablished, they do not regenerate. For example, after five or six decades
a New England forest is a rich mixture
of birches, maples, beeches, and other
species. The trees vary in size, but the
trees that dominate now are generally
taller than those dominating at earlier
stages. Mter one or two centuries, such
a forest will be composed mainly of
shade-tolerant species.
(a)
(b)
Figure 10.5 • A series of photographs of succession on an abandoned pasture in the
Poconos: (a) a second-year field; (b) young red cedar trees, grasses, and other plants some
years after a field was abandoned; (c) mature forests developed along old stone farm walls.
I 0.3 WHEN NATURE RESTORES ITSELF: THE PROCESS OF ECOLOGICAL SUCCESSION
183
Patterns in Succession
Succession occurs in most kinds of ecosystems, and when
it occurs, it follows certain general patterns. We will now
consider succession in three classic cases involving forests:
( 1) on dry sand dunes along the shores of the Great Lakes
in North America, (2) in a northern freshwater bog, and
( 3) in an abandoned farm field.
Dune Succession
Sand dunes are continually being formed along sandy
shores and then breached and destroyed by storms. In
Indiana, on the shores of Lake Michigan, soon after a dune
is formed, dune grass invades. This grass has special adaptations to the unstable dune. Just under the surface, it puts
out runners with sharp ends (if you step on one, it will
hurt). The dune grass rapidly forms a complex network of
underground runners, crisscrossing almost like a coarsely
woven mat. Above the ground, the green stems carry out
photosynthesis, and the grasses grow.
Once the dune grass is established, its runners stabilize
the sand, and seeds of other plants are less easily buried
too deep or blown away. The seeds germinate and grow,
and an ecological community of many species begins to
develop. The plants of this early stage tend to be small,
grow well in bright light, and withstand the harshness of
the environment-high temperatures in the summer, low
temperatures in the winter, and intense storms.
Slowly, larger plants, such as eastern red cedar and
eastern white pine, are able to grow on the dunes.
Eventually, a forest develops, which may include species
such as beech and maple. Such a forest can persist for
many years, but at some time a severe storm breaches
even these heavily vegetated dunes, and the process begins again (Figure 10.6).
Figure 10.6 • Dune succession on the shores ofLake Michigan.
Dune grass shoots appear scattered on the slope, where they emerge
from undergrmmd runners.
Bog Succession
A bog is an open body of water with surface inletsusually small streams-but no surface outlet. As a result,
the waters of a bog are quiet, flowing slowly if at all. Many
bogs that exist today originated as lakes that filled depressions in the land, which in turn were created by glaciers
during the Pleistocene ice age.
Succession in a northern bog, such as Livingston Bog
in Michigan (Figure 10.7), begins when a sedge (grasslike
herbs) puts out floati!lg runners (Figure 10.8a) b). These
runners form a complex matlike network, similar to that
formed by dune grass. The stems of the sedge grow on the
runners and carry out photosynthesis. Wind blows particles onto the mat, and soil, of a kind, develops. Seeds of
other plants land on the mat and do not sink into the water. They can germinate. The floating mat becomes
thicker, and small shrubs and trees, adapted to wet environments, grow. In the North, these include species of the
blueberry family.
Figure 10.7 • Livingston Bog, a
famous bog in the northern part of
Michigan lower peninsula.
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CHAPTER I 0 •
ECOLOGICAL RESTORATION
Sedge puts out floating runners
I
Open water
(a)
Sedge forms a floating mat
that supports other plants
"""
(b)
Figure 10.8 • Diagram of bog succession.
Open water (a) is transformed through
formation of a floating mat of sedge and
deposition of sediments (b) into wetland
forest (c).
(c)
The bog also fills in from the bottom as streams carry
fine particles of clay into it (Figure 10.8b) c). At the shoreward end, the floating mat and the bottom sediments
meet, forming a solid surface. But farther out, a quaking
bog occurs. You can walk on this quaking bog mat; and if
you jump up and down, all the plants around you bounce
and shake. The mat is really floating. Eventually, as the bog
fills in from the top and the bottom, trees that can withstand wetter conditions-such as northern cedar, black
spruce, and balsam fir-grow. The formerly open water
bog becomes a wetland forest.
Today, much of this land has been abandoned for farm ing and allowed to grow back to forest (see
Figure 10.5) . The first plants to enter the abandoned
farmlands are small plants adapted to the harsh and
highly variable conditions of a clearing-a wide range of
temperatures and precipitation. As these plants become
established, other, larger plants enter. Eventually, large
trees grow, such as sugar maple, beech, yellow birch, and
white pine, forming a dense forest .
Old-Field Succession
In the eastern United States, such as the states of
New England, a great deal of land was cleared and
farmed in the eighteenth and nineteenth centuries.
Reviewing these three examples of ecological succession involving forests , you can see common elements in them,
even though the environments are very different. Common
elements of succession in such cases include the following:
General Patterns of Succession
I 0.3 WHEN NATURE RESTORES ITSELF: THE PROCESS OF ECOLOGICAL SUCCESSION
185
Weeds - short-lived
herbaceous plants
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Graphs showing changes in
biomass and diversity with succession.
Soil organic matter and total ecosystem storage
of chemical elements
Figure 10.9 •
l. An initial kind of vegetation specially adapted to the
unstable conditions. These plants are typically small in
stature, with adaptations that help stabilize the physical
environment.
2. A second stage with plants still of small stature, rapidly
growing, with seeds that spread rapidly.
3. A third stage in which larger plants, including trees, enter and begin to dominate the site.
4. A fourth stage in which mature forest develops.
Although we list four stages, it is common practice to combine the first two and to speak of early, middle, and late successional stages. These general patterns of succession can be
found in most ecosystems, although the species differ. The
stages of succession are described here in terms of vegetation, but similarly adapted animals and other life-forms are
associated with each stage. We discuss other general properties of the process of succession later in this chapter.
Species characteristic of the early stages of succession
are called pioneers, or early-successional species. They
have evolved and are adapted to the environmental conditions in early stages of succession. Plant species that
dominate late stages of succession, called late-successional
species, tend to be slower-growing and longer-lived. These
species have evolved and are adapted to environmental
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CHAPTER 10 •
ECOLOGICAL RESTORATION
----Time
conditions in the late stages. For example, they grow well
in shade and have seeds that, while not as widely dispersing, can persist a rather long time.
In early stages of succession, biomass and biological diversity increase (Figure 10.9). In middle stages of succession, we find trees of many species and many sizes. Gross
and net production (see Chapter 9) change during succession as well: Gross production increases, and net production decreases. Chemical cycling also changes: The organic
material in the soil increases, as does the amount of chemical elements stored in the soils and trees.6 We look more
closely at chemical cycling changes in the next section.
10.4 Succession and Chemical Cycling
One of the important effects of succession is a change in
the storage of chemical elements necessary for life. On
land, the storage of chemical elements (including nitrogen,
phosphorus, potassium, and calcium, essential for plant
growth and function) generally increases during the progression from the earliest stages of succession to middle
succession. There are two reasons for this.
First, organic matter stores chemical elements; as long as
there is an increase in organic matter within the ecosystem,
there will be an increase in the storage of chemical ele-
ments. This is true for live and dead organic matter. In adclition, many plants have root nodules containing bacteria
that can assimilate atmospheric nitrogen, which is then used
by the plant in a process known as nitrogen fixation.
The second reason is indirect: The presence of live and
dead organic matter helps retard erosion. Both organic
and inorganic soil can be lost to erosion because of the
effects of wind and water. Vegetation tends to prevent
such losses and therefore causes an increase in total
stored material.
Organic matter in soil contributes to the storage of
chemical elements in two ways. First, it contains chemical
elements itself. Second, dead organic matter functions as
an ion-exchange column that holds on to metallic ions that
would otherwise be transported in the groundwater as dissolved ions and lost to the ecosystem. As a general rule,
the greater the volume of soil and the greater the percentage of organic matter in the soil, the more chemical
elements will be retained.
However, the amotmt of chemical elements stored in a soil
depends not only on the total volume of soil but also on its
storage capacity for each element. The chemical storage capacity of soils varies with the average size of the soil particles.
Soils composed mainly of large, coarse particles, like sand,
have a smaller total surface area and can store a smaller quantity of chemical elements. Clay, which is made up of the smallest particles, stores the greatest quantity of chemical elements.
Soils contain greater quantities of chemical elements
than do live organisms. However, much of what is stored
in a soil may be relatively unavailable, or may only become
available slowly because the elements are tied up in complex compounds that decay slowly. In contrast, the elements stored in living tissues are readily available to other
organisms through food chains.
Rates of cycling and average storage times are system characteristics (as discussed in Chapter 3). Soils store more elements than does live tissue but cycle them at a slower rate.
The increase in chemical elements that occurs in the early and
middle stages of succession does not continue indefinitely. If
an ecosystem persists for a very long period with no disturbance, it will experience a slow but definite loss of stored
chemical elements. Thus, the ecosystem will slowly run downhill and become depauperate-literally, impoverished-and
thus less able to support rapid growth, high biomass density,
and high biological diversity (Figure l 0 .l 0). 7 The changes in
chemical cycling during disturbance and successional recovery
are discussed further in A Closer Look l 0.2.
10.5
Species Change in Succession:
Do Early-Successional Species
Prepare the Way for
Later Ones?
To restore an ecosystem, it is important to understand
what causes one species to replace another during the succession process. If we understand these causes and effects,
10.5
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c
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a>
Ol
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z
succession
succession
(a)
co
a>
Co
·c:
:::::1
Total in ecosystem
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:::::1
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Time
Early
succession
--___..~
Post late
succession
(b)
(a) Hypothesized changes in soil nitrogen during the course of soil development. (b) Change in total soil phosphorus over time with soil development. [Source: P. M. Vitousek
and P. S. White, "Process Studies in Forest Succession," in D. C.
West, H. H. Shugart, and D. B. Botkin, eds., Forest Succession:
Concepts and Applications (New York: Springer-Verlag, 1981),
Figure 17.1, p. 269.]
Figure 10.10 •
we can use them to better restore ecosystems. Earlier and
later species in succession may interact in three ways:
through (l) facilitation, (2) interference, or (3) life history differences. If they do not interact, the result is
termed chronic patchiness (Table 10.2 ).8, 9
Table 10.2 • Patterns of Interaction among Earlier
and Later Species in Succession
I. Facilitation. One species can prepare the way for the
next (and may even be necessary for the occurrence of the
next).
2. Interference. Early-successional species can, for a time,
prevent the entrance of later-successional species.
3. Life history differences. One species may not affect the
time of entrance of anod1er; two species may appear at different times during succession because of differences in
transport, germination, growth, and longevity of seeds.
4. Chronic patchiness. Succession never occurs, and the
species that enters first remains until the next disturbance.
Source: J. H. Connell and R. 0. Slatyer, "Mechanisms of Succession
in Natural Communities and Their Role in Community Stability
and Organization," American Naturalist III (1977): 1119-1144;
S. T. A. Pickett, S. L. Collins, and J. J. Armesto, "Models,
Mechanisms, and Pathways of Succession," Botanical RevieJV 53
(1987): 335-371.
SPECIES CHANGE IN SUCCESSION: DO EARLY-SUCCESSIONAL SPECIES PREPARE THE WAY FOR LATER ONES?
1871
~
..
Facilitation
In dune and bog succession, the first plant species-dune
grass and floating sedge-prepare the way for other species
to grow. This is called facilitation (early-successional
species facilitate the establishment of later-successional
species; Figure lO.lla).
Facilitation has been found to take place in tropical
rain forests.lO Early-successional species speed the
reappearance of the microclimatic conditions that occur
in a mature forest. Because of the rapid growth of earlysuccessional plants, the temperature, relative humidity,
and light intensity at the soil surface in tropical forests
can reach levels similar to those of a mature rain forest after only 14 years. 6 Once these conditions are established,
species that are adapted to deep forest shade can germinate and persist.
Knowing the role of facilitation can be useful in the
restoration of damaged areas. Plants that facilitate the presence of others should be planted first. On sandy areas, for
example, dune grasses can help hold the soil before attempts are made to plant larger shrubs or trees.
Facilitation-pine provides shade
that helps oaks
1!1
(a)
Interference-grass interferes with seeds
of other species
Grass continues to re mai n grass
Bare groun d goes
to pine tree
---------1~
(b)
Chronic patchiness- no effect of one
species on another
Interference
Facilitation does not always occur. Sometimes, instead, certain early-successional species interfere with the entrance of
other species (Figure lO.llb). For example, in the old
fields of mid-Atlantic states, such as the states from
Connecticut to Virginia, among the early-successional
species are grasses that form dense cover, including the
prairie grass little bluestem. The living and dead stems of
this grass form a mat so dense that seeds of other plants
that fall onto it cannot reach the ground and therefore do
not germinate. The grass interferes with the entrance of
other plant species-a process called, naturally enough,
interference.
Interference does not last forever, however. Eventually,
some breaks occur in the grass mat-perhaps from surface
water erosion, the death of a patch of grass from disease,
or removal by fire. Breaks in the grass mat allow seeds of
trees such as red cedar to reach the ground. Red cedar is
also adapted to early succession because its seeds are spread
rapidly and widely by birds who feed on them and because
this species can grow well in the bright light and otherwise harsh conditions of early succession. Once started,
red cedar soon grows taller than the grasses, shading them
so much that they cannot grow. More ground is open, and
the grasses are eventually replaced.
In parts of Asia, interference occurs where bamboo
grows, and, in tropical areas, where another, smaller grass,
Imperata, grows. Like little bluestem in the United States,
these grasses form stands so dense that seeds of other,
later-successional species cannot reach the ground, germinate, or obtain enough light, water, and nutrients to sur-
188
CHAPTER 10 •
ECOLOGICAL RESTO RATION
3
3
(c)
Figure 10.11 • Two of the three patterns of interaction among
species in ecological succession. (a) Facilitation. As Henry David
Thoreau observed in Massachusetts more than l 00 years ago, pines
provide shade and act as "nurse trees" for oaks. Pines do well in
openings. If there were no pines, few or no oaks would survive.
Thus the pines facilitate the entrance of oaks. (b) Interference. Some
grasses that grow in open areas form dense mats that prevent seeds
of trees from reaching the soil and germinating. The grasses interfere
with the addition of trees. (c) Chronic patchiness. Earlier-entering
species neither help nor interfere with other species; instead, as in a
desert, the physical environment dominates.
vive. Imperata either replaces itself or is replaced by bamboo, which then replaces itself.lO Once established,
Imperata and bamboo appear able to persist for a long
time. Once again, when and if breaks occur in the cover of
these grasses, other species can germinate and grow, and a
forest eventually develops.
Changes in Chemical Cycling puring
a Disturbance
When an ecosystem is disturbed by fire,
storms, or human activities, changes occur
in chemical cycling. For example, when a
forest is burned, complex organic compounds, such as wood, are converted to
smaller inorganic compounds, including
carbon dioxide, nitrogen oxides, and sulfur
oxides. Some of the inorganic compounds
from the wood are lost to the ecosystem
during the fire as vapors that escape into
the atmosphere and are distributed widely
or as particles of ash that are blown away,
but some of the ash falls directly onto the
soil. These compounds are highly soluble
in water and readily available for vegetation
uptake. Therefore, immediately after a fire,
the availability of chemical elements increases. This is true even if the ecosystem as
a whole has tmdergone a net loss in total
stored chemical elements.
If sufficient live vegetation remains after a fire, tl1en the sudden, temporary in-
crease, or pulse, of newly available elements is taken up rapidly, especially if it is
followed by a moderate amount of rainfall
(enough for good vegetation growth but
not so much as to cause excessive erosion). The pulse of inorganic nutrients
can then lead to a pulse in the growth of
vegetation and an increase in the amount
of stored chemical elements in the vegetation. This in turn boosts the supply of
nutritious food for herbivores, which can
subsequently undergo a population increase. The pulse in chemical elements in
the soil can therefore have effects that extend throughout the food chain.
Other disturbances on the land produce
effects sin1ilar to those of fire. For example,
severe storms, such as hurricanes and tornadoes, knock down and kill vegetation.
The vegetation decays, increasing the concentration of chemical elements in the soil,
which are tl1en available for vegetation
Life History Differences
In other cases, species do not so much affect one another. Rather, differences in the life histories of the
species allow some to arrive first and grow quickly,
while others arrive later and grow more slowly. An example of such a life history difference is seed disbursal. The seeds of early-successional species are readily
transported by wind or animals and so reach a clearing
sooner and grow faster than seeds of late-successional
species. In many forested areas of eastern North
America, for example, birds eat the fruit of cherries and
red cedar, and their droppings contain the seeds, which
are spread widely.
In contrast, late-successional species can have life histories that bring them in much later. Although sugar
maple, for example, can grow in open areas, its seeds take
longer to travel and its seedlings can tolerate shade. Beech
produces large nuts that store a lot of food for a newly germinated seedling. This helps the seedling establish itself in
the deep shade of a forest until it is able to feed itself
I 0.5
growth. Storms also have another effect in
forests: When trees are uprooted, chemical
elements that were near the bottom of the
root zone are brought to the surface,
where they are more readily available.
Knowledge of the changes in chemical
cycling and the availability of chemical elements from the soil during succession can
be useful to us in restoring damaged lands.
We know that nutrients must be available
within the rooting depth of the vegetation. The soil must have sufficient organic
matter to hold on to nutrients.
Restoration will be more difficult where
the soil has lost its organic matter and has
been leached. A leached soil has lost nutrients as water drains through it, especially acidic water, dissolving and carrying
away chemical elements. Heavily leached
soils, such as those subjected to acid rain
and acid mine drainage, pose special challenges to the process of land restoration.
through photosynthesis. But these seeds are heavy and are
moved relatively short distances by seed-eating animals.
Chronic Patchiness
A fourth possibility is that species just do not interact and that
succession, as it has been described, does not take place. This
is called chronic patchiness (Figure lO.llc), and it is the case
in some deserts. For example, in the warm deserts of
California, Arizona, and Mexico, the major shrub species grow
in patches, which often consist of mature individuals with few
seedlings. These patches tend to persist for long periods until
there is a disturbance.ll Similarly, in highly polluted environments, a sequence of species replacement may not occur.
What kinds of changes occur during succession depends on
the complex interplay between life and its environment. Life
tends to build up, or aggrade, whereas nonbiological processes
in the environment tend to erode and degrade. In harsh environments, where energy or chemical elements required for life
are limited and disturbances are frequent, the physical, degrading environment dominates, and succession does not occur.
SPECIES CHANGE IN SUCCESSION: DO EARLY-SUCCESSIONAL SPECIES PREPARE THE WAY FOR LATER ONES?
189
L.
..
Ho\N Can We Evaluate Constructed Ecosystems?
What happens when restoring damaged ecosystems is not an
option? In such cases, those responsible for the damage may be
required to establish alternative ecosystems to replace the damaged ones.
An example involved some saltwater wetlands on the coast of
San Diego County, California. In 1984, construction of a floodcontrol channel and two projects to improve interstate freeways
damaged an area of saltwater marsh. The projects were of concern because California had lost 91% of its wetland area since
1943 and the few remaining coastal wetlands were badly fragmented. In addition, the damaged area provided habitat for three
endangered species: the California least tern, the light-footed
clapper rail, and a plant called the salt-marsh bird's beak.
The California Department of Transportation, with funding
from the Army Corps of Engineers and the Federal Highway
Administration, was required to compensate for the damage by
constructing new areas of marsh in the Sweetwater Marsh
National Wildlife Refuge. To meet these requirements, eight
islands, known as the Connector Marsh, with a total area of
4.9 hectares, were constructed in 1984. An additional ?-hectares
area, known as Marisma de Nacion, was established in 1990.
Goals for the constructed marsh, which were established by
the U.S. Fish and Wildlife Service, included the following:
l. Establishment of tide channels with sufficient fish to provide
food for the California least tern.
2. Establislunent of a stable or increasing population of salt-marsh
bird's beak for three years.
3. Selection of the Pacific Estuarine Research Laboratory
(PERL) at San Diego State University to monitor progress on
the goals and conduct research on the constructed marsh. In
1997 PERL reported that goals for the least tern and bird's beak
had been met but that attempts to establish a habitat suitable for
the rail had been only partially successful.
During the past decade, PERL scientists have conducted extensive research on the constructed marsh to determine the reasons
for its limited success. They found that rails live, forage, and nest
in cordgrass more than 60 em tall. Nests are built of dead cordgrass
attached to stems of living cordgrass so that the nests can remain
above the water as it rises and falls. If the cordgrass is too short, the
nests are not high enough to avoid being washed out during high
tides. Researchers suggested that tl1e coarse soil used to construct
the marsh did not retain the amount of nitrogen needed for cordgrass to grow tall. Adding nitrogen-rich fertilizer to the soil
resulted in taller plants in the constructed marsh, but only if the fertilizer was added on a continuing basis.
Another problem is that the diversity and numbers of large
invertebrates, which are the major food source of the rails, are
lower in the constructed marsh than in natural marshes. PERL
researchers suspect that this, too, is linked to low nitrogen levels.
Because nitrogen stimulates the growth of algae and plants,
Species
Mitigation Goals
Progress in Meeting Requirements
Status as of 2006
California
least tern
Tidal channels witl1 75% of the fish
species and 75% of the number of fish
found in natural channels
Met standards
Salt-marsh
bird's beak
Through reintroduction, at least 5
patches (20 plants each) tl1at remain
stable or increase for 3 years
Light-footed
clapper rail
Seven home ranges (82 ha), each
having tidal channels with:
a. Forage species equal to 75% of
the invertebrate species and 75%
of the number of invertebrates in
natural areas
b. High marsh areas for rails to find
refuge during high tides
c. Low marsh for nesting with 50%
coverage by tall cordgrass
Did not succeed on constructed islands but an
introduced population on natural Sweetwater
Marsh thrived for 3 years (reached 140,000
plants); continue to monitor because plant is
prone to dramatic fluctuations in population
Constructed
FWS recommended
change from
endangered to
threatened
Still listed
as endangered
d. Population of tall cordgrass that is
self-sustaining for 3 years
Note: FWS stands for US Fish and Wildlife Service
r90
CHAPTER I 0 •
ECOLOGICAL RESTORATION
Met standards
Sufficient in 1996 but two home ranges fell
short in 1997
All home ranges met low marsh acreage
requirement and all but one met cordgrass
requirement and six lacked sufficient tall cordgrass
Plant height can be increased with continual use
of fertilizer but tall cordgrass is not self-sustaining
Still listed
as endangered;
in 2005, eight
captive-raised
birds were
released
which provide food for small invertebrates, and these in turn
provide food for larger invertebrates, low nitrogen can affect the
entire food chain.
Wetlands Flunk Real-World Test." Based on the information you
have about the project, would you agree or disagree with this
judgment? Explain your answer.
Critical Thinking Questions
3. How do you think one can decide whether a constructed
ecosystem is an adequate replacement for a natural ecosystem?
I. Make a diagram of the food web in the marsh showing how
the clapper rail, cordgrass, invertebrates, and nitrogen are
related.
2. The headline of an article about the Sweetwater Marsh project in the April 17, 1998, issue of Science declared, "Restored
4. The term adaptive management refers to the use of scientific
research in ecosystem management. In what ways has adaptive
management been used in the Sweetwater Marsh project? What
lessons from the project could be used to improve similar projects in the future?
10.6 Applying Ecological Knowledge
to Restore Heavily Damaged
lands and Ecosystems
An example of how ecological succession can aid in the
restoration of heavily damaged lands is the effort being
made to undo mining damage in Great Britain, where
mining has caused widespread destruction to land. In
Great Britain, where some mines have been used since medieval times, approximately 55,000 hectares (136,000
acres) have been damaged by mining. Recently, programs
have been initiated to remove toxic pollutants from the
mines and mine tailings, to restore tl1.ese damaged lands
to useful biological production, and to restore the visual
attractiveness of the landscape.l2
One area damaged by a long history of mining lies
within the British Peak District National Park, where lead
has been mined since the Middle Ages and waste tailings
are as much as 5 m (16.4 ft) deep. The first attempts torestore this area used a modern agricultural approach: heavy
application of fertilizers and planting of fast-growing
agricultural grasses to revegetate the site rapidly. These
grasses quickly green on the good soil of a level farm
field, and it was hoped that, with fertilizer, they would do
the same in this situation. But after a short period of
growth, the grasses died. On the poor soil, leached of its
nutrients and lacking organic matter, erosion continued,
and the fertilizers that had been added were soon leached
away by water runoff. As a result, the areas were shortly
barren again.
When the agricultural approach failed, an ecological
approach was tried, using knowledge about ecological
succession. Instead of planting fast-growing but vulnerable agricultural grasses, ecologists planted slow-growing
native grasses known to be adapted to minerally deficient
soils and the harsh conditions that exist in cleared areas.
In choosing these plants·, the ecologists relied on their observations of what vegetation first appeared in areas of
Great Britain that had undergone succession naturally.l2
The result of the ecological approach has been successful
restoration of once-damaged lands (Figure 10.12).
Figure 10.12 • An old lead -mining area in Great Britain, now
undergoing restoration. Restoration involves planting of
early-successional native grasses adapted to low-nutrient soils with
little physical structure.
Heavily damaged landscapes are to be found in many
places. Restoration similar to that in Great Britain is done in
the United States to reclaim lands damaged by strip mining.
In such cases, restoration sometimes begins during the mining process rather than afterward. Similar methods could also
be used to restore areas once occupied by buildings in cities.
Summary
• Restoration of damaged ecosystems is a major new
emphasis in environmental sciences and is developing into
a new field. Restoration involves a combination of human
activities and natural processes of ecological succession.
• Disturbance, change, and variation in the environment
are natural, and ecological systems and species have
evolved in response to these changes.
• When ecosystems are disturbed, they undergo a process
of recovery known as ecological succession, the
SUMMARY
191
establishment and development of an ecosystem.
Knowledge of succession is important in the restoration
of damaged lands.
• During succession there is usually a clear, repeatable
pattern of changes in species. Some species, called
early-successional species, are adapted to the first stages,
when the environment is harsh and variable but when
necessary resources may be available in abundance. This
contrasts with late stages in succession, when biological
effects have modified the environment and reduced some
of the variability but also have tied up some resources.
Typically, early-successional species are fast-growing,
whereas late-successional species are slow-growing and
long-lived.
REEXAMINING
Biomass, production, diversity, and chemical cycling
change during succession. Biomass and diversity peak
in mid -succession, increasing at first to a maximum,
then declining and varying over time.
Changes in the kinds of species found during succession
can be due to facilitation, interference, or simply life history differences. In facilitation, one species prepares the
way for others. In interference, an early-successional
species prevents the entrance of later-successional ones.
Life history characteristics of late-successional species
sometimes slow their entrance into an area.
THEMES
AND
ISSUES
I
Human Population
If we degrade ecosystems to the point where their recovery from disturbance is
slowed or they cannot recover at all, then we reduce the local carrying capacity
of those areas for human beings. For this reason, an understanding of the factors that determine ecosystem restoration is important to developing a sustainable human population.
Sustainability
Life tends to build up; nonbiological forces in the environment tend to tear
down. By helping ecosystems to develop, we promote sustainability. Heavily
degraded land, such as land damaged by pollution or overgrazing, loses the
capacity to recover-to undergo ecological succession. Knowledge of the
causes of succession can be useful in restoring ecosystems and thereby achieving sustainability.
Global Perspective
Each degradation of land takes place locally, but such degradation has been happening around the world since the beginnings of civilization. Ecosystem degradation is therefore now a global issue.
In cities, we generally eliminate or damage the processes of succession and the
ability of ecosystems to recover. As our world becomes more and more urban,
we must learn to maintain these processes within cities as well as in the countryside. Ecological restoration is an important way to improve city life.
Urban World
People and Nature
Science and
192
Values
CHAPTER 10 •
Restoration is one of the most important ways that people can compensate for
their undesirable effects on nature.
Because ecological systems naturally undergo changes and exist in a variety of
conditions, there is no single "natural" state for an ecosystem. Rather, there is
the process of succession, with all of its stages. In addition, there are major
changes in the species composition of ecosystems over time. While science can
tell us what conditions are possible and have existed in the past, which ones we
choose to promote in any location is a question of values. Values and science are
intimately integrated in ecological restoration.
ECOLOGICAL RESTORATION
Key Terms
balance of nature 178
chronic
patchiness 189
climax state 179
early -successional
species 186
ecological succession 182
facilitation 188
interference 188
late-successional
species 186
life history difference 189
primary succession 182
restoration ecology 178
secondary succession 182
successional stages 186
Study Questions ================================
l. Farming has been described as managing land to keep
it in an early stage of succession. What does this mean,
and how is it achieved?
2. Redwood trees reproduce successfully only after disturbances (including fire and floods), yet individual
redwood trees may live more than 1,000 years. Is redwood an early- or late-successional species?
3. Why could it be said that succession does not take place
in a desert shrubland (an area where rainfall is very low
and the only plants are certain drought-adapted shrubs)?
4. Develop a plan to restore an abandoned field in your
town to natural vegetation for use as a park. The
following materials are available: bales of hay; artificial
fertilizer; and seeds of annual flowers, grasses, shrubs,
and trees.
5. Oil has leaked for many years from the gasoline tanks
of a gas station. Some of the oil has oozed to the
surface. As a result, the gas station has been abandoned and revegetation has begun to occur. What effects would you expect this oil to have on the process
of succession?
6 . Refer to the Iraqi marshes of the opening case study.
Assume that there is no hope of changing water diversion from the many dams upstream on the Tigris and
Euphrates rivers. Develop a plan to restore the marshes,
considering in particular the decrease in the area of the
marshes.
7 . Early in the twentieth century, a large meteorite collided with the earth in Siberia and destroyed boreal
forests over a large area. In what ways might restoration
and succession following this large-scale disturbance
differ from restoration and succession after a fire
burned a few hectares in the same forest? (See Chapter 8
for information about boreal forests.)
FurtherReadinQ ==============================================================
Botkin, D. B. 1992. Discordant Harmonies: A New Ecology for the 21st
Century. New York: Oxford University Press.
Botkin, D. B. 2001. No Man's Garden: Thoreau and a New Vision for
Civilization and Nature. Washington, D.C.: Island Press.
Higgs, E. 2003. Nature by Design: People, Natural Process, and Ecological
Restoration. Cambridge, Mass.: MIT Press. A book that discusses the
broader perspective on ecological restoration, including philosophical aspects.
Falk, Donald A., and Joy B. Zedler. 2005. Foundations of Restoration
Ecology. Washington, D.C.: Island Press. A new and important book,
written by two of the world's experts on ecological restoration.
FURTHER READING
193