Download - Mark Recapture
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Mark Recapture
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An application: Estimating Steller sea lion pup mortality
• Steller sea lion numbers have declined 70-90% since 1950s; now listed as endangered
• Is mortality of pups an important factor in population decline?
• Can’t safely tag newborn pups, so used natural markings of moms to “mark” individual pups
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• After each birth, “mark” mom by photographing, finding natural marks, and giving her a name
• Every day, find mom and see if she suckles a pup. If so, her pup is still alive!
• If she doesn’t suckle her pup - the pup may be dead
• Use a mark-recapture model to estimate the mortality rate
• Results: Pup mortality was generally low, but big storms killed large numbers of pups in unprotected rookeriesThese finding improved population models and helped focus conservation efforts(Kaplan et al. 2006)
“Sucka”
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Central question to this method: What proportion of the population does a
sample represent?
When do we take samples?
When we can’t observe the entire population
Mark Recapture method can tell us…
•Population size
• Population demographics (birth rate, death, movement, survival, age structure)
Nichols,Janna
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Petersen Mark-Recapture Method
Catch several animals
Survey 1:
Mark all (M) animals
Return animals to population
Catch (R) animals
Survey 2:
Count recaptures (m)
Return animals to population
What is the estimated population size (N)?
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Survey 1:
M = 12
Survey 2:
R = 15m = 4
N = ?
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R = 15m = 4
What is the total population size (N)?
Note that the proportion marked in the populationequals the proportion marked in the 2nd sample
M = 12
N = M R m
M = mN R
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N = 12 * 15 4
R = 15m = 4
What is the total population size (N)?
Note that the proportion marked in the populationequals the proportion marked in the 2nd sample
N = M R m
M = 12
N = 45(estimated)
N = 43
(actual)
M = mN R
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When would mark-recapture give you a bad estimate?
• Unequal chances of being caught
• Marks fall off or go unnoticed
• Marked animals don’t disperse evenly back into the general population
• Population is open
S. Schultz
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Closed populations
No individuals enter or leave the population between surveys
Survey 1 Survey 2
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Open populations
Individuals enter or leave the population between surveys
Survey 1 Survey 2
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N = R M m
M = mN R
N = Estimated population size
M = Number of animals marked
R = Number of animals captured in resampling event
m = Number of “R” that were already marked
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N = R M m
M = mN R
N = Estimated population size
M = Number of animals marked
R = Number of animals captured in resampling event
m = Number of “R” that were already marked
Use paper and cups to test out a mark-recapture accuracy.Test variables that increase or decrease accuracyReport results to the rest of class.