Population Ecology Population ecology is the study of populations in relation to environment –Including environmental influences on population density

Population Ecology

• Population ecology is the study of populations in relation to environment– Including environmental influences on population

density and distribution, age structure, and variations in population size

Levels of Levels of OrganizatiOrganizationon

•OrganismOrganism

•PopulationPopulation

•CommunityCommunity

•EcosystemEcosystem

•BiomeBiome

Describing Describing populationspopulations

• SizeSize: total # of individuals in a pop (N)

• DensityDensity: # / unit area

Describing populations

Estimation of Population Size

Sampling technique

• Mark and recapture


Mark and Recapture• Orgs are captured, tagged, and then

released.• Some time later, the same process is

repeated and the following is used:


• Ex: – Suppose that 50 zebra mussels are captured,

marked, and released. One week later, 100 zebra mussels are captured and 10 are found to have markings already. The estimated population would then be

• N= (50·100) / (10) = 5000/10 = 500

• N=500


Dispersion

• Describes how Describes how individuals in individuals in a population a population are are distributeddistributed

1.1.ClumpedClumped

2.2.UniformUniform

3.3.RandomRandom


Clumped dispersion Clumped dispersion (most common)

– Individuals aggregate in patches– May be influenced by resource availabilityresource availability &

behaviorbehaviorExample:Example: For many animals, such as these

wolves, living in groups

1.increases the effectiveness of hunting.

2.spreads the work of protecting and caring for young.

3.helps exclude other individuals from their territory.

Fish travel in schools – safety in numbers


Uniform dispersionUniform dispersion-Individuals are evenly distributed-May be influenced by social interactionssocial interactions such as territorialityterritoriality

Birds nesting on small islands, such as these king penguins on South Georgia Island in the South Atlantic Ocean, often exhibit uniform spacing, maintained by aggressive interactions between neighbors.

Some plants secrete

toxins that keep others

away


Random dispersionRandom dispersion– The position of each individual is

independent of other individuals

Dandelions grow from windblown seeds that land at random and later germinate.


• Biotic potential of a population= – max rate at which a population could increase under

ideal conditions

• Influenced by several factors:– Age at which reproduction begins– Life span during which the organisms are capable of

reproducing– Number of reproductive periods in the lifetime– Number of offspring the organism is capable of having

Changes in Population SizeDescribing populations

Age Structure

• Description of the abundance of individuals of each age in a population.


Age Structure Diagrams


China

India

United States

2004

Zero population

growth

1954

Animation



Survivorship Curves• Describes how the mortality of individuals

in a species varies during their lifetime

The survivorship curve for Belding’s ground squirrels– Shows that

the death rate is relatively constant

1000

100

10

1

Num

ber

of s

urvi

vors

(lo

g sc

ale)

0 2 4 6 8 10

Age (years)

Males

Females


• Survivorship curves can be classified into three general types: Type I, Type II, and Type IIIType I, Type II, and Type III

I

II

III

50 10001

10

100

1,000

Percentage of maximum life span

Num

ber

of

surv

ivors

(lo

g

scale

)

•Type I •A species in which most survive to middle age•Parenting

•Type II•Length of survivorship is random. Death rate is constant.

•Type III•Most individuals die young. •Few reach reproductive maturity.•Fish release 1000s of eggs.•No parenting

I

II

III

50 10001

10

100

1,000

Percentage of maximum life span

Num

ber

of

surv

ivors

(lo

g

scale

)

Animation

Factors Factors Affecting Affecting

PopulationPopulation

Limiting Factors Things that prevent a population

from attaining its biotic potential– 2 categories

1.Density-dependent2.Density-independent

Density Dependent

Density Independent

Factors Affecting Factors Affecting PopulationPopulation

Density DependentPopulations regulated by density-dependent

factors are affected by the numbernumber of organisms present.a) Predation b) Parasitism c) Diseased) Competition

-the more organisms crowd together, the more damaging are food shortages, parasites, and predators.

Intraspecific Competition = the struggle between members of a population for scarce resources


Density Independent• The number of organisms present does not

affect the influence of the factor. – Fire, earthquakes, storms, floods

• Density-independent factors show no correlation with the size of the population.


Describing Population Describing Population GrowthGrowth

Population ecologists describe two general patterns of populations growth:

1. Exponential2. Logistic

The growth of a population can be

described by the following equation:• r= reproductive rate (or growth rate)• N= population size at the beginning of

the interval for which the births and deaths are counted.

r =.

births-deathsN

Net increase of individuals

Can r be negative? Zero?

Types of Types of Population Population

GrowthGrowth

r =Ex:

– A population of 1000 had 60 births and 10 deaths over a one year period. What is the growth rate?

– 0.05 per year

births-deathsN


GrowthGrowth

If the rate of reproduction per individual remains constant through time, then the rate at which the population increases is a multiple of the number of individuals in the population.

In a population showing exponential growth the individuals are notare not limited by food, disease, predation or competition.

Exponential growth

r>0Types of Types of

Population Population GrowthGrowth

Exponential population growth– Results in a J-shaped curve– Usually short lived in nature- not sustainable

0 5 10 150

500

1,000

1,500

2,000

Number of generations

Pop

ula

tion

siz

e (

N)dNdt1.0N

dNdt0.5N

Pop

ula

tion s

ize (

N)

r>0Types of Types of

Population Population GrowthGrowth

Carrying Capacity (K)• Populations

usually reach a carrying capacity.

• Upper limit to the number of individuals the environment can support.

the real world


GrowthGrowth

Logistic growth

•A population can grow exponentially only for short periods of timeshort periods of time, because it would very quickly deplete all the resources necessary for its survival.

•Over long periods of time, populations tend to attain an equilibrium population size which is determined by the available resources.

In most real populations both food and disease become important as conditions become crowded.

dNdt rmaxN

Animation


GrowthGrowth

• Zero population growth – Occurs when the birth rate equals the death rate

• The population growth equation can be expressed asdN

dt rN


GrowthGrowth

Logistic and exponential growth are Logistic and exponential growth are associated with 2 kinds of life-history associated with 2 kinds of life-history

strategiesstrategies

1.1.R-selected species R-selected species (strategists)(strategists)• organisms are opportunistic; they reproduce

rapidly when the environment is uncrowned and resources are vast

2.2.K-selected species K-selected species (strategists)(strategists) • operate at a density near K

Life-history strategies

R-selected speciesR-selected speciesa. Opportunistic species, which tend to be colonizers.

b. Grasses, many insects-Usually quickly invade, reproduce many offspring and die-Offspring mature quickly and require very little parental care

c. Strategies for continued existence is based on individuals having the following traits: 1) small size 2) short life span 3) mature fast 4) produce many offspring 5) engage in little care of offspring


K-selected speciesSuch populations are equilibrium species, tend to be

specialists rather than colonizers, and may become extinct when their evolved way of life is disrupted (e.g., the grizzly bear, Florida panther, etc.).

Overall strategy for continued existence is based on having the following traits: 1) large size 2) long life span 3) slow to mature 4) produce few offspring 5) expend considerable energy in care

–sensitive to population density


http://www.bigcatrescue.org/StockPhoto/photoimages/flash/0allflashphotos/floridapanther/floridapanther01.jpg

Human Population Growth

The following made exponential growth possible

1. Increases food supply2. Reduction in disease3. Reduction in human

waste4. Expansion of habitat

As of 28 April 2010, the human population of the world is estimated by the United States Census Bureau to be 6,817,500,000.

Countries Ranked by Population: 2010

Source: U.S. Census Bureau, International Data Base.

6,817,500,000

Map of countries by population for the year 2007

http://upload.wikimedia.org/wikipedia/commons/b/b1/World_population.PNG

Documents

Population Ecology Population ecology is the study of populations in relation to environment –Including environmental influences on population density