How do ecologists measure population of animals that are all over the place like rabbits or deer?
They usually use a random sampling technique. Suppose we want to find out how many animals there are in an area of 1000 square Kilometers. We randomly select an area of 250 square K and count the animals in that area. We select other 250 square K area and count again. By doing so we find the average number of animals per 250 square K x 4 and we can determine the total in 1000 square K
This method can be used to sample anything from insects to deer.