Describe two prerequisites for determining population size or density (# indiv / area) and potential challenges?

To determine population size one must have an effective way to count individuals in a given area. This can be challenging since individuals move and may be accounted for multiple times, therefore skewing the final results. Population density requires one to also calculate the area in which the population resides. This is relatively simple to do, but if individuals then exit or enter the calculated area, then the population density changes.