Chapter 19

Population Ecology

Biology and Society:Multiplying Like Rabbits

In 1859, 12 pairs of European rabbits were released on a ranch in southern Australia.

By 1865, 20,000 rabbits were killed on just that one ranch.

By 1900, several hundred million rabbits were distributed over most of the continent.

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The European rabbits– Destroyed farming and grazing land

– Promoted soil erosion

– Made grazing treacherous for cattle and sheep

– Competed directly with native marsupials

The European red fox– Was introduced to control the rabbits

– Spread across Australia

– Ate several species of native birds and small mammals to extinction

– Had little impact on the rabbit population

Today, rabbits, foxes, and a long list of other non-native animal and plant species are

– Still entrenched in Australia

– Damaging the environment

– Costing hundreds of millions of dollars in economic losses

AN OVERVIEW OF POPULATION ECOLOGYPopulation ecology is the study of factors that affect population:

– Density

– Growth

A population is a group of individuals of a single species that occupy the same general area.

Population ecology focuses on the factors that influence a population’s

– Density

– Structure

– Size

– Growth rate

Population ecology is used to study– How to develop sustainable fisheries

– How to control pests and pathogens

– Human population growth

Population density is the number of individuals of a species per unit of area or volume. Examples include

– The number of largemouth bass per cubic kilometer (km3) of a lake

– The number of oak trees per square kilometer (km2) in a forest

– The number of nematodes per cubic meter (m3) in a forest’s soil

Population DensityHow do we measure population density?

– In most cases, it is impractical or impossible to count all individuals in a population.

– In some cases, population densities are estimated by indirect indicators, such as

– Number of bird nests

– Rodent burrows

The age structure of a population is the distribution of individuals among age groups.

The age structure of a population provides insight into– The history of a population’s survival

– Reproductive success

– How the population relates to environmental factors

Population Age Structure

11

Percent of population0 10 20 30 40 50

1

22

3

4

5

6

7

8

9

10

Age

(yea

rs)

Figure 19.3

Life Tables and Survivorship CurvesLife tables

– Track survivorship

– Help to determine the most vulnerable stages of the life cycle

Survivorship curves– Graphically represent some of the data in a life table

– Are classified based upon the rate of mortality over the life span of an organism

Table 19.1

Percentage of maximum life span0 50 100

0.1

1

10

100

Perc

enta

ge o

f sur

vivo

rs (l

og s

cale

)

Figure 19.4

Life History Traits as Evolutionary AdaptationsAn organism’s life history is the set of traits that affect the organism’s schedule of

– Reproduction

– Survival

Key life history traits are the– Age at first reproduction

– Frequency of reproduction

– Number of offspring

– Amount of parental care provided

Life history traits– Evolve

– Represent a compromise of the competing needs for

– Time

– Energy

– Nutrients

Organisms with an opportunistic life history– Take immediate advantage of favorable conditions

– Typically exhibit a Type III survivorship curve

Organisms with an equilibrial life history– Reach sexual maturity slowly

– Produce few, well cared for offspring

– Are typically large-bodied and longer lived

– Typically exhibit a Type I survivorship curve

Table 19.2

POPULATION GROWTH MODELSPopulation size fluctuates as individuals

– Are born

– Immigrate into an area

– Emigrate away

– Die

The Exponential Growth Model: The Ideal of an Unlimited Environment

Exponential population growth describes the expansion of a population in an ideal and unlimited environment.

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Exponential growth explains how– A few dozen rabbits can multiple into millions

– In certain circumstances following disasters, organisms that have opportunistic life history patterns can rapidly recolonize a habitat

Time (months)

Popu

latio

n si

ze (N

)

0 1 2 3 4 5 6 7 8 9 10 11 120

50

100

150

200

250

300

350

400

450

500

Figure 19.5

The Logistic Growth Model: The Reality of a Limited Environment

Limiting factors– Are environmental factors that hold population growth in check

– Restrict the number of individuals that can occupy a habitat

© 2010 Pearson Education, Inc.

The carrying capacity is the maximum population size that a particular environment can sustain.

Logistic population growth occurs when the growth rate decreases as the population size approaches carrying capacity.

Year

1915 1925 1935 19450

2

4

6

8

10

Bre

edin

g m

ale

fur s

eals

(thou

sand

s)

Figure 19.6

The carrying capacity for a population varies, depending on– The species

– The resources available in the habitat

Organisms exhibiting equilibrial life history patterns occur in environments where the population size is at or near carrying capacity.

The logistic model and the exponential model are theoretical ideals of population growth.

No natural population fits either one perfectly.

Exponential growth

Logistic growth

Time

Carryingcapacity

0

Num

ber o

f ind

ivid

uals

Figure 19.7

Regulation of Population GrowthDensity-Dependent Factors

The logistic model is a description of intraspecific competition, competition between individuals of the same species for the same limited resources.

© 2010 Pearson Education, Inc.

As population size increases– Competition becomes more intense

– The growth rate declines in proportion to the intensity of competition

A density-dependent factor is a population-limiting factor whose effects intensify as the population increases in density.

Density (beetles/0.5 g flour)0 20 40 60 80 100 120

20

40

60

80

100

Surv

ivor

s (%

)

(b) Decreasing survival rates with increasing density in apopulation of flour beetles

Figure 19.8b

Density-dependent factors may include– Accumulation of toxic wastes

– Limited food supply

– Limited territory

Density-Independent FactorsDensity-independent factors

– Are population-limiting factors whose intensity is unrelated to population density

– Include abiotic factors such as

– Fires, Floods, Storms

In many natural populations, density-independent factors limit population size before density-dependent factors become important.

Over the long term, most populations are probably regulated by a mixture of

– Density-independent factors

– Density-dependent factors

Exponentialgrowth

Suddendecline

Apr May Jun Jul Aug Sep Oct Nov Dec

Num

ber o

f aph

ids

Figure 19.10

Population CyclesSome populations have regular boom-and-bust cycles characterized by periods of rapid growth followed by steep population declines.

A well studied example of boom and bust cycles are the cycles of– Snowshoe hares

– One of the hares’ predators, the lynx

The cause of these hare and lynx cycles may be– Winter food shortages for the hares

– Overexploitation of hares by lynx

– A combination of both of these mechanisms

Snowshoe hare Lynx

Year1850 1900

0

3

6

9

0

40

80

120

160

Har

e po

pula

tion

(thou

sand

s)

Lynx

pop

ulat

ion

(thou

sand

s)

Figure 19.11

APPLICATIONS OF POPULATION ECOLOGYPopulation ecology is used to

– Increase populations of organisms we wish to harvest

– Decrease populations of pests

– Save populations of organisms threatened with extinction

Conservation of Endangered SpeciesThe U.S. Endangered Species Act defines

– An endangered species as one that is in danger of extinction throughout all or a significant portion of its range

– A threatened species as one that is likely to become endangered in the foreseeable future

A major factor in population decline is habitat destruction or modification.

The red-cockaded woodpecker– Requires longleaf pine forests with clear flight paths between trees

– Suffered from fire suppression, increasing the height of the vegetation on the forest floor

– Recovered from near-extinction to sustainable populations due to controlled burning and other management methods

A red-cockadedwoodpecker perchesat the entrance to itsnest in a long-leafpine tree.

High, dense undergrowth impedesthe woodpeckers’ access to feedinggrounds.

Low undergrowth offers birds aclear flight path between nest sitesand feeding grounds.

Figure 19.12

Sustainable Resource ManagementAccording to the logistic growth model, the fastest growth rate occurs when a population size is at roughly half the carrying capacity.Theoretically, populations should be harvested down to this level, assuming that growth rate and carrying capacity are stable over time.In the northern Atlantic cod fishery

– Estimates of cod stocks were too high

– The practice of discarding young cod (not of legal size) at sea caused a higher mortality rate than was predicted

– The fishery collapsed in 1992 and has not recovered

1960 1970 1980 1990 20000

100

200

300

400

500

600

700

800

900

Yiel

d (th

ousa

nds

of m

etric

tons

)

Figure 19.13

Invasive SpeciesAn invasive species

– Is a non-native species that has spread far beyond the original point of introduction

– Causes environmental or economic damage by colonizing and dominating suitable habitats

In the United States, invasive species cost about $137 billion a year.Invasive species typically exhibit an opportunistic life history pattern.

Cheatgrass– Is an invasive species in the western United States

– Currently covers more than 60 million acres of rangeland formerly dominated by native grasses and sagebrush

– Produces seeds earlier and in greater abundance than native species

– Forms highly flammable brush creating fires that native plants cannot tolerate

European starlings– Are another invasive species

– Were first released into New York in 1890

– Now number more than 200 million in the United States

Biological Control of PestsInvasive species may benefit from the absence of

– Pathogens

– Predators

– Herbivores

Biological control– Is the intentional release of a natural enemy to attack a pest population

– Is used to manage an invasive species

In 1950, the Australian government introduced a virus lethal to European rabbits into the rabbits’ Australian environment.

Although initially successful, natural selection favored– Resistant rabbits

– Variations of the virus that

– Were not fatal or

– Killed their hosts more slowly

Evolutionary changes such as these, in which an adaptation of one species leads to a counteradaptation in a second species, are known as coevolution.

The Process of Science: Can Biological Control Defeat Kudzu?

Kudzu– Is an invasive Asian vine

– Covers about 12,000 square miles of the southeastern United States

– Has a range limited by cold winters

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Many strategies to control kudzu have been considered with little success.

A fungal pathogen called Myrothecium verrucaria appears to be a promising candidate for biological control.

Observation: The fungus Myrothecium verrucaria causes severe disease in other weeds belonging to the same family as kudzu.

Question: Will the application of fungal spores of M. verrucaria control an established stand of kudzu in a natural setting?

Hypothesis: M. verrucaria treatment that was effective in small outdoor plantings would also be most effective in a natural setting.

Prediction: The greatest kudzu mortality would result from the treatment that sprayed the highest concentration of spores in combination with a wetting agent.

Results: The hypothesis was supported by the data, as indicated in the following table.

Untreated

Trea

tmen

t

Wetting agent only

Spores (2 × 107/ml) + wetting agent

Spores (2 × 106/ml) + wetting agent

Spores (2 × 107/ml) + water

Spores (2 × 106/ml) + water

Mortality (%)

0

0 20

40 60 80 100

Figure 19.17

Integrated Pest ManagementAgricultural operations create their own highly managed ecosystems that

– Have genetically similar individuals (a monoculture)

– Are planted in close proximity to each other

– Function as a “banquet” for

– Plant-eating animals

– Pathogenic bacteria

– Viruses

Like invasive species, most crop pests– Have an opportunistic life history pattern

– Can cause extensive crop damage

Pesticides may– Result in pesticide-resistant pests

– Kill the pest and their natural predators

– Kill pollinators

Integrated pest management (IPM)– Tolerates a low level of pests instead of total eradication

– Produces a sustainable control of agricultural pests

– Uses a combination of

– Biological methods

– Chemical methods

– Cultural methods

IPM methods include– Using pest-resistant varieties of crops

– Using mixed-species plantings

– Rotating crops to deprive the pest of a dependable food source

HUMAN POPULATION GROWTHThe History of Human Population Growth

From 2000 to 500 years ago (in 1500)– Mortality was high

– Births and deaths were about equal

– The world population held steady at about 300 million

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The world’s population began to grow exponentially in the 1900s due to advances in

– Nutrition

– Sanitation

– Health care

Ann

ual i

ncre

ase

(mill

ions

) Population increase

Total population size

Year

Tota

l pop

ulat

ion

(bill

ions

)

1500 1550 1600 1700 18001650 1750 1850 19501900 2000 2050

20

40

60

80

100

0

2

4

6

8

10

Figure 19.19

Worldwide population growth rates reflect a mosaic of the changes occurring in different countries.

– In the most developed nations, the overall growth rates are near zero.

– In the developing world

– Death rates have dropped

– High birth rates persist

Table 19.3

Age structures help predict a population’s future growth.

The following figure shows the estimated and projected age structures of Mexico’s population in

– 1985

– 2010

– 2035

Age Structures

1985 2010 2035

FemaleMaleFemaleMaleFemaleMale

80+75–7

970–7465–6960–6455–5950–5445–4940–4435–3930–3420–24

25–2915–1910–14 5–9

0–4

5 4 3 2 1 0 1 2 3 4 5 5 4 3 2 1 0 1 2 3 4 5 5 4 3 2 1 0 1 2 3 4 5Population in millions

Total population size = 139,457,070Population in millions

Total population size = 112,468,855Population in millions

Total population size = 76,767,225

Age

Figure 19.20

Population momentum is the continuation of population growth as girls in the prereproductive age group reach their reproductive years.

Age structure diagrams may also indicate social conditions. An expanding population needs

– Schools

– Employment

– Infrastructure

Trends in the age structure of the United States from 1983 to 2033 reveal important patterns.

1983 2008 2033

Birth years Male Femalebefore 1954

1954–581959–631964–68

1969–731974–78

1979–831984–88

1989–931994–98

1999–20032004–08

2009–132014–18

2024–282019–23

2029–33

12 10 8 6 4 2 0 2 4 6 8 10 1212 10 8 6 4 2 0 2 4 6 8 10 1212 10 8 6 4 2 0 2 4 6 8 10 12Population in millions

Total population size = 383,116,539Population in millions

Total population size = 305,548,183Population in millions

Total population size = 234,307,207

Birth yearsBirth years Male Female

Female

Male

Age

2004–081999–2003

1994–981989–931984–881979–83

1974–781969–731964–68

1959–631954–58

1949–531944–48

1939–431934–38

1929–33before 1929before 1904

1904–081909–13

1914–181919–231924–281929–331934–38

1939–431944–48

1949–531954–58

1959–631964–68

1969–731974–78

1979–830–45–9

10–1415–1920–2425–2930–3435–3940–4445–4950–5455–5960–6465–6970–7475–79

80+

Figure 19.21

Our Ecological FootprintAn ecological footprint is an estimate of the amount of land required to provide the raw materials an individual or a population consumes, including:

– Food

– Fuel

– Water

– Housing

– Waste disposal

The ecological footprint of the United States– Is 9.4 hectares per person

– Represents almost twice what the U.S. land and resources can support.

The ecological impact of affluent nations is a problem of overconsumption, not overpopulation.

Compared to a family in rural India, Americans have an abundance of possessions.

There is tremendous disparity in consumption throughout the world.

The world’s richest countries– Have 20% of the global population

– Use 86% of the world’s resources

The rest of the world– Has 80% of the population

– Uses just 14% of global resources

NorthAmerica

SouthAmerica

Africa

Europe Asia

AustraliaKey

> 5.4 global hectares per person

3.6–5.4 global hectares per person

1.8–3.6 global hectares per person

0.9–1.8 global hectares per person

< 0.9 global hectares per person

Insufficient data

Figure 19.23

Evolution Connection: Humans as an Invasive Species

Pronghorn antelope– Roamed the open plains and shrub deserts of North America millions of

years ago

– Have a top speed of 97 km/h (60 mph)

– Are the fastest mammal on the continent

Pronghorn speed is likely an adaptation to outrun American cheetahs, which went extinct about 10,000 years ago.

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About 10,000 years ago– Changes in the biotic and abiotic environments happened too rapidly for

an evolutionary response

– Many large North American mammals, in addition to American cheetahs, went extinct, including

– Lions

– Saber-toothed cats

– Towering short-faced bears

Although still hotly disputed, many scientists think that the extinction at the end of the ice age about 10,000 years ago was in part the result of the human invasion of North America, with humans serving as the damaging invasive species.

Like other invasive species, humans may change the environment at an accelerating pace and too rapidly for other species to evolve and survive.