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Oct. 6, 2010 2
Today’s topics
• What is population ecology?• Population change and regulation
– Density independence– Density dependence
• Life history traits• Alaska example
– Predator control
Oct. 6, 2010 3
Population-groups of organisms of the same species, present at the same place and time
• Population ecologists are often concerned with population dynamics: the changes that occur over time and what causes those changes.
Oct. 6, 2010 4
Population ecology questions…
• What is the the size of the population?– Census – try to count every individual– Estimate – survey a portion of the population and
extrapolate.
Oct. 6, 2010 9
Population ecology questions…
• Is the population increasing or decreasing?– Birth rates – individuals added per unit time– Death rates – individuals deleted per unit time– Immigration rates – individual moving in per unit
time– Emigration rates – individuals moving out per unit
time
Oct. 6, 2010 10
Not all individual are identical• For instance, birth rates, death rates, and
movement rates depend on age, sex, and many other characteristics of an individual and the environment.
Oct. 6, 2010 11
Senescence – decrease in fecundity and increase in mortality rate resulting from deterioration
in physiological function with age.
Age
Offspring per individual female
Oct. 6, 2010 12
Life tables – summary by age of survivorship of an individual in a population (simple version)
• Need to know how many are dying in each age interval.
• For example:
Age interval, years, x Number dying, dx
0-1 10
1-2 6
2-3 2
3-4 1
Oct. 6, 2010 13
From there, we can compute number surviving (nx) and cumulative survival
rate from birth until age x (lx)
Age interval, years, x
Number dying, dx
nx lx
0-1 10 20 1.0
1-2 6 14 0.7
2-3 2 12 0.6
3-4 2 10 0.5
Oct. 6, 2010 14
Survivorship Curves
Age interval, years, x
Number dying, dx
nx lx
0-1 10 20 1.0
1-2 6 14 0.7
2-3 2 12 0.6
3-4 2 10 0.5
If we know this, we can graphically illustrate the pattern of mortality across different age groups
Oct. 6, 2010 15
Hypothetical survivorship curves
Most mammals are type I or II. With regards to “r” and “K” selected species, which one is type I?
Oct. 6, 2010 16
More complex life tables
• Fecundity (mx) = number of offspring produced by an average female of age x during that age period
• Survival rate (sx) = survival rate at age x• Mortality rate (qx) = mortality rate at age x
Oct. 6, 2010 17
If we know change over time, then we can compute λ (lamda)
• λ = population growth rate from one point in time (t) to some future time (t + 1)
• For example, if there is 100 individuals in the population one year ago and there is 110 now, then..
N(t+1) = λN(t)
110 = λ100
λ = 1.1
λ sometimes called finite rate of population increase
Oct. 6, 2010 18
Assuming λ is constant over time
• How much will the population grow in 10 years?
Nt = λtN0
Nt = 1.110*100
Nt = ?
Important note = this equation assumes unimpeded growth (no density dependence factors operating on population)
Oct. 6, 2010 20
Density Dependence• It is impossible for an population to continue
to grow indefinitely at a constant rate.• Growth will slow as limiting factors exert
influence– Food supply– Shelter– Predators– Competitors– Parasites– Disease
• The influence often increases as the size and density of the population increases
Oct. 6, 2010 21
With density dependence
• As density increases, birth rates decrease, death rates increase, and/or emigration increases
• The logistic curve represents population change over time in a density dependent system.
• “K” plays a key role the logistic curve model.
Oct. 6, 2010 23
Logistic Equation
dN
dtrN 1
N
K
dN/dt = Population growth rateK = carrying capacity of the populationr = growth rate per individual or intrinsic rate of natural increase“r” can be calculated as individual birth rate minus individual death rate
Oct. 6, 2010 24
Logistic Equation
dN
dtrN 1
N
K
The term in parenthesis is a density dependent term that ranges from 0 to 1.
As N approaches K, then the density dependent term approaches 0.
As the density dependent term approaches 0, the growth rate slows.
Oct. 6, 2010 25
Logistic Equation
dN
dtrN 1
N
K
Simply, as the size of a mammal population approaches the maximum number that the habitat can support, the growth rate of the population slows..
Oct. 6, 2010 26
Lets try it. (hypothetically)
• “K” for moose in the Tanana Flats (just south of Fairbanks) is 2,000 individuals.
• What is the growth rate if the actual population is 500?
• What is the growth rate if the population is 1,900?
• How about 2,500?• Let “r” = 0.2
Oct. 6, 2010 28
Cycles – populations fluctuating widely in constant periods
1960 1964 1968
Lemmings in Barrow
Oct. 6, 2010 30
Increase in moose, caribou, and wolves following wolf control in Alaska (Boertje et al. 1996)
14 wolves/1,000 km2Before 1975
1975
1975-1982 4-5 wolves/1,000 km2
Predator control for 7 years
1982Stop predator control
1986 15-16 wolves/1,000 km2
Oct. 6, 2010 31
How did moose respond183 moose/1,000 km2Before 1975
1975
1975-1982 481 moose/1,000 km2
Predator control for 7 years
1982Stop predator control
λ = 1.15
1982-1994 λ = 1.051,020 moose/1,000 km2
Oct. 6, 2010 32
Why did killing wolves increase the wolf population?
Why did the moose population continue to increase after the wolf
population recovered?
Oct. 6, 2010 33
Predator Pit hypothesis – predation regulate prey at a low and stable density well below “K”
Time
Population size
Predator pit – under maximum growth potential
Oct. 6, 2010 34
Predator control allows prey to escape pit
Time
Population size
Increase growth rate of a larger prey population can sustain impact of predators without population decline
Oct. 6, 2010 36
Elevating prey base above “K” may result in habitat damage, crash the population, and potential reduce
future “K”.
Time
K
Pop
Time
K
Time
KPop
Pop