Ch. 26 Population Ecologycf.linnbenton.edu/mathsci/bio/klockj/upload/Ch26 Population Ecology… ·...

Ch. 26 Population Ecology

http://galen.metapath.org/popclk.html


Chapter 26 At a Glance

How Does Population Size Change?

How Is Population Growth Regulated?

How Are Populations Distributed in Space and

Time?

How Is the Human Population Changing?

26.1 How Does Population Size Change?

A population consists of all the members of a particular

species that live within an ecosystem

A community is group of interacting populations (Douglas

Fir forest community)

Communities exist within ecosystems (old growth forest

ecosystem in Pacific NW), which include all the living an

nonliving components of a defined geographical area

The biosphere is the enormous ecosystem that

encompasses all of Earth’s habitable surface

Ecology is the study of the interrelationships of organisms

with each other and with the nonliving environment


Population size is the outcome of opposing

forces

– Four factors determine whether and how much

the size of a population changes

–Births

–Deaths

–Migration of individuals into the population

(immigration)

–Migration of individuals out of the population

(emigration)



forces (continued)

– Birth and immigration add individuals to a

population

– Death and emigration remove individuals from

the population



forces (continued)

– A simple equation describes the change in

population size within a given time period:

–Change in population size = (births – deaths)

+ (immigrants – emigrants)

births immigration

deaths

(births deaths) (immigrants emigrants)

change in population size

emigration

Population Change

Fig. 26-1


Population size is the outcome of opposing forces

(continued)

– Two opposing forces that determine birth and death rates

are biotic potential and environmental resistance

– Biotic potential is the theoretical maximum rate at

which a population could increase, assuming a

maximum birth rate and minimal death rate

– Environmental resistance refers to the curbs on

population growth that are set by the living and

nonliving environment


Population size is the outcome of opposing forces

– Examples of environmental resistance include:

– Interactions among species, such as competition, predation, and parasitism

– The always-limited availability of nutrients, energy, and space

– Natural events, such as storms, fires, freezing weather, floods, and droughts


Biotic potential can produce exponential growth

– Evolutionarily successful organisms possess

traits that make them well adapted to their

environment

– These organisms pass these inherited traits on

to as many healthy offspring as possible

– If environmental resistance is reduced,

populations can grow extremely rapidly


Population growth is a function of the birth rate,

the death rate, and population size

– The growth rate (r) of a population, also called

the rate of natural increase, is the change in the

population size per individual per unit of time

– Growth rate is expressed by the equation:

–r (growth rate) = b (birth rate) – d (death rate)


The growth rate of a population

– Birth rate (b) is the number of births per individual during a specific unit of time, such as a month or a year

– For example, if there are 150 births among 1,000 individuals in a year, b = 0.15

– Death rate (d) is the number of deaths per individuals during a specific unit of time

– For example, if there are 50 deaths among 1,000 individuals in a year, d = 0.05 or (1000/50)

– If the birth rate exceeds the death rate, the population will grow

– If the death rate exceeds the birth rate, the population will decline


The growth rate of a population (continued)

– The growth rate of this population of 1000 is

therefore:

–r (growth rate) = 0.15 (birth rate) – 0.05 (death

rate) = 0.1 = 10% per year

–population growth (rN) = 0.1 x 1,000 = 100, so

the population has grown by one hundred

individuals in the first year


If births exceed deaths by a constant percentage, population growth produces a J-curve

– A patter of continuously accelerating increase in population size is called exponential growth

– When graphed against time, a shape called a J-curve is produced

TIME

PO

PU

LA

TIO

N S

IZE

45.3


If births exceed deaths by a constant

percentage, population growth produces a J-

curve (continued)

– This high biotic potential evolved because it

helps ensure that, in a world filled with forces of

environmental resistance, some offspring survive

to reproduce


Several factors influence biotic potential

– The age at which the organism first reproduces

– The frequency at which reproduction occurs

– The average number of offspring produced each

time

– The length of the organism’s reproductive life

span

– The death rate of individuals

– Increased death rates can slow the rate of

population growth significantly


The age at which an organism first reproduces affects the size of the future population – For example, consider two populations of golden eagles

that are followed for 30 years

– Individuals in one population begin reproducing at the age of 4 years

– Individuals in the other population begin reproducing at 6 years

– At 30 years, the earlier reproducing population would be 10 times the size of the other population

1,764 17,314 30

392 2,504 24

86 362 18

18 52 12

4 8 6

2 2 0

Time

(years)

Number

of

eagles

(pop. 1)

Number

of

eagles

(pop. 2)

At 24 years,

this population

has 392 eagles

At 24 years,

this population

has 2,504 eagles

reproduce at 4 years (pop. 1) reproduce at 6 years (pop. 2)

Exponential Growth Curves are J-Shaped

Fig. 26-2


The death rate affects population size

– As long as birth rate exceeds death rate,

population size will follow a J-shaped rate of

increase

26.2 How Is Population Growth Regulated?

Exponential growth only occurs under special

conditions

– Exponential growth cannot continue indefinitely

– All populations that exhibit exponential growth

must eventually stabilize or crash

– Exponential growth can be observed in

populations that undergo boom-and-bust

cycles, in which periods of rapid population

growth are followed by a sudden, massive die-off


Exponential growth only occurs under special

conditions (continued)

– Boom-and-bust cycles can be seen in short-lived, rapidly

reproducing species, such as microbes and insects

– Seasonal populations are linked to changes in rainfall,

temperature, or nutrient availability

– Ideal conditions encourage rapid growth; deteriorating

conditions encourage massive die-off

– Rabbits in Australia, algae in a pond


Boom-and-bust cycles can be seen in short

lived, rapidly reproducing species (continued)

– Complex factors produce four-year cycles for

small rodents, such as lemmings

–Lemming populations may grow until lack of

food, large migrations, and predators and

starvation cause sudden high mortality

Boom-and-Bust Population Cycles

Fig. 26-4b

(b) Boom-and-bust cycles in a lemming population in Alaska


Exponential growth occurs when environmental

resistance is reduced

– In populations that do not experience boom-and-bust

cycles, exponential growth may occur temporarily under

special circumstances such as:

– When food supply is increased

– When population-controlling factors, such as

predators, are reduced

– For example, the whooping crane population has grown

exponentially since they were first protected from hunting

and human disturbance in 1940

Exponential Growth of Wild Whooping Cranes

Fig. 26-5


Exponential growth can occur when individuals

invade a new habitat

– Invasive species are organisms with a high

biotic potential that are introduced into

ecosystems where they did not evolve and

where they encounter little environmental

resistance

Rubus armeniacus

Probably the most commonly eaten blackberry, the highly invasive also known as the

Himalayan . A shrubby weed native to Eurasia, the plant aggressively forms dense thickets .

http://www.kingcounty.gov/environment/animalsAndPlants/noxious-weeds/weed-identification/blackberry.aspx


Environmental resistance limits population

growth

– Many populations that exhibit exponential growth

eventually stabilize

–As resources become depleted, reproduction

slows and the growth rate would eventually

drop to zero, causing the population size to

remain constant


Environmental resistance limits population growth (continued)

– This growth pattern, where populations increase to the maximum number sustainable by their environment and then stabilize, is called logistic population growth

– The maximum population size that can be sustained by an ecosystem for an extended time without damage to the ecosystem is called its carrying capacity (K)

– When logistic growth is plotted, it results in an S-shaped growth curve, or S-curve

The S-Curve of Logistic Population Growth

Fig. 26-6a

(a) An S-shaped growth curve stabilizes at carrying capacity

Growth stops and the

population stabilizes close

to the carrying capacity

Population

grows rapidly

Growth

rate slows



growth (continued)

– If a population far exceeds the carrying capacity

of its environment, excess demands placed on

the ecosystem are likely to destroy crucial

resources

– This can permanently and severely reduce

carrying capacity, causing the population to

decline to a fraction of its former size or

disappear entirely

The S-Curve of Logistic Population Growth

Fig. 26-6b

(b) Consequences of exceeding carrying capacity

High damage; the

carrying capacity is

permanently lowered

Low damage; resources

recover, and the

population fluctuates

Extreme

damage; the

population

dies out

The population

overshoots the

carrying

capacity; the

environment

is damaged



growth (continued)

– For example, when reindeer were introduced

onto an island with no large predators, their

population increased rapidly, seriously

overgrazing the vegetation they relied on for food

– As a result, the reindeer population plummeted

exponential

growth population

crash

The Effects of Exceeding Carrying Capacity

Fig. 26-7



growth (continued)

– Logistic population growth can occur in nature

when a species moves into a new habitat

–For example, new barnacle settlers along a

rocky coast may find ideal conditions that

allow their population to grow exponentially

–As population density increases, however,

individuals begin to compete for space,

energy, and nutrients

A Logistic Curve in Nature

Fig. 26-8 time (weeks)

nu

mb

er

of

ba

rna

cle

s (

pe

r c

m2)

life span

population density

off

sp

rin

g p

er

da

y

da

ys

Density-Dependent Environmental Resistance

Fig. 26-9



growth (continued)

– As environmental resistance increases,

population growth slows and eventually stops

– In nature, conditions are never completely

stable, so both carrying capacity and the

population size will vary somewhat from year to

year

– However, environmental resistance ideally

maintains populations at or below the carrying

capacity of their environment


Environmental resistance can be classified into

two broad categories

– Density-independent factors, which limit

population size regardless of the population

density

– Density-dependent factors, which increase in

effectiveness as the population density increases

–Nutrients, energy, and space are all density-

dependent regulators of population size


Density-independent factors limit populations

regardless of their density

– The most important natural density-independent factors

are climate and weather, which are responsible for most

boom-and-bust population cycles

– For example, most insects and annual plant

populations are limited in size by the number of

individuals that can be produced before the first hard

freeze

– Hurricanes, droughts, floods, and fire can have

profound effects on local populations, particularly

small, short-lived species


Density-independent factors limit populations

regardless of their density (continued)

– Human activities can also limit the growth of

natural populations

–Pesticides and pollutants can cause drastic

declines in natural populations

–Overhunting has driven some species to

extinction


Density-dependent factors become more effective as

population density increases

– Populations of organisms with a life span of more than a

year have evolved adaptations that allow them survive

seasonal changes, such as the onset of winter

– Many mammals develop thick coats and store fat;

some hibernate

– Many birds migrate long distances to find food and a

hospitable climate

– Tree and bushes enter dormancy, dropping leaves

and slowing their metabolic activities


Important density-dependent factors limiting

population growth are:

– Predation

– Parasitism

– Competition


Predators exert density-dependent controls on populations

– Predators are organisms that eat other organisms, called their prey

– Predation becomes important as prey populations grow because predators eat a variety of prey, depending upon what is most abundant and easiest to find

– In this way, predators exert density-dependent population control over more than one prey population

Predators Help Control Prey Populations

Fig. 26-10a


Predator populations often grow as their prey

become more abundant

– The number offspring produced is determined by

the abundance of prey

–For example, snowy owls hatch up to 12

chicks when lemmings (their prey) are

abundant, but may not reproduce at all in

years when the lemming population has

crashed

Predators Help Control Prey Populations

Fig. 26-10b


An example of predator-prey population cycles are the fluctuations seen in laboratory populations of the bean weevil (prey) and its braconid wasp predator

– Wasps lay their eggs on the bean weevil larvae, which provides food for the newly hatched wasp larvae

– A large weevil population ensures a high survival rate for wasp offspring, increasing the predator population

– Under intense predation, the weevil population plummets, reducing food for the next generation of wasps

– Reduced wasp predation then allows the weevil population to increase

– The cycle thus begins again

Experimental Predator-Prey Cycles

Fig. 26-11

bean weevils (prey)

A high predator

population

reduces the prey

population

The prey population

peaks when the

predator population

is low

braconid wasp (predator)


Predation may maintain prey populations near a

density that can maintain carrying capacity, or at well

below carrying capacity

– For example, the prickly pear cactus (from South

America) was introduced into Australia in the late 1800s

and spread uncontrollably

– In the 1920s, cactus moths (predators of the prickly pear)

were introduced from Argentina to feed on the cacti

– Within a few years, the cacti were virtually eliminated

– Today, the predatory moth maintains its prey cacti at low

population densities, well below carrying capacity


Parasites spread more rapidly among dense populations

– Parasitism involves a parasite living on or in a host organism, harming it but not generally killing it because many parasites benefit by having their host remain alive

– Most parasites cannot travel long distances, so they spread more readily among hosts in dense populations

– Examples of parasites include the bacterium that causes Lyme disease, some fungi, intestinal worms, ticks, and some protists


Competition for resources helps control

populations

– Competition is defined as the interaction among

individuals who attempt to use the same limited

resource, and this interaction limits population

size in a density-dependent manner


There are two major forms of competition:

– Interspecific competition, between individuals

of different species

– Intraspecific competition, between individuals

of the same species

–Because the needs of members of the same

species for resources are almost identical,

intraspecific competition is an important

density-dependent mechanism of population

control


Organisms have evolved ways to deal with intraspecific

competition (continued)

– Many animals have evolved contest competitions,

where social or chemical interactions determine access

to important resources

– Territorial species—such as wolves, fish, rabbits, and

songbirds—defend areas that contain important

resources

– Only the best adapted individuals are able to defend

their territories, whereas those without territories may

not reproduce or may become easy prey


Organisms have evolved ways to deal with

intraspecific competition (continued)

– As population densities increase and competition

becomes more intense, some animals react by

emigrating

– Large numbers leave their homes to colonize

new areas; many die in the quest

–For example, locusts emigrate, consuming

vegetation in their path

Emigration

Fig. 26-13


Density-independent and density-dependent factors interact to regulate population size

–For example, a caribou weakened by hunger (density-dependent) and attacked by parasites (density-dependent) is more likely to be killed by an exceptionally cold winter (density-independent)

26.3 How Are Populations Distributed in Space and

Time?

Populations exhibit different spatial distributions

– There are three major types of spatial

distributions

–Clumped

–Uniform

–Random

Clumped Distribution

Fig. 26-14a

Uniform Distribution

Fig. 26-14b

Random Distribution

Fig. 26-14c


Time?

Survivorship in populations follows three basic

patterns

– Late-loss populations

– Constant-loss populations

– Early-loss populations


Time?


patterns (continued)

– Survivorship describes the pattern of survival in

a population

– Survivorship tables track groups of organisms

born at the same time throughout their life span,

and record how many continue to survive in each

succeeding year


Time?


patterns (continued)

– A survivorship curve for a population can be

produced by graphing survivorship table data

–The Y-axis logs the number of individuals

surviving to a particular age out of an initial

population size born at a specific time

–The X-axis plots increasing age categories

after a specific birth date

Survivorship Tables and Survivorship Curves

Fig. 26-15

late loss

(human)

constant loss

(American robin)

early loss

(dandelion)

100,000 0 (birth)

99,124 10

98,713 20

97,754 30

96,489 40

93,698 50

87,967 60

76,241 70

54,117 80

22,312 90

2,523 100

Age Number

of

survivors

(a) A survivorship

table

(b) Survivorship curves

Fig. 45-10a, p.805

Fig. 45-10b, p.805

Fig. 45-10c, p.805

26.4 How Is the Human Population Changing?

Demographers track changes in human populations

– Demography is the study of the changing human

population

– Demographers measure human populations, track

population changes in different countries and regions,

and make comparisons between developed and

developing countries

– Demographic data are used to formulate policies in

public health, housing, education, employment,

immigration, and environmental protection


The human population continues to grow rapidly

– In the last few centuries, the human population has grown at nearly an exponential rate following a J-shaped growth curve

– Over the last decade, however, the human population has been growing at a relatively constant rate, suggesting that it may not longer be growing exponentially

– However, Earth’s human population grows by 75 million people each year

– Are we entering the final bend of the S-shaped growth curve?

Human Population Growth

Fig. 26-16

Technical advances Agricultural advances Industrial and

medical

advances

Date

1804

1927

1960

1975

1987

1999

2012

Time to add

each billion

(years)

All of human

history

123

33

14

13

12

13

Billions

1

2

3

4

5

6

7*

*projected

1804

1975

1960

1927

1987

1999

2008

2012*

Bubonic

plague

year


A series of advances have increased Earth’s

carrying capacity to support people

– Humans have manipulated the environment to

increase the Earth’s carrying capacity

– Several technological advances have greatly

influenced the human ability to make resources

available

–Technical advances

–Agricultural advances

– Industrial and medical advances


Technical advances

– Early humans discovered fire, invented tools and

weapons, built shelters, and designed protective

clothing

–Tools and weapons allowed them to hunt

more effectively to increase their food supply

–Shelter and clothing increased the habitable

areas of the globe


Agricultural advances

– Around 8000 B.C., animals and plants were

domesticated, providing a larger and more stable

food supply

– This resulted in a longer life span and more

childbearing years, although disease continued

to restrain population growth


Industrial and medical advances

– Beginning in England in the mid-eighteenth

century, medical and industrial advances

permitted a population explosion

– Industrial advances allowed fewer people to

produce more food

–Medical advances decreased the death rate

from infectious disease


The demographic transition explains trends in

population size

– In developed countries, people benefit from a relatively

high standard of living, with access to modern technology

and medical care, including contraception

– Examples include Australia, New Zealand, Japan, and

countries in North America and Europe

Most of Earth’s people, however, live in developing

countries, which lack these advantages

– Examples include countries in Central and South

America, Africa, and much of Asia


The rate of population growth in countries that are now developed has changed over time in predictable stages, in a pattern called demographic transition – Pre-industrial stage: The population was relatively small

and stable, with high birth rates and high death rates

– Transitional stage: Food production increased and health care improved, which caused death rates to fall; because birth rates remained high, there was an explosive population increase

– Industrial stage: Birth rates fell as contraceptives were more available, and as people moved from farms to cities, where children were less important as a source of labor

– Post-industrial stage: Populations are relatively stable, with low birth and death rates

The Demographic Transition

Fig. 26-17

birth rate

death rate

population size

natural rate

of population

increase

Pre-industrial

Stage Transitional

Stage Industrial

Stage Post-industrial

Stage

Birth rate

declines

Population

grows rapidly

Death rate

declines

Population

remains low

Birth rate

remains high

Population

growth slows

Population

stabilizes

Birth and death

rates are low

Birth and death

rates are high


The demographic transition explains trends in

population size (continued)

– A population will eventually stabilize if parents

have just the number of children to replace

themselves, called replacement-level fertility

(RLF)

–Because not all children survive to maturity,

RLF is 2.1 per woman


World population growth is unevenly distributed

– Many developing countries still have rapidly

growing populations, as birth rates vastly exceed

death rates

–This results from low incomes and the need

for many children to raise family income or

produce food

– In these countries, knowledge of, and access

to, contraception are limited


World population growth is unevenly distributed

(continued)

– In spite of the population reduction of some

developing countries, zero population growth will

not be achieved globally

–The U.N. predicts a global human population

of over 9 billion, and growing, by the year

2050

–7.9 billion of those people will live in


Historical and Projected World Population

Fig. 26-18


developed countries

2009: 6.8 billion


The current age structure of a population

predicts its future growth

– Age structure diagrams show the distribution of

human populations by age and gender

– Age structure can be shown graphically

–Age is shown on the vertical axis

–The number of individuals in each age group

is shown on the horizontal axis, with males

and females placed on opposite sides


The current age structure of a population

predicts its future growth (continued)

– All age-structure diagrams peak at the maximum

life span, but the shape below the peak reveals if

the population follows one of three basic age-

structure patterns

–A growing population

–A stable population

–A shrinking population


A growing population

– If adults of reproductive age (15 to 44 years) are

having more children than are needed to replace

themselves, the population is above RLF and is

expanding

–The age-structure diagram will be roughly

triangular

–An example of a country with a growing

population is Mexico

female male

Mexico 2009

(a) Population pyramid for Mexico

Age-Structure Diagram

Fig. 26-19a


A stable population

– If adults of reproductive age have just the

number of children needed to replace

themselves, the population is at RLF and is

stable

–The age-structure diagram will have relatively

straight sides

–An example of a country with a stable

population is Sweden

female male

Sweden 2009

(b) Population pyramid for Sweden


Fig. 26-19b


A shrinking population

– If adults of reproductive age have fewer children

than are needed to replace themselves, the

population is below RLF and is shrinking

–The age-structure diagram will be narrow a

the base

–An example of a country with a shrinking

population is Italy


Fig. 26-19c

female male

Italy 2009

(c) Population pyramid for Italy


Average-age structure diagrams have been

plotted for developed and developing countries

for 2009, with predictions for 2050

– These diagrams reveal that even if developing

countries were to achieve RLF immediately, their

population increases would continue for decades

–A large population of children today create a

momentum for future growth as they enter

their reproductive years


Fertility in Europe is below replacement level

– A comparison of growth rates for various world regions shows Europe as the only one with an average rate of change in population that is negative

–The average fertility rate is 1.5, which is substantially below RLF

–Concerns about the availability of future workers and taxpayers have prompted several countries to offer incentives for couples to have children at an earlier age

Europe: 0.0%

Latin America/Caribbean: 1.5%

Asia (excluding China): 1.5%

Developing countries average: 1.5%

Africa: 2.4%

N. America: 0.6%

World average: 1.2%

Developed

countries average: 0.2%

China: 0.5%

Population Change by World Regions

Fig. 26-21


The U.S. population is growing rapidly

– With a population of over 307 million and a

growth rate of about 1% per year, the U.S.

population is the fastest growing of all developed

countries

–The U.S. fertility rate is about 2.0—actually

below RLF

–However, immigration is adding rapidly to the

population, because the fertility rate of new

immigrants is above RLF

0

25

50

75

100

125

150

175

200

225

250

275

300

325

1850 1950 year

1800 1900 2000

U.S

. p

op

ula

tio

n (

in m

illi

on

s)

U.S. Population Growth

Fig. 26-22


Rapid population growth in the U.S. may have serious implications for the environment of the U.S. and the Earth

– Americans consume far more resources and produce far more pollution than the global average

– The spread of housing, commercial establishments, and energy-extracting enterprises degrades and destroys natural habitats, reducing the carrying capacity for non-human life of ecosystems in the United states

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