Population Estimation

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Population Estimation
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Lecture Goals

• Present Overview of principles of population
  estimation
• Introduce basic methods
• Complete enumeration
• Mark-recapture
• Density estimation
• Depletion estimates

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In all methods to follow, we assume

• No significant immigration/emigration
• Population numbers are determined by birth/death
  within population

• It is important that the spatial scale that we
  examine matches that of the population!
• Population size of squirrels
• in a backyard inappropriate scale
• in a woodlot appropriate size ?
• on an island appropriate size

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Complete Enumeration

• Sometimes possible for animals or plants
• Individuals must be
• Conspicious (often due to size)
• Sessile or very slow
• Not too abundant
• Occurring in a limited area
• Do you know of some examples of this?

5

• Northern Right Whale(Eubalaena glacialis )
• Populations annual migration
• Individual identities by markings
• Only 100s in population

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Mark-Recapture Methods

• Greater numbers make enumeration impractical
• The Method
• Capture some of the population
• Uniquely mark the captured individuals
• Can be a unique natural mark coloration, DNA,
  etc.
• Release marked individuals
• Recapture a second group
• Examine ratio of marked to unmarked
• Assumes complete randomization (mixing) between
  capture recapture

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Marks
DNA?
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Principle In a larger population fewer marked
individuals are recaptured

• Capture 10 frogs, mark release
• Recapture 10 frogs, 2 are marked
• How many frogs?

• Actual methods quantify uncertainty in estimate
• Sample size effects (what if only 1 sampled
  above?)
• provide population estimate as a range

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Density Estimates

• Extend estimates beyond region sampled
• Useful for large areas or high numbers

• Procedure
• Establish study area (range of population?)
• Break area into plots (quadrats?)
• Count all individuals in plot
• Divide counted by area
• Multiply density by total area of all plots

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Depletion Estimates

• Assume that catch-rate declines with abundance
• Keep a record of catch catch rate over time
• Plot catch rate on cumulative catch

• Keep catching eventually you catch them all!
• Use regression to predict this value

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2
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Catch Rate
Cumulative Catch
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Which of the methods discussed might you apply to
estimate the population size of

• Wolves
• Burr oak
• Walleye
• Deer mice
• Morells
• Crows
• Poplar (this is a trick question)