Sampling
- counting the number of organisms in a small area and multiplying to estimate the number in a larger area
Mark & Recapture
- capture an organism & release it to remix with population. Then recapture some more in that area to determine population size. Formula is as follows: R (marked recaptures) / C (total in second sample) = M (marked initially) / N (total pop. size)
Indirect Observation -
estimating the population size by tracks, nest or other animal signs in an ecosystem.