26 population growth and regulation. how does population size change? population community ...

26Population Growth and Regulation

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How Does Population Size Change?

• Population community ecosystem biosphere

• Ecology is the study of the interrelationships of organisms with each other and with the nonliving environment


• Changes in population size result from natural increases and net migration– Population size changes through

• Births • Deaths • Net migration

How Does Population Size Change? • Changes in population size result from natural

increases and net migration (continued)– A population thus grows when the sum of natural

increase and net migration is positive and declines when this sum is negative

• A simple equation for the change is– Change in population size natural increase net migration

(births deaths) (immigration emigration)


• Populations grow based on the birth rate, the death rate, and the population size– The size of most natural populations of organisms

fluctuates over the course of a year because reproduction tends to be seasonal

– Growing populations add individuals in proportion to the population’s size, much like a bank account accumulates compound interest

– If conditions are the same, a population will grow at a constant percentage of its size over a given interval


• Populations grow based on the birth rate, the death rate, and the population size (continued)– The growth rate (r) of a population is the

percentage change in the population size per unit time

– The population growth rate is the birth rate (b) minus its death rate (d)

• r (growth rate) b (birth rate) d (death rate)

How Does Population Size Change? • Populations grow based on the birth rate, the

death rate, and the population size (continued)– If births exceed deaths, exponential growth occurs

• A constant growth rate (r) produces exponential growth• During exponential growth, an ever-larger number is

added to the population during each succeeding time period

• If the size of an exponentially growing population is graphed against time, a characteristic shape called a J-curve will be produced

• The age at which an organism first reproduces affects the size of the future population

Figure 26-1 Exponential growth curves are J-shaped

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time (years)20

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

At 24 years,this populationhas 2,504 eagles 0 2 2

6 8 4

12 52 18

18 362 86

24 2,504 392

30 17,314 1,764

Time(years)

Numberof

eagles(pop. 1)

Numberof

eagles(pop. 2)

At 24 years,this populationhas 392 eagles

How Does Population Size Change? • Populations grow based on the birth rate, the

death rate, and the population size (continued)– If births exceed deaths, exponential growth occurs

• The death rate has a major impact on population size

• As long as births exceed deaths, the population eventually becomes enormous

• As the death rate increases, it takes longer to reach any given population size

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500

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time (hours)

nu

mb

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of

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no deaths10% death rate25% death rate

It takes about 4hours to produce1,500 bacteria

It takes about 3.5hours to produce1,500 bacteria

It takes about 5.5hours to produce1,500 bacteria


• Biotic potential determines the maximum rate at which a population could increase– Several factors influence biotic potential

• The age at which the organism first reproduces • The frequency of reproduction• The average number of offspring produced each time • The length of the organism’s reproductive life span • The death rate of individuals

How Is Population Growth Regulated?

• Population size results from the interaction between biotic potential and environmental resistance, or all the curbs on population growth imposed by the living and nonliving environmentExamples:

• interactions among organisms such as predation and competition for limited resources

• natural events such as freezing weather, storms, fires, floods, and droughts


• Exponential growth only occurs under unusual conditions – Under unusual and temporary circumstances,

natural populations exhibit exponential growth, producing J-shaped growth curves


• Exponential growth only occurs under unusual conditions (continued) – Exponential growth occurs in populations with

boom-and-bust cycles• periods of rapid population growth are followed by a

sudden, massive die-off • Seasonal populations are linked to changes in rainfall,

temperature, or nutrient availability • Ideal conditions encourage rapid growth; deteriorating

conditions encourage massive die-off

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er 1

00 t

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Boom-and-bust cycles in a lemming population inthe Canadian Arctic.

0

Nutrients are depleted, andwater temperature falls

Favorable growthconditions occur

“boom” “bust”

Jan Mar Maymonth

A boom-and-bust cycle in photosynthetic bacteria

Jul Sep Nov

po

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sity


• Exponential growth only occurs under unusual conditions (continued) – Exponential growth occurs temporarily

when environmental resistance is reduced

• Populations that do not experience boom-and-bust cycles

• Special circumstances:– A reduction in predation– An increase of food supply or habitat

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• Exponential growth only occurs under unusual conditions (continued) – Exponential growth occurs temporarily when

environmental resistance is reduced (continued)• Exponential growth can occur when individuals invade

a new habitat with little competition• 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


• Environmental resistance limits population growth – Many populations that exhibit exponential growth

eventually stabilize to match the resources available to support them

• As resources become depleted, reproduction slows and the growth rate eventually drops to zero, causing the population size to remain constant

How Is Population Growth Regulated? • Environmental resistance limits population

growth (continued) – Logistic growth occurs when new populations stabilize

as a result of environmental resistance• 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)

Figure 26-5a An S-shaped growth curve stabilizes at carrying capacity

carrying capacityn

um

ber

of

ind

ivid

ual

s Growthrate slows

Populationgrows rapidly

Growth stops and thepopulation stabilizes closeto the carrying capacity

time

An S-shaped growth curve stabilizes at carrying capacity

0


• Environmental resistance limits population growth (continued) – Logistic growth (continued)

• In nature, an increase in population size (N) above carrying capacity (K) can be sustained for a short time

• 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


• Logistic growth (continued)– Occurs 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

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• Environmental resistance limits population growth (continued) – Two forms of environmental resistance usually

maintain populations at or below the carrying capacity of their environment

• Density-independent factors limit population size regardless of the population density

• Density-dependent factors increase in effectiveness as the population density increases


• Two forms of environmental resistance – 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

– Many insects and annual plant populations– Hurricanes, droughts, floods, and fire


• Two forms of environmental resistance – Density-independent factors (continued)

• Human activities can also limit the growth of natural populations

– Pesticides and pollutants can cause drastic declines– Overhunting has driven some species to extinction

• Habitat destruction by humans is the single greatest threat to wildlife worldwide


• Two forms of environmental resistance – Density-dependent factors (continued)

• Populations of organisms with a life span of more than a year have evolved adaptations that allow them to survive density-independent controls imposed by seasonal changes, such as cold and lack of food during the winter



• Density-dependent factors exert a negative feedback effect on population size, because they become increasingly effective as the population density increases

• Predators exert density-dependent controls on populations

– Prey are killed directly and eaten, but not always



• Predators exert density-dependent controls– Predation becomes important as prey populations grow

because predators eat a variety of prey, depending on what is most abundant and easiest to find

– Predator populations often grow as their prey becomes more abundant, which makes them even more effective as control agents

Predator populations oftenincrease when prey are abundant

How Is Population Growth Regulated? • Two forms of environmental resistance

– Density-dependent factors (continued)• Some predator-prey population cycles are out-of-phase

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adu

lt p

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ula

tio

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bean weevils (prey)braconid wasp (predator)

A high predatorpopulationreduces the preypopulation

The prey populationpeaks when thepredator populationis low

How Is Population Growth Regulated? • Two forms of environmental resistance

– Density-dependent factors (continued)• Parasites spread more rapidly among dense populations

– Cannot travel long distances– Parasites influence population size by weakening their hosts

and making them more susceptible to death from other causes, such as harsh weather or predators

» Organisms weakened by parasites are less likely to reproduce



• Competition for resources helps control populations – Competition is the interaction among individuals who

attempt to use the same limited resource– There are two major forms of competition

» Interspecific competition, between individuals of different species

» Intraspecific competition, between individuals of the same species


• Two forms of environmental resistance – Density-independent and density-dependent

factors interact to regulate population size • The size of a population at any given time is the result

of complex interactions between density-independent and density-dependent forms of environmental resistance

• 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)

Review

1. Describe exponential growth.2. When would exponential growth occur?3. Describe a logistic growth curve.4. What are the two major forms of

environmental resistance?

How Are Populations Distributed in Space and Age?

• Populations exhibit different spatial distributions– Spatial distribution describes how individuals within a

population are distributed within a given area

– Spatial distribution may vary with time, changing with the breeding seasons

– Ecologists recognize three major types of spatial distribution:

• Clumped

• Uniform

• Random


• Populations exhibit different spatial distributions– Populations whose members live in groups exhibit

clumped distribution • Examples include elephant herds, wolf packs, prides of

lions, flocks of birds, and schools of fish

Clumped distribution

clumped


• Populations exhibit different spatial distributions– Advantages of clumped distributions include

• Many eyes that can search for localized food sources • Movement of the group (e.g., schools of fish or flocks of

birds) can confuse predators by their sheer numbers • Predators, in turn, may hunt in groups, cooperating to bring

down larger prey


• Populations exhibit different spatial distributions– Uniform distribution maintain a relatively constant

distance between individuals • This is common among territorial animals defending scarce

resources or breeding territories

uniform

Uniform distribution


• Populations exhibit different spatial distributions– Random distribution is relatively rare

• Such individuals do not form social groups • The resources needed are more or less equally available

throughout the area they inhabit– Resources are not scarce enough to

require territorial spacing – Examples include trees and other plants

in rain forests

random

Random distribution

Figure 26-13 Survivorship tables and survivorship curves

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survivors

0 (birth) 100,000

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early loss(dandelion)

constant loss(American robin)

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percent of maximum life span

Survivorship curves

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A survivorshiptable

How Is the Human Population Changing?

• The human population continues to grow rapidly Date Billions Time to add

each billion(years)

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*projected

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All of humanhistory

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Technical advances Agricultural advances

B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C.

B.C./A.D.

A.D. A.D.

Industrialand medical

advances

1999

20117

6

14


• A series of advances has increased Earth’s carrying capacity to support people– Early humans

• Discovered fire• Invented tools and weapons• Built shelters• Designed protective clothing – small stable populations• Agricultural advances – longer life span but high death rate• Medical advances – decreased birth and death rate


• The age structure of a population predicts its future growth– Age structure diagrams show age groups on the

vertical axis and the numbers (or percentages) of individuals in each age group on the horizontal axis, with males and females shown on opposite sides

– Age structure diagrams all rise to a peak that reflects the maximum human life span

– The shape of the rest of the diagram reveals whether the population is expanding, stable, or shrinking

Figure 26-16a Africa: A rapidly growing population

Africa 2010

male

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age

4 2 0 2 4 6percent of population

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Africa: A rapidly growing population

Figure 26-16b North America: A slowly growing population

female

North America 2010

male

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North America: A slowly growing population

female

Figure 26-16c Europe: A slowly declining population

Europe 2010

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Europe: A slowly declining population

female


• The age structure of a population predicts its future growth– Median age – age at which half the population is

younger and half is older• The median age depends on the age structure• The lower the median age, the more rapidly the

population will expand• In a stable human population, fewer than 20% of

individuals are younger than age 15

Figure 26-18 United Nations world population projections

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Review

1. Explain the three basic types of distribution within populations.

2. Describe the three general types of survivorship curves.

3. How does the shape of an age structure diagram predict future changes in population sizes?

26 population growth and regulation. how does population size change? population community ...

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