How do ecoligists use the technique of sampling to estimate population size?

Answer 1

There are many many ways. One method is called mark-recapture. The simplest method involves taking 2 samples. In the first sample, all the animals that are captured are marked (this can be leg bands, ear tags, toe clipping, or even using photoID in the case of whales/tigers). The size of this sample is called M to denoted that this is now our population of Marked animals. The second sample, which is collected at a later date, will usually (hopefully) contain some of the previously marked animals, and some animals that weren't previously caught. The size of the second sample is denoted n, and the number of marked animals in it is called m.

But we want to know N - the total population size. We can assume that the ratio of marked animals in our second sample (m/n) is the same as the ratio of marked animals in the population (M/N).

Therefore:

M/N=m/n

Rearrange it:

N=(n*M)/m

We know the values for M,n, and m so we can figure out N.

This is the simplest case, and is know as the Lincoln-Peterson estimator. There are many extensions to this that allow for more samples etc.