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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.