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Lecture 9

Population Ecology

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Today’s topics

• What is population ecology?• Population change and regulation

– Density independence– Density dependence

• Life history traits• Alaska example

– Predator control

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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.

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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.

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Caribou census – aerial photographs

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Moose estimate – aerial surveys

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Deer estimate - DNA

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Spotlight counts

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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

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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.

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Senescence – decrease in fecundity and increase in mortality rate resulting from deterioration

in physiological function with age.

Age

Offspring per individual female

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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

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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

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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

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Hypothetical survivorship curves

Most mammals are type I or II. With regards to “r” and “K” selected species, which one is type I?

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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

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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

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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)

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Populations increase exponentially rather than arithmetically

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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

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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.

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Logistic curve

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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

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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.

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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..

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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

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Cycles – populations fluctuating widely in constant periods

1960 1964 1968

Lemmings in Barrow

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Alaska Example• Intensive Management (i.e., predator control)

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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

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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

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Why did killing wolves increase the wolf population?

Why did the moose population continue to increase after the wolf

population recovered?

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Predator Pit hypothesis – predation regulate prey at a low and stable density well below “K”

Time

Population size

Predator pit – under maximum growth potential

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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

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Danger! Knowing “K” is important

Time

Population size

K

Unsustainable level

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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

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The story gets even more ecologically complex and political.

• Maybe a report topic???