Hi Ronald,

It depends on the species you are working on and the size of the quadrate. Population estimating methodology varies with different species so please mention the species.

Further, Random sampling itself follows the assumption that every species and individual have equal chances of opportunity to be sampled.

Good Luck!!

You don't mention if it is an animal or plant species you are working on or how easy it is to see. I recommend

Sampling (Hardcover)

by Steven K. Thompson

Wiley

as a good starting point for designing a survey. Are the individuals in the species clustered or habitat specific. This makes a big difference in how you design the survey. Are quadrats necessary or could a distance sampling or transect based  procedure be more appropriate?

I used a distance based method to get population estimates of invasive species seedlings in fynbos many years ago. In all cases it is necessary to recognise that you will need large samples in order to get reliable results (high precision, low bias).  

Certainly, more info on what kind of species. I have a horror story from many years ago of someone wanting to sample a rare snail that he knew was present. To do it quantitatively, he sampled many hundred quadrats placed at random and searched each rather thoroughly. He found a single specimen!

When a species is both very rare and aggregated, it is probably not sensible to try to estimate its density  over a large area. The mean will be meaningless.

Thank You so much Miss Zarreen Syed, Hubertus Jacobus Van Hensbergen and Robert A D Cameron for your answers.... 

well i will be working on endangered plants in eastern Himalayas specially on medicinal plants. any suggestion on flora based study...

Since plant are usually clustered in populations and habitat specific it would be necessary to design a survey method where

1) The detection probability and group size are taken into account and

2) The sampling area is stratified in some way to reflect the habitat preference of different species

The detection probability and group size relationship will mean that there are additional variance components in the analysis based on group size and additional parameters in the estimation model based on the detection function.

Initially it is probably more efficient to conduct belt transects in order to get baseline information on the density, distribution and habitat preferences of the species concerned before designing a more formal survey methodology.

You can use the:

1. Systematic sampling Method coupled with transect based Method.

2. Or the stratified sampling Method.