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calculation tends to overestimate the population size, especially if the number of recaptures is
small. Assumption 7 is often violated, because it is difficult to generate sufficient recaptures in large
populations. To provide a better estimate, ecologists sometimes use multiple marks and
recaptures and the Schnabel index to estimate population size:
m ÂM C
i N= i i =1 m ÂR i i =1 where M i = the total number of previously marked animals at time i, Ci = the number caught at
time i, and Ri = the number of marked animals caught at time i.
As an example, suppose we caught and marked 100 animals in our first sample, captured 85
animals (15 marked and 70 unmarked) in a second sample, and then captured 105 animals (25
marked and 80 unmarked) in a third sample. We would then have the following:
Table 3.1 Example of Schnabel Index Calculation
i
1
2
3 Ci
100
85
105 Ri
0
15
25 New marked
100
70
 Mi
0
100
170 Note that Mi at time 1 is zero; this happens because we start this sampling with no marked
animals. For each subsequent time period, Mi is simply the sum of all previous values in the “New
marked” column. From the above,...
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 Spring '09
 Sundram
 Ecology, Evolution, population size, Schnabel

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