Create a brochure about human society’s impact on ecosystems

1.Create a brochure about human society’s impact on ecosystems and the costs and benefits of human enterprise. Include the following:

• Explain how ecosystem degradation and loss results from human society.
• Describe the effects of human activity on plants, animals, and ecosystem dynamics, and provide specific examples.
• Describe the economic decisions underlying conservation and exploitation. Explain the costs and benefits of human enterprise in terms of ecosystems, and provide specific examples.

Use images as appropriate.

Cite at least two references consistent with APA guidelines.

 

Use fist two attachement for this

 

 

2. write a 1,050- to 1,400-word paper about the ecosystems you have chosen and the species that make up these ecosystems. In your paper, include the following items:

• Describe your pair of ecosystems and the types of current or proposed exploitation in one or both ecosystems. Explain the past, present, and potential future consequences of overexploitation in these ecosystems. Describe the potential costs and benefits of at least two exploitation activities.
• Explain current and potential management of your ecosystems. How can modification, cultivation, or restoration alter these ecosystems? Outline at least two potential management plans beyond cessation of exploitation, and describe the costs and benefits of each.
• Prioritize conservation efforts in your pair of ecosystems based on values and the principles of conservation biology by including the following elements:
  • Prioritize species and ecosystems for protection based on the values identified in the Acting Locally Paper – Part One.
  • Describe a regional or global threat to your ecosystems, such as global climate change, and outline a plan to combat this threat.
  • Rank the priorities of continuing or ceasing exploitation activities, of alternative management plans, and of the plan to combat the regional or global threat to biodiversity.
  • Defend your prioritization based on specific values and on your plan’s overall effect on biodiversity, ecological integrity, and economic feasibility.
• Identify at least two specific practical actions and at least two specific political actions you can take to support the top priorities you have identified for the pair of ecosystems. Interpret how these specific actions can support conservation and biodiversity.

Cite at least four references.

Format your paper consistent with APA guidelines.

 

uSE the 3rd attachment for possible reference

 

 

 

3.Write a 1,050- to 1,400-word paper about genetically vigorous populations. Include the following items:

• Describe the importance of genetic diversity in populations. Explain how genetic diversity in plants and animals supports long-term viability, biodiversity, and biotic integrity. Describe specific examples of at least one plant population and at least one animal population facing challenges in genetic diversity, and explain the potential or demonstrated threats to viability posed by deficient genetic diversity.
• Describe the values underlying population management. Relate genetic diversity to the success of population management. Explain the costs and benefits in successful population management. Provide at least two specific examples of current and past population management efforts.
• Compare ex situ conservation to in situ conservation. Describe the role of zoos, aquariums, and botanical gardens in conservation. Explain the efforts of these institutions to support genetically vigorous populations.
• Explain why the Endangered Species Act was created to promote conservation of plants and animals.

Cite at least six references, including at least two scholarly sources.

Format your paper consistent with APA guidelines.

e eBook Collection chapter 8

With light streaming out of our cities at night, with roads and power lines etched

across most landscapes, any visitor from another planet would be well aware of

human activities long before arriving on earth. A conservation biologist might argue

that Homo sapiens is only one of many millions of species that constitute life on earth,

but there is no denying that people are a dominant life-form. As we have captured

more and more of the earth’s resources, allowing our population and biomass to

grow larger and larger, many other species have declined or even disappeared. Indeed,

you could build a strong argument that we have degraded the overall ability of the

earth to support life given the area occupied by human activity and the amount of

photosynthesis appropriated by people (Vitousek et al. 1997; Rojstaczer et al. 2001;

Sanderson et al. 2002a; Haberl et al. 2004; Imhoff et al. 2004) (Fig. 8.1).

In this chapter we will examine the various ways in which people diminish the

earth’s ability to support a diverse biota. To begin, we need to make some key distinctions,

starting with habitat versus ecosystem. A habitat is the physical and biological

environment used by an individual, a population, a species, or perhaps a group of

species (Hall et al. 1997). In other words, at the species level we can speak of blue

whale habitat and sequoia habitat, and perhaps waterfowl habitat. However, if the

group of species is too broad, the term becomes so general as to be almost meaningless.

What does “wild life habitat” mean if virtually every environment supports wild

organisms? Even a parking lot will have microbes and small invertebrates living in the

cracks in the pavement. An ecosystem is a group of organisms and their physical

environment (see Chapter 4), such as a lake or a forest, and it may or may not correspond

to the habitat of a species. A forest ecosystem may constitute the sole habitat of

a squirrel, but a frog’s habitat might include both the forest and a lake, and a bark

beetle’s habitat might only be certain species of trees spread widely across the forest.

We can also make a distinction between degradation and loss of habitats or ecosystems.

Habitat degradation is the process by which habitat quality for a given species is

diminished: for example, when contaminants reduce a species’s ability to reproduce in

an area. When habitat quality is so low that the environment is no longer usable by a

given species, then habitat loss has occurred. The line between habitat degradation

and loss will often be unclear. For example, if environmental changes prevent a

species from reproducing, but some individuals can still be found (e.g. dispersing juvenile

animals, or a few old trees that survive, but whose seeds never survive), is this

habitat loss or severe degradation? Sometimes, these differences can be clarified if we

describe the types of habitat use more explicitly: for example, by referring to breeding

CHAPTER 8

Ecosystem

Degradation and Loss

Ecosystem Degradation and Loss 151

habitat, foraging habitat, winter habitat, and so on. Note that habitat loss or degradation

for one species will probably constitute habitat gain or enhancement for some

other species. For example, cutting a forest is likely to degrade or destroy habitat for a

squirrel, but the resulting early successional ecosystem is likely to be new habitat for

at least one butterfly species. A more poignant example comes from the Everglades,

where managing the hydrological regime means choosing between the habitat of two

endangered species: wood storks (which need periods of very limited water to concentrate

their food in residual pools) and snail kites (which need long, wet periods)

(Bancroft et al. 1992; Beissinger 1995; Curnutt et al. 2000).

Ecosystem degradation occurs when alterations to an ecosystem degrade or destroy

habitat for many of the species that constitute the ecosystem. For example, when

warm water from a power plant increases the temperature of a river, causing many

temperature-sensitive species to disappear, this is ecosystem degradation by a conservation

biologist’s definition. In contrast, an ecosystem ecologist might focus on

changes in ecosystem function such as a reduction in productivity, rather than on

structural attributes such as the abundance and diversity of biota. Ecosystem loss

occurs when the changes to an ecosystem are so profound and when so many

species, particularly those that dominate the ecosystem, are lost that the ecosystem

is converted to another type. Deforestation and draining wetlands are just two of

many processes that destroy ecosystems.

Let us consider a hypothetical example to illustrate these distinctions. Imagine a

small forest park on the edge of city in which there are many dead and dying trees

Figure 8.1 This map shows the human footprint, a quantitative depiction of human influence on the land surface,

based on geographic data on human population density, land transformation, transportation and electrical power

infrastructure, and normalized to reflect the continuum of human influence across each terrestrial biome defined

within biogeographic realms. Further details are available at the “Atlas of the Human Footprint” website

(www.wcs.org/humanfootprint) and in Sanderson et al. (2002a). (Map provided by the Wildlife Conservation Society.)

(i.e. snags). The park manager might decide to make this forest safer for walkers by

removing all snags near paths. This would degrade the park’s value as a habitat for

the many species that require snags, such as woodpeckers and termites. If the manager

were very thorough and cut down every snag in the park, regardless of its location,

this would constitute habitat loss for snag-dependent species. Assuming

snag-associated species were more than a trivial portion of the forest’s biota, then loss

of snags would also lead to ecosystem degradation. Removing the forest to create a

golf course would constitute ecosystem loss.

There are many ways to degrade or destroy habitats or ecosystems, and in this chapter

we can only provide a broad overview. We will begin with two sections on what

humans add to natural environments: (1) substances that contaminate air, water, soil,

and biota; and (2) physical structures such as roads, dams, and buildings. The third section

covers some of the ways we modify physical environments by eroding soil, consuming

water, and changing fire regimes. In the fourth, fifth, and sixth sections we will

review three major processes by which ecosystems are destroyed or severely degraded:

deforestation, desertification, and the various processes afflicting wetlands and aquatic

ecosystems (e.g. draining and filling). We will not focus on two of the major causes of

ecosystem loss and species endangerment: conversion of ecosystems to urban areas and

agriculture (Czech et al. 2000; McKinney 2002). Their direct effects are so unsubtle

that they do not require much elaboration; we will discuss how to mitigate their impacts

in Chapter 12, “Managing Ecosystems.” Finally, we will discuss fragmentation, a process

by which ecosystem destruction can isolate the biota of those ecosystems that remain

intact. For the sake of simplicity we will cover each issue independently, but realize that

in the real world many problems occur simultaneously and interact with one another.

Two special forms of ecosystem degradation – overexploitation of biota and introduction

of exotic species – will be covered in Chapters 9 and 10, “Overexploitation” and

“Invasive Exotics.” Note that all these sundry threats are direct, proximate causes of loss

of biodiversity. As with so many problems, the ultimate cause is human overpopulation

and overconsumption, but we will reserve discussion of this topic until Part IV, “The

Human Factors.” One deadly enterprise merits special mention here: war. The human

dimensions of war’s tragedies are all too familiar, and it takes but a moment’s reflection

to extend its images – ravaged lands, shattered bodies – to all biota. As you read the following

chapter, realize that virtually all the activities described here can become part of

a war machine with dire and far-reaching consequences (Westing 1980; Dudley et al.

2002). Indeed, long after a war is over, elephants, rhinos, and any large, marketable

animals will continue to suffer from the widespread distribution of weaponry.

Contamination

One might define a pollutant or contaminant as a substance that is where we do not

want it to be. This suggests that substances often do not stay where we put them; they

move. There are three main media that can move pollutants – air, water, and living

organisms – and we will structure our overview of the topic by focusing on air pollution,

water pollution, and pesticides. Note that there is overlap among these media; for

example, acid rain begins as air pollutants and ends up contaminating a lake or causing

increased concentrations of heavy metals in biota. Pesticides can be distributed by air or

water, but we will focus on those that move from organism to organism in a food web.

152 Part II Threats to Biodiversity

Air Pollution

Every day huge quantities of materials are lofted into the atmosphere from our vehicles,

factories, and homes. Nitrogen oxides and sulfur oxides combine with water to

form nitric and sulfuric acids, the basis of acid rain. Chlorofluorocarbons (CFCs) and

halons rise to the upper atmosphere, where they reduce the concentration of ozone,

allowing more harmful ultraviolet radiation to reach the earth’s surface. Closer to

earth, ozone and a suite of other chemicals form toxic clouds called smog.

Through extensive research we know that these and other forms of air pollution

have impaired the health of people and domestic plants and animals (Holgate et al.

1999). We know less about the effects of air pollution on wild species, but given the

basic similarity in the physiology of domestic and wild species, it is likely that they are

also affected (Barker and Tingey 1992). Certainly severe air pollution has even killed

the majority of plant species downwind from some factories (Fig. 8.2). No doubt

many animal species also become locally extinct in these zones, but it would be hard

to know if they were directly eliminated by air pollution or simply disappeared

because of the loss of plant species. Even moderate levels of air pollution are known

to eradicate many lichen species; in fact this relationship is so well documented that

lichens are widely used to monitor air pollution (Gombert et al. 2005).

Chronic effects that diminish an individual’s health and vigor, and thereby reduce

reproductive success or longevity, are probably more common than acute effects that kill

organisms directly. For example, in parts of Belgium great tits have reduced reproductive

success that appears to be correlated to heavy metal contamination (Janssens et al. 2003).

Ecosystem Degradation and Loss 153

Figure 8.2 Fumes from a copper smelter killed most of the vegetation in the Copper

Basin, Tennessee. This photo was taken in 1945, about 25 years after the fumes were

controlled. (Photo from USDA Forest Service.)

154 Part II Threats to Biodiversity

Even species living far from the source of air pollution may be affected. Notably,

declines in some remote amphibian populations (Houlahan et al. 2000; Beebee and

Griffiths 2005) might be linked to air pollution because of its effects on the acidity of

aquatic ecosystems, global climate, pesticides, and ultraviolet radiation. For example,

some research indicates that certain amphibian species, especially those living at high

altitudes, are vulnerable to ultraviolet-B radiation (e.g. Blaustein et al. 2003); at least

one paper has directly implicated climate change in the loss of many frog species at a

Costa Rican site (Pounds et al. 1999; see Chapter 6); and Fig. 8.3 presents a case where

pesticides carried by the wind were implicated. Similarly, one of the major threats to

coral reefs can probably be traced to global climate change induced by air pollution;

Figure 8.3

Analysis of the

spatial patterns of

dominant winds

(arrows) and agricultural

lands

(shaded areas) indicated

that air pollution

by pesticides

is likely to have

played a major role

in the decline of

four species of

frogs in California

(Davidson et al.

2002). Two other

species seemed to

have been more

affected by direct

habitat loss; climate

change and

ultraviolet radiation

did not seem

important in this

system.

unusually warm water temperatures are thought to be a primary cause of “bleaching,”

the massive death of coral polyps (West and Salm 2003).

Water Pollution

The list of substances with which we pollute aquatic ecosystems is very diverse. It

includes innocuous materials such as mud and plant matter that may become

contaminants only when they reach such high concentrations that they smother the

bottom of aquatic ecosystems or use up all the oxygen as they decompose. The list

also includes chemicals such as nitrates and phosphates that are important nutrients

for aquatic plants, but can lead to an excessive growth of plants, upsetting the balance

of an aquatic ecosystem. On the other hand, there are chemicals such as dioxin

that endanger life at concentrations so low that they are measured in parts per billion.

Some pollutants are routinely discharged into aquatic ecosystems from factories and

sewage treatment plants. Others enter in a catastrophic deluge after an accident such

as the rupture of an oil tanker. Still others, such as sediments, pesticides, and fertilizers,

often seep in gradually, carried by the runoff from our agricultural fields, lawns,

and streets. When pollutants originate from broad areas, these places are called nonpoint

sources, in contrast to specific sites (e.g. factories), which are called point sources.

It may surprise you to know that nonpoint-source pollution, usually involving

sediments and nutrients and not highly toxic chemicals, is considered the leading

threat to endangered freshwater species in the United States (Richter et al. 1997).

Not surprisingly, aquatic species and ecosystems are more threatened by water pollution

than are terrestrial biota. On a local scale, there are many lakes, streams,

rivers, and bays where water pollution has eliminated so many species that it would

be fair to say that the aquatic ecosystem has been destroyed, even though a body of

water and a handful of species remain. One of Europe’s largest rivers, the Rhine,

exemplifies this problem; along substantial stretches the natural biota has been

severely altered by pollution (Table 8.1) (Broseliske et al. 1991).

Elimination of a species from a single water body may mean global extinction

because many aquatic species are found in a single lake or river system, having

evolved in isolation from their relatives in nearby water bodies. One of the most interesting

examples of this comes from Lake Victoria in East Africa, home to hundreds of

endemic cichlid fish species (Seehausen et al. 1997). Separation among these closely

related species is highly dependent on females choosing mates of the correct species;

however, with growing eutrophication the lake’s turbidity is increasing, and the

females cannot distinguish the colors they need to see to choose the correct mates.

Consequently, cichlid diversity is declining in eutrophic areas of the lake.

In contrast, water pollution is less likely to cause global extinction of species in

marine ecosystems than in freshwater ecosystems because marine ecosystems are

often too large to pollute in their entirety and because many marine species have large

geographic ranges, making it less likely that their entire range would be so polluted as

to be uninhabitable (Palumbi and Hedgecock 2005). Even though water pollution

may not be responsible for the global extinction of marine species, it still can have a

profound impact on marine biodiversity, particularly through local extirpations: for

example, when coral reefs are smothered in silt or overrun with macroalgae because

of excessive nutrients and eutrophication (Jompa and McCook 2003; Nugues and

Roberts 2003). Water pollution can also upset the equilibrium of marine food webs,

Ecosystem Degradation and Loss 155

such as when an excess of nutrients causes an explosive growth of toxin-producing

plankton known as “harmful algal blooms” (Anderson et al. 2002).

In recent years growing concern has focused on contamination from pharmaceutical

drugs such as ibuprofen and other anti-inflammatory analgesics that pass from

humans into aquatic ecosystems through waste water disposal (Tixier et al. 2003).

In one case a drug administered to cattle, diflonac, has caused kidney failure in three

species of vultures that feed on cattle carcasses, leading to widespread, catastrophic

declines in vulture populations in the Indian subcontinent (Oaks et al. 2004).

Pesticides

To capture a large portion of the earth’s resources people must compete against other

organisms, and pesticides are one of our preferred tools for doing this. We use enormous

quantities of insecticides and rodenticides to kill animals that would eat our

crops, herbicides to kill plants that would compete with our crop plants, and fungicides

to kill fungi that would decompose our food and fiber. Worldwide, over 50,000 different

pesticide products with active ingredients weighing over 2.6 million metric tons are

used each year (World Resources Institute et al. 1998). Some of these pesticides are

relatively benign. They kill only a small group of target organisms, they are used in

limited areas (e.g. food storage facilities), and after use they quickly break down into

harmless chemicals. Unfortunately, very few pesticides meet all these criteria, and

some, such as the notorious DDT, wreak havoc on a broad set of nontarget organisms

for a long period over large areas.

156 Part II Threats to Biodiversity

Upper Rhine Middle Rhine Lower Rhine

1916 1980 1916 1980 ~1900 1981–1987

Gastropoda (snails) 8 4 8 5 11 10

Lamellibranchiata (mussels) 11 4 10 4 14 7

Crustacea (crustaceans) 3 2 3 2 3 13

Heteroptera (true bugs) 2 1 1 0 1 1

Odonata (dragonflies) 2 1 1 0 3 2

Ephemeroptera (mayflies) 11 4 3 0 21 2

Plecoptera (stoneflies) 13 0 12 0 13 0

Trichoptera (caddisflies) 11 5 11 2 17 5

Total 61 21 49 13 83 40

Source: from Broseliske et al. (1991).

Table 8.1

Changes in species

richness of some

invertebrate taxa in

the Rhine.

Ecosystem Degradation and Loss 157

Croplands strewn with corpses can

mark the aftermath of pesticide use, but

more often the effects are not seen until

much later and in more subtle ways. One

example of this has garnered considerable

attention: pesticides and related chemicals

that mimic the action of the female

sex hormone estradiol (Colborn et al.

1996; National Research Council 1999;

Hayes 2004). Sterility, delayed sexual

maturity, abnormal sex organs, and an

array of other problems have been attributed

to these contaminants, which are

characterized as “endocrine disruptors”

or “hormonally active agents.” Longterm,

insidious effects of pesticides are

well documented because some of them

can persist in the tissue of living organisms,

accumulating in one individual, and

passing on to other individuals through a

food web. The most infamous example

involves a set of chemicals known as

chlorinated hydrocarbons (which

includes DDT, many other pesticides, and

some chemicals that are not pesticides,

such as PCBs, polychlorinated biphenyls).

They are soluble in fat and can take years,

even decades, to break down. This means

that they pass from prey to predators up a

food chain and can concentrate in top

predators, a process known as biomagnification

(Fig. 8.4). Populations of several

predatory birds (ospreys, brown pelicans,

bald eagles, peregrines, and others) were

dramatically reduced by chlorinated

hydrocarbons during the 1950s and

1960s. Use of these chemicals has been

sharply curtailed in many wealthier countries, and this has allowed populations of these

birds to recover somewhat (Sheail 1985). However, use of chlorinated hydrocarbon pesticides

continues in many less-developed countries, and, because of their persistence, a

wide variety of chlorinated hydrocarbons continue to contaminate the environment of

places where they have been banned (Berg et al. 1992; Gonzalez-Farias et al. 2002).

Accounts of the negative effects of pesticides typically focus on species that are most

similar to us – birds and mammals – because we tend to be more concerned about their

welfare, and because toxic effects on these species may portend toxic effects on us.

However, it is likely that the most serious effects of pesticides fall on organisms that are

most closely related to the target species. Consider the insect order Lepidoptera (butterflies

Figure 8.4 Persistent pesticides and similar compounds accumulate

in the tissues of one species and then are passed up the food

web to other species where they become more concentrated. This

process is called biomagnification or bioamplification. In this figure

DDT has entered the food web of Lake Kariba in Zimbabwe and

reached its highest levels in top predators such as crocodiles, tigerfish,

and cormorants. Numbers are parts per billion of DDT and its

derivatives in the fat of the species illustrated. (Redrawn by permission

from Berg et al. 1992.)

and moths), which includes many pest species, as well as many endangered species. It

seems reasonable to assume that attempts to control pest lepidoptera with insecticides

would jeopardize some rare lepidoptera, although in practice this has not been well documented

to date (New 1997; Pimentel and Raven 2000). Loss of nontarget insect populations

may have far-reaching consequences. In particular, there is growing concern about

the loss of pollinating insects and the consequences this may have for a wide range of

plants that require animal pollinators, for the other animals dependent on those plants,

and for human food production (Allen-Wardell et al. 1998; Kremen et al. 2002).

Roads, Dams, and Other Structures

Flying in a plane, you can easily see the hand of humanity; most landscapes are crisscrossed

with roads, railroads, fences, and utility corridors and dotted with buildings,

dams, mines, parking lots, and many other structures. The total area covered by such

structures is significant (about 3 million km2 worldwide; over 2% of the land area

[Wackernagel et al. 2002]) and represents a loss of habitat for virtually all wild species.

Looking beyond the immediate footprint of these structures,

one can see that a much larger area is affected. For example,

roads and their adjacent impact zones cover an estimated

20% of the area of the United States (Forman 2000; also

see Riitters and Wickham 2003). Thus we can list “construction

of human infrastructure” along with deforestation,

desertification, and other processes that destroy entire

ecosystems, all of which we will discuss later in this chapter.

In this section we will focus on the consequences of adding

these and other structures to the biota of entire landscapes,

especially on animals that move across landscapes.

Roads

The most ubiquitous structures created by people are roads,

and while roads facilitate the movement of people, they can

also serve as impediments to the movements of many animals

(Forman and Alexander 1998; Forman et al. 2003).

Some roads have curbs or lane dividers that are an absolute

barrier to small, flightless animals such as amphibians, small

reptiles, and various invertebrates. More commonly animals

are capable of crossing a road, but may be run down in the

process (Fig. 8.5). In a two-year study of a 3.6 km stretch of

highway in Ontario, Canada, over 32,000 vertebrate carcasses

were found (Ashley and Robinson 1996). Most of the

mortality fell on amphibians and reptiles; overland migrations

of these species to and from breeding sites make them

especially vulnerable (e.g. Gibbs and Shriver 2002; Gibbs

and Steen 2005). Nevertheless, just the mortality of birds

(62 species; 1302 individuals) and mammals (21 species;

282 individuals) in this study would extrapolate to billions of

carcasses on the world’s road system without even

158 Part II Threats to Biodiversity

Figure 8.5 Roads act as filters to the movements

of many animals, especially because of

collisions such as the one that killed this tayra in

Belize. (Photo from M. Hunter.)

attempting to measure the mortality of amphibians, reptiles, and invertebrates. Most of

the individual animals killed on roads may be of common species that are in no danger of

extinction, but even a few road deaths can be of great consequence for an endangered

species. For example, Florida scrub jay territories adjacent to roads are population sinks

because of traffic-induced mortality (Mumme et al. 2000). For some species roads are a

psychological filter; individuals are apparently reluctant to cross them even though physically

capable of doing so. In the Brazilian Amazon some bird species, especially those

found in the understory of interior forests, very rarely crossed roads, even roads where

regrowth formed a nearly intact canopy over the road (Laurance et al. 2004). If organisms

are unable or unwilling to cross a road, then the populations on either side of the

road may become isolated from one another; this has been demonstrated for amphibians

(Gibbs 1998) and beetles (Keller and Largiader 2003).

A second major problem associated with roads is the access they provide to people

who may overexploit organisms or destroy whole ecosystems. The roads penetrating

formerly remote areas of tropical forest, allowing access by poachers who overexploit

game populations and settlers who raze the tropical forests, are a particularly lamentable

example of this phenomenon. The effect of road access on habitat quality has

been well studied for some large carnivores such as wolves and tigers (Kerley et al.

2002; Theuerkauf et al. 2003).

Roads may also provide access to exotic organisms that can disrupt native populations

(Hansen and Clevenger 2005). Usually, these will be species carried, intentionally

or not, by people traveling along the highway. Sometimes, exotic species will move

along the road by themselves. In particular, weedy exotic plants seem to use the

disturbed ground of roadsides to invade a landscape (Gelbard and Belnap 2003)

(Fig. 8.6). Finally, roads have a variety of physical and chemical attributes that are

likely to affect adjacent aquatic and terrestrial ecosystems. These include various substances

such as dust, sediment, salt, heavy metals, hydrocarbons; a sunny, windy,

warm microclimate; blocking surface water runoff; and more (Trombulak and Frissell

2000; Angermeier et al. 2004). One of the most annoying physical aspects of roads for

human observers – traffic noise – was found to reduce bird population densities in a

band hundreds of meters wide in one study (Reijnen et al. 1995).

Dams

Worldwide, over 45,000 large dams (>15 meters high) have affected most of the

world’s major river systems (Nilsson et al. 2005). The damming of streams and rivers

destroys many aquatic ecosystems, flooding ecosystems upstream of the dam and

changing water flows to downstream ecosystems. We will return to these issues in a

later section; here the focus will be on the barrier effects of dams. Many animals move

up and down rivers during the course of a year, or during their life cycle, searching for

the best places to forage or breed. Some of them can fly or walk around dams (otters,

mergansers, mayflies, etc.), but for totally aquatic species dams can be very significant

barriers. Moving downstream these animals are likely to be churned to death or at

least highly stressed in turbines (Wertheimer and Evans 2005). Moving upstream

they encounter an insurmountable wall that may or may not have a fish ladder

around it, and even fish ladders work for only a portion of the population. The reservoir

behind a dam may also impede movement, especially if it has been stocked with

exotic, predatory fish. Of course, fish are the best known victims of dams, especially

Ecosystem Degradation and Loss 159

anadromous fish such as salmon that move long distances between riverine spawning

areas and marine foraging areas (Petrosky et al. 2001; Fig. 8.7). Some salmon populations

have been completely eliminated, largely by dams, despite millions of dollars

spent building fishways, trucking fish around dams, supplementing populations with

hatchery-reared stocks, and so on (Molony et al. 2003). A study of eight rivers in

Sweden suggested that the effects of dams and reservoirs on shoreline plants are also

shaped by dispersal issues: water-dispersed species with a limited ability to float were

strongly affected by damming (Jansson et al. 2000a; also see 2000b).

Other Barriers

Some landscapes are dissected by barriers specifically designed to inhibit the movement

of animals. Notably, rangeland fences stretch huge distances, controlling the

movement of both livestock and large wild mammals and sometimes severing

seasonal migrations (Berger 2004). For example, in Botswana, thousands of kilometers

of fences have been erected to isolate livestock from wild ungulates that might

harbor diseases. These fences have had catastrophic consequences for native ungulates,

especially wildebeest, that must migrate to access water during dry seasons

(Williamson et al. 1988).

160 Part II Threats to Biodiversity

Figure 8.6 Exotic and native plant species richness in plots 50 meters away from paved,

improved-surface, graded, and four-wheel-drive (4WD) roads through grasslands, shrublands,

and woodlands in southern Utah, USA (Gelbard and Belnap 2003). Error bars represent

1 SE. Different letters indicate significant differences (p < 0.05) among levels of road

improvement.

Utility corridors also dissect landscapes, potentially isolating the organisms on

either side. For example, a forest herb that spreads by means of vegetative reproduction

would be impeded by the dry, sunny environment of a power line running

through a forest. In one study even reindeer, a species usually found in open environments,

exhibited avoidance of power lines during winters in Norway (Nellemann et al.

2001). Pipelines and irrigation canals also have the potential to be direct barriers. In

the best known example of this issue – the Trans-Alaskan Pipeline and caribou

migrations – elevating the pipe kept it from being an absolute barrier but pipelines

still degrade caribou habitat quality (Cameron et al. 2005).

Most bird species can readily fly over human-made barriers, although some forest

birds are very reluctant to venture into the open, and some of the large, flightless birds

(e.g. emus and ostriches) are easily stopped by fences. Unfortunately, birds are often

killed by flying into human structures. Large numbers of migrating birds collide with

power lines, antennas, lighthouses, windmills, and similar structures (e.g. Barrios and

Rodriguez 2004); even local movements can result in a collision with a large window.

Trash and Other Things

In this final section on human-made structures we will list some of the other things

people make and then add to the natural environment that are detrimental to other

Ecosystem Degradation and Loss 161

Figure 8.7 Survival of wild, juvenile, chinook salmon migrating toward the sea before

(1966–8) and after (1970–5) completion of two dams on the Snake River in Washington.

(Redrawn by permission from Raymond 1979; also see Petrosky et al. 2001.)

organisms. Much of this material is trash, things discarded by people, perhaps intentionally

or perhaps not. Lost or discarded fishing gear is a major hazard (Coe and Rogers

1997; Derraik 2002). The worst offenders are probably lost gill nets – often called ghost

nets – which can drift for months or years, still catching fish, diving birds, seals, and

other creatures. It is difficult to estimate the extent of this mortality, but with about

21,300 km of nets (enough to reach more than halfway around the world) set nightly

to catch salmon and squid in the North Pacific alone, the total loss is likely to be enormous

(Laist 1987). Even a single strand of monofilament fishing line discarded by an

angler can ensnare an animal and kill it. Fishing sinkers made of lead and lead shot discharged

by waterfowl hunters accumulate on the bottoms of water bodies, where they

are likely to be swallowed by bottom-feeding birds and cause lead poisoning (Sanderson

and Bellrose 1986; Sidor et al. 2003). Lead shot in carcasses often poisons scavengers

such as California condors and various species of eagles (Pain et al. 2005). One of the

major causes of death among sea turtles appears to be ingesting marine debris, especially

plastic bags and balloons that they mistake for jellyfish (Derraik 2002).

Some of the problems we cause by putting human-made objects into natural environments

would be hard to predict. Consider a seemingly innocuous item, red plastic

insulators for electric fences. It turns out that large numbers of hummingbirds mistook

the insulators for flowers and electrocuted themselves until the manufacturer

withdrew the product. Street lights on beaches may make people feel safer, but they

can disorient hatchling sea turtles when they emerge from their nests and make an

already perilous trip to the sea even more dangerous (Tuxbury and Salmon 2005).

Given that six of the seven species of marine turtle are endangered to varying

degrees, any added source of mortality may be of some consequence.

Lastly, we can list the forms of motorized transport that travel across our lands and

waters without using roads, often crushing plants, colliding with animals, and compacting

and eroding soil. Collisions with motor boats are a major source of mortality for

manatees in Florida (Nowacek et al. 2004). In deserts and on beaches off-road vehicles

are a threat to sedentary or slow-moving species such as plants, hatchling birds, and

desert tortoises. Ironically, fences are a common way to control off-road vehicles (Brooks

1995, 1999), and these have their own ecological problems unless carefully designed.

Earth, Fire, Water

In this section we will consider some of the ways people modify physical environments

that may have negative consequences for biota. We will focus on three issues –

soil erosion, changing fire regimes, and water consumption – that usually degrade

ecosystems without destroying them.

Soil Erosion

Soil erosion is a natural process, an inevitable consequence of wind, rain, and gravity.

The problem is that the rate of soil erosion is often greatly accelerated by human use of

ecosystems (Fig. 8.8). Indeed, it has been estimated that collectively human activities,

such as agriculture, overgrazing by livestock, timber harvesting, and road and building

construction, erode soil at ten times the rate of all natural processes combined

(Wilkinson 2005). Under extreme circumstances, soils that took centuries to form can

be eroded in a matter of hours in a torrential rain storm.

162 Part II Threats to Biodiversity

Ecosystem Degradation and Loss 163

Soil erosion is a double-edged sword. Not only does it

produce sediments that can blanket other ecosystems,

leading to some of the water pollution problems discussed

above (Alin et al. 1999), but it also degrades the productivity

of the land from which the soil is eroded. As Lester

Brown has written, “Society can survive the exhaustion of

oil reserves, but not the continuing wholesale loss of topsoil.”

When a terrestrial ecosystem loses soil and its productivity

is diminished, what are the consequences for the

ecosystem’s biota? This is a difficult question, in part

because there is no simple relationship between productivity

and biodiversity. Some highly productive ecosystems

support a very diverse biota (e.g. tropical rain forests), and

some support relatively few species (e.g. salt marshes). In

the short term, most species are likely to be more affected

by the agent of soil disturbance – the plow, the chainsaw,

etc. – than by the subsequent soil erosion. In the long

term, diminishing the productivity of an ecosystem for

several centuries could be one more stressor that pushes a

species that is dependent on that type of ecosystem a bit

closer to extinction.

In some severe cases, ecosystems can be highly

degraded and species extirpated by soil erosion. For example,

on Round Island in the Indian Ocean, rabbits and

goats introduced to provide a food source for passing

mariners removed most of the vegetation and this led to

severe soil erosion. Two species of reptiles became extinct,

and ten species of plants, three reptiles, and a seabird

were at risk until the exotic herbivores were removed and

erosion was brought under control (North et al. 1994).

We will return to soil erosion below in our discussion on

desertification.

Fire Regimes

Few phenomena can match the ability of a large, hot forest fire to totally transform a

natural ecosystem in a short time. Volcanoes, nuclear bombs, and large meteorites

could readily match a fire, but, thankfully, these are rare events. The apparent devastation

wrought by severe fires has led to concerted efforts to control all fires. Smokey

the Bear’s “Only you can prevent forest fires!” is one of the best-known phrases in the

United States’ advertising media.

Unfortunately, the campaign has been too successful in many respects, especially

when humans reduce the frequency of fires in ecosystems where they are a natural

phenomenon (Van Lear et al. 2005). For example, most natural grasslands and shrublands,

and some types of forests (e.g. certain eucalypt forests in Australia and pine

forests of the southeastern and southwestern United States) are adapted to experiencing

low-intensity fires at frequent intervals (Whelan 1995; Bond and Keeley 2005;

Bond et al. 2005). Consequently, their vegetation changes dramatically without fires to

Figure 8.8 Soil erosion has profoundly degraded

ecosystem productivity in many regions, although it

is most noticeable in mountainous areas, as in this

photo from the Himalayas. (Photo from M. Hunter.)

inhibit the influx of fire-intolerant species. Furthermore, when low-intensity fires are

suppressed, fuel can accumulate and any fire that does get started is likely to be very

intensive. A well known example of the consequences of removing fire from a firedependent

ecosystem comes from Michigan, where fire suppression led to a shortage of

young jack pine stands, the sole habitat of the rare Kirtland’s warbler (Probst and

Donnerwright 2003). The Kirtland’s warbler almost became extinct before its habitat

needs were recognized and met through active forest management.

On the other hand, humans often burn ecosystems quite deliberately. This can be an

ecological problem if the frequency and intensity of the fires are too great. For example,

about 70% of the forests of New Zealand have been eliminated by fire; much of

this occurred soon after Polynesian colonization (Ogden et al. 1998). Undoubtedly,

the very earliest humans realized that fire often promotes grassy vegetation, and

therefore they set fires to produce food for their preferred prey animals and later for

their livestock. These practices continue in many places to this day and, when overdone,

can be a problem. This is most evident when fire is used as a tool to clear forest

for agriculture, as is happening in many countries with burgeoning human populations,

an issue we will discuss below. It might also be a problem in semiarid environments

where frequent burning allows little opportunity for the soil’s organic matter to

develop and thus can contribute to desertification (Woube 1998; Savory and

Butterfield 1999).

Water Use

Every year people directly use 4430 cubic kilometers of water (Postel et al. 1996): that

is over 700,000 liters per person. Some of it we drink. Far more of it we use to irrigate

our crops and lawns, to bathe, to flush our toilets, to manufacture sundry products

such as paper, and to cool our power plants. To be specific, an estimated 65% is used for

agriculture, 22% for industry, and 7% for domestic purposes, and 6% is lost to evaporation

from reservoirs (Postel et al. 1996). With 43,000 liters required to produce a kilogram

of beef it is not surprising that agriculture is the dominant use (Pimentel et al.

2004). Some of this water is returned to an aquatic ecosystem; most of it is returned to

the atmosphere through evaporation and transpiration. When large volumes of water

are removed from aquatic ecosystems, their biota is likely to be affected. Not surprisingly,

the effects are most dramatic in arid regions. Desert springs, streams, and wetlands

are usually rare and fragile ecosystems, often containing unique species that

have evolved in isolation (Minckley and Deacon 1991; Fagan et al. 2002). Obviously, if

most of their water is removed, these ecosystems will be degraded (Contreras-B. and

Lozano-V. 1994). Consider the lower Colorado River in the arid southwestern United

States, where water flow disruptions have had a major role in the decline of 45 endangered

species (Glenn et al. 2001).

Water scarcity can be an issue even in places where there is a great deal of water.

Most of the southern tip of Florida is essentially one huge wetland – the Everglades –

that covers many thousands of square kilometers in a sheet of water. Yet the

Everglades is so shallow, and the demands on its water from farmers and coastal communities

are so great, that the whole ecosystem is being profoundly changed by a

scarcity of water (Davis and Ogden 1994). Notably, the numbers of herons, egrets,

and other wading birds have declined sharply, in part because a reduction in freshwater

input has reduced the productivity of estuarine parts of the Everglades.

164 Part II Threats to Biodiversity

Deforestation

■ Forests cover less than 6% of the earth’s total surface area.

■ Forests are habitat for a majority of the earth’s known species.

■ Forests are being lost faster than they are growing.

These three facts highlight why many conservation biologists believe that deforestation

may be the most important direct threat to biodiversity. In this section we will

first review some of the causes, and then some of the consequences, of deforestation.

Causes of Deforestation

Forests tend to grow in places with reasonably fertile soils and benign climates, not too

dry and not too cold. These also tend to be good places for people to live and grow crops.

Consequently, millions of square kilometers of forests have been removed to make way

for our agriculture, homes, businesses, mines, and reservoirs since the beginning of

agriculture (Williams 2003). This process has slowed, stopped, or even reversed in some

areas that were extensively deforested many years ago, such as Europe, China, and eastern

North America. In some developed countries, the demand for forest land is less

because the human population has stabilized, or because the local economy has shifted

from agriculture (the single biggest cause of deforestation) to industry. In other places,

such as large parts of China, there are simply few forests left to remove. Unfortunately,

deforestation continues at an alarming pace in many tropical regions. The statistics

vary widely – an area the size of Switzerland every year, nearly 50,000 ha every day,

and so on – and we do not really have a good estimate, but the basic fact remains: forests

are disappearing, especially tropical forests (Williams 2003). The fundamental reasons

for the current spate of tropical deforestation are threefold. First, human populations

are increasing rapidly in most tropical areas. Second, many of these people are poor,

and clearing forest to open a small plot where crops can be grown is often their only

choice for survival. Third, corporations and wealthy individuals cut forests for wood

products with inadequate attention to regrowth (especially in Asia) and to open the

land for cattle ranching (especially in Latin America).

Unfortunately, poor farmers are often trapped in poverty because the lands they clear

are not really suitable for agriculture in the first place. After only a few years the soil’s fertility

is drained, and they must move on to another site and clear more forest. The process

of clearing a small patch of tropical forest, growing crops for a few years, and then moving

on to another site is called shifting cultivation and it is a traditional, sustainable practice

when human populations are low and the abandoned site is allowed to return to

forest. However, when populations are too high, then people stay at a site too long or

return to a previously used site too soon. Alternatively, they may sell the land to a wealthy

cattle rancher. Particularly in Latin America, much of the tropical forest initially cleared

for subsistence agriculture ends up as rangeland for cattle, while under some circumstances

the cattle ranchers raze the forest themselves (Fearnside 2005). In Asia, the direct

drivers of deforestation are often logging companies. Whatever the underlying reason,

abusive use of a site is likely to degrade the soil so badly that, even when it is abandoned, it

will probably take several centuries, or even millennia, for a rich forest to return. Tropical

forest soils are notorious for being easily degraded and difficult to reforest (Lal 1995).

In many people’s eyes timber harvesting is a major cause of deforestation. For example,

Pimm (1991, p. 136) wrote “consider the ultimate form of external environmental

Ecosystem Degradation and Loss 165

disturbance – total destruction of the habitat, such as might result from logging of a forest,

or an asteroid collision, or a nuclear holocaust.” This viewpoint needs to be scrutinized,

however. A forest can be profoundly disturbed by severe fires or windstorms, but in

time the forest will be restored by ecological succession. Similarly, when a forest is

clearcut, it will eventually return to a forest again if it is given enough time and freedom

from additional disturbances such as plows and cattle and real estate developers

(Fig. 8.9). It may or may not resemble a forest that was disturbed by natural phenomena,

but it will be a forest. Time is the critical issue here. Calling a clearcut forest deforested is

probably appropriate only if its recovery will take significantly longer than recovery from

a natural disturbance. Note that under some circumstances logging can negatively affect

a forest even if only a small portion of the trees are removed, but this is more appropriately

called degradation than deforestation. This issue will be covered in Chapter 9, and

in Chapter 12 we will address ways to harvest wood and maintain biodiversity.

Consequences of Deforestation

The extraordinary species diversity of forests is based on a number of factors (Hunter

1990); here are four key ones. First and most basically, the environmental conditions

that forests require – some soil and a reasonably benign climate – are favorable to life in

general. Contrast the places where forests grow to a tundra or desert. Second, the durability

of wood means that forests contain an enormous reservoir of organic matter, and

this material represents food and shelter to a large set of invertebrates, fungi, and

microorganisms. For example, just two families of wood-boring beetles – long-horned

beetles and metallic wood borers – contain twice as many species as all the world’s

bird, mammal, reptile, and amphibian species combined (Hunter 1990). Third, the

strength of wood makes forests taller, more three-dimensional, than other terrestrial

ecosystems. The height of a forest means that it contains many different microenvironments,

from the sunny, windy foliage at the top of the canopy to the cool, damp

recesses of a crack in the bark of a tree trunk, and each of these different microenvironments

may support a different set of small creatures. Fourth, forests are dynamic

ecosystems, frequently changing through the processes of disturbance and succession,

and many of these changes are marked by differences in species composition.

Among all forests, the most diverse are the tropical rain forests. Indeed, many biologists

believe that half of all the species on earth may occur in tropical rain forests (Wilson

1992). Our knowledge is too limited to corroborate this statement (as was explained in

Chapter 3), but we can consider many fragmentary bits of supporting evidence. For

example, 43 species of ants have been found on one tree in a Peruvian tropical forest,

about equal to the number that occur in all of Great Britain, and 1000 tree species were

found collectively in ten 1 ha plots in Borneo, far more than occur in all of the United

States and Canada (Wilson 1992). The reasons for the extraordinary diversity of tropical

forests are complex and not well understood (Hill and Hill 2001). Suffice it to say here

that the four factors mentioned above probably play a role (for example, tropical rain

forests are taller and have larger reservoirs of organic matter than many other types of

forest), as well as other factors such as long-term climate change.

Needless to say, when people convert a forest to another type of ecosystem, most of the

forest-dependent species are lost from that site for some period. It is easy to name forestdwelling

species that are threatened with extinction largely because of deforestation – giant

166 Part II Threats to Biodiversity

pandas, tigers, gorillas, and many many more – but, of course, these are just the tip of the

iceberg. With most of the earth’s biodiversity residing in insects and other small organisms,

and with many, perhaps most, of these small species living in tropical forests where they

remain unknown to science, we can only make gross estimates of the likely impact of

deforestation (Lawton et al. 1998). Fully acknowledging the extent of our uncertainty, it is

still clear that a large portion of the earth’s biodiversity is found in tropical forests and that

these forests are being lost to deforestation at a very high rate. Consequently, all conservation

biologists believe that protection of tropical forests must be a high priority.

Ecosystem Degradation and Loss 167

Figure 8.9

Clearcuts have a

dramatic effect on

forest biota but the

key issue is what

happens in the following

years; will

the forest regenerate

or will it be

converted to

another use, such

as housing or agriculture,

and thus

constitute deforestation?

We also

need to consider to

what extent a

clearcut does or

does not resemble

the natural disturbance

regime for a

particular type of

forest. (Photo from

Marc Adamus.)

Thus far we have focused on the biological consequences of deforestation, but

through changes in the physical environment, deforestation can have effects far

beyond the edge of the forest. We have already discussed soil erosion as a source of sediment

that can contaminate aquatic ecosystems. On a global scale, forests affect the

earth’s climate by acting as reservoirs of carbon, and when they are cut, much of the

carbon moves into the atmosphere as carbon dioxide, the major greenhouse gas

(Steininger 2004). More locally, because much of the water vapor in the atmosphere

above a forest is maintained by evaporation and transpiration, when a forest is cut,

rainfall may decrease. This makes the hot, dry conditions of a deforested site even hotter

and drier.

Desertification

When you envision a barren, nearly lifeless landscape, do you think of deserts? This

image ignores the myriad species that flourish in desert ecosystems, but nevertheless,

fewer species overall live in arid environments than in more humid ones. Therefore it

is of great concern to conservationists that the extent of arid land – currently about

35% of the earth’s land surface – is apparently increasing because of human activities

(Mainguet 1999). In particular, grasslands and woodlands (i.e. relatively dry

forests in which tree crowns do not meet to form a continuous canopy) are being

degraded until they are dominated by sparse, relatively unproductive vegetation

(Fig. 8.10). This process is called desertification.

Causes of Desertification

In most parts of the world desertification is closely associated with overgrazing

(Schlesinger et al. 1990; Asner et al. 2004). Too many cattle, sheep, goats, and other

livestock consume and trample too many plants, and this alters the species composition

and structure of the vegetation and reduces the overall biomass. With few plants

to protect the soil and with many animal hooves breaking and compacting the soil,

erosion is likely to increase. The excessive burning of grasslands, usually to provide

fodder for livestock, may further exacerbate the problem (Savory and Butterfield

1999), while suppression of natural fire regimes can lead to the encroachment of

shrubs (Asner et al. 2004).

Cultivation is also a major cause of desertification (Dregne 2002), particularly because

it generates soil erosion. Furthermore, croplands that require irrigation often face two

other problems: salinization and waterlogging (Contreras-B. and Lozano-V. 1994;

Mainguet 1994). Salinization is common when irrigation is used in arid environments

because large volumes of water evaporate, leaving behind salts that can reach toxic concentrations.

If farmers try to solve this problem by using enough water to leach the salts

lower into the soil, waterlogged soils can occur. Cutting trees in woodlands, usually for

fuelwood, can also contribute to desertification.

Over the long term, whenever cyclical changes in the earth’s orbit have led to

warmer and drier conditions, some grasslands and woodlands have become deserts

(see Chapter 6). Against this background, the relative importance of long-term climate

change and short-term droughts, natural erosion, and human-induced causes of

desertification is a complex and controversial topic. Some argue that anthropogenic

168 Part II Threats to Biodiversity

factors are paramount; others

argue for climate change; and

their relative role seems to depend

on what part of the globe you are

talking about (Geist and Lambin

2004).

Consequences of

Desertification

Desertification and its consequences

are often overlooked until

they become extreme, in part

because it is harder to recognize

the work of hungry livestock (the

cumulative impact of thousands of

small bites) than the work of a

hungry chainsaw (Fleischner

1994). A deforested site often looks

like a disaster, but an overgrazed

ecosystem where grasses have been

replaced by unpalatable brush may

not look degraded to the untrained

eye. Grasslands and woodlands

that are vulnerable to desertification

may not match the wealth of

biodiversity of forests, but they do

have a large set of unique species

that merit the attention of conservation

biologists, including such

well known species as African elephants,

cheetahs, black-footed ferrets,

both black and white rhinos,

great bustards, and African wild

dogs. Furthermore, in decrying the

loss of grasslands and woodlands

to desertification, it is important

not to imply that deserts lack biodiversity

value. Thousands of species

are found in deserts, and many of

them are highly endangered:

desert tortoises; Asian and African

wild asses; sundry species of cactus;

and a variety of antelopes

such as the addax, scimitar-horned oryx, and Arabian oryx, to name some of the better

known taxa.

Ecosystem Degradation and Loss 169

Figure 8.10 This photo from the Khyber Pass in Afghanistan reveals some

of the classic signs of desertification: virtually no ground vegetation (at

least of palatable plants), a browse line on the tree indicating how high

livestock can reach, and soil erosion. (Photo from M. Hunter.)

It is instructive to think of a continuum of decreasing ecosystem biomass and productivity

from forests to woodlands to grasslands to deserts. Ecosystems that already

fall in the desert part of this continuum are still vulnerable to being pushed further

down the continuum of decreasing productivity and biomass. This perspective raises

the possibility that some species adapted to the lower end of this continuum might

benefit from desertification by having larger areas of habitat (Whitford 1997). This

may be true of some common, highly adaptable species, but the species of greatest

concern are likely to be habitat specialists that cannot survive in degraded ecosystems

that are a human-created facsimile of natural desert.

One reason why desertification has had a significant impact on biodiversity is that

relatively few grasslands and woodlands have been protected as parks (Hoekstra et al.

2005). This is partly because these lands usually lack the amenities – lakes, mountains,

forests – that people seek for outdoor recreation. (A notable exception to this

generalization comes from eastern and southern Africa, where tourists visit arid and

semiarid parks to see the spectacular suite of large mammals.) Of course, establishing

some more parks would not be a complete solution; wiser management of all ecosystems

vulnerable to desertification must be the goal.

Draining, Dredging, Damming, etc.

Swamps and marshes, bogs and fens, lakes and ponds, rivers and streams, estuaries

and the ocean, and more: there is a wide variety of ecosystems – freshwater

ecosystems, marine ecosystems, and wetlands – in which water is a medium for life,

not just an essential nutriment. Similarly, there is a wide variety of ways in which

people destroy these ecosystems by changing their hydrology (Fig. 8.11). We will

begin by briefly reviewing some of these methods.

Filling a wet depression with material until the surface of the water table is well

below ground is an obvious way to turn a wet ecosystem into a dry one. This method

is usually too expensive to use for creating agricultural land, but it is routinely used to

create house lots, airports, parking lots, and other high-priced land. Small, shallow

wetlands are particularly vulnerable to being filled.

Draining a wet ecosystem (i.e. lowering the water table by moving the water somewhere

else) is a common practice. In its simplest form it involves digging ditches that

allow the water to drain away. Under the right circumstances, this method can be used

to drain large areas relatively easily. Occasionally, water is actually pumped out of a

wet ecosystem at great expense.

The primary impetus for both draining and filling is to acquire more land that is useful

for human enterprises. The single biggest use for land created in this fashion is agriculture,

except in urban and suburban areas where housing developments, shopping

malls, and other projects are often the key issue. Occasionally, sites are drained to

improve their ability to produce timber, and in some countries peatlands are drained so

that the peat can be mined for fuel. Clearly, it is easier to drain or fill a shallow basin

than a deep one, and thus wetlands are far more vulnerable to these losses than are

lakes, rivers, and estuaries.

Dredging involves digging up the bottom of a water body – the mud and a host of muddwelling

creatures – and depositing the material elsewhere, often in a wetland that someone

wants filled. The goal is usually to maintain a shipping channel in a river or harbor;

170 Part II Threats to Biodiversity

Ecosystem Degradation and Loss 171

the ecological result is a scarred bottom and sediment pollution. Sometimes, the sediments

contain high concentrations of toxins that are returned to the food web after dredging.

Channelizing rivers and streams means making them straighter, wider, and deeper

and replacing riparian (shoreline) vegetation with banks of stone or concrete. This

conversion from a complex of natural riverine communities to a barren canal may

meet engineering objectives, usually flood control, but is obviously an environmental

calamity. Sometimes canals are dug to connect separate water bodies; these can

become conduits that allow the mixing of formerly isolated biotas (Smith et al. 2004).

Figure 8.11 A

complex of aquatic

ecosystems before

and after human

alterations. In the

lower right a housing

development

that was previously

surrounded by

dikes is being

extended by filling

the wetland.

Nearby, the channel

is being

dredged. Upstream

the river has been

channelized and

the adjacent wetlands

ditched. A

tributary on the

right side of the

main river has

been dammed to

create a reservoir.

In the real world it

would be highly

unusual to have all

these activities in a

small area.

Damming rivers and streams can profoundly change ecosystems both upstream and

downstream (Nilsson and Berggren 2000; Bunn and Arthington 2002). First,

upstream of a dam, a flowing-water ecosystem (the technical term is lotic) is converted

to a standing-water (lentic) ecosystem, and wetland and upland ecosystems will also be

flooded and thus become part of a reservoir. Wetlands are especially likely to be extensively

flooded because their elevation is often close to that of a nearby river.

Additionally, many reservoirs are subject to dramatic fluctuations in water level

depending on changing demands for electricity and water. This means that the shores

of reservoirs are often quite barren because relatively few species can cope with being

inundated and then exposed in this manner (Jansson et al. 2000a). Second, downstream

of a dam, floodplain ecosystems are likely to be replaced by upland ecosystems

if the dam minimizes or eliminates the seasonal floods that are critical to the maintenance

of these ecosystems. In the river itself, species are likely to be challenged by flow

rates that are very unnatural: too much short-term fluctuation in response to demands

for water or electricity, or not enough annual fluctuation in response to rainy and dry

seasons. Also, the water temperature may be too warm (if drained from the top of the

reservoir) or too cold (if drained from the bottom of the reservoir) (Vaughn and Taylor

1999). A third issue, dams as barriers to the movements of aquatic species, was discussed

above in the section on roads and dams as barriers.

Diking consists of constructing earthen banks, usually called dikes or levees, along the

edges of water bodies to prevent flooding. Given that floods are natural phenomena vital

to the maintenance of many types of ecosystems, diking can easily destroy ecosystems,

especially because it is often linked with developing land for other purposes.

Obviously, the world’s oceans and seas are too large to be converted to other types of

ecosystems by filling, draining, etc., but they are not completely immune to these

processes. The bays and inlets that line oceans and seas (often these are estuaries where

salt and fresh waters meet) are small enough to be affected by these processes, especially

filling and dredging. Furthermore, sometimes our attempts to control currents in these

areas with breakwaters, jetties, and other structures can change marine ecosystems

profoundly. For example, shortsighted attempts to maintain sandy beaches by building

jetties often end up accelerating beach erosion and sand deposition somewhere else.

To discuss the consequences for biodiversity of filling, draining, dredging, channelizing,

damming, and diking, we will focus on the two groups of species that are most

vulnerable to these processes: those associated with wetlands and rivers. In the wake

of devastating tsunamis, hurricanes, and floods, the consequences of degrading

shoreline ecosystems is of great concern for human communities too, albeit beyond

our scope here. Suffice it to say that the impacts of natural disasters are much

less severe where shoreline ecosystems are intact enough to provide a buffer

(Danielsen et al. 2005).

Consequences for Wetland Biota

Stemming the loss of wetlands has become a major goal of conservationists for two

basic reasons: the rarity of wetlands and their ecological value. Wetlands cover a relatively

small portion of the earth’s total surface, roughly 1–2%, and this portion is

decreasing (Harcourt 1992). In the conterminous United States, roughly 53% of wetlands

were lost between the 1780s and 1990s (Dahl 2000), and worldwide figures

172 Part II Threats to Biodiversity

are probably roughly comparable (Dugan 1993; Mitsch and Gosselink 2000). These

facts alone make it imperative to protect remaining wetlands, given a goal of protecting

biodiversity at the ecosystem level (see Chapter 4). Furthermore, wetlands are

often keystone ecosystems, playing critical roles in a landscape through hydrological

processes, biomass production and export, removal of contaminants from polluted

water, and so forth. (See Mitsch and Gosselink 2000 for a review of this topic.)

At the species level of biodiversity, wetlands are important because they are habitat

for a diverse biota comprising three groups of species. First, there are species that are

primarily aquatic (such as many species of fish and insects) that can use the pools of

water often found in wetlands. Some may be permanent residents; some may be visitors,

coming only at high tide, or during spring or monsoonal high waters.

Second, many terrestrial species are facultative users of wetlands, with a portion of

their population found in wetlands. Wetlands can be particularly important refugia for

terrestrial species that are sensitive to human interference; this is because wetlands

tend to be too wet for humans to hunt, plow, or extract trees and too dry for them to

access by boat. For example, the mangrove swamps in the mouth of the Ganges River,

the Sunderbans, harbor one of the world’s largest remaining tiger populations.

Finally, there are many thousands of species that are uniquely adapted to the interface

of wet and dry environments found in wetlands. These include whole families of

plants (cattails, water lilies, bur-reeds, and many more) and insects (e.g. predaceous

diving beetles, water boatmen, and several families of damselflies and dragonflies)

that are almost exclusively found in wetlands. Among vertebrates, most amphibians

and turtles are wetland species.

Throughout the world the loss of wetlands has pushed many species toward

extinction. Nine of the world’s 15 species of cranes – birds that require wetlands for

breeding and often foraging – are in jeopardy. In recent years herpetologists have been

alarmed by precipitous drops in many frog populations, and wetland loss is a primary

cause (Houlahan et al. 2000; Beebee and Griffiths 2005).

Consequences for River Biota

Rivers and streams are often likened to the arteries of a landscape, and this metaphor

is apt from both an ecological and economic perspective. It is hard to imagine human

history without rivers – bringing water to our croplands and homes, driving waterwheels

and turbines, providing a transportation network, and carrying away our

wastes. Think about how many of the world’s cities are located on a river, and you

will appreciate their pivotal role.

Unfortunately, being the focus of so much attention has left many rivers badly

degraded by water pollution, channelization, and dredging, or converted to reservoirs

by dams (Malmqvist and Rundle 2002; Postel and Richter 2003). The victims

of this scarcity of clean, free-flowing rivers do not draw much public attention

because they are chiefly fish, mollusks, and insects, not the birds and mammals that

galvanize public support (Allan and Flecker 1993). Scores of riverine fish species are

threatened with extinction, but most of them are minnows and other small species

that are seldom seen. Only a few economically important fish species such as various

salmon species are likely to garner much attention. Even lower on the list of public

popularity are mussels, crayfish, and other invertebrates, even though hundreds,

Ecosystem Degradation and Loss 173

perhaps thousands, of species are endangered by river degradation and conversion.

One analysis of North American crayfishes and unionid mussels estimated that 63%

of the crayfish species (198 of 313) and 67% of the unionid mussels (201 of 300)

were either extinct or at some level of risk (Master 1990). Another analysis of the

freshwater fauna of North America demonstrated that the recent (since 1900)

extinction rate of these animals was about five times greater than that of terrestrial

vertebrates and that this difference was likely to persist in the future (Ricciardi and

Rasmussen 1999). A similarly dramatic story could be told for Asian rivers – home

to over half of the world’s large dams (over 15 meters tall) and a large portion of the

world’s freshwater crabs, snails, turtles, crocodilians, river dolphins, and fishes

(Dudgeon 2000, 2002). For example, there are 105 families of freshwater fishes in

Asia compared with 74 in Africa and 60 in South America. For many taxa we do not

even have enough information to evaluate rarity or endangerment. For example,

388 algal species were recorded in one stream in southern Ontario (Moore 1972),

but few streams have been inventoried this thoroughly, and thus virtually all of their

algal species could be eradicated without documentation of their disappearance.

Fragmentation

When early explorers of wild regions found a high vantage point from which to scan

the terrain, they often wrote of a “sea of green” to convey the unbroken vastness of

the forests and grasslands they traversed. A modern traveler, looking down from a

plane, is likely to describe a typical landscape as a “patchwork quilt” – a mosaic of pastures

and croplands, woodlots and house lots and parking lots. The process by which a

natural landscape is broken up into small parcels of natural ecosystems, isolated from

one another in a matrix of lands dominated by human activities, is called

fragmentation. Because fragmentation almost always involves both loss and isolation

of ecosystems, researchers would like to distinguish between the effects of these two

processes but it is not often practical to do so (Guerry and Hunter 2002; Fahrig 2003).

Fragmentation is a major focal point for conservation biologists, both because it has

degraded many landscapes and because many nature reserves have become isolated

fragments or are in danger of becoming so (Saunders et al. 1991). In addition, it captured

the interest of many conservation biologists because it was recognized as an issue

at about the same time that conservation biology was emerging as a new discipline; in

other words, it was new ground for conservation biology to plow. Furthermore, it

appeared to have a theoretical foundation in an intriguing body of ideas and observations

known as island biogeography (Box 8.1). It seemed reasonable to assume that the

effects of isolation on the biota of oceanic islands might provide a model for understanding

the effects of isolation on populations inhabiting patches of natural ecosystems that were

isolated in a sea of human-altered land.

Most conservation biologists have come to recognize that the applicability of island

biogeography theory to fragmentation issues is quite limited, primarily because fragmentation

“islands” are not nearly as isolated for most species as true oceanic islands

(Zimmerman and Bierregaard 1986; Debinski and Holt 2000; Haila 2002).

Nevertheless, island biogeography does provide a conceptual foundation for understanding

fragmentation and is the origin for two important ideas. Small fragments

(or islands) have fewer species than large fragments, and more isolated fragments

174 Part II Threats to Biodiversity

have fewer species than less isolated fragments. We will begin by considering these

two ideas further.

Fragment Size and Isolation

There are three main reasons why large fragments have more species than small

fragments (Fig. 8.13). First, a large fragment will almost always have a greater variety

of environments than a small fragment (e.g. different types of soil, a stream, a

rock outcrop, an area recently disturbed by fire), and each of these will provide

niches for some additional species.

Second, a large fragment is likely to have both common species and uncommon

species (i.e. species that occur at low densities), but a small fragment is likely to have

only common species. This idea is easy to grasp when we consider species that have

large home ranges; for example, it means that we are unlikely to find a bear in a tiny

fragment. However, it also applies to species that have rather limited home ranges but

still actively avoid small fragments. For example, certain small birds such as Sprague’s

pipits and grasshopper sparrows have home ranges of only a few hectares, but are

usually not found in habitat fragments less than 100 ha in size (Davis 2004). Species

that do not occur in small patches of habitat are called area-sensitive species and are

often of concern to conservationists. Furthermore, uncommon species that are not

area-sensitive (i.e. that can find habitat in a small fragment) are also unlikely to occur

in a small patch by chance alone. This last point is a subtle one that is often overlooked

(Haila 1999), but it is easily explained with an example. Imagine there was an

uncommon tree species that had an average density of one individual per 1000 ha;

all other things being equal, a 100 ha sample plot would have a 1:10 chance of

Ecosystem Degradation and Loss 175

BOX 8.1

Island biogeography theory

The fundamental idea of MacArthur and Wilson’s (1967) equilibrium theory

of island biogeography is that the number of species on an island represents

a balance between immigration and extinction. The rate of

immigration is determined largely by how isolated an island is; the more

isolated, the lower its immigration rate. This is represented in Fig. 8.12,

with the curve for remote islands (far) being lower than the curve for

islands that are near the mainland (near). Extinction rates are a function of

island size; populations on large islands tend to be larger and thus less vulnerable

to extinction. In Fig. 8.12 the extinction curve for large islands is

lower than the curve for small islands.

For any given island there is an extinction rate and an immigration rate

that will balance one another and keep the number of species relatively constant.

In this example, the numbers of species for four equilibria are represented

as follows: SFS, number of species on a far, small island; SFL, far,

large island; SNS, near, small island; SNL, near, large island. P is the total

number of species that could potentially immigrate to the island from a

nearby landmass.

Figure 8.12 A graphical representation

of island biogeography theory.

(From Hunter 1990, reprinted

by permission of Prentice-Hall,

Englewood Cliffs, New Jersey.)

176 Part II Threats to Biodiversity

containing this species, but a 10

ha plot would have only a 1:100

chance. This sampling effect, added

across many species, would mean

that a small fragment would have

fewer species than a large fragment

simply because it is a smaller sample.

To adjust for this phenomenon,

fragmentation studies should focus

on number of species per unit area

(e.g. Rudnicky and Hunter 1993),

but most only report the number of

species in each fragment.

Third, small fragments will, on

average, have smaller populations

of any given species than large

islands, and a small population is

more susceptible to becoming

extinct than a large population

(Henle et al. 2004). This idea was a

key point in the preceding chapter.

Fragments that are isolated from

other, similar patches by great distances

or by terrain that is especially

inhospitable are likely to

have fewer species than less isolated

fragments for two reasons.

First, relatively few individuals of a

given species will immigrate into

an isolated fragment. Immigrating

individuals are important both

because they can “rescue” a small

population from extinction and

because they can replace a population

that has already disappeared

(Brown and Kodric-Brown 1977).

Second, species that are mobile

enough to use an “archipelago” of small habitat patches to collectively comprise a

home range are less likely to use an isolated fragment simply because it is inefficient to

visit it. For example, the copperbelly water snake travels among ephemeral wetlands

foraging for frogs and it seems to fare badly when wetlands are lost and the average

distance among the remaining wetlands increases (Roe et al. 2004).

Causes of Fragmentation

The fundamental cause of fragmentation is expanding human populations converting

natural ecosystems into human-dominated ecosystems. Fragmentation typically

Figure 8.13 The number of species in a sample plot or on an island

increases as area increases, but the steepness or slope of the curve

varies considerably among taxa. Note that in these graphs for taxa on

islands in the Baltic Sea some of the y axes are linear and some are logarithmic.

All of the x axes are logarithmic. Recall from Chapter 6 that

these lines are described by the formula S = CAz, where S is number of

species, A is area, and C and z are constants. (Redrawn from Järvinen

and Ranta 1987.)

Ecosystem Degradation and Loss 177

begins when people dissect a natural landscape with roads and then perforate it by

converting some natural ecosystems into human-dominated ones (Fig. 8.14). It culminates

with natural ecosystems reduced to tiny, isolated parcels. Thus fragmentation

almost always involves both reducing the area of natural ecosystems and increasing

their isolation, although some authors have advocated reserving the term for isolation

(Fahrig 2003). As the single largest user of land, agriculture is the proximate cause of

most fragmentation. Certainly, for many terrestrial species, a large expanse of cropland

is a barrier nearly as effective as a stretch of water. Urban and suburban sprawl

may be a more effective barrier to movement, but their total area is much more limited

than that of agriculture. Some writers use “fragmentation” to describe any

process that breaks up extensive ecosystems, including natural events such as fires,

whereas other writers restrict the term to human-induced changes. In any case,

human activities are the major cause of fragmentation in most landscapes.

Sometimes, it is unclear whether human land uses cause fragmentation. Consider

clearcutting forests; if this leads to the forest’s being converted to farmland, then

clearcutting obviously contributes to fragmentation. However, if the clearcut site is

allowed to undergo succession and return to forest, this may or may not constitute

fragmentation, depending on whether the clearcut is extensive enough to constitute a

significant barrier to the movement of plants and animals (Haila 1999, 2002). Of

course, this will vary from species to species. A slow-moving, moisture-loving slug is

far more likely to be deterred by a clearcut than most birds that can fly across a

clearcut in a few seconds. Similarly, at what point on the continuum of desertification

does fragmentation occur? A plant whose seeds are dispersed long distances by wind

may cross a desertified barrier easily, whereas a short-dispersal plant may be incapable

of crossing the barrier in one trip and unable to establish a population halfway

across in the degraded habitat.

Consequences of Fragmentation

Ecosystem destruction is the driving force behind fragmentation, and thus it is

inevitable that fragmentation is associated with negative effects on biodiversity. The

reason why fragmentation elicits so much special concern from conservationists is that

its consequences are greater than we would anticipate based solely on the area of

ecosystems destroyed. Notably, remnant ecosystems that seem to have escaped destruction

may no longer be available for area-sensitive species that cannot use small patches

of habitat. Most prominent among these are large predators that need extensive home

ranges to find enough prey (Crooks 2002). Some small species with limited home

ranges also avoid small habitat patches: for example, birds (Davis 2004) and beetles

(Laurance et al. 2002). This may occur because they require the microclimate characteristic

of the interior of large habitat patches, or because they select habitat patches

large enough to support other members of their species (a type of loose coloniality)

(Stamps 1991), or because of their interactions with other biota as predators, prey, or

competitors (Gibbs and Stanton 2001).

In highly fragmented landscapes, it is difficult for individuals (usually juvenile animals,

seeds, or spores) to disperse to another suitable patch of habitat. If immigration

and emigration are very limited, then the individuals occupying a fragment may effectively

constitute a small independent population and, as we saw in Chapter 7, small

178 Part II Threats to Biodiversity

Figure 8.14

People usually initiate

fragmentation

by building a road

into a natural landscape,

thereby dissecting

it. Next,

they perforate the

landscape by converting

some natural

ecosystems into

agricultural lands.

As more and more

lands are converted

to agriculture,

these patches coalesce

and the natural

ecosystems are

isolated from one

another; at this

stage fragmentation

has occurred.

Finally, as more of

the natural patches

are converted,

becoming smaller

and farther apart,

attrition is occurring.

(Terminology

from R. Forman,

personal communication,

and 1995;

also see Collinge

and Forman 1998.)

populations are more likely to disappear. Furthermore, if a population

does disappear, a low immigration rate will mean it takes much

longer to establish a new population. Even if fragmentation only

leads to partial isolation, this may change one large population into a

metapopulation, which may also affect population viability and persistence.

The dispersal of fire is also an issue; fragmentation has

greatly disrupted natural fire regimes in regions where fires once

swept across the landscape (Van Lear et al. 2005).

The migration of animal species that travel between habitats seasonally

could be impeded by fragmentation (Hunter 1997). In practice, this

is likely to be a problem mainly for species that walk, such as large

 

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