Chapt 33

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687

Population EcologyChapter Concepts

33.1 Scope of Ecology Ecology is the study of the interactions of

organisms with each other and the physicalenvironment. 688

The scope of ecology encompasses theindividual, the population, the community, theecosystem, and the biosphere. 688

33.2 Characteristics of Populations Population size depends upon births, deaths,

immigration, and emigration. 690 Two patterns of population growth (exponential

and logistic) have been developed. 690 Mortality within a population is often illustratedby a survivorship curve. 691

33.3 Regulation of Population Size Factors that affect population size are classified

as density-independent and density-dependent.692

33.4 Life History Patterns

Life history patterns range from one in whichmany young receive little care to one in whichfew young receive much care. 694

33.5 Human Population Growth The human population is still growing

exponentially, and how long this can continue isnot known. 695 These poppies are members of a population whose size is

determined by the carrying capacity of the environment.

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how the fishes feed (Fig. 33.1). Most organisms do not existsingly; rather, they are part of a population, a functional unitthat interacts with the environment.Apopulationis definedas all the organisms within an area belonging to the samespecies. At this level of study, ecologists are interested in fac-tors that affect the growth and regulation of population size.

A communityconsists of all the various populations in-teracting at a locale. In a coral reef, there are numerous pop-ulations of fishes, crustaceans, corals, and so forth. At thislevel ecologists want to know how interactions like preda-tion and competition affect the organization of a community.An ecosystem contains a community of populations andalso the abiotic (nonliving) environment. Energy flow and

chemical cycling are significant aspects of understandinghow an ecosystem functions. The biosphereis that portionof the entire earths surface where living things exist.

Modern ecology is not just descriptive, it also developshypotheses that can be tested. Acentral goal of modern ecol-ogy is to develop models which explain and predict the dis-tribution and abundance of organisms. Ultimately, ecologyconsiders not one particular area, but the distribution andabundance of populations in the biosphere.

Ecology is the study of the interactions of

organisms with other organisms and with the

physical environment. These interactions determine

the distribution and abundance of organisms at a

particular locale and over the earths surface.

33.1 Scope of EcologyEcology is the study of the interactions of organisms witheach other and with the physical environment. Ecology, likeso many biological disciplines, is wide-ranging. At one of itslowest levels, ecologists study how the individual organismis adapted to its environment. For example, they study whyfishes in a coral reef live only in warm tropical waters and

688 Part 7 Behavior and Ecology 33-2

Imagine a watering hole that can accommodate 100 ze-

bras. If, at first, there are only two zebras, and each pair

of zebras produces only four zebras, how many more

generations could there be without overtaxing the waterhole? Youre correct if you say four and incorrect if you say

five. The problem is that you cant just consider the newly ar-

rived zebrasyou have to add the number of zebras already

there.

2 4 8 16 32 64 128

Also, notice that when its time to stop, there are only 62 ze-

bras (30 32). Thats one of the unusual things about pop-

ulation growthat one point it seems as if there is plenty of

room, and then all of a sudden, theres not enough room.

Modern ecologists now recognize that knowledge of

population growth has almost unlimited application possibil-

ities, including the management of wildlife to prevent extinc-

tion and the maintenance of food (organic nutrients) sources

for the human population.

Organism Population Community Ecosystem

Figure 33.1 Ecological levels.The study of ecology encompasses various levels, from the individual organism to the population, community, and ecosystem.

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Density and Distribution of PopulationsPopulation density is the number of individuals per unit areaor volume. If we calculated the density of the human popu-

lation, we would know how many individuals there are persquare mile, for example. From this we might get the im-pression that humans are uniformly distributed, but weknow full well that most people live in cities. Even within acity more people live in particular neighborhoods than oth-ers. Population distribution is the pattern of dispersal of indi-viduals within the area of interest.

There are three patterns of distribution: uniform, ran-dom, and clumped (Fig. 33.2). Human beings have the

clumped pattern of distribution, which is the most commonpattern. Today, ecologists want to discover what causes thespatial distribution of organisms. With reference to humanbeings, we know that many cities sprung up at the junctionof rivers or near inlets that make good harbors for ships. In astudy of the distribution of hard clams in a bay on the southshore of Long Island, New York, it was found, as discussedon page 693, that clams are apt to occur where the sedimentcontains oyster shells. Hopefully, this information can beused to transform areas of low abundance to areas of high

abundance of clams.As with clams, the distribution of organisms can be due

to abiotic factors. Physical factors like a particular inorganicnutrient (the oyster shells provide calcium carbonate for theformation of clam shells) can determine where organismsoccur. Also important are precipitation and temperature,which can be limiting factors for the distribution of an or-ganism. Limiting factors are those factors that particularlydetermine whether an organism lives in an area. Trout live

only in cool mountain streams where there is a high oxygencontent, but carp and catfish are found in rivers near thecoast because they can tolerate warm waters, which have alow concentration of oxygen. The timberline is the limit oftree growth in mountainous regions or in high latitudes.Trees cannot grow above the high timberline because of lowtemperature and the fact that water remains frozen most ofthe year.

The distribution of organisms can also be due to biotic(living) factors. In Australia, the red kangaroo does not live

outside inland areas because it is adapted to feeding on thearid grasses that grow there. And there are more humanswhere the soil is suitable for growing crops than where thesoil is rocky and poor in inorganic nutrients.

Ecology as a science includes a study of the

distribution of organisms: where and why

organisms are located in a particular place at a

particular time.

Chapter 33 Population Ecology 68933-3

a.

b.

c.

food

heat

fuel

materials waste

Figure 33.2 Patterns of distribution within a population.Members of a population may be distributed uniformly, randomly, or

usually in clumps. a. Golden eagle pair distribution is uniform over a

suitable habitat area due to the territoriality of the birds. b. The

distribution of female moose with calves is random over a suitable

habitat. c. Human beings tend to be clumped in cities where many

people take up residence. Cities take resources from and send their

waste to surrounding regions.


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33.2 Characteristics of PopulationsPopulations have a particular pattern of growth and sur-

vivorship among other possible characteristics.

Patterns of Population GrowthPopulations have a certain size and the size can stay thesame from year to year, increase, or decrease, according to a

per capita rate of increase. Suppose, for example, a humanpopulation presently has a size of 1,000 individuals, and thebirthrate is 30 per year, and the death rate is 10 per year. Theper capita rate of increase per year will be:

= 0.02 = 2.0% per year

(Notice that our per capita rate of increase disregarded eitherimmigration or emigration, which for the purpose of our dis-

30 101,000

cussion can be assumed to be equivalent.) The highest possi-ble per capita rate of increase for a population is called itsbi-otic potential (Fig. 33.3). Whether the biotic potential is highor low depends on such factors as the following:

1. usual number of offspring per reproduction

2. chances of survival until age of reproduction

3. how often each individual reproduces

4. age at which reproduction begins.

Suppose we are studying the growth of a population ofinsects that are capable of infesting and taking over an area.Under these circumstances exponential growth is expected.An exponential pattern of population growth results in a J-

shaped curve (Fig. 33.4a). This pattern of population growthcan be likened to compound interest at the bank: as yourmoney increases, the more interest you will get. If the insectpopulation has 2,000 individuals and the per capita rate ofincrease is 20% per month, then there will be 2,400 insects af-ter one month, 2,880 after two months, and 3,456 after threemonths, and so forth.

Notice that a J-shaped curve has these phases:

lag phase: during this phase, growth is slow because the

population is small.exponential growth phase: during this phase, growth is

accelerating and the population is exhibiting itsbiotic potential.

Usually, exponential growth cannot continue for longbecause of environmental resistance. Environmental resis-tance is all those environmental conditions such as a limitedsupply of food, an accumulation of waste products, in-creased competition, or predation that prevent populationsfrom achieving their biotic potential. Due to environmentalresistance, growth levels off and a pattern of populationgrowth called logistic growth is expected. Logistic growthresults in an S-shaped growth curve (Fig. 33.4b).


Figure 33.3 Biotic potential.Animal husbandry relies on biotic potential. If a single female pig has

her first litter at nine months, and produces two litters a year, each of

which contain an average of four females (which in turn reproduced at

the same rate), there would be 2,220 pigs by the end of three years.

Time

NumberofOrganisms

a. Time

NumberofOrganisms

b.

lag

exponentialgrowth

lag

exponentialgrowth

deceleration

stable equilibrium

environmentalresistance

carryingcapacity

Figure 33.4 Patterns of population growth.a. Exponential growth results in a J-shaped growth curve because the growth rate is positive. b. Logistic growth results in an S-shaped growth

curve because environmental resistance causes the population size to level off and be in a steady state.

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Notice that an S-shaped curve has these phases:

lag phase: during this phase, growth is slow because thepopulation is small.

exponential growth phase: during this phase, growth isaccelerating due to biotic potential.

deceleration phase: during this phase, the rate ofpopulation growth slows down.

stable equilibrium phase: during this phase, there is littleif any growth because births and deaths are aboutequal.

The stable equilibrium phase is said to occur at thecarrying

capacity of the environment. The carrying capacity is thenumber of individuals the environment can normallysupport.

Our knowledge of logistic growth has practical impli-cations. The model predicts that exponential growth willoccur only when population size is much lower than thecarrying capacity. So, as a practical matter, if we are using afish population as a continuous food source, it would bebest to maintain the population size in the exponentialphase of growth. Biotic potential is having its full effect

and the birth rate is the highest it can be during this phase.If we overfish, the population will sink into the lag phase,and it will be years before exponential growth recurs. Onthe other hand, if we are trying to limit the growth of apest, it is best to reduce the carrying capacity rather thanreduce the population size. Reducing the population sizeonly encourages exponential growth to begin once again.Farmers can reduce the carrying capacity for a pest by al-ternating rows of different crops rather than growing one

type of crop per the entire field.

Exponential growth produces a J-shaped curvebecause population growth accelerates over time.

Logistic growth produces an S-shaped curvebecause the population size stabilizes when the

carrying capacity of the environment has been

reached.

SurvivorshipPopulation growth patterns assume that populations aremade up of identical individuals. Actually, the individualsare in different stages of their life span. Let us consider howmany members of an original group of individuals born atthe same time, called a cohort, are still alive after certain in-tervals of time. If we plot the number surviving, a survivor-ship curve is produced.

For the sake of discussion, three types of idealized sur-vivorship curves are recognized (Fig. 33.5a). The type Icurve is characteristic of a population like humans in whichmost individuals survive well past the midpoint, and deathdoes not come until near the end of the life span. On the

other hand, the type III curve would be typical for a popula-tion of oysters in which most individuals die very young. Inthe type II curve, survivorship decreases at a constant ratethroughout the life span. This has been found typical of apopulation of song birds.

Sometimes populations do not fit any of these curves ex-actly. For example, in a cohort of Poa annua plants, most in-dividuals survive till six to nine months, and then thechances of survivorship diminish at an increasing rate.

There is much that can be learned about the life historyof a species by studying its survivorship curve. Would youpredict that most or few members of a population with atype III survivorship curve are contributing offspring to the

next generation? Obviously since death comes early formost members, only a few are living long enough to repro-duce. What about the other two types of survivorshipcurves?

Populations have a pattern of survivorship that

becomes apparent from studying the survivorship

curve of a cohort.


NumberofSurvivors

Percent of Life Span

1,000

100

10

0 50 100

I

II

III

human

Hydra

oyster

Figure 33.5 Survivorship curves.Human beings have a type I survivorship curve: the individual usually

lives a normal life span and then death is increasingly expected.Hydras have a type II curve in which chances of surviving are the

same for any particular age. Oysters have a type III curve: most

deaths occur during the free-swimming larva stage, but those that

survive to adulthood usually live a normal life span.

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33.3 Regulation of Population SizeEcologists want to determine when and if the two patterns ofpopulation growth just discussed occur in nature. They wantto know if the factors which regulate population growth are al-ways extrinsic (environmental) or whether there are also in-trinsic (based on the anatomy, physiology, or behavior of theorganism) factors which regulate population growth.

In one study, 4 male and 21 female reindeer (Rangifer)were released on St. Paul Island in the Bering Sea off Alaskain 1911. St. Paul Island had a completely undisturbed envi-ronment. The reindeer fed on lichens, which grow slowlyand cannot recover quickly from grazing. Normally, rein-

deer migrate seasonally, giving their food source a chance toregrow, but these reindeer were confined to an island. Also,there was little hunting pressure, and there were no preda-

tors. The herd grew exponentially to about 2,000 reindeer;overgrazed the habitat; and then abruptly declined to only 8animals in 1950. Such a precipitous drop is called a popula-tion crash (Fig. 33.6). In this instance it does seem as if envi-

ronmental factors, such as food source and possibly disease,are regulating population size.

For some time ecologists have recognized that the envi-ronment contains both abiotic and biotic components. Theysuggested that abiotic factors like weather and natural disas-ters were density-independent. By this they meant that thenumber of organisms present did not influence the effect ofthe factor. Fires dont necessarily kill a larger percentage ofindividuals as the population increases in size (Fig. 33.7). On

the other hand, biotic factors like parasitism, competition,and predation were designated as density-dependent. Con-sider, for example, an area in which there are only 100 holesfor crabs to hide in. The greater the number of crabs beyond100, the better the chance a shorebird will find one and eat it.

In a population study of the great tit, Parus major, it wasfound that the population size fluctuates above and belowthe carrying capacity. While density-dependent and density-independent factors are involved, the researchers believethat territoriality, an intrinsic factor, also plays a role. Terri-toriality is apparent when members of a population arespaced out more than would be expected from a random oc-cupation of the area.

Density-independent and density-dependent

factors can often explain the population dynamics

of natural populations. Both types of factors are

extrinsic to the organism; perhaps intrinsic factors

like territoriality also play a role.


1910 1920 1930 1940 19500

500

1,000

1,500

2,000

NumberofReindeer

b.

a.

decline as a resultof sudden resourcedepletion

exponentialgrowth

Figure 33.6 Density-dependent effect.a.A reindeer, Rangifer. b. On St. Paul Island, Alaska, reindeer grew

exponentially for several seasons and then underwent a sharp

decline as a result of overgrazing the available range.

Figure 33.7 Density-independent effect.A fire can start and rage out of control regardless of how many

organisms are present.

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The hard clam is a commercially harvested bivalve mollusk thatis found throughout the Great South Bay located on the south

shore of Long Island. The clam is eaten raw on the half shell,

and is also baked. Hard clams are a necessary ingredient for

New England and Manhattan clam chowder. The Great South

Bay has often been referred to as a hard clam factory because

in the 1970s, over half of the hard clams harvested in the United

States came from its waters. For the past 20 years, Jeffrey Kass-

ner (Fig. 33A) has been studying the distribution of hard clams

in the eastern third of the bay for the Town of Brookhaven.Knowing the distribution and abundance of hard clams, as

well as the responsible environmental factors of this distribu-

tion is important to Brookhaven because the hard clam industry

has an annual value of $10 million and employs 300 fishermen.

Of particular interest to Brookhaven is the possibility of using

this information to develop projects, such as utilizing aquacul-

ture technology, that will increase hard clam abundance.

Kassner first took a census of the hard clam population. For

several weeks during the summer, a barge-mounted crane witha one square meter clamshell bucket was used to take bottom

grabs at 232 stations located throughout the study area. Each

bottom sample was placed in a one square meter wire sieve and

washed with a high pressure water hose to separate the hard

clams from the sediment so that the hard clams could be

counted and measured in order to calculate various demo-

graphic parameters of the hard clam population. The fieldwork

was physically hard, and by the end of the day, the crew of stu-

dents and biologists were tired, wet, and covered with mud. The

results, however, have proven to be well worth the effort.Working with Dr. Robert Cerrato of the

State University of New York, Kassner has

drawn a composite census map showing

the distribution of and abundance of the

clams. He was surprised to find that hard

clams are not distributed uniformly

throughout the study area, but occur in dis-

tinct patches of high and low abundance. A

dense assemblage of clams is traditionallyreferred to as a clam bed and six such

beds were identified.

Kassner and students found that nearly

all of the beds coincided with areas of high

shell content sediment associated with

relict oyster reefs. This observation is of

historical note as well as biological interest

because up until the early years of this cen-

tury, the Great South Bay was a major pro-

ducer of oysters and was the source of the

world famous Blue Point Oyster. Although oysters are no

longer found in the Great South Bay because of environmental

changes, they left behind a legacy in the sediment that now sup-

ports high abundances of hard clams.

The knowledge that hard clam abundance is positively asso-

ciated with relict oyster reefs might make it possible to trans-

form low-abundance areas into high-abundance areas. Kassner

felt he needed a complete sedimentary portrait of high- and

low-abundance areas. To develop one he borrowed techniques

generally associated with other marine science disciplines: fromshipwreck hunting, he used a side-scan sonar to map the topog-

raphy of the bottom; from deep-sea research, he used a ROV

(Remotely Operated Vehicle) to photograph the bottom; from

commercial fishing, a fathometer to map the bottom; and from

pollution studies, a sediment profile camera to photograph the

sediment-water interface where hard clams live and feed.

Kassner, with the help of students, is now in the process of

putting all this different information together on a single map.

Because it will take time and perhaps more studies to developthe portrait, he is concurrently exploring the feasibility of hav-

ing shells placed on low-abundance areas in order to create new

relict oyster reefs. If this strategy works, it would be a tremen-

dous boon to the shellfish industry because nearly three-

quarters of the study area is low abundance.

Kassner feels his work is not only scientifically interesting, it

is also personally rewarding. Over the years, he has become

friends with many fishermen and knows that if he is successful,

he will be helping them to continue an occupation that in some

cases goes back generations.

693

Distribution of Hard Clams in the Great South Bay

Figure 33AJeffrey Kassner, with the help of students, is preparing a sedimentary portrait of high

and low clam abundance areas in Great South Bay, Long Island, New York. This

information will be used to increase the yield of clams for local fishermen.


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33.4 Life History PatternsWe have already had an opportunity to point out that popu-lations vary on such particulars as their rate of growth andlife span. Such particulars are a part of a species life historypattern, which is based on genetically determined variationsthat are subject to natural selection.

The logistic growth model has been used to suggest thatone possible pattern is an opportunistic pattern and the otheris an equilibrium pattern (Fig. 33.8). These are also calledr-selected and K-selected because in mathematical formulasfor population growth, r represents the per capita rate of in-crease and Krepresents the carrying capacity. Members of

opportunistic populations are small in size, mature early,and have a short life span. They tend to produce many rela-tively small offspring and to forego parental care in favor ofa greater number of offspring. The more offspring, the morelikely some of them will survive a population crash. Becauseof their short life span and ability to disperse to new locales,density-dependent mechanisms such as predation and com-petition are unlikely to play a major role in regulating popu-lation size and growth rates. Classic examples of suchopportunistic species are many insects and weeds.

In contrast, we know there are populations whose sizeremains pretty much at the carrying capacity. Resourcessuch as food and shelter are relatively scarce for these indi-viduals, and those who are best able to compete will have

the largest number of offspring. These organisms allocateenergy to their own growth and survival and to the growthand survival of their offspring. Therefore they are fairlylarge, are slow to mature, and have a fairly long life span.They are specialists rather than colonizers and tend to be-come extinct when their normal way of life is destroyed. Thebest possible examples of equilibrium species are foundamong birds and mammals. The Florida panther is thelargest of the animals in the Florida Everglades, requires avery large range, and produces few offspring, which mustbe cared for. Currently, the Florida panther is unable to com-pensate for a reduction in its range, and is therefore on theverge of extinction.

Nature is actually more complex than these two possiblelife history patterns and most populations lie somewhere inbetween these two extremes. For example a cod is a ratherlarge fish weighing 1025 pounds and measuring up to3 feet in lengthbut the cod releases gametes in vast num-bers, the zygotes form in the sea, and the parents make nofurther investment in developing offspring. Of the 6 to 7 mil-lion eggs released by a single female cod, only a few will be-come adult fish.

Differences in the environment result in different

selection pressures and a range of life history

characteristics.

Figure 33.8 Life history patterns.Dandelions are an opportunistic species with the characteristics

noted, and bears are equilibrium species with the characteristics

noted. Often the distinctions between these two possible life

history patterns are not as clear cut as they may seem.

Opportunistic Pattern

Small individualsShort life spanFast to matureMany offspringLittle or no care of offspring

Equilibrium Pattern

Large individuals

Long life spanSlow to matureFew offspringMuch care of offspring

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33.5 Human Population GrowthThe human population has an exponential pattern of growthand a J-shaped growth curve (Fig. 33.9c). It is apparent fromthe position of 1999 on the growth curve in Figure 33.9a thatgrowth is still quite rapid. The equivalent of a medium-sizedcity (200,000) is added to the worlds population every day,and 88 million (the equivalent of the combined populationsof the United Kingdom, Norway, Ireland, Iceland, Finland,and Denmark) are added every year.

The present situation can be appreciated by consideringthe doubling time. The doubling timethe length of time ittakes for the population size to doubleis now estimated to

be 47 years. Such an increase in population size will put ex-treme demands on our ability to produce and distribute re-sources. In 47 years, the world will need double the amountof food, jobs, water, energy, and so on just to maintain thepresent standard of living.

Many people are gravely concerned that the amount oftime needed to add each additional billion persons to theworld population has taken less and less time. The first billiondidnt occur until 1800; the second billion arrived in 1930; thethird billion in 1960, and today there are nearly 6 billion. Onlyif the per capita rate of increase declines can there be zeropopulation growth, when the birthrate equals the death rateand population size remains steady. The worlds populationmay level off at 8, 10.5, or 14.2 billion, depending on the speedwith which the per capita rate of increase declines.

More-Developed Versus Less-DevelopedCountriesThe countries of the world can be divided into two groups.The more-developed countries (MDCs), typified by coun-tries in North America and Europe, are those in which pop-ulation growth is low and the people enjoy a good standardof living (Fig. 33.9a). The less-developed countries (LDCs),such as countries in Latin America, Africa, and Asia, arethose in which population growth is expanding rapidly andthe majority of people live in poverty (Fig. 33.9b). (Some-times the term third-world countries is used to mean theless-developed countries. This term was introduced by

those who thought of the United States and Europe as thefirst world and the former USSR as the second world.)

The more-developed countries (MDCs) doubled theirpopulations between 1850 and 1950. This was largely due toa decline in the death rate, the development of modern med-icine, and improved socioeconomic conditions. The declinein the death rate was followed shortly thereafter by a declinein the birthrate, so that populations in the MDCs experi-enced only modest growth between 1950 and 1975. This se-quence of events (i.e., decreased death rate followed bydecreased birthrate) is termed a demographic transition.

Yearly growth of the MDCs as a whole has now stabi-lized at about 0.1%. The populations of a few of the MDCsItaly, Denmark, Hungary, Swedenare not growing or areactually decreasing in size. In contrast, there is no leveling


more-developed countries

less-developedcountries

1999

1750

0

2

4

6

8

10

12

1800 1850 1900 1950 2000 2050 2100 2150

P

opulation(inbillions)

Source: Population Reference Bureau.

b.

c.

Figure 33.9 World population growth.People in the (a) more-developed countries have a high standard of living and will

contribute least, while people in the (b) less-developed countries have a low standard of

living and will contribute most to the world population growth. c. World population growth

to 1998 with estimates to 2150.

a.

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off and no end in sight to U.S. population growth, as dis-cussed in the reading on page 698. Although yearly growthof the United States is only 0.6%, many people immigrate tothe United States each year. In addition, there was an un-

usually large number of babies born between 1947 and 1964(called a baby boom). Therefore, a large number of womenare still of reproductive age.

Although the death rate began to decline steeply inthe LDCs following World War II with the importation ofmodern medicine from the MDCs, the birthrate remainedhigh. The yearly growth of the LDCs peaked at 2.5% be-tween 1960 and 1965. Since that time, a demographic tran-sition has begun: the decline in the death rate has slowed

and the birthrate has fallen. A yearly growth of 1.8% is ex-pected by the end of the century. Still, because of expo-nential growth, the population of the LDCs may explodefrom 4.4 billion today to 10.2 billion in 2100. Most of thisgrowth will occur in Africa, Asia, and Latin America.Ways to greatly reduce the expected increase have beensuggested:

1. Establish and/or strengthen family planning programs.A decline in growth is seen in countries with good

family planning programs supported by communityleaders. Currently, 25% of women in the sub-SaharanAfrica say they would like to delay or stopchildbearing, yet they are not practicing birth control;likewise, 15% of women in Asia and Latin Americanhave an unmet need of birth control.

2. Use social progress to reduce the desire for largefamilies. Many couples in the LDCs presently desire asmany as four to six children. But providing education,raising the status of women, and reducing child

mortality are desirable social improvements that couldcause them to think differently.

3. Delay the onset of childbearing. A delay in the onset ofchildbearing and wider spacing of births could cause atemporary decline in the birthrate and reduce thepresent reproductive rate.

Age DistributionsThe age-structure diagrams of MDCs and LDCs in Figure

33.10 divide the population into three age groups: depen-dency, reproductive, and postreproductive. The LDCs areexperiencing a population momentum because they havemore women entering the reproductive years than olderwomen leaving them.

Laypeople are sometimes under the impression that ifeach couple has two children, zero population growth (noincrease in population size) will take place immediately.However, replacement reproduction, as it is called, will still

cause most countries today to continue growing due to theage structure of the population. If there are more youngwomen entering the reproductive years than there are older

women leaving them, then replacement reproduction will

still result in growth of the population.Many MDCs have a stable age structure, but most

LDCs have a youthful profilea large proportion of thepopulation is younger than the age of 15. This means thattheir populations will still expand greatly, even afterreplacement reproduction is attained. The more quicklyreplacement reproduction is achieved, however, thesooner zero population growth will result.

Currently, the less-developed countries are expandingdramatically because of exponential growth.


80+

7579

7074

6569

6064

5559

5054

45494044

3539

3034

2529

20241519

1014

59

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7579

7074

65696064

5559

5054

45494044

35393034

2529

20241519

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59

04

0

Males Females

Less Developed

More Developed

millions

postreproductive

postreproductive

reproductive

reproductive

dependency

a.

b.

300 300250 250200 200150 150100 10050 50

Figure 33.10 Age-structure diagrams (1998).The diagrams illustrate that (a) the MDCs are approaching

stabilization, whereas (b) the LCDs will expand rapidly due to their

age distributions. Source: United Nations Population Division, 1998.

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A Sustainable World

While we are sometimes quick to realize that thegrowing populations of the LDCs are putting a

strain on the environment, we should realize thatthe excessive resource consumption of the MDCsalso stresses the environment. Environmental im-pact is measured not only in terms of populationsize, but in terms of resource consumption and thepollution caused by each person in the population.An average American family, in terms of per capitaresource consumption and waste production, is theequivalent of 30 people in India (Fig. 33.11).

Before the industrial revolution, people feltbetter connected to the plants and animals onwhich they depended, and they were better able tolive in a sustainable way. After the industrial revo-lution, we especially began to think of ourselves asseparate from nature and endowed with the rightto exploit nature as much as possible. But our in-dustrial society lives on borrowed carrying capacityour cities not only borrow resources from thecountry, our entire population borrows from the

past and future. The forests of the Carboniferousperiod have become the fossil fuels that sustain our way oflife today, and the environmental degradation we cause isgoing to be paid for by our children.

Ecologists have two favorite sayings: (1) Everything isconnected to everything else, and (2) there is no free lunch.We have seen that if you affect one part of the carbon cycle,you affect the entire balance of carbon in the entire world.Ecological effects know no boundaries. Coal that is burned

in the Midwest releases acids into the atmosphere that af-fect lakes in the Northeast. And plants and animals arentthe only organisms affected. Humans are dependent onnatural cycles just as much as any organism in the bio-sphere. What we do to natural ecosystems will eventuallybe felt by us also. The second saying means that we have topay for what we do. If we build a home on a floodplain, wecan expect that it will be flooded once in a while. When weburn fossil fuels, we can expect acid rain and global warm-ing as a consequence. Many times it is difficult to predict

the particular consequences, but we can be assured thateventually they will become apparent.

Overpopulation and overconsumption account for in-creased pollution, and also for the present mass extinctionof wildlife. We are expected to lose one-third to two-thirdsof the earths species, any one of which could possiblyhave made a significant contribution to agriculture ormedicine. It should never be said, What use is this organ-ism? Aside from its contribution to the ecosystem in

which it lives, one never knows how a particular organismmight someday be useful to humans. Adult sea urchinskeletons are now used as molds for the production of

small artificial blood vessels, and armadillos are used inleprosy research.

It is clearly time for a new philosophy. In a sustain-able world, development will meet economic needs of allpeoples while protecting the environment for future gen-erations. Various organizations have singled out commu-nities to serve as models of how to balance ecological andeconomic goals. For example, in Clinch Valley of south-west Virginia, the Nature Conservancy is helping to revivethe traditional method of logging with draft horses. Thistechnique, which allows the selective cutting of trees, pre-serves the forest and prevents soil erosion, which is sodamaging to the environment. The United Nations has anestablished bioreserve system, a global network of sites

that combine preservation with research on sustainablemanagement for human welfare. More than 100 countriesare now participants in the program. However, sustain-ability is more than likely incompatible with the kinds ofconsumption/waste patterns currently practiced in devel-oped countries.

All peoples can benefit from a sustainable world

where economic development and environmental

preservation are considered complementary, ratherthan opposing, processes.

Population

Percent

0

20

40

60

80

100

Hazardouswaste

production

Consumption

fossil fuels

other industrialized countries

Source: Natural Resources Defense Council, 1993.

developing countries

United States

metals paper

5

17

78

18

10

72

25

35

40

20 20

33

42

25

60

Figure 33.11 Resource consumption for MDCs and LDCs.The populations of MDCs are smaller than LDCs. Yet, the MDCs produce most of

the hazardous wastes because their consumption of fossil fuels, metals, and

paper, for example, is much greater than the LDCs.

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Summarizing the Concepts

33.1 Scope of Ecology

Ecology is the study of the interactions of organisms with other organ-isms and with the physical environment. Ecology encompasses several

levels of study: organism, population, community, ecosystem, and fi-

nally the biosphere. Population density is simply the number of indi-

viduals per unit area or volume. Distribution of these individuals can

be uniform, random, or clumped. Most members of a population are

clumped as are the members of a human population. Limiting factors

such as water, temperature, and availability of organic nutrients often

determine a populations distribution.

33.2 Characteristics of PopulationsFuture population size is dependent upon the per capita rate of increase.

The per capita rate of increase is calculated by subtracting the number of

deaths from the number of births and dividing by the number of indi-

viduals in the population. (Immigration and emigration are usually con-

sidered to be equal.) Every population has a biotic potential, the greatest

possible per capita rate of increase under ideal circumstances.

Two possible patterns of population growth are considered. Expo-

nential growth results in a J-shaped curve because as the population

increases in size so does the expected increase in new members. Most

environments restrict growth, and exponential growth cannot continue

indefinitely. Under these circumstances logistic growth occurs and anS-shaped growth curve results. When the population reaches carrying

capacity, the population stops growing because environmental resis-

tance opposes biotic potential.

Populations tend to have one of three types of survivorship

curves, depending on whether most individuals live out the normal

life span, die at a constant rate regardless of age, or die early.

33.3 Regulation of Population SizePopulation growth is limited by density-independent (e.g., weather)

and density-dependent factors (predation, competition, and resource

availability). Do some populations have an intrinsic means of regulat-

ing population growth as opposed to density-independent and

density-dependent factors, which are extrinsic means? Territoriality is

given as an example of a possible intrinsic means of regulation.

33.4 Life History PatternsThe logistic growth model has been used to suggest that life history

patterns depend on natural selection and vary from those species that

are opportunists to those that are in equilibrium with the carrying ca-

pacity of the environment. Opportunistic species produce many young

within a short period of time and rely on rapid dispersal to new, unoc-

cupied environments. Population size is regulated by density-

independent factors. Equilibrium species produce a limited number of

young, which they nurture for a long time, and population size is reg-

ulated by density-dependent factors.

33.5 Human Population GrowthThe human population is expanding exponentially, and it is unknown

when the population size will level off. Most of the expected increasewill occur in certain LDCs (less-developed countries) of Africa, Asia,

and Latin America. Support for family planning, human development,

and delayed childbearing could help prevent an expected increase.

The answer to how to curb the expectedincrease in the worlds population liesin discovering how to curb the rapid pop-ulation growth of the less-developedcountries. In these countries, populationexperts have discovered what they call thethe virtuous cycle. Family planningleads to healthier women, and healthierwomen have healthier children, and thecycle continues. Women no longer have to

have many babies for a few to survive.More education is also helpful becausebetter educated people are more interestedin postponing childbearing and promot-ing womens rights. Women who haveequal rights with men tend to have fewerchildren.

There isnt any place where womenhave had the choice that they havent cho-sen to have fewer children, says BeverlyWinikoff at the Population Council in New

York City. Governments dont need to

resort to force. Bangladesh is a case inpoint. Bangladesh is one of the densestand poorest countries in the world. In 1990the birthrate was 4.9 children per womanand now it is 3.3. This achievement wasdue in part to the Dhaka-based GrameenBank, which loans small amounts ofmoney mostly to destitute women to starta business. The bank discovered that whenwomen start making decisions about their

lives, they also start making decisionsabout the size of their families. Familyplanning within Grameen families is twiceas common as the national average; in fact,those women who get a loan promise tokeep their families small! Also helpful has

been the network of village clinics thatcounsel women who want to use contra-ceptives. The expression contraceptivesare the best contraceptives refers to thefact that you dont have to wait for social

changes to get people to use contracep-

tives

the two feed back on each other.Unfortunately, some of the less-

developed countries faced with economiccrisis have cut back on their family plan-ning programs, and the more-developedcountries have not taken up the slack. In-deed, some foreign donors have also cut

back on aidthe U.S. by one-third.

Questions1. Do you think less-developed countries

should simply make contraceptionavailable, or should more persuasivemethods be employed? Explain.

2. Do you think that more-developedcountries should be concerned aboutpopulation growth in the less-developedcountries? Why or why not?

3. Are you in favor of foreign aid to helpcountries develop family planningprograms? Why or why not?


Go To Student OLC

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4. An S-shaped growth curvea. occurs when there is no environmental resistance.

b. includes an exponential growth phase.c. occurs if survivorship is short-lived.

d. occurs in natural populations but not laboratory ones.e. All of these are correct.5. If a population has a type I survivorship curve (most live the

entire life span), which of these would you also expect?a. a single reproductive event per adult

b. overlapping generationsc. sporadic reproductive eventsd. reproduction occurring near the end of the life spane. None of these are correct.

6. A pyramid-shaped age distribution means that thea. prereproductive group is the largest group.

b. population will grow for some time in the future.c. more young women are entering the reproductive years

than older women leaving theirs.d. country is more likely an LDC rather than an MDC.e. All of these are correct.

7. Which of these is a population-independent regulating factor?a. competition d. weather

b. predation e. resource availabilityc. size of population

8. Fluctuations in population growth can correlate to changes in

a. predation.b. weather.c. resource availability.d. population regulating factors.e. All of these are correct.

9. An equilibrium life history pattern includes all buta. large individuals.

b. long life span.c. individuals slow to mature.d. few offspring.

e. little or no care of offspring.10. The human populationa. is undergoing exponential growth.

b. is not subject to environmental resistance.c. fluctuates from year to year.d. only grows if emigration occurs.e. All of these are correct.

11. Label this S-shaped growth curve.

Time

Nu

mberofOrganisms

a.

b.

c.

d. e.

stable equilibrium

Studying the Concepts

1. What are the various levels of ecological study? 6882. What are three types of distribution patterns for a popula-

tion? Explain why the human population has a clumpedpattern. 689

3. How do you calculate the per capita rate of increase for apopulation? What is biotic potential? 690

4. What type growth curve indicates that exponential growth isoccurring? What are the environmental conditions for expo-nential growth? 690

5. What type growth curve indicates that biotic potential isbeing opposed by environmental resistance? Whatenvironmental conditions are involved in environmental

resistance? 6906. What is the carrying capacity of an area? 6917. Describe the three general types of survivorship

curves. 6918. Give examples of extrinsic density-independent and

density-dependent factors that regulate populationsize. 692

9. Give support to the belief that intrinsic factors might regulatepopulation size in some populations. 692

10. Name and give five contrasting characteristics for the twoextreme life history patterns. 694

11. Why would you expect the life histories of natural popula-tions to have a mixture of characteristics from these twopatterns? 694

12. What type of growth curve presently describes the popula-tion growth of the human population? 695

13. Distinguish between MDCs and LDCs. Include a reference toage-structure diagrams. 69596

14. Explain why the population of LDCs is expected to increasetremendously. What steps could be taken to prevent this fromoccurring? 696

Testing Yourself

Choose the best answer for each question.1. Which of these levels of ecological study involves both abiotic

and biotic components?a. organisms d. ecosystem

b. populations e. All of these are correct.c. communities

2. When phosphorus is made available to an aquatic commu-nity, the algal populations suddenly bloom. This indicatesthat phosphorus isa. a density-dependent regulating factor.

b. gaseous.c. a reproductive factor.d. a limiting factor.e. All of these are correct.

3. A J-shaped growth curve should be associated witha. exponential growth.

b. biotic potential.c. no environmental resistance.d. high per capita rate of increase.e. All of these are correct.


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Understanding the Terms

age-structure diagram 696biosphere 688

biotic potential 690carrying capacity 691cohort 691community 688demographic transition 695doubling time 695ecology 688ecosystem 688environmental resistance 690

exponential growth 690less-developed country (LDC)

695logistic growth 690more-developed country

(MDC) 695population 688replacement reproduction

696sustainable world 697zero population growth 696

Match the terms to these definitions:a. Group of organisms of the same species occupy-

ing a certain area.

b. Maximum population growth rate under ideal

conditions.c. Growth, particularly of a population, in which the

increase occurs in the same manner as compound interest.

d. Due to industrialization, a decline in the birthratefollowing a reduction in the death rate so that the populationgrowth rate is lowered.

e. Largest number of organisms of a particularspecies that can be maintained indefinitely by a givenenvironment.

Thinking Scientifically

1. Considering the population of zebras described in the firstparagraph:

a. Plot the growth curve for this zebra population usingnumber of zebras versus time.

b. How would you describe the shape of your curve?c. If the zebra population happens to outstrip the carrying

capacity of the environment, what would happen to thecurve?

2. Consider this definition of overpopulation: ...where there aremore people than can live on the earth in comfort, happiness,and health, and still leave the world a fit place for futuregenerations. 1

a. Do comfort and happiness mean the typical standard ofliving seen in developed countries or in less-developedcountries? Should everyone in the world have the samestandard of living? Why or why not?

b. What standard of health is acceptable for the developedcountries? Whose responsibility is it to achieve this end?

c. Should citizens and private industry work to find ways tomake the world an ecologically fit place for future genera-tions? Why?

d. When discussing overpopulation, should we think in

terms of the world, the country, or the area?1From George Morris, 1973. Overpopulation: Everyones Baby. London: Priory Press

Limited, p. 24.

Using Technology

Your study of population ecology is supported by theseavailable technologies:

Essential Study Partner CD-ROMEcology Populations

Human Impact

Visit the Mader web site for related ESP activities.

Exploring the Internet

The Mader Home Page provides resources and tools asyou study this chapter.

http://www.mhhe.com/biosci/genbio/mader

http://www.mhhe.com/biosci/genbio/maderinquiry9/student/olc/chap33espactivities_s.htmlhttp://www.mhhe.com/biosci/genbio/maderhttp://www.mhhe.com/biosci/genbio/maderinquiry9/student/olc/chap33espactivities_s.htmlhttp://www.mhhe.com/biosci/genbio/mader

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