• POPULATION = a group of interbreeding organisms (same species) that live in the same place at the same time and compete for the same resources.

• Resources = food, water, shelter, mates, and so on . . .

• resources pop. size

• resources pop. size

The population size of a species in a given space at a given time is determined by the interplay between BIOTIC POTENTIAL and ENVIRONMENTAL RESISTANCE.

Biotic potential = growth rate with unlimited resources.

Environmental resistance = all the factors acting jointly to limit population growth.

Four variables change population size:

1.NATALITY = birth rate

2.MORTALITY = death rate

3.IMMIGRATION = rate of organisms moving in

4.EMIGRATION = rate of organisms moving out

DENSITY DEPENDENT FACTORS = affect a populations’ size depending on its population density.

1.Predation

2.Disease

3.Availability of food and water

4.Space

Negative Feedback!!

Populations change in response to environmental stress or changes in environmental conditions.

1. In size = # of individuals2. Density = # of individual / specific space3. Age distribution = proportions / age

group4. Dispersion =

Clumped(elephants)

Uniform(creosote bush)

Random(dandelions)

DENSITY INDEPENDENT FACTORS = affect a populations’ size regardless of its population density.

1.Weather

2.Earthquakes

3.Floods

4.Fires

. . . Natural disasters

R-strategists populations are most affected by

these.

No population can grow indefinitely!

Every environment has a CARRYING CAPACITY = the maximum number of individuals of a given species that can be sustained indefinitely in a given space.

2.0

1.5

1.0

.5

Nu

mb

er o

f sh

eep

(m

illio

ns)

1800 1825 1850 1875 1900 1925Year

Factors that affect carrying capacity:

1. Competition with/in and between species.

2. Natural and human caused catastrophes.

3. Immigration and emigration.4. Seasonal fluctuations in food, water,

shelter, and nesting sites.

A population that has few if any resource limitations grows exponentially.

EXPONENTIAL GROWTH starts out slowly and then proceeds faster and faster as the population increases.

Time (t)

Po

pu

lati

on

siz

e (N

)“J” population growth curve

LOGISTIC GROWTH involves initial exponential growth and then there is a steady decrease in growth as the population encounters environmental resistance and approaches carrying capacity and levels off.

“S or sigmoid” population

growth curve

Time (t)

Po

pu

lati

on

siz

e (N

)

K

Time (t)

Po

pu

lati

on

siz

e (N

)K

Exponential phase

Transitional phase

Plateau phase

Opportunistic or r-Selected Species

cockroach dandelion

Many small offspringLittle or no parental care and protection of offspringEarly reproductive ageMost offspring die before reaching reproductive ageSmall adultsAdapted to unstable climate and environmental conditionsHigh population growth rate (r)Population size fluctuates wildly above and below carrying capacity (K)Generalist nicheLow ability to competeEarly successional species

Fewer, larger offspringHigh parental care and protection of offspringLater reproductive ageMost offspring survive to reproductive ageLarger adultsAdapted to stable climate and environmental conditionsLower population growth rate (r)Population size fairly stable and usually close to carrying capacity (K)Specialist nicheHigh ability to competeLate successional species

elephant saguaro

Competitor or K-Selected Species

Nu

mb

er o

f in

div

idu

als

Time

Carrying capacity

K species;experienceK selection

r species;experiencer selection

K

REPRODUCTIVE STRATEGIES

SURVIVORSHIP CURVES

Reindeer on St. Paul Island

• 41 mi2

• Started with 25 Reindeer (4 males and 21females)

• No predators• 200 mi from

mainland so no on or off

• No hunting

End Result

Start with 251938= 2046 deer1950= 8 deerGrowth Rate of 16%Did bounce back to a

steady 250 deer

Birth and Death Calculations

Crude Birth Rate (CBR)= # of births per 1000 individuals/yr

CBR=(Births/Pop. Size)x1000

The world CBR is 20.3

Crude Death Rate (CDR) = # of deaths per 1000 individuals/yr

CDR= (Deaths/Pop. Size)x1000

The world CDR is 9.6

More Calculations

Natural Increase Rate in % (NIR)

NIR = (CBR-CDR)/10

Doubling Rate = how many years it will take to double the population

DR= 70/NIR

Density= # of individuals in a given area

D= Pop./ Area

IB Acronyms for Countries

• MEDC= More Economically Developed Country

• LEDC= Less Economically Developed Country

• NIC= Newly Industrialized Country