In ecology why would sampling contain unreliable data?

Question

Sampling as in taking one portion of an area counting the species and then multiply that number of species by the number of the whole number/amount of the area?

Please help..confused..

Answer 1

whats true for that one area has potential to be not true for other areas. its like taking sample data when trying to answer "how much of the population is overweight?" you can go down one block in a city and see that 80% of the people on that block are overweight, but the rest of the city could be supermodels.  

Answer 2

Anytime your area is not homogenous with respect to biotic and abiotic factors you will likely get unreliable sampling results by extrapolating.  Differences in soil composition, altitude, moisture, etc. will alter species abundances between sampling areas.