Surveying Populations

1
Surveying Populations

• How many are there?
• Where are they?
• Are there more here than there?
• Are there fewer than there were before?

2

• A Caricature of the Differences Between
• Plants and Animals.
• PLANTS
• Plants are easy to count density because they do
  not move.
• The difficulty in plants often lies in estimating
  reproduction and
• dispersal of seeds.
• Body size varies strongly so the value of an
  individual, as a unit to count, is often low
  (indeterminant size)
• Individuals may be hard to assess because of
  clonal growth.

3

• A Caricature of the Differences Between
• Plants and Animals.
• ANIMALS (particularly vertebrates)
• We often know  fecundity and juvenile
  survivorship pretty well. 
• The tough part is determining population size and
  population growth rates.
• This is because animals move around and become
  hard to find.
• Detectability is a central concern in estimating
  population size.
• Animals may aggregate in different patterns to
  form groups.
• This may vary through the year
• Animals come and go from an area, changing the
  population size without birth or death.

4
A variety of methods for estimation

• Direct Indices
• Incomplete counts
• Capture indices
• Harvest indices
• Photo station counts
• Track plates
• Indirect Indices
• Scat and sign counts
• Structure (eg, nests) counts
• Auditory indices
• Territory mapping

5
Methods used to sample animal populations

• Complete census 18 individuals

6
EXAMPLE

• US Census

7
Territory mapping
8
Partial survey

• sample here to predict what is
  here

9
Observability

Thus, we develop an INDEX of abundance

10
Mark Recapture study
Mark
11
Recapture
12
Assumption

• Population closed
• - no immigration, emigration
• - no births or deaths

13
Assumption

• Population is well-mixed

14
Assumption

• Animals dont learn to avoid capture or
  observations

15
Assumption

• Markers are permanent

16
METHODS

• Simple Mark Recapture (Lincoln-Peterson method)
• Schnabel method
• Triple Catch
• Jolly-Seber repeat capture methods

17
Peterson Method

• n1 marked and released, 1st time
• n2 Total caught on 2nd occasion
• m2 marked on 2nd occasion
• Presumption The number marked in the second
  sample should represent the proportion of the
  whole population that is marked n1/ N m2/n2
• SO N n1n2/m2
• For small samples N (n11)(n21)/(m21)
• Percent relative precision can also be calculated

18
Schnabel Method

• Based on extrapolating a sample out to complete
  sampling

100
of frogs in sample that had been previously
marked
Total of frogs marked
19
Schnabel Method

• Probably does not work so well if you think you
  will catch a small of the population

100
of frogs in sample that had been previously
marked
Total of frogs marked
20
Triple Catch Method

• Three catches allows an estimate of gains and
  losses.
• N2 M21(n21)/(m211) and
• M21 (m31(r21)/(m321) m21
• Survival s2 M21/r1
• Gains g2 1 ((m31 1)n2)/((n31)m21)
• Definitions
• nx caught on each date
• rx released
• mji caught on day j marked on day i

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Jolly Seber

• Extends these methods. Mark individuals on a
  sequence of days and keep track of when you catch
  them.
• Allows estimation of gains and losses as well as
  whether there are trap happy animals