A scientist is trying to determine the population size of western fence lizards (Sceloporus occidentalis) in the Sycamore Canyon Park in Riverside using the mark-recapture method. During her first sampling event, she caught and marked 50 lizards which she then released back into the population. A week later she caught a total of 45 lizards, 7 of which were marked. What is the estimated population size of the western fence lizard?

The mark-recapture method is an indirect method to estimate population sizes. In this example, the estimated population size of the western fence lizard is N = 321.42 individuals.

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There are different methods to study population density.

There are direct methods and indirect methods.

Among these last ones, we might find the Mark-Recapture technique.

The Mark-Recapture technique assumptions are

A closed population during the sampling season, with no mortality nor natality.

Random distribution of individuals in the population,

All the individuals have the same probability of being sampled.

Marks in the individual last the whole sampling season,

Marks do not affect the individuals or their behavior.

The method consists of

1) Capturing a first sample of individuals belonging to the population under study, in this case, lizard's population.

2) Marking the captured individuals and releasing them again.

3) Taking a second sample where a new capture is done

4) Counting the number of individuals that are already marked. These marked individuals belong to the first sample.

5) Calculating the population size by using the Lincoln Index or any other index.

The Lincolns Index estates that to estimate the total number of individuals in this population, N, the following equation must be considered:

N = (n1 x n2) / m

1) The initial sample has n1 number of individuals

2) The second sample has n2 number of individuals

4) The number of already marked individuals is m

5) The total number of individuals in the population is N

In the exposed example,

n1 = 50 lizards
n2 = 45 lizards
m = 7 lizards

N = (n1 x n2) / m

N = ( 50 x 45 ) / 7

N = 2250 / 7

N = 321.428 ≅ 321.42 individuals

The estimated population size of the western fence lizard is N = 321.42 individuals.

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