Population EcologyPart 2

Discussion

population

ecosystem

community

biosphere

organism

Big Questions:

• What characterizes a population?• What important factors influence population

growth?

Population Size• Changes to

population size– adding & removing

individuals from a population

• birth• death• immigration• emigration

Births and immigration add individuals to a population.

Births Immigration

PopuIationsize

Emigration

Deaths

Deaths and emigration remove individuals from

a population.

Population growth rates• Factors affecting population growth rate

– sex ratio• how many females vs. males?

– generation time• at what age do females reproduce?

– age structure• how females at reproductive age in cohort?

Life table

Demography• Factors that affect growth & decline of

populations– vital statistics & how they change over time

Why do teenage boys pay high car insurance rates?

females males

What adaptations have led to this difference

in male vs. femalemortality?

Survivorship curves• Graphic representation of life table

Belding ground squirrel

The relatively straight lines of the plots indicate relatively constant rates of death; however, males have a lower survival rate overall than females.

Age structure• Relative number of individuals of each age

What do these data imply about population growth in these countries?

Survivorship curves

• Generalized strategiesWhat do these graphs tell about survival & strategy of a species?

0 25

1000

100

Human(type I)

Hydra(type II)

Oyster(type III)

10

150

Percent of maximum life span

10075

Su

rviv

al p

er th

ousa

nd

I. High death rate in post-reproductive years

II. Constant mortality rate throughout life span

III. Very high early mortality but the few survivors then live long (stay reproductive)

Trade-offs: survival vs. reproduction• The cost of reproduction

– increase reproduction may decrease survival• age at first reproduction • investment per offspring• number of reproductive cycles per lifetime

Natural selection favors a life history that maximizes lifetime reproductive success

Reproductive strategies• K-selected

– late reproduction– few offspring– invest a lot in raising offspring

• primates• coconut

• r-selected– early reproduction– many offspring– little parental care

• insects• many plants

K-selected

r-selected

Trade offsNumber & size of offspring

vs.Survival of offspring or parent

r-selected

K-selected

“Of course, long before you mature, most of you will be eaten.”

Life strategies & survivorship curves

0 25

1000

100

Human(type I)

Hydra(type II)

Oyster(type III)

10

1

50

Percent of maximum life span

10075

Su

rviv

al p

er th

ousa

ndK-selection

r-selection

Population growthchange in population = births – deathsExponential model (ideal conditions)

dN = riNdt

N = # of individualsr = rate of growthri = intrinsic ratet = timed = rate of change

growth increasing at constant rate

intrinsic rate = maximum rate of growth

every pair has 4 offspring

every pair has 3 offspring

Exponential growth rate• Characteristic of populations without

limiting factors – introduced to a new environment or rebounding from a

catastrophe

African elephantprotected from hunting

Whooping cranecoming back from near extinction

K =carryingcapacity

Logistic rate of growth• Can populations continue to grow

exponentially? Of course not!

effect of natural controls

no natural controls

What happens as N approaches K?

Influence of population size (N) on per capita rate of increase (r)

Maximum

Positive

Negative

0N K

Population size (N)

Per

cap

ita r

ate

of in

crea

se (r)

Population growth predicted by the logistic model

dNdt

1.0N Exponential growth

Logistic growth

dNdt

1.0N1,500 N

1,500

K 1,500

0 5 10 150

500

1,000

1,500

2,000

Number of generations

Pop

ulat

ion

size

(N

)

500

400

300

200

100

0200 10 30 5040 60

Time (days)

Num

ber

of

clad

ocer

ans

(per

20

0 m

l)

• Maximum population size that environment can support with no degradation of habitat– varies with

changes in resources

Time (years)1915 1925 1935 1945

10

8

6

4

2

0Num

ber

of

bre

edin

g m

ale

fur

se

als

(tho

usa

nds)

Carrying capacity

What’s going on with the plankton?

Changes in Carrying Capacity

• Population cycles– predator – prey

interactions

At what population level is the

carrying capacity?

K

K

2007-2008

AnyQuestions?

• What influences populations to adopt r or k life history strategies?

• Explain exponential growth.• Know what carrying capacity is.• Explain why human populations are

experiencing exponential growth.